Materials/Methods: We compared 2 volumetric modulated arc therapy (VMAT) planning methods, conventional VMAT planning, and MU-effi- cient VMAT planning on 14 SBRT/SRS plans. Treatment sites included brain, liver, spine, and lung. Both methods utilized a commercially available treatment planning system. The MU-efficient planning method used the MU objective with a strength setting of 100, a lower objective of 0, and an upper objective of 1.5 times the prescription dose per fraction. The plan normalization and the optimization objectives for organs at risk (OARs) and targets were identical between the conventional VMAT plan and the MU-efficient plan. Dosimetric quality metrics such as the con- formity index (CI), gradient measure, and planning target volume dose coverage (minimum, maximum, and mean dose) were compared between the initial clinical plan and the MU-efficient plan. MU efficiency metrics including the total MUs, IMRT factor, and delivery time were also compared between the conventional and MU-efficient plans. Results: The CI (PZ0.863) and gradient measure (PZ0.192) were statistically equivalent for both sets of plans. The average IMRT factor was 3.1 MU/cGy for the clinical plans, and 2.3 MU/cGy for the MU- efficient plans. Improvement in MU-efficient was less for targets with concavities or asymmetry (spine or multiple metastases treated with a single iso). The average reduction in IMRT factor for concave/asym- metric targets (0.4 MU/cGy) was less than other sites (0.9 MU/cGy). Assuming a dose rate of 600 MU/minute, the MU-efficient plans resulted in reduction in treatment delivery time of 5.3 minutes (0.6-14.9 minutes) per course of treatment. Conclusion: MU-efficient treatment planning facilitated by the MU objective is a simple way to reduce the IMRT factor for SBRT/SRS plans and reduce table time without sacrificing target coverage, dose gradient, conformity, or any other plan quality metric studied. Author Disclosure: R. Manger: None. A. Yock: None. D.J. Hoopes: None. 3594 On the Use of Intensity Modulated Arc Therapy for Whole-Brain Radiation Therapy With Hippocampal Avoidance and Simultaneous Integrated Boost for Multiple Brain Metastases D. Pokhrel, 1 S.S. Sood, 2 C.M. McClinton, 1 X. Shen, 1 C.E. Lominska, 1 R. Badkul, 1 H. Jiang, 1 M.P. Mitchell, 1 and F. Wang 1 ; 1 University of Kansas Medical Center, Kansas City, KS, 2 Kansas University Medical Center, Department of Radiation Oncology, Kansas City, KS Purpose/Objective(s): To retrospectively evaluate the accuracy, quality, and efficiency of intensity modulated arc therapy (IMAT) for hippo- campal avoidance whole-brain radiation therapy (HA-WBRT) with simultaneous integrated boost (SIB) in patients with multiple brain metastases (m-BM). Materials/Methods: After obtaining approval for NRG-CC001 benchmark submission from ACR, 5 patients with m-BM were retrospectively replanned for HA-WBRT with SIB using IMAT planning. The hippo- campus, organs at risk (OAR), and m-BM gross tumor volumes (up to 10 m-BM total/patient) were contoured on MRI which had been fused with the planning CT. The hippocampal avoidance zone (HAZ) consisted of a 5- mm margin around the paired hippocampi. A 1-mm margin was used to generate the m-BM planning target volume (PTV). The whole-brain PTV (WB-PTV) was defined as the whole brain tissue volume minus HAZ and m-BM PTVs. Prescription dose was 30 Gy for WB-PTV and 45 Gy for each m-BM in 10 fractions. Three full coplanar arcs with orbit avoidance sectors were used. Treatment plans were evaluated using homogeneity (HI) and conformity indices (CI) for target coverage and dose to OAR. Dose delivery efficiency and accuracy of each IMAT plan was assessed using a modulated radiation therapy measurement device. Results: The dosimetric results for all 5 WB-IMAT with SIB plans met WB-PTV D 2% ,D 98% , and V30 Gy NRG-CC001 requirements. The plans demonstrated highly conformal and homogenous coverage of the WB- PTV with mean HI and CI values of 0.33  0.05 (range 0.27-0.38) and 0.97 0.01 (range 0.96-0.97), respectively. Two patients could not un- dergo HA due to the presence of hippocampal metastases. The 3 HA patients met protocol guidelines with maximum dose and dose to 100% of hippocampus less than 16 Gy and 9 Gy, respectively. One patient who had 8 m-BM was treated with WBRT with SIB. Highly conformal radiosurgical dose distributions (CI<1.2) were achieved for all 5 pa- tients with a total of 39 m-BM. The m-BM PTVs had a mean HI of 0.09  0.03 (range 0.037-0.19) and a mean CI of 1.01  0.07 (range 0.91 1.2). Total monitor unit were, on average, 1702  103 and average beam on time was 4.2  0.2 min. The IMAT plans demonstrated ac- curate dose delivery of 95.5  0.3%, on average, for clinical gamma passing rate of 2%/2 mm criteria. Conclusion: All HA plans met NRG-CC001 criteria and IMAT planning provided highly conformal and homogenous dose distributions for the WB- PTV and m-BM PTVs with lower doses to OAR such as the hippocampus. These results suggest that HA-WBRT with SIB is a clinically feasible, fast, and effective treatment option for patients even with a relatively large numbers of m-BM. In addition to reduced neurotoxicity, WBRT with SIB strategy may positively affect patient quality of life by utilizing one course of treatment that only requires one treatment time, one treatment machine, and potentially one treatment center. Author Disclosure: D. Pokhrel: None. S.S. Sood: None. C.M. McClinton: None. X. Shen: None. C.E. Lominska: None. R. Badkul: None. H. Jiang: None. M.P. Mitchell: None. F. Wang: None. 3595 Volumetric Modulated Arc Therapy (VMAT) Treatment Planning for Thoracic Vertebral Metastases Using Stereotactic Body Radiation Therapy (SBRT) M. Mallory, D. Pokhrel, R.K. Badkul, H. Jiang, F. Wang, and C.E. Lominska; University of Kansas Medical Center, Kansas City, KS Purpose/Objective(s): To retrospectively evaluate the plan quality, treatment efficiency, and accuracy of volumetric modulated arc therapy (VMAT) plans for thoracic spine metastases using stereotactic body radi- ation therapy (SBRT). Materials/Methods: Seven patients with thoracic vertebral metastases treated with noncoplanar hybrid arcs (NCHA) (1 to 2 3D-conformal partial arcs + 7 to 9 intensity modulated radiation therapy [IMRT] beams) were reoptimized with VMAT plans using 3 coplanar arcs. Tu- mors were located between T2-T7 and planning target volumes (PTVs) were between 24.3 and 240.1 mL (median 48.1 mL). All prescriptions were 30 Gy in 5 fractions with 6-MV beams using a treatment platform with micro-multileaf collimators. MR images were fused with planning CTs for target and OAR contouring. Plans were compared for target coverage using conformality index (CI), homogeneity index (HI), D90, D98, D2, and Dmedian. Normal tissue sparing was evaluated by comparing doses to the spinal cord (Dmax, D0.35cc, and D1.2cc), esophagus (Dmax and D5cc), heart (Dmax, D15cc), and lung (V5 and V10). Data analysis was performed with a 2-sided t test for each set of parameters. Dose delivery efficiency and accuracy of each VMAT plan was assessed via quality assurance (QA) with a modulated radiation therapy measurement device. Beam-on time was recorded, and a gamma index was used to compare dose agreement between the planned and measured doses. Results: VMAT plans resulted in improved CI (1.02 vs 1.36, PZ0.05), HI (0.14 vs 0.27, PZ0.01), D98 (28.4 vs 26.8 Gy, PZ0.03), D2 (32.9 vs 36.0 Gy, PZ0.02), and Dmedian (31.4 vs 33.7 Gy, PZ0.01). D90 was improved but not statistically significant (30.4 vs 31.0 Gy, PZ0.38). VMAT resulted in statistically significant improvements in normal tissue sparing: esophagus max (22.5 vs 27.0 Gy, PZ0.03), Esophagus 5cc (17.6 vs 21.5 Gy, PZ0.02), and heart max (13.1 vs 15.8 Gy, PZ0.03). Improvements were seen in cord and lung sparing as well but were not statistically significant. The average beam-on time was 4.7  0.6 min for VMAT vs 7.1  1 min for NCHA (not accounting for couch kicks). VMAT plans demonstrated an accurate dose delivery of 95.5  1.0% for clinical gamma passing rate of 2%/2 mm criteria, which was similar to NCHA plans. Conclusion: VMAT plans resulted in improved dose distributions and normal tissue sparing compared to NCHA plans. Significant reductions in treatment time could potentially minimize patient discomfort and International Journal of Radiation Oncology  Biology  Physics E650