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Spinal Cord Motion Considerations for Spine Stereotactic Body Radiation Therapy (SBRT): Does the Cord Move?
Volume 84 Number 3S Supplement 2012 convert cases of conventionally-fractionated adjuvant RT to 1-5 fraction radiosurgical cases. Author Disclosure: I.J. Barani: E. Research Grant; BrainLab. F. Honoraria; BrainLab. A.T. Parsa: E. Research Grant; BrainLab. F. Honoraria; BrainLab.
2159 Low-dose Conformal Radiation Therapy for Ipilimumab-induced Hypophysitis P.L. Menzel,1 A. Geng,1 C. Marquez,1 M. Scala,2 D. Minor,1 and R. Abendroth1; 1California Pacific Medical Center, San Francisco, CA, 2Memorial Sloan-Kettering Cancer Center, New York, NY Purpose/Objective(s): Ipilimumab is an immunostimulatory anti-CTLA 4 antibody that has been shown to improve survival in patients with metastatic melanoma. Unfortunately, 10-15% of patients treated with ipilimumab develop hypophysitis, which is often treated with systemic steroids or ipilimumab cessation or dose reduction. As these treatments are not optimal for patients who may be responding to ipilimumab, we present here our initial experience treating ipilimumab-induced hypophysitis with low-dose conformal radiation therapy. Materials/Methods: This is a single institution retrospective review of metastatic melanoma patients with ipilimumab-induced hypophysitis who were treated with low-dose pituitary radiation therapy. All patients had biochemical evidence of pituitary dysfunction and MRI features of hypophysitis, including diffuse pituitary enlargement and enhancement. Radiation therapy was linear accelerator-based and consisted of three daily 150 cGy fractions delivered using 6- or 15-MV photons. The PTV consisted of the pituitary gland and a 0.5-1 cm margin, which was treated via a three or five field conformal plan. A thermoplastic mask was employed for immobilization. Clinical follow-up as well as pre- and post-treatment MRIs were used to evaluate treatment efficacy, with pituitary volumes calculated using the volumetric formula for an ellipsoid (4/3 x p x R1 x R2 x R3). Mean pre- and post-radiation therapy pituitary volumes were compared using the Student’s t-test. Results: Between September 10, 2010 and February 24, 2012, seven patients (3 women, 4 men) with a median age of 65 years (range, 39-88) were treated. At a median follow-up of 31 weeks (range, 1-77), no patients experienced acute radiation therapy toxicity or developed new symptoms following treatment. Mean pre- and post-radiation therapy pituitary volumes were 796.7 (range, 433.1-962.5) and 380.8 (range, 266.8-569.7) mm3, respectively (p Z 0.005). Radiation therapy was delivered over a median of 5 days (range, 3-5) and without interruption in any patient. All patients who presented with generalized symptoms of fatigue, anorexia, and headache reported symptomatic improvement following treatment. Every patient who presented with a hormonal deficiency had persistent endocrine dysfunction at last follow-up. Conclusion: Low-dose conformal radiation therapy results in significant clinical improvement and radiographic response in patients with ipilimumab-induced hypophysitis, although there is no amelioration of endocrine dysfunction following treatment. This treatment represents a promising therapeutic option for patients with ipilimumab-induced hypophysitis whose survival may be adversely affected by treatment with systemic steroids or interference of ipilimumab administration. Additional exploration and follow-up is necessary to determine whether early treatment initiation can reverse endocrine dysfunction. Author Disclosure: P.L. Menzel: None. A. Geng: None. C. Marquez: None. M. Scala: None. D. Minor: None. R. Abendroth: None.
2160 Long-term Outcomes After Fractionated External Bream Radiation (EBRT) for Pituitary Adenoma: A 25-year Update L.L. Ochoa, D.F. Mullen, J.R. Simpson, and C.G. Robinson; Washington University, St. Louis, MO Purpose/Objective(s): We have previously published outcomes of 121 patients treated with fractionated radiation therapy for pituitary adenoma
Poster Viewing Abstracts S279 treated from 1954 through 1982 at this medical center. The purpose of this study was to update the next 25 years of experience with EBRT, particularly in light of a shift toward Gamma-knife (GK) at our institution during this time period. Materials/Methods: Patients treated with radiation therapy (EBRT, GK) for pituitary adenoma from 1980 to 2005 were evaluated as part of an IRB approved protocol. A retrospective chart review was conducted and patient, tumor, and both surgical and radiation therapy treatment data was extracted from the medical record. Tumor control was determined based on clinical notes and radiographic imaging if available. Results: A total of 141 patients (89 EBRT, 52 GK) were treated during the time period. Median follow up was 7.6 years (range, 0-29). Of the 89 patients treated with EBRT, 54% were male (N Z 48) and the median age was 50.8 years (range, 17-78). The cohort consisted of 67% White (N Z 60), 31% African American (N Z 28) and .01% (N Z 1) other. There were 97% pituitary adenoma and 3% atypical pituitary adenoma. Functioning pituitary adenomas comprised 34% (N Z 29), and 64% (N Z 64) were nonfunctioning. Of the nonfunctioning adenomas, 59% presented with visual symptoms. Eighty-six of the patients were treated initially with external beam radiation therapy to a median dose of 54 Gy (range, 41.4 Gy - 60 Gy) at 1.8 Gy per fraction (range, 1.6 - 2 Gy). The most frequently prescribed dose was 54 Gy (22%), and the next most frequently prescribed was 50.4 Gy (21%). After initial EBRT, 6 patients had recurrences requiring retreatment, for a cumulative local control rate of 93.2%. Of these patients 1 required surgery, 1 required retreatment with EBRT and 4 were salvaged with GK. In comparison, 10y LC for 50-54 Gy was 94.1% and 50 Gy was 85% from 1954-1982. There were no treatment related deaths, and no reported late toxicity. Conclusions: For patients with pituitary adenoma, treatment with EBRT provides excellent long-term control and survival with low toxicity. Outcomes after GK will be reported separately. Author Disclosure: L.L. Ochoa: None. D.F. Mullen: None. J.R. Simpson: None. C.G. Robinson: None.
2161 Spinal Cord Motion Considerations for Spine Stereotactic Body Radiation Therapy (SBRT): Does the Cord Move? C. Tseng,1 M. Sussman,2 A. Simeonov,1 D. Letourneau,1 E. Yu,1 and A. Sahgal1; 1Princess Margaret Hospital, Toronto, ON, Canada, 2 Toronto General Hospital, Toronto, ON, Canada Purpose/Objective(s): An accurate assessment of spinal cord motion is an important consideration when evaluating the appropriateness of a spinal cord planning organ at risk margin (PRV) for spine SBRT. Materials/Methods: We analyzed spinal cord motion in 13 patients treated with spine SBRT (1 cervical, 10 thoracic, and 2 lumbar vertebrae) for metastatic tumors using dynamic axial and sagittal magnetic resonance imaging (MRI) sequences (3T). The scan interval was over 137 seconds and focused on the treated vertebral level. Patients were not in an immobilization device during the MRI. We independently considered oscillatory and random bulk motion. Oscillatory motion was defined as periodic displacements associated with physiologic phenomena, such as cardiac and respiratory cycles, and its magnitude assessed by computing the standard deviation of displacements over time. Random bulk shifts were defined as those associated with gross patient motion, and determined by calculating the magnitude between the largest displacements in opposing directions. Using a correlation coefficient template-matching algorithm, we evaluated displacement in the anteroposterior (AP), lateral (LR), and superoinferior (SI) directions. For each patient, we executed the motion-tracking algorithm over 3 separate trials, each with the template placed at a slightly different location around the cord, to quantify the random motion measure error, and to provide an estimate of true displacement at each time point. Results: The median oscillatory spinal cord motion in the AP, LR, and SI directions were 0.11mm, 0.12mm, and 0.12mm, respectively, and maximal oscillatory motion were 0.28mm, 0.46mm, and 0.48mm, respectively. In the AP, LR, and SI directions, oscillatory motion was less than 0.25mm in 92%, 85%, and 85% of patients, respectively. The median bulk
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displacements in the AP, LR, and SI directions were 0.44mm, 0.59mm and 0.55mm, and maximal displacements were 1.54mm, 2.56mm, and 2.21mm, respectively. In the AP, LR, and SI directions, bulk displacements were less than 0.25 mm in 8%, 0%, and 0%, respectively and more than 1.5mm in 8% of patients in the AP and LR directions, and 15% of patients in the SI direction. Conclusions: There is spinal cord motion; however, physiologic oscillatory motion was generally minor. In select patients, random bulk motion may be significant and more than 1.5mm, which reinforces the role of near-rigid body immobilization during spine SBRT. Author Disclosure: C. Tseng: None. M. Sussman: None. A. Simeonov: None. D. Letourneau: None. E. Yu: None. A. Sahgal: None.
Oncology, Providence, RI, 3Rhode Island Hospital, Department of Neurosurgery, Providence, RI
2162 Reirradiation With Stereotactic Body Radiation Therapy (SBRT): Analysis of Human Spinal Cord Tolerance Using the Generalized Linear-quadratic Model Z. Huang,1 N.A. Mayr,2 W.T. Yuh,2 A. Sahgal,3 J.Z. Wang,2 and S.S. Lo4; 1 East Carolina University, Greenville, NC, 2Ohio State University, Columbus, OH, 3University of Toronto, Toronto, ON, Canada, 4Case Western Reserve University, Cleveland, OH Purpose/Objective(s): Optimal dose delivery, while preventing devastating neurologic complications, is of utmost importance for spinal SBRT, particularly in the re-treatment setting; and defining spinal cord dose constraints is challenging. The recently described generalized linearquadratic (gLQ) model provides more consistent response modeling across conventional through ablative-dose ranges, but the gLQ model has not been used clinically in spinal SBRT. Using the gLQ model, we reanalyzed published dosimetric data of patients with radiation myelopathy (RM) after spinal SBRT, who had been previously treated with conventionally fractionated radiation (RTconv). Materials/Methods: Based on thecal sac (TS) contours for SBRT, spinal cord dose-volume histograms (DVHs) of 5 RM patients (5 spinal segments) and 14 no-RM patients (16 spinal segments) were reanalyzed. The maximum dose point (Pmax) within the TS and doses to 0.1-, 1.0-, and 2.0-cc TS volumes were obtained. The gLQ-based biologically effective doses were calculated and normalized to 2-Gy equivalent dose (nBEDgLQ Z Gy2/2_gLQ). The initial RTconv dose, converted to Gy2/2_gLQ, was added. Results: For the pre-treatment RTconv, nBEDgLQ was similar among noRM and RM patients (27.8-52.5, median: 38.6; and 20.0-50.8, median: 38.8 Gy2/2_gLQ, respectively).The mean Pmax nBEDgLQ to the TS from SBRT reirradiation was significantly lower in no-RM patients compared to RM patients: 18.0 vs 57.0 Gy2/2_gLQ (p Z 1E-6). Similarly, total (RTconv+ SBRT) mean Pmax nBEDgLQ was lower in no-RM than RM patients: 62.2 (range: 42.9-82.4) Gy2/2_gLQ vs. 94.8 (range: 70.2-133.4) Gy2/2_gLQ (p Z 0.005). The proportion of total Pmax nBEDgLQ accounted for by the hypofractionated SBRT Pmax nBEDgLQ was 0.52-0.74 (median: 0.58) for RM patients, compared to 0.04-0.49 (median: 0.28) for no-RM patients. In RM patients, mean nBEDgLQ to Pmax, 0.1-, 1.0- and 2.0-cc was 57.0, 48.3, 34.3, and 26.8 Gy2/2_gLQ. No RMs were seen below a total (RTconv+ SBRT) spinal cord nBEDgLQ of 70 Gy2/2_gLQ. Conclusions: Based on this clinical data, the gLQ-derived spinal cord dose threshold for total nBEDgLQ, incorporating both conventional-fraction pretreatment and ablative-fraction re-treatment, was 70 Gy2/2_gLQ. However, with the current scarcity of clinical data, this approach must be further clinically validated. Author Disclosure: Z. Huang: None. N.A. Mayr: None. W.T. Yuh: None. A. Sahgal: None. J.Z. Wang: None. S.S. Lo: None.
2163 Dosimetric Accuracy of Ray-tracing Algorithm for Treatment of Thoracic Spine Lesions Using Robotic Radiosurgery R.L. Ebeling III,1 J. Hiatt,2 O. Adetokunbo,3 M. Goldman,3 B. Curran,2 T. DiPetrillo,1 T. Kinsella,2 and J.T. Hepel2; 1Tufts University Affiliated Hospitals, Boston, MA, 2Rhode Island Hospital, Department of Radiation
Purpose/Objective(s): Frequently, dose adjustments or optimization using Monte Carlo are employed to correct for tissue homogeneity inaccuracies of the ray-tracing dose calculation algorithm used for robotic radiosurgery treatment planning of lung tumors. Spine radiosurgery patients are required to be treated in the supine position to allow for accurate intra-treatment spine tracking; yet posterior oriented beams are not possible with this system. As such, in the treatment of thoracic spine lesions the majority of treatment beams traverse the lungs. In addition, high-density surgical hardware is often present in and around the target lesion. The dosimetric impact of inaccuracies in correcting for these marked differences in tissue density for the treatment planning of thoracic spine lesions is not known. Materials/Methods: Thirty-one patients with 43 thoracic vertebral lesions treated in our department using robotic radiosurgery between 2009 and 2012 were identified. All patients were planned using the robotic radiosurgery Multiplan system employing ray-tracing algorithm (RTA) for tissue heterogeneity correction to a median dose of 24 Gy in 1-5 fractions. For each patient, a comparison plan was generated using Monte Carlo dose calculation algorithm (MCA) using identical beam arrangement and monitor units. Plans were compared for differences in GTV coverage by the prescription isodose line, GTV D90%, spinal cord maximum dose, and dose to 0.25cc of spinal cord. Results: Mean GTV coverage was 26.3% lower for the MCA as compared with RTA, 60.3% vs 86.6%, respectively (p < 0.0001). Similarly, mean GTV D90% was 2.7% lower for MCA (range: -7.34% to 9.26%, p Z 0.001). Conversely, mean doses to the spinal cord were significantly higher using MCA by 6.9% (range: -9.8% to 24.1%) for maximum dose and by 1.2% (range: -8.9% to 10.6%) for 0.25cc; p < 0.0001 and p < 0.01, respectively. Presence of surgical hardware (n Z 8) had only a small effect with mean differences of -5.4% for GTV coverage (p Z 0.6), -2.2% for GTV D90% (p Z 0.3), 4.3% for cord max (p Z 0.2), and 5.4% for cord 0.25cc (p Z 0.04). Conclusion: Ray-tracing dose calculation algorithm for the thoracic spine overestimates GTV coverage and underestimates spinal cord dose. This can result in significant impact on both disease control and potential for cord injury, especially in the retreatment setting where treatment doses and cord constraints are pushed to tolerance levels. Optimization of treatment plans using Monte Carlo dose calculation algorithm is, thus, warranted in these patients. Author Disclosure: R.L. Ebeling, III: None. J. Hiatt: None. O. Adetokunbo: None. M. Goldman: None. B. Curran: None. T. DiPetrillo: None. T. Kinsella: None. J.T. Hepel: None.
2164 Analysis of Robustness of a Combined Cranial Photon and MLCbased Spinal Proton Field Matching Technique for Delivery of Craniospinal Irradiation H. Zhai, C. Hill-Kayser, P. James, R. Lustig, H. Lin, J. Mcdonough, Z. Tochner, and S. Both; The Hospital of University of Pennsylvania, Philadelphia, PA Purpose/Objective(s): In general, many patients requiring craniospinal irradiation (CSI) are children, for whom radiation therapy is frequently associated with severe side effects, such as fertility dysfunction, cardiac morbidity, and secondary malignancies. Proton beam treatment (PBT) has increased our ability to maximize the dose to the tumor while sparing normal structures, reducing the risk of acute as well as late and long-term side effects. CSI patients are treated in a prone position and careful junctions are created between cranial photon and posterior spinal proton fields. In this study, we examine the robustness of this novel technique. Materials/Methods: Nine patients were treated with opposed lateral cranial photon beams (6 MV) and two to four posterior passive scattered spinal proton beams. The patients were positioned prone and immobilized using a thermoplastic frame and vacuum bag. Daily orthogonal kV imaging was used for alignment based on bony anatomy. Weekly verification CTs were acquired during the treatment. The verification CTs were
2159 Low-dose Conformal Radiation Therapy for Ipilimumab-induced Hypophysitis P.L. Menzel,1 A. Geng,1 C. Marquez,1 M. Scala,2 D. Minor,1 and R. Abendroth1; 1California Pacific Medical Center, San Francisco, CA, 2Memorial Sloan-Kettering Cancer Center, New York, NY Purpose/Objective(s): Ipilimumab is an immunostimulatory anti-CTLA 4 antibody that has been shown to improve survival in patients with metastatic melanoma. Unfortunately, 10-15% of patients treated with ipilimumab develop hypophysitis, which is often treated with systemic steroids or ipilimumab cessation or dose reduction. As these treatments are not optimal for patients who may be responding to ipilimumab, we present here our initial experience treating ipilimumab-induced hypophysitis with low-dose conformal radiation therapy. Materials/Methods: This is a single institution retrospective review of metastatic melanoma patients with ipilimumab-induced hypophysitis who were treated with low-dose pituitary radiation therapy. All patients had biochemical evidence of pituitary dysfunction and MRI features of hypophysitis, including diffuse pituitary enlargement and enhancement. Radiation therapy was linear accelerator-based and consisted of three daily 150 cGy fractions delivered using 6- or 15-MV photons. The PTV consisted of the pituitary gland and a 0.5-1 cm margin, which was treated via a three or five field conformal plan. A thermoplastic mask was employed for immobilization. Clinical follow-up as well as pre- and post-treatment MRIs were used to evaluate treatment efficacy, with pituitary volumes calculated using the volumetric formula for an ellipsoid (4/3 x p x R1 x R2 x R3). Mean pre- and post-radiation therapy pituitary volumes were compared using the Student’s t-test. Results: Between September 10, 2010 and February 24, 2012, seven patients (3 women, 4 men) with a median age of 65 years (range, 39-88) were treated. At a median follow-up of 31 weeks (range, 1-77), no patients experienced acute radiation therapy toxicity or developed new symptoms following treatment. Mean pre- and post-radiation therapy pituitary volumes were 796.7 (range, 433.1-962.5) and 380.8 (range, 266.8-569.7) mm3, respectively (p Z 0.005). Radiation therapy was delivered over a median of 5 days (range, 3-5) and without interruption in any patient. All patients who presented with generalized symptoms of fatigue, anorexia, and headache reported symptomatic improvement following treatment. Every patient who presented with a hormonal deficiency had persistent endocrine dysfunction at last follow-up. Conclusion: Low-dose conformal radiation therapy results in significant clinical improvement and radiographic response in patients with ipilimumab-induced hypophysitis, although there is no amelioration of endocrine dysfunction following treatment. This treatment represents a promising therapeutic option for patients with ipilimumab-induced hypophysitis whose survival may be adversely affected by treatment with systemic steroids or interference of ipilimumab administration. Additional exploration and follow-up is necessary to determine whether early treatment initiation can reverse endocrine dysfunction. Author Disclosure: P.L. Menzel: None. A. Geng: None. C. Marquez: None. M. Scala: None. D. Minor: None. R. Abendroth: None.
2160 Long-term Outcomes After Fractionated External Bream Radiation (EBRT) for Pituitary Adenoma: A 25-year Update L.L. Ochoa, D.F. Mullen, J.R. Simpson, and C.G. Robinson; Washington University, St. Louis, MO Purpose/Objective(s): We have previously published outcomes of 121 patients treated with fractionated radiation therapy for pituitary adenoma
Poster Viewing Abstracts S279 treated from 1954 through 1982 at this medical center. The purpose of this study was to update the next 25 years of experience with EBRT, particularly in light of a shift toward Gamma-knife (GK) at our institution during this time period. Materials/Methods: Patients treated with radiation therapy (EBRT, GK) for pituitary adenoma from 1980 to 2005 were evaluated as part of an IRB approved protocol. A retrospective chart review was conducted and patient, tumor, and both surgical and radiation therapy treatment data was extracted from the medical record. Tumor control was determined based on clinical notes and radiographic imaging if available. Results: A total of 141 patients (89 EBRT, 52 GK) were treated during the time period. Median follow up was 7.6 years (range, 0-29). Of the 89 patients treated with EBRT, 54% were male (N Z 48) and the median age was 50.8 years (range, 17-78). The cohort consisted of 67% White (N Z 60), 31% African American (N Z 28) and .01% (N Z 1) other. There were 97% pituitary adenoma and 3% atypical pituitary adenoma. Functioning pituitary adenomas comprised 34% (N Z 29), and 64% (N Z 64) were nonfunctioning. Of the nonfunctioning adenomas, 59% presented with visual symptoms. Eighty-six of the patients were treated initially with external beam radiation therapy to a median dose of 54 Gy (range, 41.4 Gy - 60 Gy) at 1.8 Gy per fraction (range, 1.6 - 2 Gy). The most frequently prescribed dose was 54 Gy (22%), and the next most frequently prescribed was 50.4 Gy (21%). After initial EBRT, 6 patients had recurrences requiring retreatment, for a cumulative local control rate of 93.2%. Of these patients 1 required surgery, 1 required retreatment with EBRT and 4 were salvaged with GK. In comparison, 10y LC for 50-54 Gy was 94.1% and 50 Gy was 85% from 1954-1982. There were no treatment related deaths, and no reported late toxicity. Conclusions: For patients with pituitary adenoma, treatment with EBRT provides excellent long-term control and survival with low toxicity. Outcomes after GK will be reported separately. Author Disclosure: L.L. Ochoa: None. D.F. Mullen: None. J.R. Simpson: None. C.G. Robinson: None.
2161 Spinal Cord Motion Considerations for Spine Stereotactic Body Radiation Therapy (SBRT): Does the Cord Move? C. Tseng,1 M. Sussman,2 A. Simeonov,1 D. Letourneau,1 E. Yu,1 and A. Sahgal1; 1Princess Margaret Hospital, Toronto, ON, Canada, 2 Toronto General Hospital, Toronto, ON, Canada Purpose/Objective(s): An accurate assessment of spinal cord motion is an important consideration when evaluating the appropriateness of a spinal cord planning organ at risk margin (PRV) for spine SBRT. Materials/Methods: We analyzed spinal cord motion in 13 patients treated with spine SBRT (1 cervical, 10 thoracic, and 2 lumbar vertebrae) for metastatic tumors using dynamic axial and sagittal magnetic resonance imaging (MRI) sequences (3T). The scan interval was over 137 seconds and focused on the treated vertebral level. Patients were not in an immobilization device during the MRI. We independently considered oscillatory and random bulk motion. Oscillatory motion was defined as periodic displacements associated with physiologic phenomena, such as cardiac and respiratory cycles, and its magnitude assessed by computing the standard deviation of displacements over time. Random bulk shifts were defined as those associated with gross patient motion, and determined by calculating the magnitude between the largest displacements in opposing directions. Using a correlation coefficient template-matching algorithm, we evaluated displacement in the anteroposterior (AP), lateral (LR), and superoinferior (SI) directions. For each patient, we executed the motion-tracking algorithm over 3 separate trials, each with the template placed at a slightly different location around the cord, to quantify the random motion measure error, and to provide an estimate of true displacement at each time point. Results: The median oscillatory spinal cord motion in the AP, LR, and SI directions were 0.11mm, 0.12mm, and 0.12mm, respectively, and maximal oscillatory motion were 0.28mm, 0.46mm, and 0.48mm, respectively. In the AP, LR, and SI directions, oscillatory motion was less than 0.25mm in 92%, 85%, and 85% of patients, respectively. The median bulk
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displacements in the AP, LR, and SI directions were 0.44mm, 0.59mm and 0.55mm, and maximal displacements were 1.54mm, 2.56mm, and 2.21mm, respectively. In the AP, LR, and SI directions, bulk displacements were less than 0.25 mm in 8%, 0%, and 0%, respectively and more than 1.5mm in 8% of patients in the AP and LR directions, and 15% of patients in the SI direction. Conclusions: There is spinal cord motion; however, physiologic oscillatory motion was generally minor. In select patients, random bulk motion may be significant and more than 1.5mm, which reinforces the role of near-rigid body immobilization during spine SBRT. Author Disclosure: C. Tseng: None. M. Sussman: None. A. Simeonov: None. D. Letourneau: None. E. Yu: None. A. Sahgal: None.
Oncology, Providence, RI, 3Rhode Island Hospital, Department of Neurosurgery, Providence, RI
2162 Reirradiation With Stereotactic Body Radiation Therapy (SBRT): Analysis of Human Spinal Cord Tolerance Using the Generalized Linear-quadratic Model Z. Huang,1 N.A. Mayr,2 W.T. Yuh,2 A. Sahgal,3 J.Z. Wang,2 and S.S. Lo4; 1 East Carolina University, Greenville, NC, 2Ohio State University, Columbus, OH, 3University of Toronto, Toronto, ON, Canada, 4Case Western Reserve University, Cleveland, OH Purpose/Objective(s): Optimal dose delivery, while preventing devastating neurologic complications, is of utmost importance for spinal SBRT, particularly in the re-treatment setting; and defining spinal cord dose constraints is challenging. The recently described generalized linearquadratic (gLQ) model provides more consistent response modeling across conventional through ablative-dose ranges, but the gLQ model has not been used clinically in spinal SBRT. Using the gLQ model, we reanalyzed published dosimetric data of patients with radiation myelopathy (RM) after spinal SBRT, who had been previously treated with conventionally fractionated radiation (RTconv). Materials/Methods: Based on thecal sac (TS) contours for SBRT, spinal cord dose-volume histograms (DVHs) of 5 RM patients (5 spinal segments) and 14 no-RM patients (16 spinal segments) were reanalyzed. The maximum dose point (Pmax) within the TS and doses to 0.1-, 1.0-, and 2.0-cc TS volumes were obtained. The gLQ-based biologically effective doses were calculated and normalized to 2-Gy equivalent dose (nBEDgLQ Z Gy2/2_gLQ). The initial RTconv dose, converted to Gy2/2_gLQ, was added. Results: For the pre-treatment RTconv, nBEDgLQ was similar among noRM and RM patients (27.8-52.5, median: 38.6; and 20.0-50.8, median: 38.8 Gy2/2_gLQ, respectively).The mean Pmax nBEDgLQ to the TS from SBRT reirradiation was significantly lower in no-RM patients compared to RM patients: 18.0 vs 57.0 Gy2/2_gLQ (p Z 1E-6). Similarly, total (RTconv+ SBRT) mean Pmax nBEDgLQ was lower in no-RM than RM patients: 62.2 (range: 42.9-82.4) Gy2/2_gLQ vs. 94.8 (range: 70.2-133.4) Gy2/2_gLQ (p Z 0.005). The proportion of total Pmax nBEDgLQ accounted for by the hypofractionated SBRT Pmax nBEDgLQ was 0.52-0.74 (median: 0.58) for RM patients, compared to 0.04-0.49 (median: 0.28) for no-RM patients. In RM patients, mean nBEDgLQ to Pmax, 0.1-, 1.0- and 2.0-cc was 57.0, 48.3, 34.3, and 26.8 Gy2/2_gLQ. No RMs were seen below a total (RTconv+ SBRT) spinal cord nBEDgLQ of 70 Gy2/2_gLQ. Conclusions: Based on this clinical data, the gLQ-derived spinal cord dose threshold for total nBEDgLQ, incorporating both conventional-fraction pretreatment and ablative-fraction re-treatment, was 70 Gy2/2_gLQ. However, with the current scarcity of clinical data, this approach must be further clinically validated. Author Disclosure: Z. Huang: None. N.A. Mayr: None. W.T. Yuh: None. A. Sahgal: None. J.Z. Wang: None. S.S. Lo: None.
2163 Dosimetric Accuracy of Ray-tracing Algorithm for Treatment of Thoracic Spine Lesions Using Robotic Radiosurgery R.L. Ebeling III,1 J. Hiatt,2 O. Adetokunbo,3 M. Goldman,3 B. Curran,2 T. DiPetrillo,1 T. Kinsella,2 and J.T. Hepel2; 1Tufts University Affiliated Hospitals, Boston, MA, 2Rhode Island Hospital, Department of Radiation
Purpose/Objective(s): Frequently, dose adjustments or optimization using Monte Carlo are employed to correct for tissue homogeneity inaccuracies of the ray-tracing dose calculation algorithm used for robotic radiosurgery treatment planning of lung tumors. Spine radiosurgery patients are required to be treated in the supine position to allow for accurate intra-treatment spine tracking; yet posterior oriented beams are not possible with this system. As such, in the treatment of thoracic spine lesions the majority of treatment beams traverse the lungs. In addition, high-density surgical hardware is often present in and around the target lesion. The dosimetric impact of inaccuracies in correcting for these marked differences in tissue density for the treatment planning of thoracic spine lesions is not known. Materials/Methods: Thirty-one patients with 43 thoracic vertebral lesions treated in our department using robotic radiosurgery between 2009 and 2012 were identified. All patients were planned using the robotic radiosurgery Multiplan system employing ray-tracing algorithm (RTA) for tissue heterogeneity correction to a median dose of 24 Gy in 1-5 fractions. For each patient, a comparison plan was generated using Monte Carlo dose calculation algorithm (MCA) using identical beam arrangement and monitor units. Plans were compared for differences in GTV coverage by the prescription isodose line, GTV D90%, spinal cord maximum dose, and dose to 0.25cc of spinal cord. Results: Mean GTV coverage was 26.3% lower for the MCA as compared with RTA, 60.3% vs 86.6%, respectively (p < 0.0001). Similarly, mean GTV D90% was 2.7% lower for MCA (range: -7.34% to 9.26%, p Z 0.001). Conversely, mean doses to the spinal cord were significantly higher using MCA by 6.9% (range: -9.8% to 24.1%) for maximum dose and by 1.2% (range: -8.9% to 10.6%) for 0.25cc; p < 0.0001 and p < 0.01, respectively. Presence of surgical hardware (n Z 8) had only a small effect with mean differences of -5.4% for GTV coverage (p Z 0.6), -2.2% for GTV D90% (p Z 0.3), 4.3% for cord max (p Z 0.2), and 5.4% for cord 0.25cc (p Z 0.04). Conclusion: Ray-tracing dose calculation algorithm for the thoracic spine overestimates GTV coverage and underestimates spinal cord dose. This can result in significant impact on both disease control and potential for cord injury, especially in the retreatment setting where treatment doses and cord constraints are pushed to tolerance levels. Optimization of treatment plans using Monte Carlo dose calculation algorithm is, thus, warranted in these patients. Author Disclosure: R.L. Ebeling, III: None. J. Hiatt: None. O. Adetokunbo: None. M. Goldman: None. B. Curran: None. T. DiPetrillo: None. T. Kinsella: None. J.T. Hepel: None.
2164 Analysis of Robustness of a Combined Cranial Photon and MLCbased Spinal Proton Field Matching Technique for Delivery of Craniospinal Irradiation H. Zhai, C. Hill-Kayser, P. James, R. Lustig, H. Lin, J. Mcdonough, Z. Tochner, and S. Both; The Hospital of University of Pennsylvania, Philadelphia, PA Purpose/Objective(s): In general, many patients requiring craniospinal irradiation (CSI) are children, for whom radiation therapy is frequently associated with severe side effects, such as fertility dysfunction, cardiac morbidity, and secondary malignancies. Proton beam treatment (PBT) has increased our ability to maximize the dose to the tumor while sparing normal structures, reducing the risk of acute as well as late and long-term side effects. CSI patients are treated in a prone position and careful junctions are created between cranial photon and posterior spinal proton fields. In this study, we examine the robustness of this novel technique. Materials/Methods: Nine patients were treated with opposed lateral cranial photon beams (6 MV) and two to four posterior passive scattered spinal proton beams. The patients were positioned prone and immobilized using a thermoplastic frame and vacuum bag. Daily orthogonal kV imaging was used for alignment based on bony anatomy. Weekly verification CTs were acquired during the treatment. The verification CTs were