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IMRT of large fields: whole abdomen irradiation
124
I. J. Radiation Oncology
● Biology ● Physics
Volume 51, Number 3, Supplement 1, 2001
clinical decision using dosimetric criteria, EUD, TCP, and NTCP. IMRT plans were also compared to the 3D conformal radiotherapy clinical treatment plan. Results: Based on clinical CT/MR scan evaluation, we predicted that both optic nerves could be spared without significant PTV compromise (4 cases) while in 11 cases we predicted that only the contralateral optic nerve could be spared without signficant PTV compromise. However, the initial CT/MR evaluation was insufficient to correctly predict the tradeoff associated with sparing both optic pathways. When analyzing optimized IMRT plans (sparing one or both OP), DVH comparisons of PTV coverage did not demonstrate any major differences: PTV compromise expected from plans sparing both OP was only observed in one case. Only upon closer inspection (using the following dosimetric endpoints) did we observe the clinical tradeoffs we initially expected.(Table) PTV minimum dose and V95 (volume of target receiving 95% of the prescription dose) decreased in plans where both optic nerves were spared (p⫽.0001), a compromise which may have clinical relevance for tumor control. The tradeoff is that the NTCP for the ipsilateral optic nerve was significantly improved by the plan sparing both optic nerves (p⫽0.0001). Conclusion: Our study shows that the initial CT/MR evaluation was insufficient in predicting correctly the tradeoff of sparing both OP in IMRT for paranasal sinus tumors. In addition, we found that DVH comparisons alone are also insufficient in choosing among competing IMRT solutions requiring a tradeoff between organ sparing and small compromises in PTV coverage. Quantative information about specific tradeoffs is important to the clinical decision-making process. Using multiple metrics to evaluate different IMRT plans (as opposed to reliance on DVH comparisons alone), facilitates the analysis of these tradeoffs.
Structure PTV
Ipsil. Optic Nerve
221
Metric Min Dose V95 EUD TCP Max Dose NTCP
Spare Contralateral OP 59.8 ⫾ 1.6 90.0 ⫾ 2.3 71.4 ⫾ 0.6 70 ⫾ 4.4 77.3 ⫾ 2.0 62.1 ⫾ 11.6
Spare Bilateral OP
P
55.1 ⫾ 1.5 80.1 ⫾ 8.3 70.3 ⫾ 0.8 66 ⫾ 4.5 56.0 ⫾ 1.3 11.4 ⫾ 5.2
0.0001 0.009 0.004 0.001 0.0001 0.0001
IMRT of Large Fields: Whole Abdomen Irradiation 1
L. Hong , K. Alektiar2, C. Chui1, T. LoSasso1, S. Spirou1, J. Yang1, M. Hunt1, H. Amols, C. Ling1, Z. Fuks2, S. Leibel2 1 Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY, 2Radiation Oncology, Memorial SloanKettering Cancer Center, New York, NY
Purpose: To develop an approach to planning and delivery of intensity-modulated whole abdomen irradiation fields with dynamic multi-leaf collimators (DMLC) to spare kidneys and bone marrow. Because of the large field sizes required, the impact of scattered dose on the inverse planning process and dosimetry verification was also investigated. Methods to circumvent field size restrictions of current linear accelerators were presented. Materials and Methods: CT simulation and 3D treatment planning were performed for 10 patients treated in a supine position. The GTV included the entire peritoneal cavity. The PTV encompassed an axial margin of 5mm around the GTV, and extended 1cm beyond the GTV in both the superior and the inferior direction. The kidneys, liver, spinal column, pelvis and femurs were also contoured. Among these patients, the ranges of PTV dimensions were: length 35 - 46cm (median 44cm), width 27 - 35cm (median 30cm), and depth 17 - 23cm (median 19cm). One isocenter was defined for patients with field length ⬍ 40cm. For patients with fields longer than 40cm, two isocenters were defined: one in the abdominal region and the other in the pelvic region. Five-field IMRT plans using 15MV photons were generated at gantry angles of 180°, 105°, 45°, 315° and 255°. Optimization was designed to spare kidneys and bones. For the prescription dose of 30 Gy, an 18 Gy dose constraint to the kidneys created highly modulated intensity profiles for some beams, necessitating ⬃ 30 calculation points per cc to reduce noise in the calculated intensity profiles. Although the scattered dose contribution was significant due to the large PTV (⬃ 8000cc), its inclusion during the inverse planning process would be impractical for clinical use because of the prohibitive computer memory and computation power needed for the large number (⬃ 240,000) of calculation points. To circumvent this problem, only primary radiation was included during the optimization, and the dose distribution was recalculated with full scatter contributions at the end of each optimization cycle. The difference between the actual dose and the desired dose was then used for next optimization cycle, and the process was iterated until further improvement became minimal. Due to the present limitation of the DMLC delivery system, a field with width ⬎ 15 cm was split into two sub-fields, and one with width ⬎ 27 cm into three. To minimize field match errors, adjacent sub-fields overlapped by at least 2 cm, with intensity ‘feathering‘ in the junction region. For patients with two isocenters, fields were overlapped lengthwise by at least 3 cm with ‘feathering‘ incorporated in the junctions through intensity modulation. Radiographic film dosimetry was performed for dose verification. Results: Homogeneous dose distributions in the target volumes, and kidney and bone sparing, were achieved with IMRT. Optimization time varied from 20 min to 80 min with the use of a 500MHz DEC alpha workstation. The average total number of DMLC beams, after splitting, was 11 and 21 for patients with one and two isocenters, respectively. The combined dose distributions of split sub-fields reproduced those of the original fields. To delivery 150 cGy, on average the total DMLC MU was 1150 and 1630 for patients with one and two isocenters respectively. The need for sub-fields caused some loss of efficiency on DMLC delivery, with increase in MU of 5% to 30%. Due to overlapping of sub-fields, scatter and leakage dose from the multi-leaf collimators of one sub-field contributed to adjacent sub-fields, and these doses were proportional to the monitor units. For very large fields, scattered dose from the MLC contributed about 4% to the total dose. To achieve agreement between measurement and calculation of 5% or better for these fields, the MLC scatter must be considered. Conclusion: We have developed and clinically implemented methods to plan and deliver whole abdomen IMRT using current linear accelerators. These methods can be applied to other sites requiring large field irradiation.
I. J. Radiation Oncology
● Biology ● Physics
Volume 51, Number 3, Supplement 1, 2001
clinical decision using dosimetric criteria, EUD, TCP, and NTCP. IMRT plans were also compared to the 3D conformal radiotherapy clinical treatment plan. Results: Based on clinical CT/MR scan evaluation, we predicted that both optic nerves could be spared without significant PTV compromise (4 cases) while in 11 cases we predicted that only the contralateral optic nerve could be spared without signficant PTV compromise. However, the initial CT/MR evaluation was insufficient to correctly predict the tradeoff associated with sparing both optic pathways. When analyzing optimized IMRT plans (sparing one or both OP), DVH comparisons of PTV coverage did not demonstrate any major differences: PTV compromise expected from plans sparing both OP was only observed in one case. Only upon closer inspection (using the following dosimetric endpoints) did we observe the clinical tradeoffs we initially expected.(Table) PTV minimum dose and V95 (volume of target receiving 95% of the prescription dose) decreased in plans where both optic nerves were spared (p⫽.0001), a compromise which may have clinical relevance for tumor control. The tradeoff is that the NTCP for the ipsilateral optic nerve was significantly improved by the plan sparing both optic nerves (p⫽0.0001). Conclusion: Our study shows that the initial CT/MR evaluation was insufficient in predicting correctly the tradeoff of sparing both OP in IMRT for paranasal sinus tumors. In addition, we found that DVH comparisons alone are also insufficient in choosing among competing IMRT solutions requiring a tradeoff between organ sparing and small compromises in PTV coverage. Quantative information about specific tradeoffs is important to the clinical decision-making process. Using multiple metrics to evaluate different IMRT plans (as opposed to reliance on DVH comparisons alone), facilitates the analysis of these tradeoffs.
Structure PTV
Ipsil. Optic Nerve
221
Metric Min Dose V95 EUD TCP Max Dose NTCP
Spare Contralateral OP 59.8 ⫾ 1.6 90.0 ⫾ 2.3 71.4 ⫾ 0.6 70 ⫾ 4.4 77.3 ⫾ 2.0 62.1 ⫾ 11.6
Spare Bilateral OP
P
55.1 ⫾ 1.5 80.1 ⫾ 8.3 70.3 ⫾ 0.8 66 ⫾ 4.5 56.0 ⫾ 1.3 11.4 ⫾ 5.2
0.0001 0.009 0.004 0.001 0.0001 0.0001
IMRT of Large Fields: Whole Abdomen Irradiation 1
L. Hong , K. Alektiar2, C. Chui1, T. LoSasso1, S. Spirou1, J. Yang1, M. Hunt1, H. Amols, C. Ling1, Z. Fuks2, S. Leibel2 1 Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, NY, 2Radiation Oncology, Memorial SloanKettering Cancer Center, New York, NY
Purpose: To develop an approach to planning and delivery of intensity-modulated whole abdomen irradiation fields with dynamic multi-leaf collimators (DMLC) to spare kidneys and bone marrow. Because of the large field sizes required, the impact of scattered dose on the inverse planning process and dosimetry verification was also investigated. Methods to circumvent field size restrictions of current linear accelerators were presented. Materials and Methods: CT simulation and 3D treatment planning were performed for 10 patients treated in a supine position. The GTV included the entire peritoneal cavity. The PTV encompassed an axial margin of 5mm around the GTV, and extended 1cm beyond the GTV in both the superior and the inferior direction. The kidneys, liver, spinal column, pelvis and femurs were also contoured. Among these patients, the ranges of PTV dimensions were: length 35 - 46cm (median 44cm), width 27 - 35cm (median 30cm), and depth 17 - 23cm (median 19cm). One isocenter was defined for patients with field length ⬍ 40cm. For patients with fields longer than 40cm, two isocenters were defined: one in the abdominal region and the other in the pelvic region. Five-field IMRT plans using 15MV photons were generated at gantry angles of 180°, 105°, 45°, 315° and 255°. Optimization was designed to spare kidneys and bones. For the prescription dose of 30 Gy, an 18 Gy dose constraint to the kidneys created highly modulated intensity profiles for some beams, necessitating ⬃ 30 calculation points per cc to reduce noise in the calculated intensity profiles. Although the scattered dose contribution was significant due to the large PTV (⬃ 8000cc), its inclusion during the inverse planning process would be impractical for clinical use because of the prohibitive computer memory and computation power needed for the large number (⬃ 240,000) of calculation points. To circumvent this problem, only primary radiation was included during the optimization, and the dose distribution was recalculated with full scatter contributions at the end of each optimization cycle. The difference between the actual dose and the desired dose was then used for next optimization cycle, and the process was iterated until further improvement became minimal. Due to the present limitation of the DMLC delivery system, a field with width ⬎ 15 cm was split into two sub-fields, and one with width ⬎ 27 cm into three. To minimize field match errors, adjacent sub-fields overlapped by at least 2 cm, with intensity ‘feathering‘ in the junction region. For patients with two isocenters, fields were overlapped lengthwise by at least 3 cm with ‘feathering‘ incorporated in the junctions through intensity modulation. Radiographic film dosimetry was performed for dose verification. Results: Homogeneous dose distributions in the target volumes, and kidney and bone sparing, were achieved with IMRT. Optimization time varied from 20 min to 80 min with the use of a 500MHz DEC alpha workstation. The average total number of DMLC beams, after splitting, was 11 and 21 for patients with one and two isocenters, respectively. The combined dose distributions of split sub-fields reproduced those of the original fields. To delivery 150 cGy, on average the total DMLC MU was 1150 and 1630 for patients with one and two isocenters respectively. The need for sub-fields caused some loss of efficiency on DMLC delivery, with increase in MU of 5% to 30%. Due to overlapping of sub-fields, scatter and leakage dose from the multi-leaf collimators of one sub-field contributed to adjacent sub-fields, and these doses were proportional to the monitor units. For very large fields, scattered dose from the MLC contributed about 4% to the total dose. To achieve agreement between measurement and calculation of 5% or better for these fields, the MLC scatter must be considered. Conclusion: We have developed and clinically implemented methods to plan and deliver whole abdomen IMRT using current linear accelerators. These methods can be applied to other sites requiring large field irradiation.