1992, The British Journal of Radiology, 65, 409-416
Quality assurance of computer controlled radiotherapy treatments* By tHelen M. Morgan MSc, MlnstP, M I P S M Department of Medical Physics and Computing, Royal Free Hospital, Hampstead, London NW3 2QG, UK
{Received 22 February 1991 and in revised form 5 August 1991, accepted 3 October 1991) Keywords: Radiotherapy, Quality assurance, Conformation
Abstract. There is a need in conformal therapy, as in any radiation therapy, for adequate quality assurance of the treatment plan and the delivery of the treatment. This paper examines quality assurance of two methods of conformal treatment, on a cobalt treatment unit using computer control. Each of the two methods demonstrates a different aspect of computer controlled treatments. Following completion of each treatment plan, an additional "quality assurance plan" is prepared. This is used to assess the integrity of the treatment plan, and the precision with which the computer controls the treatment unit. A simple method, using solid state detectors in a Perspex phantom, is used to validate the dosimetry of the "quality assurance plan". Quality assurance of the computer control is performed daily prior to treatments. At each treatment, parameters identifying the start position and final position of the computer controlled couch movements and the exposure time are noted by the radiographers. Comparison of the recorded movement of the treatment couch and the exposure time with that intended during each course of treatment has demonstrated, inter alia, limitations on couch speed control at speeds of less than 10 mm per min.
Quality assurance in radiotherapy, for dosimetry and mechanical alignment of the treatment unit, has always been recognized as an important part of radiotherapy physics (Hospital Physicists' Association (HPA), 1971; Institute of Physical Sciences in Medicine (IPSM), 1988). With the advent of computer controlled treatment units there is need for even greater vigilance, since the treatment plan is more complicated than for conventional treatment. In computer controlled conformation therapy, during which the computer is programmed to control many machine movements with the radiation on, any errors that do occur are likely to be more complicated to correct. This paper describes the quality assurance for two types of treatment under computer control on a cobalt unit (Brace et al, 1981) at the Royal Free Hospital. The main difference between these two types of treatment is whether the couch is driven in position mode or in rate mode. In position mode the couch is driven to a specified position between exposures (i.e. with the radiation off). Rate mode is used to drive the couch when the radiation is on; in this mode the final position and the exposure time are specified. Treatment planning
An example of using the couch in position mode is treatment of the pelvis and para-aortic nodes. Target volumes that include these sites are of the order of *Presented at: "Computers for the control of radiotherapy treatment units", February 1988, London. fCurrent address: Wessex Regional Medical Physics Service, Royal United Hospital, Bath BA1 3NG, UK.
Vol. 65, No. 773
350 mm long. The target is divided into short sections for treatment planning purposes (Fig. 1) and the couch is driven in position mode from one section to the next between exposures. The plan for each section is calculated on a commercial treatment planning computer using computed tomographic (CT) information. An arcing arrangement of radiation fields is used, as this is the most time efficient on a cobalt unit. Each section may have a different rotation centre. Owing to the complex shape of the target volumes, some sections may have up to three rotation centres to get the best fit of the isodose distribution to the target (Fig. 2). Rate mode is used to drive the couch to treat long shallow target volumes such as those involving the spinal cord. The treatment planning of these volumes has been automated and details were reported by Shentall at a meeting "Computers for the control of radiotherapy treatment units", February 1988 in London. There are three phases to be planned: the opening and closing phases and the intermediate phase. In each phase the couch is driven in rate mode with the shutter open. Both treatment techniques have been described (Davy, 1983). For both types of treatment, once the treatment plan details have been completed, a computer file is prepared. Data preparation for computer control
To deliver a computer controlled treatment it is first necessary to determine a "start" position for the treatment couch. The start position is that required to set up to a reference point (tattoo) on the patient, which has been determined at their CT planning session. The patient location on the treatment couch will vary 409
H. M. Morgan
Projection angle 0 Figure 2. A treatment plan for a section of a pelvic target volume. There are three rotation centres (©) about which the gantry delivers an arcing field. Each of the two anterior rotation centres also has a static field. The target outline is represented by a dashed line and the projection of the start position onto this plane (A) is shown. The plan is normalized to 100% at the most posterior rotation centre.
50 mm Projection angle 270° Figure 1. A typical pelvis and para-aortic node target volume displayed at orthogonal projection angles (AP and lateral views). The volume is divided into sections as shown, and a treatment plan calculated for each section.
slightly from day to day, and consequently the longitudinal, lateral (and vertical) coordinates of the treatment couch at the start position will vary similarly. All computer controlled movements are then relative to these start position coordinates. For treatment to a pelvis and para-aortic nodes target, the couch is displaced from the start position to place the rotation centre of the first section at the treatment unit isocentre by driving the couch longitudinally, laterally and vertically in position mode. The radiation is delivered according to the treatment plan and then the couch is driven to position the rotation centre of the next section at the treatment unit isocentre. This continues until the last section of the target has been treated. If there is more than one rotation centre for a section, then the vertical and lateral couch drives are used to position the next rotation centre at the machine isocentre without operating the longitudinal drive. The complete set of treatment plans with the necessary couch displacements from the start position are translated into a set of computer instructions, which are identified by the patient's name. This is called the Patient File. The spine treatment plans are carried out by driving the couch longitudinally in rate mode whilst the shutter 410
is open. The gantry is usually held at 0° for the whole treatment. Again there may be couch vertical and lateral movements so that the centre of the target volume is driven through the machine isocentre along its whole course. A computer program has been developed to create a Patient File for these treatments (reported by Shentall at the meeting "Computers for the control of radiotherapy treatment units", February 1988). For both treatment types a quality assurance (QA) file is also prepared on the computer. This QA file is used for quality assurance of dosimetry and to check reproducibility, and is identical to the Patient File except that when implemented (1) the gantry position is fixed at 0°, and (2) there are no couch lateral or vertical displacements from the start position. The field size, couch longitudinal displacement and exposure time vary identically to the Patient File. The lateral and vertical couch movements are excluded to simplify the measurement and calculation of the dose along the locus of the isocentre. Quality assurance
Dosimetry From the set of treatment plans or treatment phases for a particular target volume, the average depth of the isocentre within the patient for all treatment fields is determined. For an arcing field, the depth is determined at 15° intervals and the mean depth of each arc used to determine the overall average depth of the isocentre for the target volume. This depth is converted to an equivalent depth in Perspex and then rounded to the nearest integral 5 mm. This is called the QA depth. The tissue equivalent of the QA depth is used to calculate the dose along the locus of the isocentre, when the treatment unit is controlled by the QA file. This dose profile is then measured with cylindrical solid state detectors (Therados SDS-C) in a Perspex phantom. The Perspex phantom consists of sheets of The British Journal of Radiology, May 1992
QA of computer controlled radiotherapy treatments
Dose, Gy 3.0
/°
2.0
1.0
t
100
200 Distance, mm
i
300
400
Figure 4. The calculated dose profile (—) in the Perspex phantom from a QA file for a pelvic treatment, together with the total doses measured from the solid state detectors (O), along the isocentre locus. The calculated direct dose to a single section ( + ) is shown for comparison with the direct dose measured by the solid state detectors ( # ) . The junctions between adjacent treatment sections are marked (f).
Figure 3. The Perspex phantom with solid state detectors in position. This is used to measure the dose delivered along the locus of the isocentre when the QA file is run (see text).
Perspex that interlock on top of each other. One 20 mm thick sheet is drilled to take the solid state detectors at 10 mm intervals along its midline (Fig. 3). The phantom is placed on the treatment couch, so that the solid state detectors lie along the rotation axis of the treatment unit. The start position of the treatment is set on the phantom and solid state detectors are inserted at positions to correspond with the planned superior and inferior borders of the target volume, the rotation centres and the borders between adjacent treatment regions to within 5 mm. Sheets of Perspex are placed over the detectors to achieve the QA depth, as calculated above. The measured doses are then compared with those calculated. This procedure is to confirm that the measured doses and overall treatment length are as predicted, to avoid gross discrepancies and to ensure that in the case of pelvic treatments there are no large dose discontinuities at the junction of adjacent treatment regions. For spinal treatments, which are planned in three phases, it is necessary to check that there are no large variations in dose between the three phases. Large discontinuities at field joins could be due to incorrect Vol. 65, No. 773
couch displacement, speed or field length. The criterion for acceptance is that the total doses measured with the solid state detectors should be within 5% of the calculated values (Fig. 4). Larger discrepancies require further investigation to search out errors in the computer file or treatment plan. For a plan to treat the pelvis and para-aortic nodes, 5-20% of the total dose at a rotation centre is due to scatter from the treatment of adjacent sections. The amount of scatter depends on the position of the section within the target length: sections at each end of the target volume will receive a smaller proportion of scattered radiation than those sections at the centre. Dose measurements from the solid state detectors are normally recorded when the QA file has finished running. However, the QA file may be interrupted at any time during its execution. By interrupting the running of the file between treatment regions (when the radiation is off), and recording dose measurements during the interruptions, the contribution of direct and scattered radiation to each section or phase from each region of the treatment can be measured. When this test is performed there are differences between the measured and calculated doses. The solid state detectors underestimate the direct dose by an average of 7%, overestimate the dose from scattered radiation by an average of 11%, and arefield-sizedependent. Reproducibility of the QAfileis checked by repeating the measurements with the solid state detectors without changing their positions in the phantom. This dosimetry check has demonstrated that the QA file is reproducible to within 1%. A significantly larger variation in reproducibility would require investigation. To validate this method of dosimetry, this QAfilehas also been run with thermoluminescence dosemeters (TLD) (Harshaw TLD-100 chips), positioned along the 41
Dose, Gy r°
3.0
1 2.0
1.0
0
f f
1
t
100
!
t
200 Distance, mm
1
t
300
1 1 1 400
Figure 6. A chart used to demonstrate the required couch vertical and longitudinal displacements from the start position ( x ) to the positions of the rotation centres ( + ) for each treatment section of a pelvic target volume, in the vertical plane. A similar chart is used to demonstrate the required couch lateral and longitudinal displacements in the horizontal plane.
Figure 5. The calculated dose profile (—) in the Perspex phantom, for the same QA file as shown in Fig. 4, together with the total doses measured by TLD (CO- The junctions between adjacent treatment sections are marked (|).
drawn to show the required couch displacements in the vertical and horizontal plane. The vertical plane (Fig. 6) shows the required couch height and couch longitudinal locus of the isocentre in the same phantom (Fig. 5). displacements from the start position; the horizontal Individually calibrated TLD were positioned at 5 mm plane shows similarly the required lateral and longiintervals in a sheet of Perspex drilled to contain them. tudinal couch displacements. Where the solid state detectors and TLD were in the The charts are positioned on the treatment couch with same position in the phantom, relative to the start a simple jig, ensuring that the laser positioning beams position, the ratio of their readings was calculated. The are centred on the start position (Fig. 7). The Patient mean of all the ratios was 0.999 with a coefficient of File is then activated in the "test mode", which enables variation of 0.02. the shutter of the treatment unit to be locked in the At the start of each QA programme, the solid state closed position. A physicist remains in the treatment detectors are individually calibrated in a water phantom room whilst the file is running, and confirms that the in a 100 mm x 100 mm field at a depth of 50 mm on the required couch movements are being carried out cobalt treatment unit against a 0.6 cm3 thimble ion correctly, by visual examination of the crosswires chamber (N.E. Technology, model 2571) with an elec- (projected by the field defining lamp in the treatment trometer (N.E. Technology, Ionex Electrometer 2500/3). unit head) and the laser beams against the charts. This check ensures that the treatment plans have been The thimble ion chamber and electrometer are calibrated in accordance with the current code of practice converted into computer instructions correctly and that (HPA, 1983; IPSM, 1990). A consistency check is then the couch drives are not "hunting" excessively to reach carried out in the Perspex phantom, with all the solid the required positions. If excessive hunting occurs, state detectors within a large field size, on the cobalt adjustments to the servo systems must be made before unit. This consistency check is repeated each day before patient treatments are carried out. use of the solid state detectors with the QA file. Despite the problems discussed above, the solid state Daily check on treatment unit control Each day, prior to computer controlled treatments, a detectors are a useful tool to guard against gross errors and large dose discontinuities, and also check the repro- special program (the run-up program) is executed to ducibility of the QA file, i.e. control of field size and check that the computer can control the treatment unit couch displacement. The method using TLDs is too correctly (Brace, 1982) (Table I). The first part of the time consuming to perform on a routine basis. Both sequence checks for noise on the various signals. The methods measured total (direct plus scatter) doses lower computer then drives the couch and treatment unit in than those calculated with a mean ratio of 0.973 for position mode to specified positions in both positive and measured to calculated doses for both the solid state negative directions. Having checked the position mode, detectors and the TLD. The coefficient of variation was the rate mode is then checked. The couch and gantry are 0.017 for the solid state detectors and 0.013 for the driven at specified speeds, again in both the positive and negative directions. The shutter open and close TLD. commands are also tested. Control of couch position At the end of this series of tests the computer analyses In addition to checking the dosimetry with the QA how successfully the movements were carried out. If any file, the computer control of the couch movements in the tolerance is exceeded a "fail" message is given. The Patient File are checked. The locus of the isocentre is tolerances on the position drives for the couch and determined from the treatment plan, and charts are gantry are equivalent to 1 mm and 0.4°, respectively. On 412
The British Journal of Radiology, May 1992
QA of computer controlled radiotherapy treatments
When all sections of the run-up program are within tolerance, the unit may be used for computer controlled treatments. If the run-up program has failed, an error message will be given in order to prevent patient treatment.
Figure 7. For each Patient File, the computer control of the required couch movements from the start position to each rotation centre is checked visually using the image of the crosswires and the alignment lasers.
the speed tests an error of 5% is allowed before the test is failed. In the event of a failure, further computer programs are available to the electronics engineer, computer scientist and physicist to pin-point the problem.
Patient treatment Whilst the patient is on treatment the following parameters are recorded manually by the radiographers at each attendance, independently of the computer: (1) the couch longitudinal start and end positions, and hence the total travel; and (2) the total exposure time recorded by the timer on the cobalt unit treatment console. These parameters may differ from those expected from the treatment file. Figure 8 shows the difference in mean recorded couch longitudinal travel compared with that demanded, for a series of 11 treatment courses, where the couch was driven in the position mode. The results are shown in date order of patient treatment course. The difference between recorded and demanded total travel does not correlate with the number of demands for longitudinal movement. However, there is a slight trend up to treatment course number 6, for the travel to fall short of that demanded. Overhauling the couch movement mechanism resolved this problem. If the total exposure time per treatment fraction exceeds the maximum time that can be set on the treatment unit timer (17.99 min), the instructions are split into two files. In normal circumstances the second file is run immediately after the first, without the radiographers entering the treatment room, but the exposure time of each file is recorded separately. The treatment unit timer detects the shutter opening and closing at different positions during source transit from the computer. So although the recorded exposure time may be different from the demanded time, it should be reproducible for each treatment fraction. Figure 9
2
(dr-
mm
dd),
1 Q
\
.
0 -1
Table I. The run-up program used to check the computer control of the treatment unit on a daily basis. The tolerance for each test is given Test
Tolerance
Noise Move to start position Position test: + ve drive Position test: — ve drive Couch speed: +50 mm min" Couch speed: —50 mm min" Arm speed: +360° min" 1 Arm speed: —360° min" 1
0 noisy readings
I
c '
Quality assurance of computer controlled radiotherapy treatments* By tHelen M. Morgan MSc, MlnstP, M I P S M Department of Medical Physics and Computing, Royal Free Hospital, Hampstead, London NW3 2QG, UK
{Received 22 February 1991 and in revised form 5 August 1991, accepted 3 October 1991) Keywords: Radiotherapy, Quality assurance, Conformation
Abstract. There is a need in conformal therapy, as in any radiation therapy, for adequate quality assurance of the treatment plan and the delivery of the treatment. This paper examines quality assurance of two methods of conformal treatment, on a cobalt treatment unit using computer control. Each of the two methods demonstrates a different aspect of computer controlled treatments. Following completion of each treatment plan, an additional "quality assurance plan" is prepared. This is used to assess the integrity of the treatment plan, and the precision with which the computer controls the treatment unit. A simple method, using solid state detectors in a Perspex phantom, is used to validate the dosimetry of the "quality assurance plan". Quality assurance of the computer control is performed daily prior to treatments. At each treatment, parameters identifying the start position and final position of the computer controlled couch movements and the exposure time are noted by the radiographers. Comparison of the recorded movement of the treatment couch and the exposure time with that intended during each course of treatment has demonstrated, inter alia, limitations on couch speed control at speeds of less than 10 mm per min.
Quality assurance in radiotherapy, for dosimetry and mechanical alignment of the treatment unit, has always been recognized as an important part of radiotherapy physics (Hospital Physicists' Association (HPA), 1971; Institute of Physical Sciences in Medicine (IPSM), 1988). With the advent of computer controlled treatment units there is need for even greater vigilance, since the treatment plan is more complicated than for conventional treatment. In computer controlled conformation therapy, during which the computer is programmed to control many machine movements with the radiation on, any errors that do occur are likely to be more complicated to correct. This paper describes the quality assurance for two types of treatment under computer control on a cobalt unit (Brace et al, 1981) at the Royal Free Hospital. The main difference between these two types of treatment is whether the couch is driven in position mode or in rate mode. In position mode the couch is driven to a specified position between exposures (i.e. with the radiation off). Rate mode is used to drive the couch when the radiation is on; in this mode the final position and the exposure time are specified. Treatment planning
An example of using the couch in position mode is treatment of the pelvis and para-aortic nodes. Target volumes that include these sites are of the order of *Presented at: "Computers for the control of radiotherapy treatment units", February 1988, London. fCurrent address: Wessex Regional Medical Physics Service, Royal United Hospital, Bath BA1 3NG, UK.
Vol. 65, No. 773
350 mm long. The target is divided into short sections for treatment planning purposes (Fig. 1) and the couch is driven in position mode from one section to the next between exposures. The plan for each section is calculated on a commercial treatment planning computer using computed tomographic (CT) information. An arcing arrangement of radiation fields is used, as this is the most time efficient on a cobalt unit. Each section may have a different rotation centre. Owing to the complex shape of the target volumes, some sections may have up to three rotation centres to get the best fit of the isodose distribution to the target (Fig. 2). Rate mode is used to drive the couch to treat long shallow target volumes such as those involving the spinal cord. The treatment planning of these volumes has been automated and details were reported by Shentall at a meeting "Computers for the control of radiotherapy treatment units", February 1988 in London. There are three phases to be planned: the opening and closing phases and the intermediate phase. In each phase the couch is driven in rate mode with the shutter open. Both treatment techniques have been described (Davy, 1983). For both types of treatment, once the treatment plan details have been completed, a computer file is prepared. Data preparation for computer control
To deliver a computer controlled treatment it is first necessary to determine a "start" position for the treatment couch. The start position is that required to set up to a reference point (tattoo) on the patient, which has been determined at their CT planning session. The patient location on the treatment couch will vary 409
H. M. Morgan
Projection angle 0 Figure 2. A treatment plan for a section of a pelvic target volume. There are three rotation centres (©) about which the gantry delivers an arcing field. Each of the two anterior rotation centres also has a static field. The target outline is represented by a dashed line and the projection of the start position onto this plane (A) is shown. The plan is normalized to 100% at the most posterior rotation centre.
50 mm Projection angle 270° Figure 1. A typical pelvis and para-aortic node target volume displayed at orthogonal projection angles (AP and lateral views). The volume is divided into sections as shown, and a treatment plan calculated for each section.
slightly from day to day, and consequently the longitudinal, lateral (and vertical) coordinates of the treatment couch at the start position will vary similarly. All computer controlled movements are then relative to these start position coordinates. For treatment to a pelvis and para-aortic nodes target, the couch is displaced from the start position to place the rotation centre of the first section at the treatment unit isocentre by driving the couch longitudinally, laterally and vertically in position mode. The radiation is delivered according to the treatment plan and then the couch is driven to position the rotation centre of the next section at the treatment unit isocentre. This continues until the last section of the target has been treated. If there is more than one rotation centre for a section, then the vertical and lateral couch drives are used to position the next rotation centre at the machine isocentre without operating the longitudinal drive. The complete set of treatment plans with the necessary couch displacements from the start position are translated into a set of computer instructions, which are identified by the patient's name. This is called the Patient File. The spine treatment plans are carried out by driving the couch longitudinally in rate mode whilst the shutter 410
is open. The gantry is usually held at 0° for the whole treatment. Again there may be couch vertical and lateral movements so that the centre of the target volume is driven through the machine isocentre along its whole course. A computer program has been developed to create a Patient File for these treatments (reported by Shentall at the meeting "Computers for the control of radiotherapy treatment units", February 1988). For both treatment types a quality assurance (QA) file is also prepared on the computer. This QA file is used for quality assurance of dosimetry and to check reproducibility, and is identical to the Patient File except that when implemented (1) the gantry position is fixed at 0°, and (2) there are no couch lateral or vertical displacements from the start position. The field size, couch longitudinal displacement and exposure time vary identically to the Patient File. The lateral and vertical couch movements are excluded to simplify the measurement and calculation of the dose along the locus of the isocentre. Quality assurance
Dosimetry From the set of treatment plans or treatment phases for a particular target volume, the average depth of the isocentre within the patient for all treatment fields is determined. For an arcing field, the depth is determined at 15° intervals and the mean depth of each arc used to determine the overall average depth of the isocentre for the target volume. This depth is converted to an equivalent depth in Perspex and then rounded to the nearest integral 5 mm. This is called the QA depth. The tissue equivalent of the QA depth is used to calculate the dose along the locus of the isocentre, when the treatment unit is controlled by the QA file. This dose profile is then measured with cylindrical solid state detectors (Therados SDS-C) in a Perspex phantom. The Perspex phantom consists of sheets of The British Journal of Radiology, May 1992
QA of computer controlled radiotherapy treatments
Dose, Gy 3.0
/°
2.0
1.0
t
100
200 Distance, mm
i
300
400
Figure 4. The calculated dose profile (—) in the Perspex phantom from a QA file for a pelvic treatment, together with the total doses measured from the solid state detectors (O), along the isocentre locus. The calculated direct dose to a single section ( + ) is shown for comparison with the direct dose measured by the solid state detectors ( # ) . The junctions between adjacent treatment sections are marked (f).
Figure 3. The Perspex phantom with solid state detectors in position. This is used to measure the dose delivered along the locus of the isocentre when the QA file is run (see text).
Perspex that interlock on top of each other. One 20 mm thick sheet is drilled to take the solid state detectors at 10 mm intervals along its midline (Fig. 3). The phantom is placed on the treatment couch, so that the solid state detectors lie along the rotation axis of the treatment unit. The start position of the treatment is set on the phantom and solid state detectors are inserted at positions to correspond with the planned superior and inferior borders of the target volume, the rotation centres and the borders between adjacent treatment regions to within 5 mm. Sheets of Perspex are placed over the detectors to achieve the QA depth, as calculated above. The measured doses are then compared with those calculated. This procedure is to confirm that the measured doses and overall treatment length are as predicted, to avoid gross discrepancies and to ensure that in the case of pelvic treatments there are no large dose discontinuities at the junction of adjacent treatment regions. For spinal treatments, which are planned in three phases, it is necessary to check that there are no large variations in dose between the three phases. Large discontinuities at field joins could be due to incorrect Vol. 65, No. 773
couch displacement, speed or field length. The criterion for acceptance is that the total doses measured with the solid state detectors should be within 5% of the calculated values (Fig. 4). Larger discrepancies require further investigation to search out errors in the computer file or treatment plan. For a plan to treat the pelvis and para-aortic nodes, 5-20% of the total dose at a rotation centre is due to scatter from the treatment of adjacent sections. The amount of scatter depends on the position of the section within the target length: sections at each end of the target volume will receive a smaller proportion of scattered radiation than those sections at the centre. Dose measurements from the solid state detectors are normally recorded when the QA file has finished running. However, the QA file may be interrupted at any time during its execution. By interrupting the running of the file between treatment regions (when the radiation is off), and recording dose measurements during the interruptions, the contribution of direct and scattered radiation to each section or phase from each region of the treatment can be measured. When this test is performed there are differences between the measured and calculated doses. The solid state detectors underestimate the direct dose by an average of 7%, overestimate the dose from scattered radiation by an average of 11%, and arefield-sizedependent. Reproducibility of the QAfileis checked by repeating the measurements with the solid state detectors without changing their positions in the phantom. This dosimetry check has demonstrated that the QA file is reproducible to within 1%. A significantly larger variation in reproducibility would require investigation. To validate this method of dosimetry, this QAfilehas also been run with thermoluminescence dosemeters (TLD) (Harshaw TLD-100 chips), positioned along the 41
Dose, Gy r°
3.0
1 2.0
1.0
0
f f
1
t
100
!
t
200 Distance, mm
1
t
300
1 1 1 400
Figure 6. A chart used to demonstrate the required couch vertical and longitudinal displacements from the start position ( x ) to the positions of the rotation centres ( + ) for each treatment section of a pelvic target volume, in the vertical plane. A similar chart is used to demonstrate the required couch lateral and longitudinal displacements in the horizontal plane.
Figure 5. The calculated dose profile (—) in the Perspex phantom, for the same QA file as shown in Fig. 4, together with the total doses measured by TLD (CO- The junctions between adjacent treatment sections are marked (|).
drawn to show the required couch displacements in the vertical and horizontal plane. The vertical plane (Fig. 6) shows the required couch height and couch longitudinal locus of the isocentre in the same phantom (Fig. 5). displacements from the start position; the horizontal Individually calibrated TLD were positioned at 5 mm plane shows similarly the required lateral and longiintervals in a sheet of Perspex drilled to contain them. tudinal couch displacements. Where the solid state detectors and TLD were in the The charts are positioned on the treatment couch with same position in the phantom, relative to the start a simple jig, ensuring that the laser positioning beams position, the ratio of their readings was calculated. The are centred on the start position (Fig. 7). The Patient mean of all the ratios was 0.999 with a coefficient of File is then activated in the "test mode", which enables variation of 0.02. the shutter of the treatment unit to be locked in the At the start of each QA programme, the solid state closed position. A physicist remains in the treatment detectors are individually calibrated in a water phantom room whilst the file is running, and confirms that the in a 100 mm x 100 mm field at a depth of 50 mm on the required couch movements are being carried out cobalt treatment unit against a 0.6 cm3 thimble ion correctly, by visual examination of the crosswires chamber (N.E. Technology, model 2571) with an elec- (projected by the field defining lamp in the treatment trometer (N.E. Technology, Ionex Electrometer 2500/3). unit head) and the laser beams against the charts. This check ensures that the treatment plans have been The thimble ion chamber and electrometer are calibrated in accordance with the current code of practice converted into computer instructions correctly and that (HPA, 1983; IPSM, 1990). A consistency check is then the couch drives are not "hunting" excessively to reach carried out in the Perspex phantom, with all the solid the required positions. If excessive hunting occurs, state detectors within a large field size, on the cobalt adjustments to the servo systems must be made before unit. This consistency check is repeated each day before patient treatments are carried out. use of the solid state detectors with the QA file. Despite the problems discussed above, the solid state Daily check on treatment unit control Each day, prior to computer controlled treatments, a detectors are a useful tool to guard against gross errors and large dose discontinuities, and also check the repro- special program (the run-up program) is executed to ducibility of the QA file, i.e. control of field size and check that the computer can control the treatment unit couch displacement. The method using TLDs is too correctly (Brace, 1982) (Table I). The first part of the time consuming to perform on a routine basis. Both sequence checks for noise on the various signals. The methods measured total (direct plus scatter) doses lower computer then drives the couch and treatment unit in than those calculated with a mean ratio of 0.973 for position mode to specified positions in both positive and measured to calculated doses for both the solid state negative directions. Having checked the position mode, detectors and the TLD. The coefficient of variation was the rate mode is then checked. The couch and gantry are 0.017 for the solid state detectors and 0.013 for the driven at specified speeds, again in both the positive and negative directions. The shutter open and close TLD. commands are also tested. Control of couch position At the end of this series of tests the computer analyses In addition to checking the dosimetry with the QA how successfully the movements were carried out. If any file, the computer control of the couch movements in the tolerance is exceeded a "fail" message is given. The Patient File are checked. The locus of the isocentre is tolerances on the position drives for the couch and determined from the treatment plan, and charts are gantry are equivalent to 1 mm and 0.4°, respectively. On 412
The British Journal of Radiology, May 1992
QA of computer controlled radiotherapy treatments
When all sections of the run-up program are within tolerance, the unit may be used for computer controlled treatments. If the run-up program has failed, an error message will be given in order to prevent patient treatment.
Figure 7. For each Patient File, the computer control of the required couch movements from the start position to each rotation centre is checked visually using the image of the crosswires and the alignment lasers.
the speed tests an error of 5% is allowed before the test is failed. In the event of a failure, further computer programs are available to the electronics engineer, computer scientist and physicist to pin-point the problem.
Patient treatment Whilst the patient is on treatment the following parameters are recorded manually by the radiographers at each attendance, independently of the computer: (1) the couch longitudinal start and end positions, and hence the total travel; and (2) the total exposure time recorded by the timer on the cobalt unit treatment console. These parameters may differ from those expected from the treatment file. Figure 8 shows the difference in mean recorded couch longitudinal travel compared with that demanded, for a series of 11 treatment courses, where the couch was driven in the position mode. The results are shown in date order of patient treatment course. The difference between recorded and demanded total travel does not correlate with the number of demands for longitudinal movement. However, there is a slight trend up to treatment course number 6, for the travel to fall short of that demanded. Overhauling the couch movement mechanism resolved this problem. If the total exposure time per treatment fraction exceeds the maximum time that can be set on the treatment unit timer (17.99 min), the instructions are split into two files. In normal circumstances the second file is run immediately after the first, without the radiographers entering the treatment room, but the exposure time of each file is recorded separately. The treatment unit timer detects the shutter opening and closing at different positions during source transit from the computer. So although the recorded exposure time may be different from the demanded time, it should be reproducible for each treatment fraction. Figure 9
2
(dr-
mm
dd),
1 Q
\
.
0 -1
Table I. The run-up program used to check the computer control of the treatment unit on a daily basis. The tolerance for each test is given Test
Tolerance
Noise Move to start position Position test: + ve drive Position test: — ve drive Couch speed: +50 mm min" Couch speed: —50 mm min" Arm speed: +360° min" 1 Arm speed: —360° min" 1
0 noisy readings
I
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