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Potential Advantages of a New Irradiation Technique: Three-dimensional Unicursal Irradiation with MHI-TM2000
I. J. Radiation Oncology d Biology d Physics
S94
201
Volume 78, Number 3, Supplement, 2010
Clinical Implementation and Initial Experience of the First Non-commercial VMAT
Y. Song, C. Obcemea, B. Mueller, C. Burman, B. Mychalczak Memorial Sloan-Kettering Cancer Center, Sleepy Hollow, NY Purpose/Objective(s): The beamlet-based IMRT has two pronounced deficiencies: an elevated MU and a reduced MU-toGy coefficient. The former increases the scattered dose to uninvolved areas and the latter decreases the delivery and energy efficiency. As more and more patients treated with IMRT have shown long-term survival, there has been an increasing concern over the risk of developing secondary malignancies and other long-term morbidities as a direct result of these high MU treatments. To address these issues, we developed our own version of VMAT in late 2009. So far, several prostate patients have been treated with this new modality. A thorough literature search indicates that this is the first non-commercial VMAT in the field. Our objectives are 1) to minimize MU; 2) to maximize delivery efficiency and 3) to provide IMRT-comparable plans. Materials/Methods: We used the aperture-based inverse optimization methodology. The aperture-based optimization can completely eliminate small MU and small aperture segments that are commonly seen in beamlet-based IMRT plans. These segments contribute little useful dose to the PTV coverage, but greatly decrease the delivery efficiency and increase the probability of producing hot spots. This reduction in fine intensity modulation is partially compensated for by increasing the beam sampling frequency to 180. To minimize the integral dose, an exponential normal tissue objective function (NTO) was included in the total quadratic cost function. Our VMAT was designed to optimize both MLC aperture shapes and corresponding MU weights simultaneously using a multi-level source sampling optimization scheme (MSSO). Unlike the beamlet-based IMRT, where intensity modulation is achieved through leaf motion alone, our VMAT produces intensity modulation through a synergistic combination of MLC aperture, temporal dose rate, and gantry angular speed modulations. Results: Our VMAT plans were delivered on Varian RapidArc-enabled Trilogy machines at the fastest gantry angular speed (4.8 /sec). For a 360 plan consisting of 180 beams, the total delivery time was 75 seconds. Four prostate cancer patients have been recruited in our initial trial. All the VMAT plans met our institutional plan acceptance criteria and were comparable or close to the IMRT plans in key dosimetric parameters. However, the VMAT plans offered better target dose homogeneity. The mean beam on time was 283.4±7.67 MU for the VMAT plans and 522.7 ± 39.6 MU for the IMRT plans. The mean MU-to-c Gy coefficient was 1.58 ± 0.04 for the VMAT plans and 2.91 ± 0.22 for the IMRT plans, respectively. Conclusions: We have successfully developed the first non-commercial VMAT. Our initial clinical experience indicates that our VMAT significantly improves the MU-to-c Gy coefficient and is a competitive and viable treatment modality. Author Disclosure: Y. Song, None; C. Obcemea, None; B. Mueller, None; C. Burman, None; B. Mychalczak, None.
202
Potential Advantages of a New Irradiation Technique: Three-dimensional Unicursal Irradiation with MHI-TM2000
T. Mizowaki1, K. Takayama1, Y. Miyabe1, S. Kaneko2, M. Kokubo3, M. Hiraoka1 1
Kyoto University, Kyoto, Japan, 2Mitsubishi Heavy Industries, Hiroshima, Japan, 3Institute of Biomedical Research and Innovation, Kobe, Japan Purpose/Objective(s): MHI-TM2000 (Mitsubishi Heavy Industries) is a newly-designed image-guided radiotherapy system. This unit has some unique features as follows: there is a rigid ring-shaped gantry on which a compact C-band 6-megavolt LINAC mounted on a gimbaled X-ray head, and the ring-shaped gantry itself can rotate around the vertical axis of the ring as well as the X-ray head unit can rotate 360 degrees around the inner circumference of the ring. To make the maximum use of this unique system, we got the idea of a new irradiation technique: three-dimensional unicursal irradiation. The purpose of this study is to present a concept and estimated potential advantages of this new technique. Materials/Methods: MHI-TM2000 has a potential to put a new irradiation technique into practice, in which both the gantry head and the ring itself continuously and simultaneously rotate during irradiation around the inner circumference of the ring and the axis of the ring gantry, respectively. This technique, as it were, three-dimensionally rotational dynamic-arc conformal irradiation (3D-DCAT): which we denominated three-dimensional unicursal irradiation. A collision map between the ring and couch/patients was created based on phantom studies assuming a tumor located in the center of the body such as pancreatic cancer. Thereafter, 3D-DCAT plan and conventional two-dimensionally rotational dynamic arc conformal irradiation (2DDCAT) plan were experimentally created to a pancreatic cancer case on iPlan 4.0 for MHI-TM2000 in reference to the collision map. Then, dosimetric comparisons among the 2D-DCAT, 3D-DCAT and intensity-modulated radiation therapy (IMRT) plan clinically applied to the same patient created with Eclipse 8.6 for Clinac iX (Varian). The width of the multileaf collimators of both machines was 5 mm. Results: Doses in 3D-DCAT plan were comparable to or even better than those 2D-DCAT and IMRT plans with respect to both the target and organs at risk. D95 of the PTV for 3D-DCAT, 2D-DCAT and IMRT were 95.5%, 95.8% and 97.0%, respectively. V20% was 11.5%, 31.3% and 9.4% for the right kidney, and 9.6%, 27.8% and 14.4% for the left kidney, respectively. V90% was 4.7%, 4.3%, and 3.2% for the small intestine, and 3.6%, 4.1%, and 3.2% for the stomach, respectively. V30% for the spinal cord was 6.0%, 0.5% and 24.7%, respectively. Ratios of the monitor unit were 107%, 100%, and 137%, respectively.
Conclusions: 3D-DCAT has some potential advantages: lower monitor units and shorter treatment time as well as the excellent dose concentration to the target. Because the current version (MHI-TM2000) does not support this new irradiation technique, those potential advantages should be validated with the future version implementing the capability of 3D-DCAT under varying conditions. Author Disclosure: T. Mizowaki, None; K. Takayama, Research contract with Mitsubishi Heavy Industries, C. Other Research Support; Y. Miyabe, Research contract with Varian Medical Systems, G. Other; S. Kaneko, Mitsubishi Heavy Industries, A. Employment; M. Kokubo, None; M. Hiraoka, Sponsored Research Program of Mitsubishi Heavy Industries, C. Other Research Support; Sponsored Research Program of Varian Medical Systems, C. Other Research Support.
S94
201
Volume 78, Number 3, Supplement, 2010
Clinical Implementation and Initial Experience of the First Non-commercial VMAT
Y. Song, C. Obcemea, B. Mueller, C. Burman, B. Mychalczak Memorial Sloan-Kettering Cancer Center, Sleepy Hollow, NY Purpose/Objective(s): The beamlet-based IMRT has two pronounced deficiencies: an elevated MU and a reduced MU-toGy coefficient. The former increases the scattered dose to uninvolved areas and the latter decreases the delivery and energy efficiency. As more and more patients treated with IMRT have shown long-term survival, there has been an increasing concern over the risk of developing secondary malignancies and other long-term morbidities as a direct result of these high MU treatments. To address these issues, we developed our own version of VMAT in late 2009. So far, several prostate patients have been treated with this new modality. A thorough literature search indicates that this is the first non-commercial VMAT in the field. Our objectives are 1) to minimize MU; 2) to maximize delivery efficiency and 3) to provide IMRT-comparable plans. Materials/Methods: We used the aperture-based inverse optimization methodology. The aperture-based optimization can completely eliminate small MU and small aperture segments that are commonly seen in beamlet-based IMRT plans. These segments contribute little useful dose to the PTV coverage, but greatly decrease the delivery efficiency and increase the probability of producing hot spots. This reduction in fine intensity modulation is partially compensated for by increasing the beam sampling frequency to 180. To minimize the integral dose, an exponential normal tissue objective function (NTO) was included in the total quadratic cost function. Our VMAT was designed to optimize both MLC aperture shapes and corresponding MU weights simultaneously using a multi-level source sampling optimization scheme (MSSO). Unlike the beamlet-based IMRT, where intensity modulation is achieved through leaf motion alone, our VMAT produces intensity modulation through a synergistic combination of MLC aperture, temporal dose rate, and gantry angular speed modulations. Results: Our VMAT plans were delivered on Varian RapidArc-enabled Trilogy machines at the fastest gantry angular speed (4.8 /sec). For a 360 plan consisting of 180 beams, the total delivery time was 75 seconds. Four prostate cancer patients have been recruited in our initial trial. All the VMAT plans met our institutional plan acceptance criteria and were comparable or close to the IMRT plans in key dosimetric parameters. However, the VMAT plans offered better target dose homogeneity. The mean beam on time was 283.4±7.67 MU for the VMAT plans and 522.7 ± 39.6 MU for the IMRT plans. The mean MU-to-c Gy coefficient was 1.58 ± 0.04 for the VMAT plans and 2.91 ± 0.22 for the IMRT plans, respectively. Conclusions: We have successfully developed the first non-commercial VMAT. Our initial clinical experience indicates that our VMAT significantly improves the MU-to-c Gy coefficient and is a competitive and viable treatment modality. Author Disclosure: Y. Song, None; C. Obcemea, None; B. Mueller, None; C. Burman, None; B. Mychalczak, None.
202
Potential Advantages of a New Irradiation Technique: Three-dimensional Unicursal Irradiation with MHI-TM2000
T. Mizowaki1, K. Takayama1, Y. Miyabe1, S. Kaneko2, M. Kokubo3, M. Hiraoka1 1
Kyoto University, Kyoto, Japan, 2Mitsubishi Heavy Industries, Hiroshima, Japan, 3Institute of Biomedical Research and Innovation, Kobe, Japan Purpose/Objective(s): MHI-TM2000 (Mitsubishi Heavy Industries) is a newly-designed image-guided radiotherapy system. This unit has some unique features as follows: there is a rigid ring-shaped gantry on which a compact C-band 6-megavolt LINAC mounted on a gimbaled X-ray head, and the ring-shaped gantry itself can rotate around the vertical axis of the ring as well as the X-ray head unit can rotate 360 degrees around the inner circumference of the ring. To make the maximum use of this unique system, we got the idea of a new irradiation technique: three-dimensional unicursal irradiation. The purpose of this study is to present a concept and estimated potential advantages of this new technique. Materials/Methods: MHI-TM2000 has a potential to put a new irradiation technique into practice, in which both the gantry head and the ring itself continuously and simultaneously rotate during irradiation around the inner circumference of the ring and the axis of the ring gantry, respectively. This technique, as it were, three-dimensionally rotational dynamic-arc conformal irradiation (3D-DCAT): which we denominated three-dimensional unicursal irradiation. A collision map between the ring and couch/patients was created based on phantom studies assuming a tumor located in the center of the body such as pancreatic cancer. Thereafter, 3D-DCAT plan and conventional two-dimensionally rotational dynamic arc conformal irradiation (2DDCAT) plan were experimentally created to a pancreatic cancer case on iPlan 4.0 for MHI-TM2000 in reference to the collision map. Then, dosimetric comparisons among the 2D-DCAT, 3D-DCAT and intensity-modulated radiation therapy (IMRT) plan clinically applied to the same patient created with Eclipse 8.6 for Clinac iX (Varian). The width of the multileaf collimators of both machines was 5 mm. Results: Doses in 3D-DCAT plan were comparable to or even better than those 2D-DCAT and IMRT plans with respect to both the target and organs at risk. D95 of the PTV for 3D-DCAT, 2D-DCAT and IMRT were 95.5%, 95.8% and 97.0%, respectively. V20% was 11.5%, 31.3% and 9.4% for the right kidney, and 9.6%, 27.8% and 14.4% for the left kidney, respectively. V90% was 4.7%, 4.3%, and 3.2% for the small intestine, and 3.6%, 4.1%, and 3.2% for the stomach, respectively. V30% for the spinal cord was 6.0%, 0.5% and 24.7%, respectively. Ratios of the monitor unit were 107%, 100%, and 137%, respectively.
Conclusions: 3D-DCAT has some potential advantages: lower monitor units and shorter treatment time as well as the excellent dose concentration to the target. Because the current version (MHI-TM2000) does not support this new irradiation technique, those potential advantages should be validated with the future version implementing the capability of 3D-DCAT under varying conditions. Author Disclosure: T. Mizowaki, None; K. Takayama, Research contract with Mitsubishi Heavy Industries, C. Other Research Support; Y. Miyabe, Research contract with Varian Medical Systems, G. Other; S. Kaneko, Mitsubishi Heavy Industries, A. Employment; M. Kokubo, None; M. Hiraoka, Sponsored Research Program of Mitsubishi Heavy Industries, C. Other Research Support; Sponsored Research Program of Varian Medical Systems, C. Other Research Support.