A Unique Volumetric Modulated Arc Stereotactic Radiosurgery (SRS)Treatment Planning Approach for Volume Staged SRS to Large Arteriovenous Malformation.

Purpose/Objective(s) The management of large Arteriovenous Malformation AVMs (> 10cc) using volume staged Stereotactic Radiosurgery (VS SRS) usually requires more than 2 VS SRS procedures. VS SRS downsizes the treatment volume per stage allowing effective higher dose of SRS to relative smaller volumes of target to reduce risk of SRS induced brain necrosis. It is imperative to maintain the SRS treatment plan quality matrix of conformity and dose gradient at optimal values to minimize the adverse radiation effects to surrounding brain tissues. However, VS SRS plans can present a challenge at treatment planning given the angioarchitecture of staged volumes and total volume, delivery technique and modality limitations. Materials/Methods A 37-year-old female patient has history of headaches, intraventricular hemorrhage, and cerebral arteriovenous malformation AVM in the right parietal lobe. This large AVM (16.5cc) scored 4 on Spetzler–Martin grading system. Patient was simulated with large board CT simulator using an SRS mask. 1 mm thickness of CT images with IV contrast per CTA protocol were obtained and imported into the TPS. MRA images data sets and Angio CBCT scans from arteriogram were registered to CT Simulation scans for contouring AMV nidus. The nidus was divided into 3 sub volumes of 8 cc, 4.5cc and 4 cc, respectively. The dose of 20 Gy was prescribed to entire volume of nidus and portions of total dose were delivered to sub volumes with interval of 2-4 weeks. The VMAT inverse treatment plans were generated for each volume/isocenter using non-coplanar arcs. to be delivered on HDMLC. Collimator and arc angles were configured geometrically to minimize dose spillage between staged volumes. Planning objectives including generalized uniform equivalent dose gEUD were assigned to planning volumes and OARs. Normal tissue objective was enabled, structure resolution was set to 1.25mm, and AAA calculation grid was set to 1mm. Individual and composite plans isodoses and DVHs were examined repetitively during the planning and optimization iterations to confirm meeting the acceptable VS SRS plan quality matrix or to adjust if needed. All three individual sub volume plans passed IMRT QA (> 90%, DTA 1mm, %DD 2%) Results The composite plan maximum dose was 33.3 Gy, minimum dose for total nidus volume was 94%, and 95.2%, 100%, 109% for three sub volumes of nidus, respectively. V95%was > 95% and D95% was > 95% for all three sub volumes and total volume in the composite plan. The mean dose was 27.7 Gy. CI was 0.63, PITV was 1.57, R50 was 4.36 while GI was 2.77, V12 was 52.9cc (3.9%). Conclusion The proposed unique VMAT VS SRS for large AVM achieves adequate and comparable treatment planning quality parameters when evaluated with published literature covering VS SRS delivery with different treatment modalities such as robotic radiosurgery systems. Our preliminary results from previous treated patients using same planning technique showed good response rate and very low rate of brain necrosis. Author Disclosure F. Wang: None. D. Soultan: None. H. Jiang: None. N. Demez: None.