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The application of electron beam delivery using dose rate variation and dynamic couch motion in conformal treatment of the cranial-spinal axis
Medical Dosimetry, Vol. 27, No. 4, pp. 265–268, 2002 Copyright © 2002 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/02/$–see front matter
PII: S0958-3947(02)00145-0
THE APPLICATION OF ELECTRON BEAM DELIVERY USING DOSE RATE VARIATION AND DYNAMIC COUCH MOTION IN CONFORMAL TREATMENT OF THE CRANIAL-SPINAL AXIS JULIE CHAPEK, C.M.D., GORDON WATSON, M.D., PH.D., LYNN M. SMITH, M.D., and DENNIS D. LEAVITT, PH.D. University of Utah Health Science Center, Salt Lake City, UT (Accepted 15 April 2001)
Abstract—Radiation therapy to the cranial-spinal axis is typically targeted to the spinal cord and to the cerebrospinal fluid (CSF) in the subarachnoid space adjacent to the spinal cord and brain. Standard techniques employed in the treatment of the whole central nervous system do little to compensate for the varying depths of spinal cord along the length of the spinal field. Lateral simulation films, sagittal magnetic resonance imaging (MRI), or computerized tomography (CT) are used to estimate an average prescription depth for treatment along the spine field. However, due to the varying depth of the target along the spinal axis, even with the use of physical compensators, there can be considerable dose inhomogeneity along the spine field. With the advent of treatment machines that have full dynamic capabilities, a technique has been devised that will allow for more conformal dose distribution along the full length of the spinal field. This project simulates this technique utilizing computer-controlled couch motion to deliver multiple small electron beams of differing energies and intensities. CT planning determines target depth along the entire spine volume. The ability to conform dose along the complete length of the treatment field is investigated through the application of superpositioning of the fields as energies and intensities change. The positioning of each beam is registered with the treatment couch dynamic motion. This allows for 1 setup in the treatment room rather than multiple setups for each treatment position, which would have been previously required. Dose-volume histograms are utilized to evaluate the dose delivered to structures in the beam exit region. This technique will allow for precise localization and delivery of a homogeneous dose to the entire CSF space. © 2002 American Association of Medical Dosimetrists. Key Words:
Treatment planning, Dynamic couch motion, Dynamic dose rate, Electrons.
irradiation to the target volume while minimizing exit dose through the anterior spine, chest, and abdomen. This method utilizes an electron beam with dynamic couch motion, dose rate, and energy variation to achieve conformal treatment to the spine. Others have utilized stationary electron beams to treat the spine.1–3 Problems with a stationary technique include dose inhomogenity within the treatment field and the need for several stationary electron fields to encompass the entire spine. This leads to hot or cold spots where fields abut. Inhomogenity can be minimized through the use of individualized compensators placed on the patient’s skin surface. However, this leads to an increase in skin dose.1,4 One group has outlined a method for accurately achieving a homogeneous dose between the photon brain fields and electron spine fields.1
INTRODUCTION Radiation therapy of the cranial-spinal axis is utilized in the treatment of cancers that have either spread or have the potential to spread throughout the cerebrospinal fluid (CSF), such as medulloblastoma or supratentorial primitive neuroectodermal tumors (PNETs). The target volume is the subarachnoid space, where the CSF bathes the brain and spinal cord. The most commonly employed technique to deliver cranial-spinal irradiation incorporates lateral-opposed photon beams to treat the brain matched to a direct posterior photon spine field. Technical problems encountered with this treatment technique include dose inhomogenity along the spine field, exit dose from the upper spine field through the neck, thorax, and abdomen and, in some patients, the inability to incorporate the spine into one radiation field. Treatmentrelated side effects of such therapy can include gastrointestinal symptoms of nausea, vomiting, and diarrhea during treatment, and bone growth abnormalities in children. To overcome some of these technical- and therapyrelated side effects of cranial-spinal irradiation, we describe a method for achieving a homogenous dose of
METHODS AND MATERIALS Positioning and treatment planning A RANDO (Alderson Research Laboratory, Stamford, CT) phantom was placed in a prone position in an alpha cradle. (Smithers Medical Systems) A treatment planning CT was obtained at 0.5-cm intervals through the entire length of the spine target volume. Target volumes were defined on the scans by a physician. A
Reprint requests to: J. Chapek, University of Utah, Department of Radiation Oncology, Health Science Center, 50 North Medical Drive, Salt Lake City, UT 84132. E-mail: [email protected] 265
266
Medical Dosimetry
Fig. 1. Diagrammatic representation of field superpositioning. The horizontal lines posterior to the patient each represent a small electron beam. In this project, we used 64 superpositioned beams to cover the target length. The first field in the treatment volume is 1-cm long, the next 2 cm, the next 3 cm and so on, until 5 cm was reached. The first 5 fields have a shared contiguous (superior) border. From this point on, the field is shifted 1-cm inferior. At the conclusion of the treatment sequence, this routine is repeated in the reverse order to achieve a contiguous border.
sagittal contour was reconstructed from these transverse CT images. To simulate dynamic couch motion, multiple small individual electron fields were superpositioned along the full length of the target volume (Fig. 1). This superposition of beams simulates the “overlap” of beams as the dynamic couch motion moves the target volume through the path of the beam. In this case, a spinal field length of 60 cm was treated. Each field was of a width required to cover the lateral dimensions of the target volume. At the commencement of the treatment sequence, the first field was 1 cm in length, the next field 2 cm, the next 3 cm, and so on, until a length of 5 cm was reached. These first 5 beams all shared a contiguous edge to create the superior field border. Through the central part of the treatment length, the 5-cm fields were sequentially shifted 1 cm inferior. At the conclusion of the treatment sequence, the same procedure was carried out as at the beginning, in the reverse order to produce a contiguous border at the termination (inferior border) of the treat-
Volume 27, Number 4, 2002
ment volume. For this case, we used 64 segments to cover the target volume. A source-to-surface distance (SSD) of 100 cm was set to the most posterior point on the contour along the length of the target volume. Because this study simulates couch motion to move the patient through the electron beam with no vertical motion of the couch, the relative SSD of the individual electron fields varied slightly with differences in the sagittal contour. The energy for each beam was determined by measuring the depth to the anterior edge of the target volume at intervals along the target length and selecting the electron energy that would ensure coverage of that point by the 95% isodose line for each individual segment. Variation in dose rate was simulated by varying the individual beam weights along the length of the field. RESULTS The dose distribution achieved using this technique is shown in Fig. 2. The 100% isodose line covers the entire target volume with a high degree of conformality. Figure 3 shows a comparison dose distribution for 2 matched posterior photon spine fields. The photon technique consists of one 40-cm-long 6-MV photon field matched to a 6-MV photon field of length required to encompass the remaining length of target volume. The match occurs at depth at the anterior aspect of the spinal cord using the appropriate gap calculation to establish the gap required on the skin to achieve this match. Comparing these 2 distributions shows that the electron technique has a superior conformity to the target volume with rapid dose fall off anteriorly, thus minimizing exit dose to critical structures. The maximum dose in this plane for the photon treatment is 170%, occurring in the beam match region, compared to the maximum dose of 130% for the electron plan. A higher relative dose is required with the photon beam to ensure coverage of the target volume by the 100% isodose line (Fig. 4). This explains the higher value isodose lines posterior to the target volume in the photon distribution (Fig. 3) compared to the electron distribution (Fig. 2). The electron
Fig. 2. Isodose distribution for electron technique. Note conformity of the 100% line. Maximum dose in this plane is 130%. Note rapid falloff of dose anterior to target volume.
Conformal electron cranial spinal irradiation ● J. CHAPEK et al.
267
Fig. 3. Isodose distribution of photon technique. Note larger 120% volume than electron technique. Maximum dose in this plane is 170%, occurring in the match region of the beams.
technique allows for customized normalization of the dose at different depths at each segment of treatment to conform 100% to the target. The conformation of the 100% line in the photon technique (Fig. 3) could be improved through the use of compensators. The dosimetry at the junction of the 2 fields can be improved to some extent through the use of matchline moves. Nothing can be done to decrease the
exit dose to critical structures. For both the electron and photon techniques, sophisticated algorithms in treatment planning systems will be able to accurately calculate the dose corrections for the heterogeneity of the spinal processes. Figure 5 shows the dose-volume histogram for the heart. For the electron plan, 50% of the heart is receiving 2.5% of prescription dose, compared to 52.5% of prescription dose from the photon technique. This shows that the electron beam is far superior to the photon beam in limiting the dose to the heart and other normal structures.
DISCUSSION
Fig. 4. Relative dose, photons vs. electrons. A higher relative dose is required with the photon technique to ensure coverage of the target by 100%. The electron technique allows for customized normalization of the dose at each segment of treatment to conform 100% to the target, hence a lower relative dose is delivered.
This study shows that the use of multiple small electron beams to treat the spine field of the cranialspinal axis allows for precise conformation of dose along the entire length of the target volume. To achieve this, dose rate and energy can be adjusted along the length of the target. Because conformation of dose is achieved through the adjustment of energy and dose rate rather than by placement of a compensator on the skin surface, there is no increase in skin dose. With the use of dynamic couch motion, it will be possible for therapists to set up 1 field in the treatment room with positioning of each subsequent beam controlled by the computerized dynamic couch motion. With this technique, there will be no need to abut or match any of the electron fields, as the dynamic couch motion will ensure that the entire length of the spinal field is treated as 1 continuous field. Thus, the potential for treatment delivery errors due to incorrect field matching or junction shifts will be eliminated. The significant reduction in dose to normal tissues such as the heart will benefit patients by decreasing treatmentrelated morbidity. Acute side effects such as esophagitis, cystitis, nausea, and pneumenitis should be reduced. More importantly, late toxicities such as cardiac dysfunction and risk of myocardial infarction, thyroid dysfunc-
268
Medical Dosimetry
Volume 27, Number 4, 2002
effectively 1 continuous field is equal to the limit of couch motion available. On our Varian 2100CD Clinac (Varian Medical Systems, Palo Alto, CA) there is a range of 91 cm, and if the couch was rotated to 270° or 90°, this full range of translation would be available. CONCLUSIONS The benefits of this dynamic electron technique in the treatment of the spinal field in cranial-spinal axis irradiation are: 1. precise localization and delivery of a homogeneous dose to the entire CSF space along the spine; 2. easier and less error prone setup, potentially reducing treatment time; and 3. decreased dose to healthy tissue anterior to the target volume. This will potentially minimize longterm treatment effects in pediatric patients, such as growth abnormalities in the spinal column, and low-dose effects to the heart and other organs. Further developments that will enable smooth clinical application of the technique will include more refined techniques for dynamic couch control along with utilization of record and verify systems. Fig. 5. DVH (heart), electrons vs. photons. There is significant decrease in heart dose with the electron technique. As seen in Figs. 2 and 3, the dose to the heart from the electron technique is limited to the bremstrahlung dose only (less than 5% of maximum dose), while the heart receives 50% to 80% of prescription dose from the photon technique.
tion, pulmonary fibrosis, and bone growth abnormalities may be reduced. We simulated a target length of 60 cm with this technique. It would not be possible to treat this length of field with a single static beam without using an extended distance technique. With this technique, the factor limiting the length of target that can be treated with what is
REFERENCES 1. Li, C.; Muller-Runkel, R.; Vijayakumar, S.; Myrianthopoulos, L.C.; Kuchinir, F.T. Craniospinal axis irradiation: An improved electron technique for irradiation of the spinal axis. Br. J. Radiol. 67:186 – 93; 1994. 2. Maor, M.H.; Fields, R.S.; Hogstrom, K.R.; Van Eys, J. Improving the therapeutic ratio of craniospinal irradiation in medulloblastoma. Int. J. Radiat. Oncol. Biol. Phys. 11:687; 1985. 3. Dewit L., Van Dam J., Rijnders A., Van De Velde G., Kian Ang K., Van Der Schueren E. A modified radiotherapy technique in the treatment of medulloblastoma. Int. J. Radiat. Oncol. Biol. Phys. 10:213– 41; 1984. 4. Low, D.A.; Starkschall, G.; Bunjnowski, S.W.; Wang, L.L.; Hogstrom, K.R. Electron bolus design for radiotherapy treatment planning: Bolus design algorithms. Med. Phys. 19:115–24; 1992.
PII: S0958-3947(02)00145-0
THE APPLICATION OF ELECTRON BEAM DELIVERY USING DOSE RATE VARIATION AND DYNAMIC COUCH MOTION IN CONFORMAL TREATMENT OF THE CRANIAL-SPINAL AXIS JULIE CHAPEK, C.M.D., GORDON WATSON, M.D., PH.D., LYNN M. SMITH, M.D., and DENNIS D. LEAVITT, PH.D. University of Utah Health Science Center, Salt Lake City, UT (Accepted 15 April 2001)
Abstract—Radiation therapy to the cranial-spinal axis is typically targeted to the spinal cord and to the cerebrospinal fluid (CSF) in the subarachnoid space adjacent to the spinal cord and brain. Standard techniques employed in the treatment of the whole central nervous system do little to compensate for the varying depths of spinal cord along the length of the spinal field. Lateral simulation films, sagittal magnetic resonance imaging (MRI), or computerized tomography (CT) are used to estimate an average prescription depth for treatment along the spine field. However, due to the varying depth of the target along the spinal axis, even with the use of physical compensators, there can be considerable dose inhomogeneity along the spine field. With the advent of treatment machines that have full dynamic capabilities, a technique has been devised that will allow for more conformal dose distribution along the full length of the spinal field. This project simulates this technique utilizing computer-controlled couch motion to deliver multiple small electron beams of differing energies and intensities. CT planning determines target depth along the entire spine volume. The ability to conform dose along the complete length of the treatment field is investigated through the application of superpositioning of the fields as energies and intensities change. The positioning of each beam is registered with the treatment couch dynamic motion. This allows for 1 setup in the treatment room rather than multiple setups for each treatment position, which would have been previously required. Dose-volume histograms are utilized to evaluate the dose delivered to structures in the beam exit region. This technique will allow for precise localization and delivery of a homogeneous dose to the entire CSF space. © 2002 American Association of Medical Dosimetrists. Key Words:
Treatment planning, Dynamic couch motion, Dynamic dose rate, Electrons.
irradiation to the target volume while minimizing exit dose through the anterior spine, chest, and abdomen. This method utilizes an electron beam with dynamic couch motion, dose rate, and energy variation to achieve conformal treatment to the spine. Others have utilized stationary electron beams to treat the spine.1–3 Problems with a stationary technique include dose inhomogenity within the treatment field and the need for several stationary electron fields to encompass the entire spine. This leads to hot or cold spots where fields abut. Inhomogenity can be minimized through the use of individualized compensators placed on the patient’s skin surface. However, this leads to an increase in skin dose.1,4 One group has outlined a method for accurately achieving a homogeneous dose between the photon brain fields and electron spine fields.1
INTRODUCTION Radiation therapy of the cranial-spinal axis is utilized in the treatment of cancers that have either spread or have the potential to spread throughout the cerebrospinal fluid (CSF), such as medulloblastoma or supratentorial primitive neuroectodermal tumors (PNETs). The target volume is the subarachnoid space, where the CSF bathes the brain and spinal cord. The most commonly employed technique to deliver cranial-spinal irradiation incorporates lateral-opposed photon beams to treat the brain matched to a direct posterior photon spine field. Technical problems encountered with this treatment technique include dose inhomogenity along the spine field, exit dose from the upper spine field through the neck, thorax, and abdomen and, in some patients, the inability to incorporate the spine into one radiation field. Treatmentrelated side effects of such therapy can include gastrointestinal symptoms of nausea, vomiting, and diarrhea during treatment, and bone growth abnormalities in children. To overcome some of these technical- and therapyrelated side effects of cranial-spinal irradiation, we describe a method for achieving a homogenous dose of
METHODS AND MATERIALS Positioning and treatment planning A RANDO (Alderson Research Laboratory, Stamford, CT) phantom was placed in a prone position in an alpha cradle. (Smithers Medical Systems) A treatment planning CT was obtained at 0.5-cm intervals through the entire length of the spine target volume. Target volumes were defined on the scans by a physician. A
Reprint requests to: J. Chapek, University of Utah, Department of Radiation Oncology, Health Science Center, 50 North Medical Drive, Salt Lake City, UT 84132. E-mail: [email protected] 265
266
Medical Dosimetry
Fig. 1. Diagrammatic representation of field superpositioning. The horizontal lines posterior to the patient each represent a small electron beam. In this project, we used 64 superpositioned beams to cover the target length. The first field in the treatment volume is 1-cm long, the next 2 cm, the next 3 cm and so on, until 5 cm was reached. The first 5 fields have a shared contiguous (superior) border. From this point on, the field is shifted 1-cm inferior. At the conclusion of the treatment sequence, this routine is repeated in the reverse order to achieve a contiguous border.
sagittal contour was reconstructed from these transverse CT images. To simulate dynamic couch motion, multiple small individual electron fields were superpositioned along the full length of the target volume (Fig. 1). This superposition of beams simulates the “overlap” of beams as the dynamic couch motion moves the target volume through the path of the beam. In this case, a spinal field length of 60 cm was treated. Each field was of a width required to cover the lateral dimensions of the target volume. At the commencement of the treatment sequence, the first field was 1 cm in length, the next field 2 cm, the next 3 cm, and so on, until a length of 5 cm was reached. These first 5 beams all shared a contiguous edge to create the superior field border. Through the central part of the treatment length, the 5-cm fields were sequentially shifted 1 cm inferior. At the conclusion of the treatment sequence, the same procedure was carried out as at the beginning, in the reverse order to produce a contiguous border at the termination (inferior border) of the treat-
Volume 27, Number 4, 2002
ment volume. For this case, we used 64 segments to cover the target volume. A source-to-surface distance (SSD) of 100 cm was set to the most posterior point on the contour along the length of the target volume. Because this study simulates couch motion to move the patient through the electron beam with no vertical motion of the couch, the relative SSD of the individual electron fields varied slightly with differences in the sagittal contour. The energy for each beam was determined by measuring the depth to the anterior edge of the target volume at intervals along the target length and selecting the electron energy that would ensure coverage of that point by the 95% isodose line for each individual segment. Variation in dose rate was simulated by varying the individual beam weights along the length of the field. RESULTS The dose distribution achieved using this technique is shown in Fig. 2. The 100% isodose line covers the entire target volume with a high degree of conformality. Figure 3 shows a comparison dose distribution for 2 matched posterior photon spine fields. The photon technique consists of one 40-cm-long 6-MV photon field matched to a 6-MV photon field of length required to encompass the remaining length of target volume. The match occurs at depth at the anterior aspect of the spinal cord using the appropriate gap calculation to establish the gap required on the skin to achieve this match. Comparing these 2 distributions shows that the electron technique has a superior conformity to the target volume with rapid dose fall off anteriorly, thus minimizing exit dose to critical structures. The maximum dose in this plane for the photon treatment is 170%, occurring in the beam match region, compared to the maximum dose of 130% for the electron plan. A higher relative dose is required with the photon beam to ensure coverage of the target volume by the 100% isodose line (Fig. 4). This explains the higher value isodose lines posterior to the target volume in the photon distribution (Fig. 3) compared to the electron distribution (Fig. 2). The electron
Fig. 2. Isodose distribution for electron technique. Note conformity of the 100% line. Maximum dose in this plane is 130%. Note rapid falloff of dose anterior to target volume.
Conformal electron cranial spinal irradiation ● J. CHAPEK et al.
267
Fig. 3. Isodose distribution of photon technique. Note larger 120% volume than electron technique. Maximum dose in this plane is 170%, occurring in the match region of the beams.
technique allows for customized normalization of the dose at different depths at each segment of treatment to conform 100% to the target. The conformation of the 100% line in the photon technique (Fig. 3) could be improved through the use of compensators. The dosimetry at the junction of the 2 fields can be improved to some extent through the use of matchline moves. Nothing can be done to decrease the
exit dose to critical structures. For both the electron and photon techniques, sophisticated algorithms in treatment planning systems will be able to accurately calculate the dose corrections for the heterogeneity of the spinal processes. Figure 5 shows the dose-volume histogram for the heart. For the electron plan, 50% of the heart is receiving 2.5% of prescription dose, compared to 52.5% of prescription dose from the photon technique. This shows that the electron beam is far superior to the photon beam in limiting the dose to the heart and other normal structures.
DISCUSSION
Fig. 4. Relative dose, photons vs. electrons. A higher relative dose is required with the photon technique to ensure coverage of the target by 100%. The electron technique allows for customized normalization of the dose at each segment of treatment to conform 100% to the target, hence a lower relative dose is delivered.
This study shows that the use of multiple small electron beams to treat the spine field of the cranialspinal axis allows for precise conformation of dose along the entire length of the target volume. To achieve this, dose rate and energy can be adjusted along the length of the target. Because conformation of dose is achieved through the adjustment of energy and dose rate rather than by placement of a compensator on the skin surface, there is no increase in skin dose. With the use of dynamic couch motion, it will be possible for therapists to set up 1 field in the treatment room with positioning of each subsequent beam controlled by the computerized dynamic couch motion. With this technique, there will be no need to abut or match any of the electron fields, as the dynamic couch motion will ensure that the entire length of the spinal field is treated as 1 continuous field. Thus, the potential for treatment delivery errors due to incorrect field matching or junction shifts will be eliminated. The significant reduction in dose to normal tissues such as the heart will benefit patients by decreasing treatmentrelated morbidity. Acute side effects such as esophagitis, cystitis, nausea, and pneumenitis should be reduced. More importantly, late toxicities such as cardiac dysfunction and risk of myocardial infarction, thyroid dysfunc-
268
Medical Dosimetry
Volume 27, Number 4, 2002
effectively 1 continuous field is equal to the limit of couch motion available. On our Varian 2100CD Clinac (Varian Medical Systems, Palo Alto, CA) there is a range of 91 cm, and if the couch was rotated to 270° or 90°, this full range of translation would be available. CONCLUSIONS The benefits of this dynamic electron technique in the treatment of the spinal field in cranial-spinal axis irradiation are: 1. precise localization and delivery of a homogeneous dose to the entire CSF space along the spine; 2. easier and less error prone setup, potentially reducing treatment time; and 3. decreased dose to healthy tissue anterior to the target volume. This will potentially minimize longterm treatment effects in pediatric patients, such as growth abnormalities in the spinal column, and low-dose effects to the heart and other organs. Further developments that will enable smooth clinical application of the technique will include more refined techniques for dynamic couch control along with utilization of record and verify systems. Fig. 5. DVH (heart), electrons vs. photons. There is significant decrease in heart dose with the electron technique. As seen in Figs. 2 and 3, the dose to the heart from the electron technique is limited to the bremstrahlung dose only (less than 5% of maximum dose), while the heart receives 50% to 80% of prescription dose from the photon technique.
tion, pulmonary fibrosis, and bone growth abnormalities may be reduced. We simulated a target length of 60 cm with this technique. It would not be possible to treat this length of field with a single static beam without using an extended distance technique. With this technique, the factor limiting the length of target that can be treated with what is
REFERENCES 1. Li, C.; Muller-Runkel, R.; Vijayakumar, S.; Myrianthopoulos, L.C.; Kuchinir, F.T. Craniospinal axis irradiation: An improved electron technique for irradiation of the spinal axis. Br. J. Radiol. 67:186 – 93; 1994. 2. Maor, M.H.; Fields, R.S.; Hogstrom, K.R.; Van Eys, J. Improving the therapeutic ratio of craniospinal irradiation in medulloblastoma. Int. J. Radiat. Oncol. Biol. Phys. 11:687; 1985. 3. Dewit L., Van Dam J., Rijnders A., Van De Velde G., Kian Ang K., Van Der Schueren E. A modified radiotherapy technique in the treatment of medulloblastoma. Int. J. Radiat. Oncol. Biol. Phys. 10:213– 41; 1984. 4. Low, D.A.; Starkschall, G.; Bunjnowski, S.W.; Wang, L.L.; Hogstrom, K.R. Electron bolus design for radiotherapy treatment planning: Bolus design algorithms. Med. Phys. 19:115–24; 1992.