Medwal Doson('tO'. Vol. 17, pp. 225-228
0739-0211/92 $6.00 + .00 Copyright © 1993 American Association of Medical Dosimetrists
Printed in the U.S.A. All rights reserved.
PHOTON
BEAM
ATTENUATION
FOR
A
PATIENT
ASSEMBLY DURING ARC THERAPY FOR MEDICAL LINEAR ACCELERATOR
SUPPORT A
S. C. S H A R M A a n d M . W . J O H N S O N Department of Radiation Oncology, E. W. Sparrow Hospital, Lansing, MI 48909 Abstract--Are therapy is one of the treatment techniques for small, centrally located deep-seated tumors. However, care must be taken to remove any components that would interfere with the beam as the gantry rotates around the patient. One such component that may interfere with the beam is the patient support assembly (PSA) or treatment table. Beam attenuation factors due to the presence of the couch side-rails and the centerspine bar of the PSA are presented for both 6 MV and 18 MV photon beams of a Clinac 1800 during a 360 ° rotation. Dose perturbations arising from these obstructions are displayed using an Alderson Rando phantom. A method is described to calculate the start and stop angles for the largest unobstructed arc for a given field size (FS), centerspine bar to the isocenter height (H), centerspine bar width (W), and SAD. As an illustration, for an SAD of 100 cm and a W of 4 cm, the start and stop angles for arc therapy with a FS of l0 × l0 cm2 and an H of 12.7 cm would be 29 ° and 331 °. Key Words: Arc therapy, Transmission factors, Linear accelerator, Treatment table.
These measurements were made for 360 ° rotation, first with no obstructions in the beam and then with the couch side-rails and the centerspine bar obstructing the beam. The centerspine was measured two ways, with and without the couch inserts in place. The dose perturbation measurements were made using an Alderson Rando anthropomorphic pelvic phantom. K o d a k XV2* verification film was sandwiched between the p h a n t o m slices and was exposed during a 360 ° rotation with the couch side-rails and then the centerspine bar obstructing the beam. The developed films were then scanned using the Multidata Film Dosimetry System*, where the optical density readings of the films were converted to dose measurements. For comparison a CT scan of the pelvic p h a n t o m was performed to generate an isodose computer plan from a G E T A R G E T ~ treatment planning computer. In addition, a method to find the optimal start and stop angles was derived using basic trigonometric principles. The start angle (O) can be calculated from the following equation:
INTRODUCTION Arc therapy, a standard technique used in radiation therapy, is an appropriate treatment choice for small, centrally located deep-seated tumors. However, care must be taken in using this technique to remove any c o m p o n e n t s that would interfere with the beam as the gantry rotates around the patient. One of these components is the patient support assembly (PSA) or treatment table of the teletherapy unit. We verified this interference by measuring the beam attenuation due to the couch side-rails and the centerspine bar of the PSA, for 6 MV and 18 MV photon beams o f a Clinac 1800* during a 360 ° rotation. Secondly, we looked at how these obstructions m a y perturb the dose to the treatment volume, using a 6 MV photon beam rotating 360 ° around an anthropomorphic phantom. These perturbations have been previously described by Krithivas and Rao from for a Clinac 4/100" PSA, using a 60 ° Arc Rotation. ~ A method is presented to solve this problem where the start and stop angles of an arc are calculated for the beam to avoid the centerspine bar. METHODS
O= tan-~(2~H)+
AND MATERIALS
The beam attenuation due to the PSA was measured in air, using a farmer-type ion chamber with a buildup cap, at the isocenter of the linear accelerator.
sin-~{SAD• sin(tan-~[FS/2"sa°]) ~ l-~+ ] -tan
Originally presented as a poster presentation 33rd Annual Meeting ofthe American Association of Physicists in Medicine, San Francisco, CA July 21-25, 1991. * Varian, Palo Alto, CA 94303.
_,[~ FS \ )
* Eastman Kodak Co., Rochester, NY 14692. t Multidata International Systems, St. Louis, MO 63119. § GE Medical Systems, Milwaukee, W! 53201. 225
(1)
226
Medical Dosimetry
Volume 17, Number 4, 1992 Table 1. Table correction factors for 360 ° rotation on a Clinac 1800 linear accelerator
FS
Centerspine bar Photon beam energy
Couchside-rails
Inserts
No inserts
6 MV 18 MV
0.971 0.976
0.971 0.982
0.984 0.991
H
SAD
ator with an SAD of 100 cm and a W of 4 cm, start and stop angles, f o r a patient with a FS of l0 × 10 cm 2 and an H of 12.7 cm, would be 29 ° and 331 °.
W RESULTS
Fig. 1. Dependent variables for calculating the start angle (0).
where the following dependent variables are shown in Figure 1: field size (FS); treatment machine's SAD: height from the centerspine bar to the isocenter (H): and width of the centerspine bar (W). The stop angle (~) is the difference between 360 ° and the start angle (O). As an illustration, for a Clinac 1800 linear acceler-
The obstructed beam attenuation measurements were normalized to the nonobstructed measurements and the results are shown in Table 1 for both 6 and 18 MV photon beams and are listed as table correction factors. These factors can then be applied to compensate for the beam attenuation during treatment planning for monitor unit settings. The results of the dose perturbation analysis can be seen in Fig. 2 (couch side-rails) and Fig. 3 (centerspine bar). These figures can be c o m p a r e d to the isodose c o m p u t e r plan (Fig. 4), which does not include the PSA obstructions. Typically, pelvic treatments are prescribed to a target volume described by the 95% or 98% isodose line. A comparison of this region shows little difference between isodose curves derived from the films and the c o m p u t e r plan. Small variations
K L-
t-
l:l~],l~[,[,I,I,l,l,l,l~lJ]Jl,l,l~i~l~l~lLl,l,l,l,l,l,l,l,l,I,],l*l
Fig. 2. Film isodose study for 360 ° rotation including the couch side-rails for a Clinac 1800, 6 M V x-rays.
Photon beam attenuation for PSA • S. C. SHARMAand M. W. JOHNSON
227
v
, I , I , I , I , I,
I , I , I ~ I , I , I , I ,I
, I ,1
, I , I , 1(
[ , I , I , I t I , I , I ,_L.d.)
[ ) [ , I i I ~]~-
Fig. 3. Film isodose study for 360 ° rotation including the centerspine bar for a Clinac 1800, 6 MV x-rays.
compared to the 360 ° rotation technique treating through the centerspine bar (Fig. 2).
may be seen in the lower isodose lines (30% or 50%), where the couch side-rails tend to decrease the dose laterally (Fig. 2) and where the centerspine bar decreases the dose posteriorly (Fig. 3). However, the target volume is essentially unaffected. The results of the isodose c o m p u t e r plan and its corresponding film study, using equation (1) to find the optimal arc, can be seen in Fig. 5 and 6. The dose can be seen to have shifted anteriorly and laterally as
./-i'
DISCUSSION The beam attenuation of both the couch siderails and centerspine bar seem to be of the same order as the transmission factors found for beam blocking trays. If the treatment beam passes through the PSA
."
.........
'
........... '--
e.
llll)i),l,l,I)l,l,l,l,l,l,l,l,l,l,l,l,l,ltl,l,l,l,l'l'l'l'l'l'l'l'l
Fig. 4. Isodose computer plan for 360 ° rotation excluding the PSA obstructions for a Clinac 1800, 6 MV x-rays.
228
Medical Dosimetry
\\
Volume 17, Number 4, 1992
//
cM
Fig. 5. lsodose computer plan for an arc, calculated to avoid the centerspine bar tbr a Clinac 1800, 6 MV x-rays.
F
I~l~l~lJl,l,l~l~l,l,l,l,l,[,I,l,l,l,l,l,l,l~l~l~l~l,l~l,l,l,l,I,
Fig. 6. Film isodose study for an arc, calculated to avoid the centerspine bar for a Clinac 1800, 6 MV x-rays.
t h e n we r e c o m m e n d that a t r a n s m i s s i o n factor for the PSA be used for m o n i t o r unit calculations. If t r e a t i n g t h r o u g h the PSA is not desirable, e q u a t i o n (1) calculates an o p t i m a l start a n d s t o p angle for an arc p r o v i d ing e q u i v a l e n t coverage o f the t r e a t m e n t v o l u m e .
REFERENCE 1. Krithivas, G.; Rao, S.N. Therapy dose perturbation due to the center bar in a Clinac 4/100 couch assembly. Med. Dos. 13:1518; 1988.
0739-0211/92 $6.00 + .00 Copyright © 1993 American Association of Medical Dosimetrists
Printed in the U.S.A. All rights reserved.
PHOTON
BEAM
ATTENUATION
FOR
A
PATIENT
ASSEMBLY DURING ARC THERAPY FOR MEDICAL LINEAR ACCELERATOR
SUPPORT A
S. C. S H A R M A a n d M . W . J O H N S O N Department of Radiation Oncology, E. W. Sparrow Hospital, Lansing, MI 48909 Abstract--Are therapy is one of the treatment techniques for small, centrally located deep-seated tumors. However, care must be taken to remove any components that would interfere with the beam as the gantry rotates around the patient. One such component that may interfere with the beam is the patient support assembly (PSA) or treatment table. Beam attenuation factors due to the presence of the couch side-rails and the centerspine bar of the PSA are presented for both 6 MV and 18 MV photon beams of a Clinac 1800 during a 360 ° rotation. Dose perturbations arising from these obstructions are displayed using an Alderson Rando phantom. A method is described to calculate the start and stop angles for the largest unobstructed arc for a given field size (FS), centerspine bar to the isocenter height (H), centerspine bar width (W), and SAD. As an illustration, for an SAD of 100 cm and a W of 4 cm, the start and stop angles for arc therapy with a FS of l0 × l0 cm2 and an H of 12.7 cm would be 29 ° and 331 °. Key Words: Arc therapy, Transmission factors, Linear accelerator, Treatment table.
These measurements were made for 360 ° rotation, first with no obstructions in the beam and then with the couch side-rails and the centerspine bar obstructing the beam. The centerspine was measured two ways, with and without the couch inserts in place. The dose perturbation measurements were made using an Alderson Rando anthropomorphic pelvic phantom. K o d a k XV2* verification film was sandwiched between the p h a n t o m slices and was exposed during a 360 ° rotation with the couch side-rails and then the centerspine bar obstructing the beam. The developed films were then scanned using the Multidata Film Dosimetry System*, where the optical density readings of the films were converted to dose measurements. For comparison a CT scan of the pelvic p h a n t o m was performed to generate an isodose computer plan from a G E T A R G E T ~ treatment planning computer. In addition, a method to find the optimal start and stop angles was derived using basic trigonometric principles. The start angle (O) can be calculated from the following equation:
INTRODUCTION Arc therapy, a standard technique used in radiation therapy, is an appropriate treatment choice for small, centrally located deep-seated tumors. However, care must be taken in using this technique to remove any c o m p o n e n t s that would interfere with the beam as the gantry rotates around the patient. One of these components is the patient support assembly (PSA) or treatment table of the teletherapy unit. We verified this interference by measuring the beam attenuation due to the couch side-rails and the centerspine bar of the PSA, for 6 MV and 18 MV photon beams o f a Clinac 1800* during a 360 ° rotation. Secondly, we looked at how these obstructions m a y perturb the dose to the treatment volume, using a 6 MV photon beam rotating 360 ° around an anthropomorphic phantom. These perturbations have been previously described by Krithivas and Rao from for a Clinac 4/100" PSA, using a 60 ° Arc Rotation. ~ A method is presented to solve this problem where the start and stop angles of an arc are calculated for the beam to avoid the centerspine bar. METHODS
O= tan-~(2~H)+
AND MATERIALS
The beam attenuation due to the PSA was measured in air, using a farmer-type ion chamber with a buildup cap, at the isocenter of the linear accelerator.
sin-~{SAD• sin(tan-~[FS/2"sa°]) ~ l-~+ ] -tan
Originally presented as a poster presentation 33rd Annual Meeting ofthe American Association of Physicists in Medicine, San Francisco, CA July 21-25, 1991. * Varian, Palo Alto, CA 94303.
_,[~ FS \ )
* Eastman Kodak Co., Rochester, NY 14692. t Multidata International Systems, St. Louis, MO 63119. § GE Medical Systems, Milwaukee, W! 53201. 225
(1)
226
Medical Dosimetry
Volume 17, Number 4, 1992 Table 1. Table correction factors for 360 ° rotation on a Clinac 1800 linear accelerator
FS
Centerspine bar Photon beam energy
Couchside-rails
Inserts
No inserts
6 MV 18 MV
0.971 0.976
0.971 0.982
0.984 0.991
H
SAD
ator with an SAD of 100 cm and a W of 4 cm, start and stop angles, f o r a patient with a FS of l0 × 10 cm 2 and an H of 12.7 cm, would be 29 ° and 331 °.
W RESULTS
Fig. 1. Dependent variables for calculating the start angle (0).
where the following dependent variables are shown in Figure 1: field size (FS); treatment machine's SAD: height from the centerspine bar to the isocenter (H): and width of the centerspine bar (W). The stop angle (~) is the difference between 360 ° and the start angle (O). As an illustration, for a Clinac 1800 linear acceler-
The obstructed beam attenuation measurements were normalized to the nonobstructed measurements and the results are shown in Table 1 for both 6 and 18 MV photon beams and are listed as table correction factors. These factors can then be applied to compensate for the beam attenuation during treatment planning for monitor unit settings. The results of the dose perturbation analysis can be seen in Fig. 2 (couch side-rails) and Fig. 3 (centerspine bar). These figures can be c o m p a r e d to the isodose c o m p u t e r plan (Fig. 4), which does not include the PSA obstructions. Typically, pelvic treatments are prescribed to a target volume described by the 95% or 98% isodose line. A comparison of this region shows little difference between isodose curves derived from the films and the c o m p u t e r plan. Small variations
K L-
t-
l:l~],l~[,[,I,I,l,l,l,l~lJ]Jl,l,l~i~l~l~lLl,l,l,l,l,l,l,l,l,I,],l*l
Fig. 2. Film isodose study for 360 ° rotation including the couch side-rails for a Clinac 1800, 6 M V x-rays.
Photon beam attenuation for PSA • S. C. SHARMAand M. W. JOHNSON
227
v
, I , I , I , I , I,
I , I , I ~ I , I , I , I ,I
, I ,1
, I , I , 1(
[ , I , I , I t I , I , I ,_L.d.)
[ ) [ , I i I ~]~-
Fig. 3. Film isodose study for 360 ° rotation including the centerspine bar for a Clinac 1800, 6 MV x-rays.
compared to the 360 ° rotation technique treating through the centerspine bar (Fig. 2).
may be seen in the lower isodose lines (30% or 50%), where the couch side-rails tend to decrease the dose laterally (Fig. 2) and where the centerspine bar decreases the dose posteriorly (Fig. 3). However, the target volume is essentially unaffected. The results of the isodose c o m p u t e r plan and its corresponding film study, using equation (1) to find the optimal arc, can be seen in Fig. 5 and 6. The dose can be seen to have shifted anteriorly and laterally as
./-i'
DISCUSSION The beam attenuation of both the couch siderails and centerspine bar seem to be of the same order as the transmission factors found for beam blocking trays. If the treatment beam passes through the PSA
."
.........
'
........... '--
e.
llll)i),l,l,I)l,l,l,l,l,l,l,l,l,l,l,l,l,ltl,l,l,l,l'l'l'l'l'l'l'l'l
Fig. 4. Isodose computer plan for 360 ° rotation excluding the PSA obstructions for a Clinac 1800, 6 MV x-rays.
228
Medical Dosimetry
\\
Volume 17, Number 4, 1992
//
cM
Fig. 5. lsodose computer plan for an arc, calculated to avoid the centerspine bar tbr a Clinac 1800, 6 MV x-rays.
F
I~l~l~lJl,l,l~l~l,l,l,l,l,[,I,l,l,l,l,l,l,l~l~l~l~l,l~l,l,l,l,I,
Fig. 6. Film isodose study for an arc, calculated to avoid the centerspine bar for a Clinac 1800, 6 MV x-rays.
t h e n we r e c o m m e n d that a t r a n s m i s s i o n factor for the PSA be used for m o n i t o r unit calculations. If t r e a t i n g t h r o u g h the PSA is not desirable, e q u a t i o n (1) calculates an o p t i m a l start a n d s t o p angle for an arc p r o v i d ing e q u i v a l e n t coverage o f the t r e a t m e n t v o l u m e .
REFERENCE 1. Krithivas, G.; Rao, S.N. Therapy dose perturbation due to the center bar in a Clinac 4/100 couch assembly. Med. Dos. 13:1518; 1988.
