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Experimental Validation of Monte Carlo Simulations Based on a Virtual Source Model for Tomotherapy in a Rando Phantom
E608
International Journal of Radiation Oncology Biology Physics
during which patient repositioning was performed were identified. The percentage of set-ups requiring repositioning (i.e., number of fractions with repositioning over total number of fractions treated) was calculated for the whole population and for cervical and non-cervical cases. A Chi-square test was done to evaluate the difference of repositioning rates between cervical and non-cervical cases. Results: 362 patients with 480 metastatic sites treated with sSBRT were included in this analysis. There were 225 males and 137 females, with mean age 58 y (ranged, 13-93 y). There were 56 cervical and 424 noncervical targets treated. Patients were treated in one to five fractions. Total number of fractions treated was 1006, of which170 were cervical and 836 were non-cervical. The overall repositioning rate for the entire population was 6.7%. There is no statistically significant difference in repositioning rates between cervical and non-cervical targets (8.8% vs. 6.1%, pZ0.19). Conclusion: The observed rate of repositioning for sSBRT treatments based on stereotactic x-ray image-guidance at our institution was low. No significant difference in intrafraction motion was noted for cervical versus non-cervical spine targets. Author Disclosure: Z. Zhao: None. J.N. Yang: None. X. Wang: None. D. Luo: None. T. Briere: None. M. McAleer: None. J. Li: None. P.D. Brown: None. A.J. Ghia: None.
Author Disclosure: J. Yuan: None. D. Albani: None. Y. Zheng: None. B.W. Wessels: None. S.S. Lo: None. M. Yao: None.
3524 Experimental Validation of Monte Carlo Simulations Based on a Virtual Source Model for Tomotherapy in a Rando Phantom J. Yuan,1 D. Albani,1 Y. Zheng,1 B.W. Wessels,1 S.S. Lo,1 and M. Yao2; 1 University Hospitals Case Medical Center, Cleveland, OH, 2University Hospitals Seidman Cancer Center and Case Western Reserve University, Cleveland, OH Purpose/Objective(s): A virtual source model (VSM) for Monte Carlo simulations of helical tomotherapy has been developed previously by the authors. The purpose of this work is to perform experiments in an anthropomorphic phantom with the same order of complexity as clinical treatments to validate the VSM and the Monte Carlo simulation tool as a whole. The goal is to provide a complementary utility for the routine second check on tomotherapy patient dose distributions. Materials/Methods: The in-house MC system uses the VSM based on the commissioning data of helical tomotherapy unit exported from the TPS. To account for the difference in penumbra and fluence of different jaw size, a 1D transverse profile and a 2D longitudinal profile for each individual jaw size were kept as part of the VSM. The effect of MLC modulation has been modeled by a transfer function called the “leaf filter.” Taking the beam limiting devices (primary collimator, jaws and MLC) into account, for each projection the 2D fluence map under MLC can be modeled as fjs(s,x,y) Z C(x) Jjs(x,y) LF(s,x), where C(x) is the 1D transverse profile which is independent of the Y direction, and Jjs(x,y) is the 2D longitudinal profile. LF(s,x) is the fluence profile of a leaf configuration sZ (s1, s2,... si) for a given projection. To evaluate the accuracy of the VSM and our MC system, experiments were performed in an anthropomorphic RANDO phantom which presents inhomogeneities as in a clinical situation. A treatment plan in the RANDO phantom by mimicking a clinical base of tongue squamous cell carcinoma cancer plan was created. Twenty TLDs and a self-developing dosimetry film were placed at the slice having a large air cavity. Results: The treatment plan was delivered three times with TLDs and selfdeveloping dosimetry film measurement at the same location. The average 95% confidence interval (CI) for all TLD measurement was 2.67. The TLD range divided by mean value, which indicates the variability of measurement, was within 5%, except for two TLDs (9%) which were shown at the high dose gradient region. Compared with measurement, the MC result and TPS result has average relative difference of 1.2% and 2.3% with the largest difference being 3.9% and 7.1%, respectively. For the cumulative DVHs, the MC result agrees with the TPS well in the PTVs and ROIs, although 1.5% difference relative to the prescribed dose for D95 was seen in the PTVs. Conclusion: The experimental results demonstrate that the VSM and the MC system can be a viable option for the accurate MC simulations of helical TomoTherapy and well suitable to be used as a utility for second dose check.
3525 Using MRI to Characterize Small Anatomic Structures Critical to Pelvic Floor Stability in Gynecologic Cancer Patients Undergoing Radiation Therapy P. Soni,1 K.E. Maturen,1 J.I. Prisciandaro,1 J. Zhou,2 Y. Cao,1 J. Balter,1 and S. Jolly1; 1University of Michigan, Ann Arbor, MI, 2Rush University Medical Center, Chicago, IL Purpose/Objective(s): Half of gynecologic cancer survivors report some level of urinary incontinence, while many report additional symptoms of pelvic organ prolapse. The effect of radiation therapy (RT) in causing urinary and pelvic floor dysfunction is not known. In order to understand the role that RT may play in pelvic functional outcomes, we utilized high resolution 3T MRI scans to characterize small pelvic structures important in maintaining pelvic floor stability, and then determined the doses that these structures inadvertently receive during pelvis RT. Materials/Methods: Thirty-two CT and MRI simulation planning scans were obtained from 16 patients diagnosed with uterine or cervix cancer prior to initiating pelvis RT. The volumes at risk were defined for each patient based on standard contouring guidelines on the CT simulation scan and an intensity modulated radiation therapy (IMRT) plan normalized to 45 Gy in 25 fractions was generated. Pelvic anatomic structures known to be critical in urinary and pelvic floor dysfunction (urethra, levator ani, internal anal sphincter, perineal membrane and perineal body) were contoured on various MRI sequences and an experienced radiologist verified the volumes. The MRI simulation scan images were registered to the CT planning scan using bony anatomy. Incidental radiation doses to these pelvic structures were determined. On each plan, the volume, mean dose, D2cc, D1cc and D0.1cc for each structure were established. Results: The T2 weighted axial and sagittal MRI sequences proved to be best in identifying the urethra. The remaining structures were best visualized on T2 weighted coronal images. D0.1cc was highest in urethra and levator ani with a mean D0.1cc of 44.1 Gy and 45.0 Gy respectively. The levator ani, urethra and internal anal sphincter received the highest doses with mean D1cc of 43.3 Gy, 39.3 Gy and 35.6 Gy with a D2cc of 42.3 Gy, 36.9 Gy and 34.3 Gy respectively. The urethra, perineal body and perineal membrane were the smallest structures with mean volumes of 3.3 cc, 1.4 cc, and 0.8 cc respectively. In some cases, the entirety of these small structures receives the full prescription dose of 45 Gy or higher. Conclusion: During pelvis IMRT, anatomic structures critical to pelvic floor stability appear to routinely receive significant RT doses. The clinical implication of these doses needs to be better understood using correlations with patient reported outcomes. MRI-based treatment planning with IMRT may allow for sparing of these structures as deemed clinically necessary. Author Disclosure: P. Soni: None. K.E. Maturen: None. J.I. Prisciandaro: None. J. Zhou: None. Y. Cao: None. J. Balter: Research Grant; NIH. S. Jolly: None.
3526 Quantification of Organ Volume Changes and Impact of Interfraction Motion on Planning Contours Using Deformable Image Registration for Head and Neck Radiation Therapy J. Kim,1 C. Liu,1 A.D. Kumarasiri,1 M. Chetvertkov,2 J.J. Gordon,1 I.J. Chetty,1 and F. Siddiqui1; 1Henry Ford Health System, Detroit, MI, 2 Wayne State University, Detroit, MI Purpose/Objective(s): To quantify organ volumes and impact of interfraction motion on organ contours over the course of head and neck radiation therapy using daily CBCT images and a deformable image registration (DIR) algorithm. Materials/Methods: Ten stage-IV oropharyngeal cancer patients were retrospectively selected. All patients were treated with volumetric modulated arc therapy (VMAT) with two full gantry rotations. A total dose of 60-70 Gy was delivered in 30-35 fractions. CBCT imaging was used for
International Journal of Radiation Oncology Biology Physics
during which patient repositioning was performed were identified. The percentage of set-ups requiring repositioning (i.e., number of fractions with repositioning over total number of fractions treated) was calculated for the whole population and for cervical and non-cervical cases. A Chi-square test was done to evaluate the difference of repositioning rates between cervical and non-cervical cases. Results: 362 patients with 480 metastatic sites treated with sSBRT were included in this analysis. There were 225 males and 137 females, with mean age 58 y (ranged, 13-93 y). There were 56 cervical and 424 noncervical targets treated. Patients were treated in one to five fractions. Total number of fractions treated was 1006, of which170 were cervical and 836 were non-cervical. The overall repositioning rate for the entire population was 6.7%. There is no statistically significant difference in repositioning rates between cervical and non-cervical targets (8.8% vs. 6.1%, pZ0.19). Conclusion: The observed rate of repositioning for sSBRT treatments based on stereotactic x-ray image-guidance at our institution was low. No significant difference in intrafraction motion was noted for cervical versus non-cervical spine targets. Author Disclosure: Z. Zhao: None. J.N. Yang: None. X. Wang: None. D. Luo: None. T. Briere: None. M. McAleer: None. J. Li: None. P.D. Brown: None. A.J. Ghia: None.
Author Disclosure: J. Yuan: None. D. Albani: None. Y. Zheng: None. B.W. Wessels: None. S.S. Lo: None. M. Yao: None.
3524 Experimental Validation of Monte Carlo Simulations Based on a Virtual Source Model for Tomotherapy in a Rando Phantom J. Yuan,1 D. Albani,1 Y. Zheng,1 B.W. Wessels,1 S.S. Lo,1 and M. Yao2; 1 University Hospitals Case Medical Center, Cleveland, OH, 2University Hospitals Seidman Cancer Center and Case Western Reserve University, Cleveland, OH Purpose/Objective(s): A virtual source model (VSM) for Monte Carlo simulations of helical tomotherapy has been developed previously by the authors. The purpose of this work is to perform experiments in an anthropomorphic phantom with the same order of complexity as clinical treatments to validate the VSM and the Monte Carlo simulation tool as a whole. The goal is to provide a complementary utility for the routine second check on tomotherapy patient dose distributions. Materials/Methods: The in-house MC system uses the VSM based on the commissioning data of helical tomotherapy unit exported from the TPS. To account for the difference in penumbra and fluence of different jaw size, a 1D transverse profile and a 2D longitudinal profile for each individual jaw size were kept as part of the VSM. The effect of MLC modulation has been modeled by a transfer function called the “leaf filter.” Taking the beam limiting devices (primary collimator, jaws and MLC) into account, for each projection the 2D fluence map under MLC can be modeled as fjs(s,x,y) Z C(x) Jjs(x,y) LF(s,x), where C(x) is the 1D transverse profile which is independent of the Y direction, and Jjs(x,y) is the 2D longitudinal profile. LF(s,x) is the fluence profile of a leaf configuration sZ (s1, s2,... si) for a given projection. To evaluate the accuracy of the VSM and our MC system, experiments were performed in an anthropomorphic RANDO phantom which presents inhomogeneities as in a clinical situation. A treatment plan in the RANDO phantom by mimicking a clinical base of tongue squamous cell carcinoma cancer plan was created. Twenty TLDs and a self-developing dosimetry film were placed at the slice having a large air cavity. Results: The treatment plan was delivered three times with TLDs and selfdeveloping dosimetry film measurement at the same location. The average 95% confidence interval (CI) for all TLD measurement was 2.67. The TLD range divided by mean value, which indicates the variability of measurement, was within 5%, except for two TLDs (9%) which were shown at the high dose gradient region. Compared with measurement, the MC result and TPS result has average relative difference of 1.2% and 2.3% with the largest difference being 3.9% and 7.1%, respectively. For the cumulative DVHs, the MC result agrees with the TPS well in the PTVs and ROIs, although 1.5% difference relative to the prescribed dose for D95 was seen in the PTVs. Conclusion: The experimental results demonstrate that the VSM and the MC system can be a viable option for the accurate MC simulations of helical TomoTherapy and well suitable to be used as a utility for second dose check.
3525 Using MRI to Characterize Small Anatomic Structures Critical to Pelvic Floor Stability in Gynecologic Cancer Patients Undergoing Radiation Therapy P. Soni,1 K.E. Maturen,1 J.I. Prisciandaro,1 J. Zhou,2 Y. Cao,1 J. Balter,1 and S. Jolly1; 1University of Michigan, Ann Arbor, MI, 2Rush University Medical Center, Chicago, IL Purpose/Objective(s): Half of gynecologic cancer survivors report some level of urinary incontinence, while many report additional symptoms of pelvic organ prolapse. The effect of radiation therapy (RT) in causing urinary and pelvic floor dysfunction is not known. In order to understand the role that RT may play in pelvic functional outcomes, we utilized high resolution 3T MRI scans to characterize small pelvic structures important in maintaining pelvic floor stability, and then determined the doses that these structures inadvertently receive during pelvis RT. Materials/Methods: Thirty-two CT and MRI simulation planning scans were obtained from 16 patients diagnosed with uterine or cervix cancer prior to initiating pelvis RT. The volumes at risk were defined for each patient based on standard contouring guidelines on the CT simulation scan and an intensity modulated radiation therapy (IMRT) plan normalized to 45 Gy in 25 fractions was generated. Pelvic anatomic structures known to be critical in urinary and pelvic floor dysfunction (urethra, levator ani, internal anal sphincter, perineal membrane and perineal body) were contoured on various MRI sequences and an experienced radiologist verified the volumes. The MRI simulation scan images were registered to the CT planning scan using bony anatomy. Incidental radiation doses to these pelvic structures were determined. On each plan, the volume, mean dose, D2cc, D1cc and D0.1cc for each structure were established. Results: The T2 weighted axial and sagittal MRI sequences proved to be best in identifying the urethra. The remaining structures were best visualized on T2 weighted coronal images. D0.1cc was highest in urethra and levator ani with a mean D0.1cc of 44.1 Gy and 45.0 Gy respectively. The levator ani, urethra and internal anal sphincter received the highest doses with mean D1cc of 43.3 Gy, 39.3 Gy and 35.6 Gy with a D2cc of 42.3 Gy, 36.9 Gy and 34.3 Gy respectively. The urethra, perineal body and perineal membrane were the smallest structures with mean volumes of 3.3 cc, 1.4 cc, and 0.8 cc respectively. In some cases, the entirety of these small structures receives the full prescription dose of 45 Gy or higher. Conclusion: During pelvis IMRT, anatomic structures critical to pelvic floor stability appear to routinely receive significant RT doses. The clinical implication of these doses needs to be better understood using correlations with patient reported outcomes. MRI-based treatment planning with IMRT may allow for sparing of these structures as deemed clinically necessary. Author Disclosure: P. Soni: None. K.E. Maturen: None. J.I. Prisciandaro: None. J. Zhou: None. Y. Cao: None. J. Balter: Research Grant; NIH. S. Jolly: None.
3526 Quantification of Organ Volume Changes and Impact of Interfraction Motion on Planning Contours Using Deformable Image Registration for Head and Neck Radiation Therapy J. Kim,1 C. Liu,1 A.D. Kumarasiri,1 M. Chetvertkov,2 J.J. Gordon,1 I.J. Chetty,1 and F. Siddiqui1; 1Henry Ford Health System, Detroit, MI, 2 Wayne State University, Detroit, MI Purpose/Objective(s): To quantify organ volumes and impact of interfraction motion on organ contours over the course of head and neck radiation therapy using daily CBCT images and a deformable image registration (DIR) algorithm. Materials/Methods: Ten stage-IV oropharyngeal cancer patients were retrospectively selected. All patients were treated with volumetric modulated arc therapy (VMAT) with two full gantry rotations. A total dose of 60-70 Gy was delivered in 30-35 fractions. CBCT imaging was used for