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Feasibility of Using the Novel SBRT System for Radiation Therapy and SRS of Intracranial Lesions
S824
International Journal of Radiation Oncology Biology Physics
DTMR. On the other hand, the isocenter dose difference between HCP and PBHC showed logarithmic relationship with PTV volume. For almost all cases, the isocenter doses calculated for HCP were greater for NHCP plans because the effect of attenuation impacted the calculation more greatly than that of scatter. The dose covering 95% volume of PTV (D95) and the volume receiving 100% prescribed dose (V100) were also analyzed. The net D95 and V100 differences were +2.9% and +8.4%, respectively. The D95 difference between PBHC and NHCP showed linear correlation with DTMR. The D95 difference between HCP and PBHC weakly correlated with PTV volume; although some cases with small PTV showed negative net D95 difference, most cases showed positive values. Conclusion: In this study, the effects of heterogeneity were separated into attenuation and scatter components. Heterogeneity corrected plans resulted in increased isocenter dose and PTV coverage. The impact of scatter is target volume dependent. For small PTV, PBHC should be discouraged. For clinical trials, the prescription dose should be carefully tailored in the context of heterogeneity correction. Author Disclosure: Y. Akino: E. Research Grant; JSPS Core-to-Core Program No. 23003. I.J. Das: None. T. Teshima: E. Research Grant; JSPS Core-to-Core Program No. 23003. H.R. Cardenes: None. C. DesRosiers: None.
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3611 VMAT for Lung SABR With a New 160 Leaf MLC J. Lilley, S.J. Derbyshire, C.M. Thompson, K.N. Franks, and V.P. Cosgrove; SJIO, Leeds, United Kingdom Purpose/Objective(s): Using VMAT to deliver lung SABR should increase treatment delivery efficiency, and may also provide improvements in critical structure sparing. The feasibility of planning VMAT lung SABR treatments using a new 160 leaf MLC was investigated. Treatment plans were compared with the current 3DCRT technique and plans for a standard 80-leaf MLC. Materials/Methods: An alpha release of Monte Carlo based planning software was used for VMAT planning, working to dose criteria defined by the UK SABR consortium: 55Gy in five fractions (prescribed to the 80% isodose) for a peripheral lung tumor. From previous work planning with a standard 80-leaf MLC, good results were obtained using an 180o arc, to spare the contra-lateral lung. VMAT plans using a single 180o arc (gantry rotation from 0o to 180o or from 180o to 360o depending on tumor location) were therefore produced for five test patients (PTV range 17-81cc), and these were compared with 80-leaf MLC plans and the clinical 3DCRT plans. All plans used a midline isocenter technique and 6MV photons. A cylindrical diode array phantom was used for dosimetric validation, together with a small ionization chamber placed at the centre of the highdose region in a thorax phantom. Results: Target coverage for all plans was comparable, with no significant differences noted between VMAT and 3DCRT plans. Mean lung doses were the same or lower, with V10 and V5 consistently lower for VMAT compared to 3DCRT. For the contra-lateral lung, V10 and V5 were reduced in all cases with VMAT. Maximum dose to heart and airways was reduced with VMAT in four patients, and maximum esophagus dose was lower in all five. VMAT delivery times ranged from 4-5 mins. For VMAT plans, ion chamber measurements agreed with planned dose calculations to within 2%. Gamma analyses of VMAT planned dose distributions with measurements showed 95% of points above the 5% isodose level agreed within 3%/3mm. Conclusion: VMAT planning methods have been developed for a new 160leaf MLC, to meet lung SABR planning aims using a single one-sided 180o arc. Plans have been successfully delivered, demonstrating good agreement with measurements. VMAT provided some advantages for OARs, however, when constraints were met, OAR optimization settings were not further adjusted from the starting values. Therefore, further sparing may be possible using more challenging constraints. Author Disclosure: J. lilley: None. S.J. Derbyshire: None. C.M. Thompson: None. K.N. Franks: None. V.P. Cosgrove: None.
Feasibility of Using the Novel SBRT System for Radiation Therapy and SRS of Intracranial Lesions M. De Ridder, M. Buleteanu, T. Gevaert, T. Depuydt, K. Poels, and D. Verellen; UZ Brussel, Free University of Brussels, Brussels, Belgium Purpose/Objective(s): The novel system, designed for SBRT, was benchmarked in a planning study against an SRS system for quality of delivered dose distributions to intracranial lesions. Materials/Methods: The SBRT system consists of a 6 MV linac mounted on an O-ring gantry. It is equipped with a 5 mm leaf width MLC. The Oring can execute rotations around the horizontal axis, and unlike C-arm gantries also around the vertical axis (60 ). This allows non-coplanar plan delivery without couch rotations. Two kV imaging systems are attached at 45 from the beam allowing simultaneous X-rays and CBCT. The SRS system is a purpose-built system for SRS, equipped with a 3mm leaf width MLC and image-guided system composed of two X-ray tubes build into the floor and two flat panel detectors mounted on the ceiling. The infrared marker-based image guided positioning system is integrated in both systems. A total of 10 patients with brain lesions treated on the SRS System were re-planned for SBRT using the identical prescription and surrounding organ sparing dose constraints. The SRS treatment planning included vertex fields, using 90 couch rotation. These vertex fields cannot be reproduced with SBRT due to the mechanical limitations of the O-ring gantry. As such alternative noncoplanar arcs class solutions were investigated for the SBRT system to treat cranial lesions. Quality of the achieved dose distributions was expressed in the conformity index (CI), homogeneity index (HI) and gradient index (GI) and compared using a paired Student t-test and statistical significance for p-values 0.05. Results: Lower HI values for the SBRT system indicate a tendency to more homogeneous target dose but the difference was not statistical significant (p Z 0.338). The median GI for the two systems, SBRT and SRS were 3.7, and 3.4 respectively indicating sharper dose fall-off outside the target volume for the SBRT system (p Z 0.036). Better dose conformity and normal tissue sparing was obtained for the SRS system in the majority of the cases in comparison to the SBRT System. After stratification of the patients in two groups based on target volume, a statistically significant advantage of SRS was observed between the two techniques only for volumes smaller than 5 ccm (p Z 0.001), while the difference for larger lesions was not significant (p Z 0.070). Conclusions: The present study compared the SBRT system with the SRS System, a “golden standard” for SRS, for treatment of intracranial cases. In terms of target dose homogeneity and dose fall-off, the SBRT system showed some advantages. The dose conformity for SBRT was comparable with SRS for lesions with volumes above 5 ccm, corresponding to 42% of the cranial cases in our hospital. For smaller lesions SRS showed significant better conformity, mainly due to a 5 mm leaf width compared to the 3 mm for SRS. Author Disclosure: M. De Ridder: None. M. Buleteanu: None. T. Gevaert: None. T. Depuydt: None. K. Poels: None. D. Verellen: None.
3613 Quantification of the Magnetic Susceptibility Effects During MRI Guided Radiosurgery of Hemorrhagic Brain Metastases T. Stanescu, C. Chung, A. Simeonov, M. van Prooijen, B. Millar, and C. Menard; Princess Margaret Hospital, Toronto, ON, Canada Purpose/Objective(s): Image geometric integrity is paramount for applications in stereotactic radiosurgery. MR images may introduce geometric uncertainties regarding the location and boundaries of target lesions via magnetic susceptibility-induced effects, which are due to differences in the magnetic properties of neighboring tissues (e.g., hemorrhagic metastases and adjacent healthy tissue). This study investigates the mechanism of the susceptibility distortions for hemorrhagic
International Journal of Radiation Oncology Biology Physics
DTMR. On the other hand, the isocenter dose difference between HCP and PBHC showed logarithmic relationship with PTV volume. For almost all cases, the isocenter doses calculated for HCP were greater for NHCP plans because the effect of attenuation impacted the calculation more greatly than that of scatter. The dose covering 95% volume of PTV (D95) and the volume receiving 100% prescribed dose (V100) were also analyzed. The net D95 and V100 differences were +2.9% and +8.4%, respectively. The D95 difference between PBHC and NHCP showed linear correlation with DTMR. The D95 difference between HCP and PBHC weakly correlated with PTV volume; although some cases with small PTV showed negative net D95 difference, most cases showed positive values. Conclusion: In this study, the effects of heterogeneity were separated into attenuation and scatter components. Heterogeneity corrected plans resulted in increased isocenter dose and PTV coverage. The impact of scatter is target volume dependent. For small PTV, PBHC should be discouraged. For clinical trials, the prescription dose should be carefully tailored in the context of heterogeneity correction. Author Disclosure: Y. Akino: E. Research Grant; JSPS Core-to-Core Program No. 23003. I.J. Das: None. T. Teshima: E. Research Grant; JSPS Core-to-Core Program No. 23003. H.R. Cardenes: None. C. DesRosiers: None.
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3611 VMAT for Lung SABR With a New 160 Leaf MLC J. Lilley, S.J. Derbyshire, C.M. Thompson, K.N. Franks, and V.P. Cosgrove; SJIO, Leeds, United Kingdom Purpose/Objective(s): Using VMAT to deliver lung SABR should increase treatment delivery efficiency, and may also provide improvements in critical structure sparing. The feasibility of planning VMAT lung SABR treatments using a new 160 leaf MLC was investigated. Treatment plans were compared with the current 3DCRT technique and plans for a standard 80-leaf MLC. Materials/Methods: An alpha release of Monte Carlo based planning software was used for VMAT planning, working to dose criteria defined by the UK SABR consortium: 55Gy in five fractions (prescribed to the 80% isodose) for a peripheral lung tumor. From previous work planning with a standard 80-leaf MLC, good results were obtained using an 180o arc, to spare the contra-lateral lung. VMAT plans using a single 180o arc (gantry rotation from 0o to 180o or from 180o to 360o depending on tumor location) were therefore produced for five test patients (PTV range 17-81cc), and these were compared with 80-leaf MLC plans and the clinical 3DCRT plans. All plans used a midline isocenter technique and 6MV photons. A cylindrical diode array phantom was used for dosimetric validation, together with a small ionization chamber placed at the centre of the highdose region in a thorax phantom. Results: Target coverage for all plans was comparable, with no significant differences noted between VMAT and 3DCRT plans. Mean lung doses were the same or lower, with V10 and V5 consistently lower for VMAT compared to 3DCRT. For the contra-lateral lung, V10 and V5 were reduced in all cases with VMAT. Maximum dose to heart and airways was reduced with VMAT in four patients, and maximum esophagus dose was lower in all five. VMAT delivery times ranged from 4-5 mins. For VMAT plans, ion chamber measurements agreed with planned dose calculations to within 2%. Gamma analyses of VMAT planned dose distributions with measurements showed 95% of points above the 5% isodose level agreed within 3%/3mm. Conclusion: VMAT planning methods have been developed for a new 160leaf MLC, to meet lung SABR planning aims using a single one-sided 180o arc. Plans have been successfully delivered, demonstrating good agreement with measurements. VMAT provided some advantages for OARs, however, when constraints were met, OAR optimization settings were not further adjusted from the starting values. Therefore, further sparing may be possible using more challenging constraints. Author Disclosure: J. lilley: None. S.J. Derbyshire: None. C.M. Thompson: None. K.N. Franks: None. V.P. Cosgrove: None.
Feasibility of Using the Novel SBRT System for Radiation Therapy and SRS of Intracranial Lesions M. De Ridder, M. Buleteanu, T. Gevaert, T. Depuydt, K. Poels, and D. Verellen; UZ Brussel, Free University of Brussels, Brussels, Belgium Purpose/Objective(s): The novel system, designed for SBRT, was benchmarked in a planning study against an SRS system for quality of delivered dose distributions to intracranial lesions. Materials/Methods: The SBRT system consists of a 6 MV linac mounted on an O-ring gantry. It is equipped with a 5 mm leaf width MLC. The Oring can execute rotations around the horizontal axis, and unlike C-arm gantries also around the vertical axis (60 ). This allows non-coplanar plan delivery without couch rotations. Two kV imaging systems are attached at 45 from the beam allowing simultaneous X-rays and CBCT. The SRS system is a purpose-built system for SRS, equipped with a 3mm leaf width MLC and image-guided system composed of two X-ray tubes build into the floor and two flat panel detectors mounted on the ceiling. The infrared marker-based image guided positioning system is integrated in both systems. A total of 10 patients with brain lesions treated on the SRS System were re-planned for SBRT using the identical prescription and surrounding organ sparing dose constraints. The SRS treatment planning included vertex fields, using 90 couch rotation. These vertex fields cannot be reproduced with SBRT due to the mechanical limitations of the O-ring gantry. As such alternative noncoplanar arcs class solutions were investigated for the SBRT system to treat cranial lesions. Quality of the achieved dose distributions was expressed in the conformity index (CI), homogeneity index (HI) and gradient index (GI) and compared using a paired Student t-test and statistical significance for p-values 0.05. Results: Lower HI values for the SBRT system indicate a tendency to more homogeneous target dose but the difference was not statistical significant (p Z 0.338). The median GI for the two systems, SBRT and SRS were 3.7, and 3.4 respectively indicating sharper dose fall-off outside the target volume for the SBRT system (p Z 0.036). Better dose conformity and normal tissue sparing was obtained for the SRS system in the majority of the cases in comparison to the SBRT System. After stratification of the patients in two groups based on target volume, a statistically significant advantage of SRS was observed between the two techniques only for volumes smaller than 5 ccm (p Z 0.001), while the difference for larger lesions was not significant (p Z 0.070). Conclusions: The present study compared the SBRT system with the SRS System, a “golden standard” for SRS, for treatment of intracranial cases. In terms of target dose homogeneity and dose fall-off, the SBRT system showed some advantages. The dose conformity for SBRT was comparable with SRS for lesions with volumes above 5 ccm, corresponding to 42% of the cranial cases in our hospital. For smaller lesions SRS showed significant better conformity, mainly due to a 5 mm leaf width compared to the 3 mm for SRS. Author Disclosure: M. De Ridder: None. M. Buleteanu: None. T. Gevaert: None. T. Depuydt: None. K. Poels: None. D. Verellen: None.
3613 Quantification of the Magnetic Susceptibility Effects During MRI Guided Radiosurgery of Hemorrhagic Brain Metastases T. Stanescu, C. Chung, A. Simeonov, M. van Prooijen, B. Millar, and C. Menard; Princess Margaret Hospital, Toronto, ON, Canada Purpose/Objective(s): Image geometric integrity is paramount for applications in stereotactic radiosurgery. MR images may introduce geometric uncertainties regarding the location and boundaries of target lesions via magnetic susceptibility-induced effects, which are due to differences in the magnetic properties of neighboring tissues (e.g., hemorrhagic metastases and adjacent healthy tissue). This study investigates the mechanism of the susceptibility distortions for hemorrhagic