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Is IMRT of lung possible without respiratory gating? Clinical evaluation of a true dose received by a moving target volume
Proceedings of the 43rd Annual ASTRO Meeting
27
enables a patient to turn radiation beam on and off freely and repeatedly during a term designated by a radiation technologist. Ten real-time portal images were taken for each patient during irradiation under a patient’s self-breath-hold and self-switching radiationbeam, and the accuracy of reproducibility of the tumor position in the radiation field was visually evaluated. Results: The average of maximum differences of the tumor position in three series of 20 patients under the breath-hold by a radiation technologist’s instruction was 3.0 mm in cranio-caudal direction, 2.1mm in antero-posterior direction and 2.3mm in right-left direction. And that in three series under the breath-hold by a patient’s self-estimation was 2.3mm in cranio-caudal direction, 1.4mm in antero-posterior direction and 1.4mm in right-left direction, respectively. A standard deviation of these differences was larger in the method of breath-hold by a radiation technologist’s instruction than in the method of breath-hold by a patient’s self-estimation. And these differences were larger in tumors of lower lung field than upper lung field. There was statistically significant difference between two methods of breath-hold. The actual switching of the radiation beam was delayed less than 0.1 second behind the patient’s switching. All portal images of 20 patients had a visually sufficient accuracy of tumor position in the radiation field with a difference less than 3mm between a planned position and actual tumor positions. Conclusion: The reproducibility of tumor position under patient’s self breath-hold without any respiratory monitoring devices had a satisfactory accuracy with a difference of tumor position less than 3mm in all directions. The method of breath-hold by a patient’s self-estimation was more accurate than that by a radiation technologist’s instruction. Our newly developed switch which enabled patients to turn the radiation-beam on and off was useful with a good reproducibility under a breath-hold at patients’ pace. And it has an excellent time-efficiency because the irradiation starts at the initiation of patients’ breath-hold and continues irradiation as long as patients keep the breath-hold. This new irradiation system is simple (no need for a respiratory monitoring device) and useful for irradiation of lung cancer with reduced PTV and a sufficient reproducibility.
42
Is IMRT of Lung Possible without Respiratory Gating? Clinical Evaluation of a True Dose Received by a Moving Target Volume
P. Zygmanski, J.H. kung, S.B. Jiang, N. Choi, G. Chen Radiation Oncology Cox-3, Massachusetts General Hospital, Boston, MA Purpose: Respiratory lung motion up to (1-2) cm has been reported in the literature. In Intensity Modulated Radiation Therapy (IMRT) of lung tumor, because of simultaneous motion of Dynamic Multileaf Collimator (DMLC) and tumor volume, Planning Target Volume to Clinical Target Volume margin (PTV-CTV) is necessary but not sufficient to ensure proper dose delivery to tumor volume. Therefore in an IMRT treatment of lung tumor, gating is necessary not only to improve sparing of normal tissues but also to ensure accurate dose coverage of tumor volume. In the literature, the following questions have not been addressed: a) Without gating, what is the dose error to target volume, b) for a gated treatment, what is an acceptable triggering window. In this work, we address these questions by proposing a novel technique for calculating a 3D dose error that would result if a lung IMRT plan is delivered without gating. The method is then generalized for a gated treatment at arbitrary triggering window. As a clinical application, we apply the method to three lung IMRT cases. Materials and Methods: An IMRT treatment planning system first calculates optimized fluence map MU(x,y) for each port, which is then converted into a sequence file, e.g., DMLC file. A DMLC file contains information about MLC subfield shapes and fractional Monitor Units (MU) to be delivered with each subfield. In IMRT of lung tumor, because of respiratory motion, a tumor volume does not receive MU(x,y) but an Effective Incident Fluence EIF(x,y). We numerically evaluate EIF(x,y) from a DMLC file by projecting individual MLC subfield shape and a corresponding MU onto a calculation plane that executes a periodic motion representative of a patient respiration. EIF(x,y) is then fed-back into dose calculation engine of a treatment planning system to recalculate the true dose to a moving tumor. Three lung cancer patients were studied as a part of dose escalation with IMRT. The IMRT plans were made on HELIOS 6.15 for delivery with 6 MV CL2100 Varian 26 pair MLC. We used the concept of to calculate a true 3D dose to a moving target volume that would result if an IMRT plan were delivered without gating. In the calculation of EIF(x,y) for each DMLC file, we modeled the respiratory motion as a sinusoidal function with an amplitude of 10 mm in the superior-inferior direction and a period of 5 seconds. Results: The table below summarizes the results. We define Vol(Rx) as volume covered by a prescription isodose line. For the patient 1, DVH with respiration had a softer shoulder at 97% to 103% dose level. For the patient 2, the DVH for primary site with respiration has softer shoulder from 90%-100% dose level. For patient 2, the low Vol(Rx) was necessary because the tumor volume was in the vicinity of spinal cord. For the patient 3, the DVH for the primary site with respiration had a softer shoulder at 96% to 106% dose level. Conclusion: We proposed a novel technique for calculating a 3D dose error to a moving tumor that would result if a lung IMRT plan is delivered without gating. We concluded that for each of these HELIOS lung IMRT plan, the dose error to a target volume was negligible. Therefore for these plans, the need for gating can be guided solely by the need to improve sparing of normal tissues. The method can be generalized to calculating 3D dose error for lung IMRT plan delivered with gating at an arbitrary triggering window.
Patient 1
RX Dose (Gy)
Gy/Fr
# fields
Vol(RX) no motion
3D(max)/D(Rx) no motion
Vol (Rx) with motion
3D(max)/D(Rx) with motion
1 2 3
80 70 78.5
2.5 2 2
5 5 5
95% 68% 95%
103% 105% 105%
925 50% 90%
107% 105% 105%
27
enables a patient to turn radiation beam on and off freely and repeatedly during a term designated by a radiation technologist. Ten real-time portal images were taken for each patient during irradiation under a patient’s self-breath-hold and self-switching radiationbeam, and the accuracy of reproducibility of the tumor position in the radiation field was visually evaluated. Results: The average of maximum differences of the tumor position in three series of 20 patients under the breath-hold by a radiation technologist’s instruction was 3.0 mm in cranio-caudal direction, 2.1mm in antero-posterior direction and 2.3mm in right-left direction. And that in three series under the breath-hold by a patient’s self-estimation was 2.3mm in cranio-caudal direction, 1.4mm in antero-posterior direction and 1.4mm in right-left direction, respectively. A standard deviation of these differences was larger in the method of breath-hold by a radiation technologist’s instruction than in the method of breath-hold by a patient’s self-estimation. And these differences were larger in tumors of lower lung field than upper lung field. There was statistically significant difference between two methods of breath-hold. The actual switching of the radiation beam was delayed less than 0.1 second behind the patient’s switching. All portal images of 20 patients had a visually sufficient accuracy of tumor position in the radiation field with a difference less than 3mm between a planned position and actual tumor positions. Conclusion: The reproducibility of tumor position under patient’s self breath-hold without any respiratory monitoring devices had a satisfactory accuracy with a difference of tumor position less than 3mm in all directions. The method of breath-hold by a patient’s self-estimation was more accurate than that by a radiation technologist’s instruction. Our newly developed switch which enabled patients to turn the radiation-beam on and off was useful with a good reproducibility under a breath-hold at patients’ pace. And it has an excellent time-efficiency because the irradiation starts at the initiation of patients’ breath-hold and continues irradiation as long as patients keep the breath-hold. This new irradiation system is simple (no need for a respiratory monitoring device) and useful for irradiation of lung cancer with reduced PTV and a sufficient reproducibility.
42
Is IMRT of Lung Possible without Respiratory Gating? Clinical Evaluation of a True Dose Received by a Moving Target Volume
P. Zygmanski, J.H. kung, S.B. Jiang, N. Choi, G. Chen Radiation Oncology Cox-3, Massachusetts General Hospital, Boston, MA Purpose: Respiratory lung motion up to (1-2) cm has been reported in the literature. In Intensity Modulated Radiation Therapy (IMRT) of lung tumor, because of simultaneous motion of Dynamic Multileaf Collimator (DMLC) and tumor volume, Planning Target Volume to Clinical Target Volume margin (PTV-CTV) is necessary but not sufficient to ensure proper dose delivery to tumor volume. Therefore in an IMRT treatment of lung tumor, gating is necessary not only to improve sparing of normal tissues but also to ensure accurate dose coverage of tumor volume. In the literature, the following questions have not been addressed: a) Without gating, what is the dose error to target volume, b) for a gated treatment, what is an acceptable triggering window. In this work, we address these questions by proposing a novel technique for calculating a 3D dose error that would result if a lung IMRT plan is delivered without gating. The method is then generalized for a gated treatment at arbitrary triggering window. As a clinical application, we apply the method to three lung IMRT cases. Materials and Methods: An IMRT treatment planning system first calculates optimized fluence map MU(x,y) for each port, which is then converted into a sequence file, e.g., DMLC file. A DMLC file contains information about MLC subfield shapes and fractional Monitor Units (MU) to be delivered with each subfield. In IMRT of lung tumor, because of respiratory motion, a tumor volume does not receive MU(x,y) but an Effective Incident Fluence EIF(x,y). We numerically evaluate EIF(x,y) from a DMLC file by projecting individual MLC subfield shape and a corresponding MU onto a calculation plane that executes a periodic motion representative of a patient respiration. EIF(x,y) is then fed-back into dose calculation engine of a treatment planning system to recalculate the true dose to a moving tumor. Three lung cancer patients were studied as a part of dose escalation with IMRT. The IMRT plans were made on HELIOS 6.15 for delivery with 6 MV CL2100 Varian 26 pair MLC. We used the concept of to calculate a true 3D dose to a moving target volume that would result if an IMRT plan were delivered without gating. In the calculation of EIF(x,y) for each DMLC file, we modeled the respiratory motion as a sinusoidal function with an amplitude of 10 mm in the superior-inferior direction and a period of 5 seconds. Results: The table below summarizes the results. We define Vol(Rx) as volume covered by a prescription isodose line. For the patient 1, DVH with respiration had a softer shoulder at 97% to 103% dose level. For the patient 2, the DVH for primary site with respiration has softer shoulder from 90%-100% dose level. For patient 2, the low Vol(Rx) was necessary because the tumor volume was in the vicinity of spinal cord. For the patient 3, the DVH for the primary site with respiration had a softer shoulder at 96% to 106% dose level. Conclusion: We proposed a novel technique for calculating a 3D dose error to a moving tumor that would result if a lung IMRT plan is delivered without gating. We concluded that for each of these HELIOS lung IMRT plan, the dose error to a target volume was negligible. Therefore for these plans, the need for gating can be guided solely by the need to improve sparing of normal tissues. The method can be generalized to calculating 3D dose error for lung IMRT plan delivered with gating at an arbitrary triggering window.
Patient 1
RX Dose (Gy)
Gy/Fr
# fields
Vol(RX) no motion
3D(max)/D(Rx) no motion
Vol (Rx) with motion
3D(max)/D(Rx) with motion
1 2 3
80 70 78.5
2.5 2 2
5 5 5
95% 68% 95%
103% 105% 105%
925 50% 90%
107% 105% 105%