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I 117-l
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0 Special Feature RTOG QUALITY ASSURANCE GUIDELINES FOR INTERSTITIAL HYPERTHERMIA B. EMAMI, M.D.,* P. STAUFFER, M.S.E.E.,+M. W. DEWHIRST, D.V.M., PH.D.,$ S. PRIONAS, PH.D.,+ T. RYAN, M.S.,+ P. CORRY, PH.D.,+ T. HERMAN, M.D.,+ D. S. KAPP, PH.D., M.D.,+ R. J. MYERSON, M.D., PH.D.,+ T. SAMULSKI, PH.D.,+ S. SAPARETO, PH.D.,+ M. SAPOZINK, M.D., PH.D.,+ P. SHRIVASTAVA, PH.D.+ AND F. WATERMAN, PH.D.+ This document specifies the current recommendations for quality assurance for hyperthermia administration with interstitial techniques as specified by the Radiation Therapy Oncology Group (RTOG). The document begins by providing a brief description of the physical principles behind the use of the three most commonly used methods of interstitial hyperthermia: radiofrequency (RF-LCF), microwave antennas, and ferromagnetic seeds. Emphasis is placed on features that effect quality assurance. Specific recommendations are provided for: a) Pretreatment planning and equipment performance checks, b) Implant considerations and documentation, c) Thermometry, and d) Safety procedures. Specific details regarding quality assurance issues that are common to all local and regional hyperthermia methods are outlined in previous documents sponsored by the RTOG. It is anticipated that technological advances may lead to future modifications of this document. Interstitial hyperthermia,
Thermometry,
Quality assurance.
INTRODUCTION
microwaves (MW), etc., and makes recommendations in relation to requirements for implantation, thermometry, and documentation.
Experience with hyperthermia in a clinical setting is largely based on the treatment of superficial, accessible tumors with noninvasive external microwave (MW) and ultrasound (US) applicators. Recent studies have shown, however, that there are obstacles to the optimal clinical use of noninvasively induced hyperthermia ( 11, 18). The most significant limiting factors are the inability of the external applicators to achieve uniform temperature, insufficient localization of temperature rise within the target volume, and inadequate depth of penetration to deeper body structures. This often results in critical portions of a tumor not reaching the desired temperatures. Because of these problems, several interstitial hyperthermia techniques have been developed as alternatives for clinical situations which allow invasive implantation of the tumor. This report is a summary of the RTOG Task Force’s current recommendations for quality assurance with interstitial hyperthermia. This document is an extension of the previously published reports relating to equipment performance quality assurance (26), quality assurance associated with treatment documentation (31, 35) and thermometry requirements (10). This document specifically deals with the specifications of various methods of interstitial hyperthermia, that is, radiofrequency (RF),
PHYSICS
OF
INTERSTITIAL
HYPERTHERMIA
Quality assurance for interstitial hyperthermia requires knowledge of the physical principles of each of the methods. Hence, a brief overview of the physics principles is provided. Further details on the physics of these techniques are available in a number of review articles (12, 15, 38, 41, 42). This discussion is intended to provide insight into how to make decisions regarding selection of the most appropriate device for a specific disease presentation as well as selection of appropriate thermometry. Radiofrequency interstitial heating Local Current Field (RF-LCF) heating is normally performed using arrays of needle electrodes connected in pairs to an RF power source. Source frequency is generally in the range of 0.5 to 1 MHz where the impedance of the tissue load is primarily resistive. Heating of the tissue occurs between electrodes in direct proportion to the tissue resistivity and according to the square of current density.
* Chairman, RTOG Hyperthermia Committee. +Member, RTOG QA Task Force.
University School of Medicine, 4939 Audubon, Suite 5500, St. Louis, MO 63 110. Supported by NC1 Grant No. CA2 166 1. Accepted for publication 18 December 1990.
* Chairman, RTOG Hyperthennia QA Task Force. Reprint requests to: Bahman Emami, M.D., Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington 1 I17
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May 1, 1991, Volume 20, Number 5
The specific absorption rate in this system will be affected by operational factors (voltage, frequency), physical characteristics and spacing of the electrodes, and electrical resistivity of the implanted tissue (Table 1). LCF heating utilizes either bare stainless steel electrodes (trocars) or partially insulated needle electrodes. Flexible, partially insulated, electrically conductive afterloading catheters have also been used ( 14, 15), and multiple-element (segmented) electrodes are being developed to provide some control over the longitudinal SAR (power absorption rate) distribution (27). Pretreatment planning of RF electrode placement and electrical connections to the power source(s) is extremely important. Equally crucial to the resulting temperature distribution is the proper placement of temperature monitoring and control probes which are required because the electrode temperatures alone do not give reliable estimates of minimum tumor temperature (39,41). Another critical issue with LCF heating is the necessity of having the implant electrodes parallel to each other. Failure to do so will lead to excessive heating where the electrodes are closest together and relatively little heating where they are farthest apart. Heating does not occur outside the volume of tissue encompassed by the implant. Thus, the implant must extend to the edge, or preferably beyond the edge of the desired target volume.
power at some distance away from the antennas; thus relatively few sources are needed to heat a given region and it is not necessary to implant outside the desired target volume. A potentially useful feature of microwave antennas is the capability of using phase control. A coherently phased system of N antennas deposits N* times more power at the array center than a single antenna (if all antennas are equi-distant from the reference point). In contrast, the same antennas driven incoherently deposit only N times the power at the reference point (42). The simplest antenna consists of a miniature co-axial cable with a discontinuity in the outer conductor and an electrical connection between the inner and outer conductor of the tip section (19). The voltage across the gap between the two sections (antenna junction) produces an electric field, oriented predominantly along the antenna, that radiates out into the surrounding tissue (19, 40, 42, 44,45). There are certain constraints in the proper use of these antennas (Table 2). First, the heating length is determined by the frequency (i.e., 4.5 cm at 9 15 MHz) and cannot be changed (16, 32, 33, 46). This limitation requires careful planning for tumors that are larger or smaller in diameter than the heating length. The second constraint is that the shape and location of the heated region are dependent upon the depth of insertion in tissue (8, 16, 30, 32-34, 38, 45, 46). At 9 15 MHz, for example, insertion depths of less than 8 cm can lead to propagation of the E-field along the coaxial cable, which can cause hot spots where the cable exits the skin. Therefore, there are guidelines relating to the minimum insertion depth of antennas operating at various frequencies (Table 2). An-
Microwave interstitial heating This technique uses a radiative antenna, usually operated between 300 to 2450 MHz (typically 915 MHz). Microwave antennas have the capability of depositing Table 1. Factors which influence
temperature
distributions
for interstitial
hyperthermia
techniques
Method
Factor
RF-LCF
Tissue thermal properties Blood perfusion Spacing of sources Tissue electrical properties Orientation of sources Source construction Operating frequency Probe temperature monitoring and power control locations Implants must extend beyond edge of target volume Unique features
f, ++, +++
electrodes
Microwave antennas
Hot wires
Hot water tubes
+ + +t +
+ + tt +
++ ++ ++
++ ++ ++
++ ++ ++
0
0
0
+++ + + +
++ ++ ++
+ + +
+ +
+ +
-c
0
0 +
0 0
+t
1. Voltage between electrode pairs
= order of increasing
Ferromagnetic seeds
importance;
fS
1. Antenna phase-power relationships 2. Insertion depth
0 = not important.
1. Seed radius 2. Type of alloy 3. Magnetic and electrical properties 4. Self-regulating temperature
++
+t
1. Flow rate 2. Temperature water
of
Quality
Table 2. Technical
assurance
factors regarding
1119
0 B. EMAMI et al.
use of different
microwave
Factor
Dipole
Choked
Heating length dependent on driving frequency Heating along feed line SAR pattern dependent on insertion depth* Poor heating at the tip Methods for phase control are developed
Yes Yes Yes No Yes
Yes Reduced+ Yes No Yes
antenna
Enlarged
Inductively heated ferromagnetic seeds This technique for interstitial hyperthermia is quite different from the previous two in that no power is deposited directly in the tissue. The technique consists of implantation of totally passive devices that can be selectively heated by magnetic coupling into an external magnetic induction field operating at 0.1 to 10 MHz. For interstitial heating the frequency must be chosen so that the power density in the ferromagnetic material is significantly higher than the power density absorbed by the tissue (usually
tip
Yes Yes Yes Some+ Yes
* Minimum insertion depth recommended: 2450 MHz, 2.5 cm; 915 MHz, 7.0 cm; 433 MHz, + Factor is less important than for other designs.
other clinical restraint relates to the region located close to the tip of the antenna. Since the current is zero at the end of the dipole antenna, the electromagnetic fields in that region are weak and the SAR is small (20, 43). This constraint means that the antenna must be inserted beyond the deep margin of the target volume. Sometimes, this cannot be easily accomplished because of anatomic constraints. The above problems are being addressed with recent design improvements including enlarged tips, choke sections, modified dipoles, and helical coil configurations (Table 2) (13, 20, 2 1, 29, 30, 38, 45, 48). Since multiple antennas (implanted in an array) are required to heat a typical tumor volume, the interrelationship between neighboring antennas is also an important consideration. Treatment planning is essential for multiple antenna arrays to determine the proper spacing, orientation, and phase relationship of energized antennas as well as to optimize placement of probes for temperature monitoring and power control. Additional factors are often critical in establishing the actual heating pattern of the array such as the catheter material and the power efficiency of each antenna. Limited treatment planning can be accomplished manually, but computer-based modeling has recently become available (24, 25, 47). It is recommended that such programs be used when possible to assist in the treatment planning process. However, they are intended as a guide and cannot replace invasive thermometry, which is required for all hyperthermia procedures. Note that helical antennas avoid most of the technical constraints associated with dipoles, but they cannot yet be driven in phase to yield predictable peak regions of SAR because of the complex nature of the E-field.
designs
Helical antenna No No No Yes No
12 cm.
50-500 KHz) (2, 27, 36). To control the temperature rise in these seeds, special alloys are used that have Curie point temperature within the range of 49 to 65°C. This is the temperature at which the magnetic properties of the alloy are reduced to zero and the implanted material is no longer preferentially heated (3, 4, 7, 9). This property of the ferromagnetic materials allows them to be thermally selfregulating. Because of the mechanism of tissue heating from thermal conduction hot sources, this modality is especially sensitive to the thermal properties of the tumor (thermal conductivity and blood flow variation within the tumor). These factors will influence the choice of Curie point temperature and seed spacing and usually require implantation outside the target volume in order to assure heating of the tumor edge. Several computer programs which mode1 temperature distributions within arrays of thermal hot sources have been published which could assist in the treatment planning process (5, 6, 15, 22, 23, 24). Other hot source techniques have similar heating patterns to those obtained with ferroseed implants.
SPECIFIC
RECOMMENDATIONS
Equipment performance tests (Table 3) Equipment safety checks and operational guidelines of hyperthermia equipment and thermometry will be performed as per AAPM report #26 (Performance Evaluation of Hyperthermia Equipment) (1). For operational quality assurance procedures in relation to radiotherapy with brachytherapy techniques refer to AAPM report # 13 (30). Additional quality assurance checks specific to the type of interstitial method used are discussed below. RF interstitial techniques. In hyperthermia treatments using RF interstitial techniques, the following tests should be performed in addition to genera1 safety and operational checks: 1) The continuity of the leads from the RF generator to the electrodes should be checked to verify that no open circuits exist. 2) The output of all RF generators should be checked with a matched resistive load and a power meter to make sure that they are transmitting the specified power levels.
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0 Biology 0 Physics
Table 3. Specific equipment
performance
RF - LCF
checks* to be performed
Microwave
1. Lead continuity
from generator to electrodes 2. Verify output power of rfgenerator(s) 3. Verify parallel implant and exclude crossed needles from heating
* These tests are in addition calibrations, etc. (I, 33).
-
-
-
-
SAR CHECK
-
5mm 1 I
:
TEMPERATURE
PROBE
px
X
3r
X
X
5
prior to a therapy
session
Ferromagnetic
seeds
1. Choose appropriate coil configuration for intended implant seed orientation 2. Determine if thermometry is needed in regions of expected soft tissue high field
to other QA checks which should be done for all hyperthermia
APPLICATOR
MWANTENNA
20. Number
antennas
Check cables for connectors for power loss/or reflected power Check SAR pattern of antennas at intended insertion depth using catheters For antenna arrays, choose antennas that have similar SAR patterns 4. Choose antenna configurations which match tumor dimensions
Interstitial microwave antennas. Before each treatment, coaxial power cables and passive microwave connecting hardware should be checked for power loss or reflected power. Readings should be compared to when the equipment was new. Documentation of measured forward and reflected power to the antennas should be corrected for any measured line losses. All microwave antennas should be checked at the intended insertion depth using appropriate antenna/catheter combinations by any one of the following methods: (a) In a clear liquid phantom with liquid crystals to display color change with temperature or, preferably (b) in a phantom with nonperturbing temperature sensors at the point of maximum SAR and at 1 cm intervals along the length of the antenna. Power at approximately I5 Watts per antenna should be turned on
MICROWAVE
May 1, 199 1. Volume
treatments
such as the thermometry
for 30 seconds and the heating rate measured at each sensor location. The heating rates may be plotted as shown in Figure I, which provides an SAR check as a verification of the approximate heating pattern of the antenna. For predictable performance inside antenna arrays, antennas should be selected with similar heating efficiencies and heating patterns which match each other as well as the tumor dimensions. Since the catheter that is used to house the antenna frequently affects the power deposition, it is necessary that testing of the antennas be done inside the catheters intended to be used in a given clinical situation. It is important to test each antenna on a regular basis because heating patterns can change as a result of routine handling or from damage. If the goal is to operate the antennas coherently, the use of individual attenuators or tuners will change the phase between antennas and the array may not function as predicted ( 17). Also, it is important to ensure that equal length phased matched cables and identical cable connections (in terms of type) are used, or coherent phase operation will not be possible. Ferromagnetic seeds. The magnetic field strength distribution inside and outside of the heating coil should be characterized. The operator should ensure that the coil can produce a sufficient magnetic field in the region of the implant without exceeding tolerable limits in the surrounding normal tissues. Any tissue regions remote from the implant site that are likely to be in the zone of high field strengths should be documented. Depending on the field frequency, implant permeability, and relative diameters of the ferroseeds and implanted tissue region, it
X
Table 4. Minimum
-2
-4
-3
DISTANCE
ALOND ANTENNA
-1 (cm
0
)
Fig. 1. Microwave applicator SAR check forms to be used for periodic QA testing of each interstitial antenna. The depth of insertion and catheter material used should be documented on such forms.
No. of heat sources 3-8 9-16 17-32 >32
Implant center 1 I 1 1
number of thermometry Center of 4 heat sources 0
probes Periphery of array
1
1 1
2 3
2
1
1121
Quality assurance 0 B. EMAMI et al. Table 5. Source and thermometry RF-LCF
Factor
implant considerations
MW-antennas
Ferroseeds
Hot water or hot wire
1. Tumor coverage 2. Heat source spacing 3. Thermometry probes (see Table 4) 4. Heat source’orientation
~_____________________________ Asforroutinebrachytherapy ______________________________t lo-15 mm 12-20 mm lo-13 mm lo-13 mm + _______ ________ ________ ___------ 2-6 additional catheters ~____----_____----______--_____~ Parallel
Parallel
5. Other
1. Pairs same length and depth 2. Insulate normal tissues
1. Proper antenna length for tumor 2. Coherent dipoles same depth
is advisable to consider inserting a subcutaneous temperature probe to ensure that there is no hazardous direct tissue power absorption when the field and normal tissue exceeds approximately 10 times the field around the ferroseeds (36, 37).
Pretreatment planning and implantation Implant strategies. It is necessary to determine
the tumor volume and its relation to adjacent normal structures by physical examination and radiological studies in order to preplan the various parameters prior to the implant procedure. Pretreatment verification of tumor extent requires CT. Verification of the tumor extent once the implant has been performed, could use conventional orthogonal radiographs with the tumor margins and other important anatomic details outlined based on the previously acquired CT scans. During the implant procedure the determination of the tumor volume, that is covered by the implant, should be carried out by the operating physician in consultation with the hyperthermia and radiation physicists. At no time should the implant be done in such a way as to compromise the best geometry for brachytherapy. Specific considerations regarding implant strategies for the various methods are shown in Tables 4 and 5.
Thermometry The types of thermometers that are acceptable for the three different heating methods are shown in Table 6. To sample the 3-dimensional temperature distribution within the target volume, placement of one or more thermometry
*30° of magnetic field
Parallel
catheters in the center of the implant, the periphery of the implant, and the center of each independently controlled applicator group is recommended. The minimum required number of thermometry probes for monitoring tissue temperature will be determined by the implant size (Table 4 and Fig. 2). If clinically possible, placement of a perpendicular thermometry catheter at the central plane of the tumor midway between two rows of electrodes and an additional perpendicular catheter immediately adjacent to one row of the electrodes is recommended (Fig. 2). After completion of the implant, a set of AP and lateral films should be obtained to document the geometry of the implant. Dummy seeds should be placed inside the thermometry catheters for this procedure. If there are areas of potential hot or cold spots, appropriate corrective measures shall be taken such as insertion of additional electrodes or thermometry catheters. On both of these films, the target volume and all hyperthermia electrodes will be identified with the target volume being confirmed from CT scans taken previously. On every electrode and thermometry catheter the intratumoral and normal tissue sections shall be marked. When applicable, the location of control sensors for each electrode shall be also identified. This information shall be transferred to electrode and thermometry schema forms. The schema forms will succinctly summarize the following: 1. Cross sectional geometry indicating implant spacing; 2. Source connections to power; 3. Thermometry probe placement in the implant crosssection;
Table 6. Choice of thermometers RF-LCF electrodes 1. Fiber optic probes 2. High resistance lead thermistors 3. RF shielded thermocouples-inside electrodes onlv
Microwave antennas 1. Fiber optic probes 2. High resistance lead thermistors
Note: For thermal mapping, plastic catheters are recommended, field perturbation.
Ferromagnetic
seeds
1. Fiber optic probes 2. High resistance lead thermistors 3. RF shielded thermocouples
as opposed to metal, to reduce thermal smearing artifacts and E
I. J. Radiation Oncology
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May 1, 1991, Volume 20, Number
5
ping will be required. Each sensor location should be measured at least every 15 minutes during the treatment session. Spacing between map points along any catheter shall be every 5 mm for tumors less than 5 cm or every 10 mm for tumors greater than 5 cm in diameter. In concordance with the previously published guidelines, the portion of catheters which pass through normal tissues and into tumor shall be documented. Following power off, continuous temperature recording will be carried out for 3 to 5 minutes with probes and positions of maximum temperature but not closer than 5 mm to any heat source. If a second heating session with the same implant is planned, a second set of orthogonal x-ray films should be taken to demonstrate whether any changes in geometry have occurred. If changes are noted, corrective measures may be required.
Electromagnetic hazards
RECOMMENDED
THERMOMETRY
LOCATIONS
FOR A TWO PLANE
IMPLANT
x0x0x0x
0
0
x
0
0
0
x
0
x
0
0
x
Fig. 2(A). Gross sectional diagram of a 16 catheter implant array. Potential sites for the additional thermometry probe catheters (minimum number specified in Table 4) are identified. (B) 3 plane implant: non-rectangular geometry implants are often used
to conform to the tumor volume. These irregular geometry arrays can easily be accommodated with little effect on recommended probe positions. (C) 2 plane implant.
4. Location of any stationary and/or control sensors in 3-dimensional space; 5. Tumor boundaries relative to the implant, active electrodes, and thermometry sensors.
For microwave techniques measurements with an RF hazard meter should be conducted during each new treatment setup to ensure that the operator and critical tissues of the patient such as the eyes and gonads are below the 5 mW/cm’ exposure limit (6 minute average exposure) as measured 15 cm from any chassis or exposed metal conductors. When ferroseed techniques are used, leakage fields in the vicinity of the patient and the operator should be monitored (magnetic and electric field components separatel y). RF exposure standards for 100 KHz range from 50 to 1500 V/m (electric field) and 0.5 to 75 A/m (magnetic field) for 24-hour exposure. The IRPA (International Radiation Protection Association) guidelines for g-hour occupational exposure are 194 V/m and 0.5 1 A/m (28). For large volume treatments with large open induction coils, treatment room personnel should avoid unnecessary patient contact and wear examination gloves to block unintended current paths from patient skin to ground. For some coil designs, Faraday shielding of treatment coils may be necessary to minimize undesired electric field coupling to the patient (37).
Treatment documentation In addition to patient and radiotherapy data, specific information on hyperthermia treatments shall be documented. The type of data which should be recorded has been previously discussed in prior publications (10, 3 1). Specific data required for the interstitial method of heating which is separate from the previously discussed information is shown in Table 7.
SUMMARY
QA procedures during treatment Temperature monitoring shall be done as defined previously (10). Briefly, multisensor probes or thermal map-
This communication summarizes the basic quality assurance guidelines for the most commonly used interstitial
1123
Quality assurance 0 B. EMAMI et al. Table 7. Treatment Common
documentation
documentation* for all types of implants
1. Cross sectional implant schema showing: a) Heat source spacing and tumor boundaries b) Temperature monitoring probe positions 2. Radiotherapy isodose plots at multiple levels with tumor/target volumes identified 3. Orthogonal x-ray films with heat sources, thermometry probes and tumor borders identified Unique RF-LCF
Microwave
electrodes
I. Types used 2. Active vs. insulated
lengths 3. Accounting of electrode connections to generator(s) 4. Location of control sensors, relative to each electrode pair
* This table lists unique
hyperthermia requirements hyperthermia
features,
documentation Ferromagnetic
antennas
1. Alloy type and seed dimensions 2. Curie point temperature 3. Magnetic field strength 4. Ferroseed length in tumor
1. Type 2. SAR pattern at intended insertion depth 3. Insertion depth 4. Length in tumor 5. Grouping of antennas to power channels 6. Location of control sensors for each power channel not already discussed
in previous
techniques. As indicated, the thermometry are the minimum needed for an adequate treatment. It is anticipated that with con-
seeds
documents
Hot water or hot wires 1. Type 2. Active vs. insulated length 3. Electrical or water connections 4. Water or source temperature
(9, 29, 33).
tinued technological progress and methods for interstitial hyperthermia, be updated in the future.
availability of new these guidelines will
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33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
crowave interstitial antennas for local hyperthermia. Int. J. Radiat. Oncol. Biol. Phys. (In press) 1990. Satoh, T.; Stauffer, P. R.; Fike, J. R. Thermal dosimetry studies of helical coil microwave antennas for interstitial hyperthermia. Int. J. Radiat. Oncol. Biol. Phys. 15: 12091218; 1988. Satoh, T.; Stauffer, P. R. Implantable helical coil microwave antenna for interstitial hyperthermia. Int. J. Hyperther. 4: 497-512; 1988. Shrivastava, P.; Luk, K.; Oleson, J.; Dewhirst, M.; Pajak, T.; Paliwal, B.; Perez, C.; Sapareto, S.: Saylor, T.; Sleeves, R. Hyperthermia quality assurance guidelines. Int. J. Radiat. Oncol. Biol. Phys. 16:57 I-587; 1989. Stauffer, P. R.; Fletcher, A. M.; DeYoung, D. W.; Dewhirst. M. W.; Oleson, J. R.; Cetas, T. C. Observations of the use of ferromagnetic implants for inducing hyperthermia. IEEE Trans. Biomed. Eng. BME-3 1:76-90; 1984. Stauffer, P. R.; Cetas, T. C.; Jones, R. C. Magnetic induction heating of ferromagnetic implants for inducing localized hyperthermia in deep seated tumors. IEEE Trans. Biomed. Eng. BME-31:235-25 1; 1984. Stauffer, P. R. Techniques for interstitial hyperthermia. In: Field, S. B.; Hand, J., eds. Introduction to the Practical Aspects of Clinical Hyperthermia. London and Philadelphia: Taylor and Francis; 1990:344-370. Strohbehn, J. W. Temperature distributions from interstitial RF electrode hyperthermia systems: Theoretical predictions. Int. J. Radiat. Oncol. Biol. Phys. 9:1655-1667; 1983. Strohbehn, J. W.; Mechling, J. A. Interstitial techniques for clinical hyperthermia. In: Hand, J. W.; James, J. R., eds. Physical techniques in clinical hyperthermia. New York, NY: John Wiley and Sons; 1986. Strohbehn, J. W. Interstitial techniques for hyperthermia. In: Field, S. B.; Franconi, C., eds. Physics and technology of hyperthermia. Boston, MA: Martinus Nijhoff Publishers; 1987. Trembly, B. S. The effects of driving frequency and antenna length on power deposition within a microwave antenna array for hyperthermia. IEEE Trans. Biomed. Eng. BME32:152-157; 1985. Trembly, B. S.; Wilson, A. M.; Sullivan, M. J.; Stein, A. D.; Wong, T. Z.; Strohbehn, J. W. Control of the SAR pattern within an interstitial microwave array through variation of antenna driving phase. IEEE Trans. Microwave Theory Technique MTT-34:568-57 1; 1986. Trembly, B. S.; Ryan, T. P.; Strohbehn, J. W. Physics of interstitial hyperthermia-microwave. In: Urano, M., Douple, E., eds. Hyperthermia and oncology, interstitial hyperthermia, Vol. 3. Utrecht, The Netherlands: VSP Press. (In press) 1991. Turner, P. F. Interstitial EM applicator/temperature probes. Proceedings of the 8th Annual Conference of the IEEE Engineering in Medicine and Biology Society, Vol. 3; 1986: 1454-1457. Visser, A. G.; Deurloo, I. K. K.; Levendag, P. C.; Tuifrok, A. C. C.; Cornet, B.; Van Rhoon, G. C. An interstitial hyperthermia system at 27 MHz. Int. J. Hyperther. 5:265276; 1989. Wong, T. Z.; Mechling, J. A.; Jones, E. L.; Strohbehn, J. W. Transient finite element analysis of thermal methods used to estimate SAR and blood how in homogeneously and inhomogeneously perfused tumor models. Int. J. Hyperther. 4:571-592; 1988. Wu, A.; Watson, M. L.; Sternick, E. S.; Bielawa, R. J.; Carr, K. C. Performance characteristics of a helical microwave interstitial antenna for local hyperthermia. Med. Phys. 14: 235-237; 1987.
20, pp.
I 117-l
124
0360.3016/91 $3.00 + .oO Copyright 0 I99 Pergamon Press plc
I
0 Special Feature RTOG QUALITY ASSURANCE GUIDELINES FOR INTERSTITIAL HYPERTHERMIA B. EMAMI, M.D.,* P. STAUFFER, M.S.E.E.,+M. W. DEWHIRST, D.V.M., PH.D.,$ S. PRIONAS, PH.D.,+ T. RYAN, M.S.,+ P. CORRY, PH.D.,+ T. HERMAN, M.D.,+ D. S. KAPP, PH.D., M.D.,+ R. J. MYERSON, M.D., PH.D.,+ T. SAMULSKI, PH.D.,+ S. SAPARETO, PH.D.,+ M. SAPOZINK, M.D., PH.D.,+ P. SHRIVASTAVA, PH.D.+ AND F. WATERMAN, PH.D.+ This document specifies the current recommendations for quality assurance for hyperthermia administration with interstitial techniques as specified by the Radiation Therapy Oncology Group (RTOG). The document begins by providing a brief description of the physical principles behind the use of the three most commonly used methods of interstitial hyperthermia: radiofrequency (RF-LCF), microwave antennas, and ferromagnetic seeds. Emphasis is placed on features that effect quality assurance. Specific recommendations are provided for: a) Pretreatment planning and equipment performance checks, b) Implant considerations and documentation, c) Thermometry, and d) Safety procedures. Specific details regarding quality assurance issues that are common to all local and regional hyperthermia methods are outlined in previous documents sponsored by the RTOG. It is anticipated that technological advances may lead to future modifications of this document. Interstitial hyperthermia,
Thermometry,
Quality assurance.
INTRODUCTION
microwaves (MW), etc., and makes recommendations in relation to requirements for implantation, thermometry, and documentation.
Experience with hyperthermia in a clinical setting is largely based on the treatment of superficial, accessible tumors with noninvasive external microwave (MW) and ultrasound (US) applicators. Recent studies have shown, however, that there are obstacles to the optimal clinical use of noninvasively induced hyperthermia ( 11, 18). The most significant limiting factors are the inability of the external applicators to achieve uniform temperature, insufficient localization of temperature rise within the target volume, and inadequate depth of penetration to deeper body structures. This often results in critical portions of a tumor not reaching the desired temperatures. Because of these problems, several interstitial hyperthermia techniques have been developed as alternatives for clinical situations which allow invasive implantation of the tumor. This report is a summary of the RTOG Task Force’s current recommendations for quality assurance with interstitial hyperthermia. This document is an extension of the previously published reports relating to equipment performance quality assurance (26), quality assurance associated with treatment documentation (31, 35) and thermometry requirements (10). This document specifically deals with the specifications of various methods of interstitial hyperthermia, that is, radiofrequency (RF),
PHYSICS
OF
INTERSTITIAL
HYPERTHERMIA
Quality assurance for interstitial hyperthermia requires knowledge of the physical principles of each of the methods. Hence, a brief overview of the physics principles is provided. Further details on the physics of these techniques are available in a number of review articles (12, 15, 38, 41, 42). This discussion is intended to provide insight into how to make decisions regarding selection of the most appropriate device for a specific disease presentation as well as selection of appropriate thermometry. Radiofrequency interstitial heating Local Current Field (RF-LCF) heating is normally performed using arrays of needle electrodes connected in pairs to an RF power source. Source frequency is generally in the range of 0.5 to 1 MHz where the impedance of the tissue load is primarily resistive. Heating of the tissue occurs between electrodes in direct proportion to the tissue resistivity and according to the square of current density.
* Chairman, RTOG Hyperthermia Committee. +Member, RTOG QA Task Force.
University School of Medicine, 4939 Audubon, Suite 5500, St. Louis, MO 63 110. Supported by NC1 Grant No. CA2 166 1. Accepted for publication 18 December 1990.
* Chairman, RTOG Hyperthennia QA Task Force. Reprint requests to: Bahman Emami, M.D., Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington 1 I17
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The specific absorption rate in this system will be affected by operational factors (voltage, frequency), physical characteristics and spacing of the electrodes, and electrical resistivity of the implanted tissue (Table 1). LCF heating utilizes either bare stainless steel electrodes (trocars) or partially insulated needle electrodes. Flexible, partially insulated, electrically conductive afterloading catheters have also been used ( 14, 15), and multiple-element (segmented) electrodes are being developed to provide some control over the longitudinal SAR (power absorption rate) distribution (27). Pretreatment planning of RF electrode placement and electrical connections to the power source(s) is extremely important. Equally crucial to the resulting temperature distribution is the proper placement of temperature monitoring and control probes which are required because the electrode temperatures alone do not give reliable estimates of minimum tumor temperature (39,41). Another critical issue with LCF heating is the necessity of having the implant electrodes parallel to each other. Failure to do so will lead to excessive heating where the electrodes are closest together and relatively little heating where they are farthest apart. Heating does not occur outside the volume of tissue encompassed by the implant. Thus, the implant must extend to the edge, or preferably beyond the edge of the desired target volume.
power at some distance away from the antennas; thus relatively few sources are needed to heat a given region and it is not necessary to implant outside the desired target volume. A potentially useful feature of microwave antennas is the capability of using phase control. A coherently phased system of N antennas deposits N* times more power at the array center than a single antenna (if all antennas are equi-distant from the reference point). In contrast, the same antennas driven incoherently deposit only N times the power at the reference point (42). The simplest antenna consists of a miniature co-axial cable with a discontinuity in the outer conductor and an electrical connection between the inner and outer conductor of the tip section (19). The voltage across the gap between the two sections (antenna junction) produces an electric field, oriented predominantly along the antenna, that radiates out into the surrounding tissue (19, 40, 42, 44,45). There are certain constraints in the proper use of these antennas (Table 2). First, the heating length is determined by the frequency (i.e., 4.5 cm at 9 15 MHz) and cannot be changed (16, 32, 33, 46). This limitation requires careful planning for tumors that are larger or smaller in diameter than the heating length. The second constraint is that the shape and location of the heated region are dependent upon the depth of insertion in tissue (8, 16, 30, 32-34, 38, 45, 46). At 9 15 MHz, for example, insertion depths of less than 8 cm can lead to propagation of the E-field along the coaxial cable, which can cause hot spots where the cable exits the skin. Therefore, there are guidelines relating to the minimum insertion depth of antennas operating at various frequencies (Table 2). An-
Microwave interstitial heating This technique uses a radiative antenna, usually operated between 300 to 2450 MHz (typically 915 MHz). Microwave antennas have the capability of depositing Table 1. Factors which influence
temperature
distributions
for interstitial
hyperthermia
techniques
Method
Factor
RF-LCF
Tissue thermal properties Blood perfusion Spacing of sources Tissue electrical properties Orientation of sources Source construction Operating frequency Probe temperature monitoring and power control locations Implants must extend beyond edge of target volume Unique features
f, ++, +++
electrodes
Microwave antennas
Hot wires
Hot water tubes
+ + +t +
+ + tt +
++ ++ ++
++ ++ ++
++ ++ ++
0
0
0
+++ + + +
++ ++ ++
+ + +
+ +
+ +
-c
0
0 +
0 0
+t
1. Voltage between electrode pairs
= order of increasing
Ferromagnetic seeds
importance;
fS
1. Antenna phase-power relationships 2. Insertion depth
0 = not important.
1. Seed radius 2. Type of alloy 3. Magnetic and electrical properties 4. Self-regulating temperature
++
+t
1. Flow rate 2. Temperature water
of
Quality
Table 2. Technical
assurance
factors regarding
1119
0 B. EMAMI et al.
use of different
microwave
Factor
Dipole
Choked
Heating length dependent on driving frequency Heating along feed line SAR pattern dependent on insertion depth* Poor heating at the tip Methods for phase control are developed
Yes Yes Yes No Yes
Yes Reduced+ Yes No Yes
antenna
Enlarged
Inductively heated ferromagnetic seeds This technique for interstitial hyperthermia is quite different from the previous two in that no power is deposited directly in the tissue. The technique consists of implantation of totally passive devices that can be selectively heated by magnetic coupling into an external magnetic induction field operating at 0.1 to 10 MHz. For interstitial heating the frequency must be chosen so that the power density in the ferromagnetic material is significantly higher than the power density absorbed by the tissue (usually
tip
Yes Yes Yes Some+ Yes
* Minimum insertion depth recommended: 2450 MHz, 2.5 cm; 915 MHz, 7.0 cm; 433 MHz, + Factor is less important than for other designs.
other clinical restraint relates to the region located close to the tip of the antenna. Since the current is zero at the end of the dipole antenna, the electromagnetic fields in that region are weak and the SAR is small (20, 43). This constraint means that the antenna must be inserted beyond the deep margin of the target volume. Sometimes, this cannot be easily accomplished because of anatomic constraints. The above problems are being addressed with recent design improvements including enlarged tips, choke sections, modified dipoles, and helical coil configurations (Table 2) (13, 20, 2 1, 29, 30, 38, 45, 48). Since multiple antennas (implanted in an array) are required to heat a typical tumor volume, the interrelationship between neighboring antennas is also an important consideration. Treatment planning is essential for multiple antenna arrays to determine the proper spacing, orientation, and phase relationship of energized antennas as well as to optimize placement of probes for temperature monitoring and power control. Additional factors are often critical in establishing the actual heating pattern of the array such as the catheter material and the power efficiency of each antenna. Limited treatment planning can be accomplished manually, but computer-based modeling has recently become available (24, 25, 47). It is recommended that such programs be used when possible to assist in the treatment planning process. However, they are intended as a guide and cannot replace invasive thermometry, which is required for all hyperthermia procedures. Note that helical antennas avoid most of the technical constraints associated with dipoles, but they cannot yet be driven in phase to yield predictable peak regions of SAR because of the complex nature of the E-field.
designs
Helical antenna No No No Yes No
12 cm.
50-500 KHz) (2, 27, 36). To control the temperature rise in these seeds, special alloys are used that have Curie point temperature within the range of 49 to 65°C. This is the temperature at which the magnetic properties of the alloy are reduced to zero and the implanted material is no longer preferentially heated (3, 4, 7, 9). This property of the ferromagnetic materials allows them to be thermally selfregulating. Because of the mechanism of tissue heating from thermal conduction hot sources, this modality is especially sensitive to the thermal properties of the tumor (thermal conductivity and blood flow variation within the tumor). These factors will influence the choice of Curie point temperature and seed spacing and usually require implantation outside the target volume in order to assure heating of the tumor edge. Several computer programs which mode1 temperature distributions within arrays of thermal hot sources have been published which could assist in the treatment planning process (5, 6, 15, 22, 23, 24). Other hot source techniques have similar heating patterns to those obtained with ferroseed implants.
SPECIFIC
RECOMMENDATIONS
Equipment performance tests (Table 3) Equipment safety checks and operational guidelines of hyperthermia equipment and thermometry will be performed as per AAPM report #26 (Performance Evaluation of Hyperthermia Equipment) (1). For operational quality assurance procedures in relation to radiotherapy with brachytherapy techniques refer to AAPM report # 13 (30). Additional quality assurance checks specific to the type of interstitial method used are discussed below. RF interstitial techniques. In hyperthermia treatments using RF interstitial techniques, the following tests should be performed in addition to genera1 safety and operational checks: 1) The continuity of the leads from the RF generator to the electrodes should be checked to verify that no open circuits exist. 2) The output of all RF generators should be checked with a matched resistive load and a power meter to make sure that they are transmitting the specified power levels.
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0 Biology 0 Physics
Table 3. Specific equipment
performance
RF - LCF
checks* to be performed
Microwave
1. Lead continuity
from generator to electrodes 2. Verify output power of rfgenerator(s) 3. Verify parallel implant and exclude crossed needles from heating
* These tests are in addition calibrations, etc. (I, 33).
-
-
-
-
SAR CHECK
-
5mm 1 I
:
TEMPERATURE
PROBE
px
X
3r
X
X
5
prior to a therapy
session
Ferromagnetic
seeds
1. Choose appropriate coil configuration for intended implant seed orientation 2. Determine if thermometry is needed in regions of expected soft tissue high field
to other QA checks which should be done for all hyperthermia
APPLICATOR
MWANTENNA
20. Number
antennas
Check cables for connectors for power loss/or reflected power Check SAR pattern of antennas at intended insertion depth using catheters For antenna arrays, choose antennas that have similar SAR patterns 4. Choose antenna configurations which match tumor dimensions
Interstitial microwave antennas. Before each treatment, coaxial power cables and passive microwave connecting hardware should be checked for power loss or reflected power. Readings should be compared to when the equipment was new. Documentation of measured forward and reflected power to the antennas should be corrected for any measured line losses. All microwave antennas should be checked at the intended insertion depth using appropriate antenna/catheter combinations by any one of the following methods: (a) In a clear liquid phantom with liquid crystals to display color change with temperature or, preferably (b) in a phantom with nonperturbing temperature sensors at the point of maximum SAR and at 1 cm intervals along the length of the antenna. Power at approximately I5 Watts per antenna should be turned on
MICROWAVE
May 1, 199 1. Volume
treatments
such as the thermometry
for 30 seconds and the heating rate measured at each sensor location. The heating rates may be plotted as shown in Figure I, which provides an SAR check as a verification of the approximate heating pattern of the antenna. For predictable performance inside antenna arrays, antennas should be selected with similar heating efficiencies and heating patterns which match each other as well as the tumor dimensions. Since the catheter that is used to house the antenna frequently affects the power deposition, it is necessary that testing of the antennas be done inside the catheters intended to be used in a given clinical situation. It is important to test each antenna on a regular basis because heating patterns can change as a result of routine handling or from damage. If the goal is to operate the antennas coherently, the use of individual attenuators or tuners will change the phase between antennas and the array may not function as predicted ( 17). Also, it is important to ensure that equal length phased matched cables and identical cable connections (in terms of type) are used, or coherent phase operation will not be possible. Ferromagnetic seeds. The magnetic field strength distribution inside and outside of the heating coil should be characterized. The operator should ensure that the coil can produce a sufficient magnetic field in the region of the implant without exceeding tolerable limits in the surrounding normal tissues. Any tissue regions remote from the implant site that are likely to be in the zone of high field strengths should be documented. Depending on the field frequency, implant permeability, and relative diameters of the ferroseeds and implanted tissue region, it
X
Table 4. Minimum
-2
-4
-3
DISTANCE
ALOND ANTENNA
-1 (cm
0
)
Fig. 1. Microwave applicator SAR check forms to be used for periodic QA testing of each interstitial antenna. The depth of insertion and catheter material used should be documented on such forms.
No. of heat sources 3-8 9-16 17-32 >32
Implant center 1 I 1 1
number of thermometry Center of 4 heat sources 0
probes Periphery of array
1
1 1
2 3
2
1
1121
Quality assurance 0 B. EMAMI et al. Table 5. Source and thermometry RF-LCF
Factor
implant considerations
MW-antennas
Ferroseeds
Hot water or hot wire
1. Tumor coverage 2. Heat source spacing 3. Thermometry probes (see Table 4) 4. Heat source’orientation
~_____________________________ Asforroutinebrachytherapy ______________________________t lo-15 mm 12-20 mm lo-13 mm lo-13 mm + _______ ________ ________ ___------ 2-6 additional catheters ~____----_____----______--_____~ Parallel
Parallel
5. Other
1. Pairs same length and depth 2. Insulate normal tissues
1. Proper antenna length for tumor 2. Coherent dipoles same depth
is advisable to consider inserting a subcutaneous temperature probe to ensure that there is no hazardous direct tissue power absorption when the field and normal tissue exceeds approximately 10 times the field around the ferroseeds (36, 37).
Pretreatment planning and implantation Implant strategies. It is necessary to determine
the tumor volume and its relation to adjacent normal structures by physical examination and radiological studies in order to preplan the various parameters prior to the implant procedure. Pretreatment verification of tumor extent requires CT. Verification of the tumor extent once the implant has been performed, could use conventional orthogonal radiographs with the tumor margins and other important anatomic details outlined based on the previously acquired CT scans. During the implant procedure the determination of the tumor volume, that is covered by the implant, should be carried out by the operating physician in consultation with the hyperthermia and radiation physicists. At no time should the implant be done in such a way as to compromise the best geometry for brachytherapy. Specific considerations regarding implant strategies for the various methods are shown in Tables 4 and 5.
Thermometry The types of thermometers that are acceptable for the three different heating methods are shown in Table 6. To sample the 3-dimensional temperature distribution within the target volume, placement of one or more thermometry
*30° of magnetic field
Parallel
catheters in the center of the implant, the periphery of the implant, and the center of each independently controlled applicator group is recommended. The minimum required number of thermometry probes for monitoring tissue temperature will be determined by the implant size (Table 4 and Fig. 2). If clinically possible, placement of a perpendicular thermometry catheter at the central plane of the tumor midway between two rows of electrodes and an additional perpendicular catheter immediately adjacent to one row of the electrodes is recommended (Fig. 2). After completion of the implant, a set of AP and lateral films should be obtained to document the geometry of the implant. Dummy seeds should be placed inside the thermometry catheters for this procedure. If there are areas of potential hot or cold spots, appropriate corrective measures shall be taken such as insertion of additional electrodes or thermometry catheters. On both of these films, the target volume and all hyperthermia electrodes will be identified with the target volume being confirmed from CT scans taken previously. On every electrode and thermometry catheter the intratumoral and normal tissue sections shall be marked. When applicable, the location of control sensors for each electrode shall be also identified. This information shall be transferred to electrode and thermometry schema forms. The schema forms will succinctly summarize the following: 1. Cross sectional geometry indicating implant spacing; 2. Source connections to power; 3. Thermometry probe placement in the implant crosssection;
Table 6. Choice of thermometers RF-LCF electrodes 1. Fiber optic probes 2. High resistance lead thermistors 3. RF shielded thermocouples-inside electrodes onlv
Microwave antennas 1. Fiber optic probes 2. High resistance lead thermistors
Note: For thermal mapping, plastic catheters are recommended, field perturbation.
Ferromagnetic
seeds
1. Fiber optic probes 2. High resistance lead thermistors 3. RF shielded thermocouples
as opposed to metal, to reduce thermal smearing artifacts and E
I. J. Radiation Oncology
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May 1, 1991, Volume 20, Number
5
ping will be required. Each sensor location should be measured at least every 15 minutes during the treatment session. Spacing between map points along any catheter shall be every 5 mm for tumors less than 5 cm or every 10 mm for tumors greater than 5 cm in diameter. In concordance with the previously published guidelines, the portion of catheters which pass through normal tissues and into tumor shall be documented. Following power off, continuous temperature recording will be carried out for 3 to 5 minutes with probes and positions of maximum temperature but not closer than 5 mm to any heat source. If a second heating session with the same implant is planned, a second set of orthogonal x-ray films should be taken to demonstrate whether any changes in geometry have occurred. If changes are noted, corrective measures may be required.
Electromagnetic hazards
RECOMMENDED
THERMOMETRY
LOCATIONS
FOR A TWO PLANE
IMPLANT
x0x0x0x
0
0
x
0
0
0
x
0
x
0
0
x
Fig. 2(A). Gross sectional diagram of a 16 catheter implant array. Potential sites for the additional thermometry probe catheters (minimum number specified in Table 4) are identified. (B) 3 plane implant: non-rectangular geometry implants are often used
to conform to the tumor volume. These irregular geometry arrays can easily be accommodated with little effect on recommended probe positions. (C) 2 plane implant.
4. Location of any stationary and/or control sensors in 3-dimensional space; 5. Tumor boundaries relative to the implant, active electrodes, and thermometry sensors.
For microwave techniques measurements with an RF hazard meter should be conducted during each new treatment setup to ensure that the operator and critical tissues of the patient such as the eyes and gonads are below the 5 mW/cm’ exposure limit (6 minute average exposure) as measured 15 cm from any chassis or exposed metal conductors. When ferroseed techniques are used, leakage fields in the vicinity of the patient and the operator should be monitored (magnetic and electric field components separatel y). RF exposure standards for 100 KHz range from 50 to 1500 V/m (electric field) and 0.5 to 75 A/m (magnetic field) for 24-hour exposure. The IRPA (International Radiation Protection Association) guidelines for g-hour occupational exposure are 194 V/m and 0.5 1 A/m (28). For large volume treatments with large open induction coils, treatment room personnel should avoid unnecessary patient contact and wear examination gloves to block unintended current paths from patient skin to ground. For some coil designs, Faraday shielding of treatment coils may be necessary to minimize undesired electric field coupling to the patient (37).
Treatment documentation In addition to patient and radiotherapy data, specific information on hyperthermia treatments shall be documented. The type of data which should be recorded has been previously discussed in prior publications (10, 3 1). Specific data required for the interstitial method of heating which is separate from the previously discussed information is shown in Table 7.
SUMMARY
QA procedures during treatment Temperature monitoring shall be done as defined previously (10). Briefly, multisensor probes or thermal map-
This communication summarizes the basic quality assurance guidelines for the most commonly used interstitial
1123
Quality assurance 0 B. EMAMI et al. Table 7. Treatment Common
documentation
documentation* for all types of implants
1. Cross sectional implant schema showing: a) Heat source spacing and tumor boundaries b) Temperature monitoring probe positions 2. Radiotherapy isodose plots at multiple levels with tumor/target volumes identified 3. Orthogonal x-ray films with heat sources, thermometry probes and tumor borders identified Unique RF-LCF
Microwave
electrodes
I. Types used 2. Active vs. insulated
lengths 3. Accounting of electrode connections to generator(s) 4. Location of control sensors, relative to each electrode pair
* This table lists unique
hyperthermia requirements hyperthermia
features,
documentation Ferromagnetic
antennas
1. Alloy type and seed dimensions 2. Curie point temperature 3. Magnetic field strength 4. Ferroseed length in tumor
1. Type 2. SAR pattern at intended insertion depth 3. Insertion depth 4. Length in tumor 5. Grouping of antennas to power channels 6. Location of control sensors for each power channel not already discussed
in previous
techniques. As indicated, the thermometry are the minimum needed for an adequate treatment. It is anticipated that with con-
seeds
documents
Hot water or hot wires 1. Type 2. Active vs. insulated length 3. Electrical or water connections 4. Water or source temperature
(9, 29, 33).
tinued technological progress and methods for interstitial hyperthermia, be updated in the future.
availability of new these guidelines will
REFERENCES 1. American Associates of Physicists in Medicine Task Group # 1. Performance evaluation of hyperthermia equipment. Amer. Assoc. Phys. Med. Report #26; 1989. 2. Atkinson, W. J.; Brezovich, I. A.; Chakraborty, D. P. Usable frequencies in hyperthennia with thermal seeds. IEEE-BME 3 1:70-75; 1984. 3. Brezovich, I. A.; Atkinson, W. J.; Chakraborty, D. P. Temperature distributions in tumor models heated by self-regulating nickel-copper alloy thermoseeds. Med. Phys. 11: 145152; 1984. 4. Burton, C. V.; Hill, M.; Walker, A. E. The RF thermoseedA thermally self-regulating implant for the production of brain lesions. IEEE Trans. Biomed. Engr. BME- 18: 104- 109; 1971. 5. Chin, R. B.; Stauffer, P. R. Treatment planning for ferromagnetic seed heating. Int. J. Radiat. Oncol. Biol. Phys. (In press) 1991. 6. Deford, J. A.; Babbs, C. F.; Patel, U. H.; Fearnot. N. E.; Marchosky, J. A.; Moran, C. J. Accuracy and precision of computer simulated tissue temperatures in individual hu-
man intracranial tumors treated with interstitial hyperthermia. Int. J. Hyperther. 6:755-770; 1990. Demer, L. J.; Chen, J. S.; Buechler, D. N.; Damento, M. A.; Poirer, D. R.; Cetas, T. C. Ferromagnetic thermoseed materials for tumor hyperthermia. Proceedings of the 8th Annual Conference of the IEEE Engineering in Medicine and Biology Society, Vol. 3; 1986:1448-1453. Denman, D. L.; Foster, A. F.; Lewis, G. C.; Redmond, K. P.; Elson, H. R.; Breneman, J. C.; Kereiakes, J. G.; Aron, B. S. The distribution of power and heat produced by interstitial microwave antenna arrays: II. The role of antenna spacing and insertion depth. Int. J. Radiat. Oncol. Biol. Phys. 14:537-545; 1988.
9. Deshmukh, R.; Damento, M.; Demer, L.; Forsyth, K.; DeYoung, D.; Dewhirst, M.; Cetas, T. C. Ferromagnetic alloys with curie temperatures near 50°C for use in hyperthermic therapy. In: Overgaard, J. eds. Hyperthermic oncology, Vol. 1. London and Philadelphia: Taylor and Francis; 1984:599-602. 10. Dewhirst, M. W.; Phillips, T. L.; Samulski, T. V.; Stauffer, P.; Shrivastava, P.; Paliwal, B.; Pajak, T.; Gillin, M.; Sapozink, M.; Myerson, R.; Waterman, F. M.; Sapareto, S. A.; Cony, P.; Cetas, T. C.; Leeper, D. B.; Fessenden, P.; Kapp, D.; Oleson, J. R.; Emami, B. RTOG quality assurance guidelines for clinical trials using hyperthermia. Int. J. Radiat. Oncol. Biol. Phys. 18: 1249- 1259; 1990. 11. Emami, B. Quality assurance in hyperthermia clinical trials. Jpn. J. Hypertherm. Oncol. 5:229-235; 1989. 12. Emami, B. Applied techniques and clinical practice of local external and interstitial hyperthermia. Refresher Course. The 3 1st Annual Scientific Meeting of the American Society for Therapeutic Radiology and Oncology, October 2-6, San Francisco, CA, 1989. 13. Eppert, V. E.; Trembly, B. S.; Richter, H. J. Air cooling for an interstitial microwave antenna: theory and experiment. IEEE Trans. Biomed. Eng. (In press) 199 1. 14. Goffinet, D. R.; Prionas, S. D.; Kapp, D. S.; Samulski, T. V.; Fessenden, P.; Hahn, G. M.; Lee, E. R.; Lohrbach, A. W.; Mariscal, J. M.; Bagshaw, M. A. Interstitial 19’Ir flexible catheter radiofrequency hyperthermia treatments of head and neck and recurrent pelvic carcinomas. Int. J. Radiat. Oncol. Biol. Phys. 18: 199-210; 1990. 15. Hand, J.; Trembly, B. S.; Prior, M. V. Physics of interstitial hyperthermia: RF and hot water techniques. In: Urano, M., Douple, E., eds. Hyperthermia and oncology, Vol. 3. Inter-
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16.
17.
18.
19.
20.
21. 22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
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