S.A. Burton

Comparison of whole versus partial vertebral body stereotactic body radiation therapy for spinal metastases.

The purpose of this study is to evaluate the difference in clinical outcomes for patients with metastatic spine disease treated with a whole versus partial vertebral body contouring approach. A retrospective study was performed for the clinical outcomes of 154 metastatic lesions to the spine in 117 patients treated with stereotactic body radiation therapy (SBRT) using the Cyberknife™ Robotic Radiosurgery System. Each patient was treated with a single session of radiotherapy using either a whole (WB) or a partial vertebral body contour approach (PB). The primary endpoint was re-treatment rate and the secondary endpoints were pain status, neurologic status, toxicity, tumor control, and survival. The WB group had a lower re-treatment rate (11% (WB) vs. 18.6% (PB), p = 0.285). Prior surgery status (P = 1.953, OR = 7.052, p< 0.001) was correlated to the re-treatment rate. Trends for local tumor control were distinct for both treatment groups (X2 = 3.380, p-value = 0.066). Treatment group (P = −1.1017, OR = 0.362, p = 0.029) was significantly correlated to the local tumor control rate. The 2-year survival was 25.7% in WB and 20.9% in PB (p = 0.741). Contouring the whole vertebral body for stereotactic body radiation therapy treatment of metastatic spinal lesions shows potential benefits by reducing the risk of recurrence, improving symptomatic relief and providing improved local tumor control.

Combined Endoscopic Endonasal Surgery and Fractionated Stereotactic Radiosurgery (fSRS) for Complex Cranial Base Tumors—Early Clinical Outcomes

Endoscopic endonasal surgery (EES) has been shown to be a feasible approach to cranial base tumors while reducing post-operative morbidity. Using the endoscopic endonasal approach alone or in combination with open approaches may provide advantages over conventional approaches. However, the balance between maximal resection and minimal injury to neurovascular structures frequently precludes gross total resection (GTR). Consequently, adjuvant radiation therapy may be an important option to improve local control (LC) of residual disease. In this retrospective series, we report clinical outcomes, morbidity, and LC of 40 patients with cranial base tumors treated with EES +/- combined open approach followed by fSRS (CyberKnife, Accuray Inc.). 26 patients had benign disease, 7 had newly diagnosed malignant disease, and 7 had previously resected malignant disease. Surgical outcomes were evaluable in all patients. LC after fSRS was evaluable in 39 patients and defined as no evidence of regrowth by MRI, CT, & physical examination. GTR was achieved in 12/40. Median post-operative length of stay (LOS) was 3 days. In multivariable analysis controlling for anatomic location and malignant histology, post-operative complications (n = 10) were significantly associated with patients having combined open and EES (p < 0.01, OR = 16.9). SRS was delivered in 1–5 sessions to a median marginal dose of 24.9 Gy. Median follow-up was 24.7 months (range, 1.5 to 61 months). LC was achieved in 89.7% (35/39) of evaluable patients. LC was achieved in 11/12 patients who had GTR. Median progression-free survival was 19.7 months (21.0 months for benign tumors (n = 26), 5.8 months for previously resected malignant disease (n = 7), and 21.2 months for newly diagnosed malignant disease (n = 7). Of the 31 patients who had symptomatic disease at presentation, 18 (58%) reported complete symptom resolution, 9 partial, and 4 no improvement. One patient who received two prior courses of radiation therapy developed osteosclerosis (grade III). Other adverse events were erythema (grade I, n = 5), nausea (grade II, n = 2), conjunctivitis (grade II, n = 1). EES followed by fSRS is a safe and effective management strategy for selected cranial base tumors. EES combined with an open surgical approach may result in increased complications. However, initial follow-up offers encouraging results indicating shorter time to recovery, acceptable LC rates compared to conventional approaches, and similar median time to progression for benign and newly diagnosed malignant disease.

Mediastinal/Hilar Stereotactic Ablative Radiation Therapy for Primary and Oligorecurrent Non–Small Cell Lung Cancer

Purpose/Objective(s): To maximize the outcomes of metastatic nonesmall cell lungcancer (NSCLC), clinicians aim to exposepatients sequentially to3 to 4 lines of systemic therapy regimens, but treatment attrition can occur due to toxicity, advanced age, or patient preference. Our study goals are to characterize patterns of treatment attrition in metastatic NSCLC patients and to identify major clinical factors associated with attrition. Materials/Methods: We conducted a retrospective review of medical and pharmacy records of lung cancer patients seen at a Canadian institute from 2012 to 2013, and who received at least 1 cycle of systemic therapy. Patients were classified into 3 mutually exclusive groups: (1) patients who completed all lines of systemic therapy; (2) patients who stopped treatment due to death; and (3) patients who stopped treatment due to other clinical factors (eg, drug toxicity). Chi-square tests were conducted to identify predictors of attrition followed by multivariate analyses that adjusted for potential confounders. Results: We identified 291 eligible metastatic NSCLC patients with complete records: median age 63 years, 38%men, 75%Caucasian, 69% baseline ECOG0or 1, 31%never-smokers, and 65%received radiation therapy. In the entire cohort, only 22% completed all lines of systemic therapy, while 40% chose to discontinue treatment due to various clinical and toxicity-related reasons. A quarter of patients died of their disease, which prohibited further therapy. In our analyses, several clinical factors were strongly associated with increased likelihood of treatment attrition, including Caucasians (OR 5.95, 95% CI 1.95-20.41, P<.001), ECOG 2+ (OR 2.78, 95% CI 1.05-8.27, PZ.038), and radiation therapy (OR 2.00, 95% CI 0.89-4.50, PZ.012). In addition, former and current smokersweremore likely to experience attrition than never smokers (OR 1.72, 95%CI 0.55-6.02,PZ.006 andOR 1.70, 95% CI 0.31-8.85, PZ.006, respectively). However, age, marital status, and history of previous malignancies did not correlate with attrition (all P>.05). Conclusion: Attrition is common in the treatment of metastatic NSCLC. Some of the factors leading to treatment attrition may be modifiable. Interventions to maximize exposure to all therapies can improve outcomes. Author Disclosure: F. Yang: None. L. Chen: None. W. Cheung: None.

SU-E-T-420: Impact of Different Prescription Isodose Lines On Plan Quality for Brain Metastases Using Multiplan System

PURPOSE With the sequential optimization algorithm in MultiPlan system, clinical objectives (homogeneity, PTV coverage, conformity, normal tissue protection) can be optimized in sequence. However, the prescription isodose line (RxIDL) varies widely among institutions, which can influence the optimized dose distribution. The aim of this study is to investigate the impact of different prescription isodose lines on plan quality for the treatment of brain metastases using CyberKnife Multiplan system. METHODS Ten patients with multiple metastases were selected for this study. Four plans were generated for each patient such that 100% of the target volume receives the prescribed dose of 18 Gy, which was 50%, 60%, 70%, and 80% prescription Isodose line, separately. The prescription isodose was calculated as the ratio of the prescription dose and the maximum dose in target volume. The dosimetric parameters, including PTV coverage, conformity index (CI), gradient index (GI) and the volume covered by 12 Gy (V12Gy) were analyzed. The plan Monitor Units (MU) and treatment time were also compared. RESULTS All plans can provide the same target coverage (100%) and similar conformity index (1.26, 1.30, 1.32, and 1.29 on average for 80%, 70%, 60%, and 50% RxIDL plans, separately); there was no difference in critical structure dose. The 50% RxIDL plans have much lower GI (4.21±1.79 for 50% and 5.56±2.92 for 80% RxIDL plans) and V12Gy (13.36±10.31cc for 50% and 15.87±11.85cc for 80% RxIDL plans). The variation in estimated treatment delivery time was insignificant. CONCLUSION The dose falloff is much faster for the lower RxIDL plans in terms of GI and V12Gy. For 50% RxIDL plans, the average V12Gy decreases by 16% compared to 80% RxIDL plans, which indicates that the normal tissue can be better protected by decreasing the prescription Isodose line.

SU-E-J-185: Gated CBCT Imaging for Positioning Moving Lung Tumor in Lung SBRT Treatment.

PURPOSE Lung stereo-tactic body radiotherapy(SBRT) treatment requires high accuracy of lung tumor positioning during treatment, which is usually accomplished by free breathing Cone-Beam computerized tomography (CBCT) scan. However, respiratory motion induced image artifacts in free breathing CBCT may degrade such positioning accuracy. The purpose of this study is to investigate the feasibility of gated CBCT imaging for lung SBRT treatment. METHODS Six Lung SBRT patients were selected for this study. The respiratory motion of the tumors ranged from 1.2cm to 3.5cm, and the gating windows for all patients were set between 35% and 65% of the respiratory phases. Each Lung SBRT patient underwent free-breathing CBCT scan using half-fan scan technique. The acquired projection images were transferred out for off-line analyses. An In-house semi-automatic algorithm was developed to trace the diaphragm movement from those projection images to acquire a patients specific respiratory motion curve, which was used to correlate respiratory phases with each projection image. Afterwards, a filtered back-projection algorithm was utilized to reconstruct the gated CBCT images based on the projection images only within the gating window. RESULTS Target volumes determined by free breathing CBCT images were 71.9%±72% bigger than the volume shown in gated CBCT image. On the contrary, the target volume differences between gated CBCT and planning CT images at exhale stage were 5.8%±2.4%. The center to center distance of the targets shown in free breathing CBCT and gated CBCT images were 9.2±8.1mm. For one particular case, the superior boundary of the target was shifted 15mm between free breathing CBCT and gated CBCT. CONCLUSION Gated CBCT imaging provides better representation of the moving lung tumor with less motion artifacts, and has the potential to improve the positioning accuracy in lung SBRT treatment.

SU‐GG‐T‐540: Intensity‐Modulated Arc Therapy for Stereotactic Radiotherapy of Spinal & Paraspinal Tumors

Purpose: Stereotactic radiosurgery of metastatic spinal tumor with intensity‐modulated radiotherapy(IMRT) technique requires a long treatment time due to an extensive monitor units (MU) resulting from multiple highly intensity‐modulated beams in order to sparing adjacent spinal cord and other critical structures. This study investigates the feasibility of using intensitymodulated arc therapy (IMAT) as an alternative modality with a shorter treatment time while maintaining a compatible dosimetric performance as IMRT technique. Methods/Materials: 8 patients with spinal or paraspinal tumor were recruited in this study. All those patients were previously treated with IMRT technique, in which 18Gy or 24Gy doses were delivered in a single fraction with 11 to 13 coplanar radiation beams. Single arc and 2‐arc IMAT plans were retrospectively generated for each patients using RapidArcTM treatment planning system (Varian Medical System, Sunnyvale, CA). The previous delivered IMRT plans were chosen as a reference. The differences of following parameters between IMAT and IMRT plans were used to evaluate the plan performance: the volumes of PTV receiving 95% and 100% of prescribed dose(V95, V100), the maximum spinal cord dose (MSPDOSE) and the total monitor units (TMU). Results: For all 8 patients, the differences of V95 and V100 between single arc IMAT and IMRT plans are −5.3%±4.8% and −9.3%±7.8% , while the difference of MSPDOSE is 0.23Gy±0.87Gy. In contrary, the differences of V95 and V100 between 2‐arc IMAT and IMRT plans are −0.67%±2.01%, −1.1%±2.23% , while the difference of MSPDOSE is 0.38Gy±0.47GY. The ratios of TMU of single arc and 2‐arc IMAT plans over IMRT plan are 55%±19% and 65%±17%. Conclusion: For stereotactic radiosurgery of spinal tumor,IMRT plan provide better dose coverage than single arc IMAT plan, but 2‐arc IMAT plan is capable of providing a compatible dosimetric performance as IMRT plan while significantly reducing the treatment time.

SU-FF-J-42: Phase Lag Measurements of Abdominal Organs Relative to An External Marker Block Using Retrospective 4D CT Imaging

Purpose: The purpose of this study is to quantify the phase lag of superior-inferior abdominal organ motion relative to an external marker block used to monitor respiratory motion. The diaphragm, liver, spleen, and kidneys were studied. Method and Materials: A 4DCT (GE Medical System, Waukesha, Wisconsin) scan correlated with respiratory motion using the Real-Time Position Management (RPM) Respiratory Gating System (Varian Medical Systems, Palo Alto, CA) was used to acquire scans of 10 patients. Up to 10 images at each slice location within one breathing cycle were acquired and sorted into respiratory phases evenly distributed in time. The superior and inferior edge of each organ was identified, and the average of these positions was used as the S-I position of the organ. The anterior edge of the external marker block was also recorded. These positions were identified for all respiratory phases. The data was then fit with a cosine squared function. The argument of the function was the observed respiratory phase plus a starting phase. The starting phase was then adjusted until the value generating the least square deviation among all measurements of the particular organ, diaphragm, or marker block for all phases was found. The difference of starting phase minus that of the marker block is then recorded as the phase lag relative to the marker block. Results: No phase lag is greater than 36° which is the minimum difference between successive phase for a respiratory cycle divided into 10 phases. Conclusion: The external marker block used to monitor respiration is observed to be in phase the motion of the abdominal organs and diaphragm within the measurement accuracy. Conflict of Interest: Software provided by GE Medical Systems.