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How Much Margin Is Adequate for Single-fraction Stereotactic Radiosurgery Treatments?
Poster Viewing Abstracts S821
Volume 84 Number 3S Supplement 2012
3601 WITHDRAWN
3602 How Much Margin Is Adequate for Single-fraction Stereotactic Radiosurgery Treatments? Q. Zhang, Y. Song, M. Chan, and C. Burman; MSKCC, New York, NY Purpose/Objective(s): Setting a proper margin is crucial for delivering the right dose to the clinical target volume (CTV) and sparing of organs at risk. This study is to determine the necessary margin expansion from CTV to planning target volume (PTV) for linac-based single-fraction stereotactic radiosurgery (SRS). Materials/Methods: Whereas in previous publications, a Gaussian function was assumed for systematic errors, we assumed a Dirac delta function for the systematic error of a specific machine. A Gaussian function was assumed for residual setup errors. Differences between a cone-beam computed tomography (CBCT) isocenter and linac isocenter were measured for a Varian Trilogy linear accelerator for two months. Formulas for calculating the CTV-PTV margin for single-fraction SRS cases were proposed. Margins for single-fraction treatment are derived, such that the CTV receives the prescribed dose in 95% of the treated patients. To validate our margin formalism, we used software to re-plan 10 previously treated patients. For each plan, we shifted the isocenter of the plan in each direction that simulated the setup error in that direction. Specifically, we shifted the isocenter setup error 3 mm in all directions, which generated a total of 8 specific clinical situations for each plan. Subsequently, the CTV coverage as a function of setup error was calculated and analyzed. Results: We have found that the proper margin for single fraction frameless SRS cases was around 3 mm for the machine investigated in this study. This value is machine-dependent and will vary with different machine. The differences between the CBCT isocenter and radiation isocenter remained almost constant in two months for this machine. This confirmed our assumption on the systematic error distribution as being a delta function. This definition is especially relevant to a single-fraction treatment. The average V80% for all the patients was 96.15.5% for isocenter with 3 mm setup error in all directions. It was found that V80% was mostly affected when the tumor was in the posterior part of the head, giving a minimum V80% of about 72%. This was entirely due to the special geometry of the posterior head. Hence, in order to achieve V80% Z 99.9%, the average additional margin beyond 3 mm should be 0.650.30 mm. However, 3 mm setup errors rarely occur simultaneously in all directions under normal clinical situations and the probability for this to happen is 8/1000000. Therefore, for practical purpose, we essentially do not need this extra 0.65 mm margin. Conclusion: A margin formula was proposed for the linac-based frameless SRS brain cases, which attempted to determine an adequate CTV- PTV margin, thereby improving the treatment outcome. This margin is machine specific and for the specific machine studied, the margin was 3 mm and validated by simulation studies. Author Disclosure: Q. Zhang: None. Y. Song: None. M. Chan: None. C. Burman: None.
3603 WITHDRAWN
3604 Feasibility, Detectability, and Experience With Platinum Seed Internal Fiducial Markers for CT-MRI Fusion and Real-time Tumor Tracking During Stereotactic Ablative Radiation Therapy V. Janardanan Nair,1,2 J. Szanto,1,2 E. Vandervoort,1,2 E. Henderson,1,2 L. Avruch,1,2 S. Malone,1,2 and J. Pantarotto1,2; 1The Ottawa Hospital Cancer Centre, Ottawa, ON, Canada, 2University of Ottawa, Ottawa, ON, Canada
Purpose/Objective(s): Despite their popularity, gold fiducials have the disadvantage of not being vividly seen in MRI sequences which give optimal tumor definition. We hypothesize that due to their differences in magnetic susceptibility platinum fiducials are more useful than gold in performing fiducial-based fusion for CT-MRI registration. Similar electron densities, however, should ensure that platinum seeds can be tracked just as well as gold. The purpose of this study is to review our pilot project experience with platinum fiducials in terms of feasibility, migration rates and detectability by orthogonal x-ray images used in robotic stereotactic ablative radiation therapy (SABR). Materials/Methods: Twenty-nine consecutive SABR patients (30 tumors) treated using fiducial tracking between January 2011 and February 2012 were reviewed. A total of 108 fiducials (18 gold seeds (GS), 5 gold coils (GC), 85 platinum seeds (PS)) implanted in or around various tumor sites (liver, kidney, prostate, pancreas, adrenals, soft tissue) were identified. Seeds were all 1mm x 3mm and coils were 0.5-0.75mm by 5mm long. The optimum MRI sequences for simultaneous visualization of PS and primary tumors, as determined by an experienced MR radiologist, were recorded. The contrast ratio (CR) of fiducials (defined as gray scale difference between background and fiducial / gray scale value of background) seen on MRI mages and treatment unit x-ray images were measured using open-source software. MRI images of platinum seeds implanted in both gel and chicken breast, and x-ray images of fiducials implanted in an acrylic pelvic phantom were also analyzed as control samples. Results: Seventy-seven percent of fiducials were in the liver. Migration rates were similar for PS versus GS and GC (6.2%). No grade 2 procedure-related complication was reported. In liver MRI, the mean CR was superior for platinum (58.615) in comparison to gold seeds (237) or gold coils (7.91) (p <0.0001). No difference was noted between the mean CR in cirrhotic versus non-cirrhotic liver (60.4 vs. 47.9; p Z 0.074). MRI sequences for tumors in other organs revealed a mean CR for platinum superior to that of gold (p<0.001). No CR difference was seen between gold and platinum on analysis of the treatment unit x-rays. Conclusion: Despite having similar electron density, platinum seeds provide a superior contrast ratio in comparison to gold seeds or coils on MRI images, making them a better choice for fiducial-based CT-MRI registration. Platinum seeds have similar migration rates and complication rates to gold and are detected equally well by an image guidance system using orthogonal x-rays. Author Disclosure: V. Janardanan Nair: None. J. Szanto: None. E. Vandervoort: None. E. Henderson: None. L. Avruch: None. S. Malone: None. J. Pantarotto: None.
3605 Dose-Volume Effects on Hearing Preservation in Gamma-knife Radiosurgery of Vestibular Schwannomas J. Xue,1 J. Grimm,2 P. Potrebko,1 A. Youssef,1 Y. Chen,1 T. LaCouture,1 L. Hughes,1 and W. Goldman1; 1Cooper University Hospital, Camden, NJ, 2 Holy Redeemer Hospital, Meadowbrook, PA Purpose/Objective(s): The volume of cochlea receiving a specified dose can be an important predictor for hearing preservation in stereotactic radiosurgery (SRS) of vestibular schwannomas (VS). This study evaluates those dosimetric parameters as computed by different dose algorithms in Gamma-knife radiosurgery. Materials/Methods: Hearing loss is the major risk of complication in SRS of VS. Radiation dose to the cochlea can be a significant factor that causes deterioration in hearing after radiosurgery. A statistical correlation has been observed between some cochlea dose parameters and hearing preservation in gamma-knife treatments. Kato et al found a cutoff dose of 4.2 Gy to the central cochlea for hearing preservation. Brown et al reported the dose-volume predictor for hearing preservation to be a mean cochlea dose of 5.1 Gy, and the mean percentage of the cochlea volume receiving a dose greater than 3.7 Gy, 4.7 Gy and 5.3 Gy to be 64%, 45% and 36%, respectively. In addition, they also discovered that every 10%
Volume 84 Number 3S Supplement 2012
3601 WITHDRAWN
3602 How Much Margin Is Adequate for Single-fraction Stereotactic Radiosurgery Treatments? Q. Zhang, Y. Song, M. Chan, and C. Burman; MSKCC, New York, NY Purpose/Objective(s): Setting a proper margin is crucial for delivering the right dose to the clinical target volume (CTV) and sparing of organs at risk. This study is to determine the necessary margin expansion from CTV to planning target volume (PTV) for linac-based single-fraction stereotactic radiosurgery (SRS). Materials/Methods: Whereas in previous publications, a Gaussian function was assumed for systematic errors, we assumed a Dirac delta function for the systematic error of a specific machine. A Gaussian function was assumed for residual setup errors. Differences between a cone-beam computed tomography (CBCT) isocenter and linac isocenter were measured for a Varian Trilogy linear accelerator for two months. Formulas for calculating the CTV-PTV margin for single-fraction SRS cases were proposed. Margins for single-fraction treatment are derived, such that the CTV receives the prescribed dose in 95% of the treated patients. To validate our margin formalism, we used software to re-plan 10 previously treated patients. For each plan, we shifted the isocenter of the plan in each direction that simulated the setup error in that direction. Specifically, we shifted the isocenter setup error 3 mm in all directions, which generated a total of 8 specific clinical situations for each plan. Subsequently, the CTV coverage as a function of setup error was calculated and analyzed. Results: We have found that the proper margin for single fraction frameless SRS cases was around 3 mm for the machine investigated in this study. This value is machine-dependent and will vary with different machine. The differences between the CBCT isocenter and radiation isocenter remained almost constant in two months for this machine. This confirmed our assumption on the systematic error distribution as being a delta function. This definition is especially relevant to a single-fraction treatment. The average V80% for all the patients was 96.15.5% for isocenter with 3 mm setup error in all directions. It was found that V80% was mostly affected when the tumor was in the posterior part of the head, giving a minimum V80% of about 72%. This was entirely due to the special geometry of the posterior head. Hence, in order to achieve V80% Z 99.9%, the average additional margin beyond 3 mm should be 0.650.30 mm. However, 3 mm setup errors rarely occur simultaneously in all directions under normal clinical situations and the probability for this to happen is 8/1000000. Therefore, for practical purpose, we essentially do not need this extra 0.65 mm margin. Conclusion: A margin formula was proposed for the linac-based frameless SRS brain cases, which attempted to determine an adequate CTV- PTV margin, thereby improving the treatment outcome. This margin is machine specific and for the specific machine studied, the margin was 3 mm and validated by simulation studies. Author Disclosure: Q. Zhang: None. Y. Song: None. M. Chan: None. C. Burman: None.
3603 WITHDRAWN
3604 Feasibility, Detectability, and Experience With Platinum Seed Internal Fiducial Markers for CT-MRI Fusion and Real-time Tumor Tracking During Stereotactic Ablative Radiation Therapy V. Janardanan Nair,1,2 J. Szanto,1,2 E. Vandervoort,1,2 E. Henderson,1,2 L. Avruch,1,2 S. Malone,1,2 and J. Pantarotto1,2; 1The Ottawa Hospital Cancer Centre, Ottawa, ON, Canada, 2University of Ottawa, Ottawa, ON, Canada
Purpose/Objective(s): Despite their popularity, gold fiducials have the disadvantage of not being vividly seen in MRI sequences which give optimal tumor definition. We hypothesize that due to their differences in magnetic susceptibility platinum fiducials are more useful than gold in performing fiducial-based fusion for CT-MRI registration. Similar electron densities, however, should ensure that platinum seeds can be tracked just as well as gold. The purpose of this study is to review our pilot project experience with platinum fiducials in terms of feasibility, migration rates and detectability by orthogonal x-ray images used in robotic stereotactic ablative radiation therapy (SABR). Materials/Methods: Twenty-nine consecutive SABR patients (30 tumors) treated using fiducial tracking between January 2011 and February 2012 were reviewed. A total of 108 fiducials (18 gold seeds (GS), 5 gold coils (GC), 85 platinum seeds (PS)) implanted in or around various tumor sites (liver, kidney, prostate, pancreas, adrenals, soft tissue) were identified. Seeds were all 1mm x 3mm and coils were 0.5-0.75mm by 5mm long. The optimum MRI sequences for simultaneous visualization of PS and primary tumors, as determined by an experienced MR radiologist, were recorded. The contrast ratio (CR) of fiducials (defined as gray scale difference between background and fiducial / gray scale value of background) seen on MRI mages and treatment unit x-ray images were measured using open-source software. MRI images of platinum seeds implanted in both gel and chicken breast, and x-ray images of fiducials implanted in an acrylic pelvic phantom were also analyzed as control samples. Results: Seventy-seven percent of fiducials were in the liver. Migration rates were similar for PS versus GS and GC (6.2%). No grade 2 procedure-related complication was reported. In liver MRI, the mean CR was superior for platinum (58.615) in comparison to gold seeds (237) or gold coils (7.91) (p <0.0001). No difference was noted between the mean CR in cirrhotic versus non-cirrhotic liver (60.4 vs. 47.9; p Z 0.074). MRI sequences for tumors in other organs revealed a mean CR for platinum superior to that of gold (p<0.001). No CR difference was seen between gold and platinum on analysis of the treatment unit x-rays. Conclusion: Despite having similar electron density, platinum seeds provide a superior contrast ratio in comparison to gold seeds or coils on MRI images, making them a better choice for fiducial-based CT-MRI registration. Platinum seeds have similar migration rates and complication rates to gold and are detected equally well by an image guidance system using orthogonal x-rays. Author Disclosure: V. Janardanan Nair: None. J. Szanto: None. E. Vandervoort: None. E. Henderson: None. L. Avruch: None. S. Malone: None. J. Pantarotto: None.
3605 Dose-Volume Effects on Hearing Preservation in Gamma-knife Radiosurgery of Vestibular Schwannomas J. Xue,1 J. Grimm,2 P. Potrebko,1 A. Youssef,1 Y. Chen,1 T. LaCouture,1 L. Hughes,1 and W. Goldman1; 1Cooper University Hospital, Camden, NJ, 2 Holy Redeemer Hospital, Meadowbrook, PA Purpose/Objective(s): The volume of cochlea receiving a specified dose can be an important predictor for hearing preservation in stereotactic radiosurgery (SRS) of vestibular schwannomas (VS). This study evaluates those dosimetric parameters as computed by different dose algorithms in Gamma-knife radiosurgery. Materials/Methods: Hearing loss is the major risk of complication in SRS of VS. Radiation dose to the cochlea can be a significant factor that causes deterioration in hearing after radiosurgery. A statistical correlation has been observed between some cochlea dose parameters and hearing preservation in gamma-knife treatments. Kato et al found a cutoff dose of 4.2 Gy to the central cochlea for hearing preservation. Brown et al reported the dose-volume predictor for hearing preservation to be a mean cochlea dose of 5.1 Gy, and the mean percentage of the cochlea volume receiving a dose greater than 3.7 Gy, 4.7 Gy and 5.3 Gy to be 64%, 45% and 36%, respectively. In addition, they also discovered that every 10%