ESTRO 36 Abstract Book

S805 ESTRO 36 2017 _______________________________________________________________________________________________

MCsquare (Souris et al. Med Phys 2016) able to compute a final dose in less than 1 minute. The hybrid optimization strategy calculates the optimal spot weights ( w ) using the analytical beamlets matrix ( P PB ) and a correction term C . After a first optimization where C = 0, the method alternates optimization of w using P PB with updates of C = D MC – D PB , where D MC results from a regular MC computation (using 10 8 protons to ensure good statistical accuracy) and D PB = P PB * w . Updates of C can be triggered as often as necessary by running the MC engine with the last corrected values of w as input. The performance of the method is illustrated on two extreme cases: prostate (relatively easy case) and lung (considered to be complex due to the high heterogeneity). For simplicity, we created PTV-based plans but the findings can be equally applied to robust optimized plans. Results For the prostate case, the recomputed MC dose after initial optimization ( C =0, before correction) revealed a decreased target coverage (D95=90% of the prescribed dose, D p ) that improved significantly after just one correction (D95 corrected =97%D p ). For the lung case, the difference between MC and PB doses before correction was very large: D95=63%D p and D5=137%D p . But still the hybrid strategy was able to partially improve target coverage (D95 corrected = 84%D p ) as well as reducing overdose (D5 corrected = 111%D p ), after two updates of C. In both cases, further corrections did not lead to better results. The results proved that the hybrid method allows us to improve dose accuracy even for very complicated cases as lung tumors. However, the success of the correction is limited by the order of magnitude of the term C, i.e, very large difference between MC and PB doses are only partially corrected.

Conclusion The results showed medium to large differences between the PB and MC doses which could be addressed totally or partially by adding a correction term during the optimization. Since MC beamlets calculation remains time-consuming, this hybrid PB-MC optimization seems a good compromise between accuracy and speed. EP-1520 Stereotactic body radiation therapy treatment planning using target volume partitioning J. Robar 1 1 Dalhousie University, Radiation Oncology, Halifax, Canada Purpose or Objective The aim of this study was to evaluate a novel approach to Volumetric Modulated Arc Therapy (VMAT) plan optimization for stereotactic body radiation therapy of the spine involving partitioning of the Planning Target Volume (PTV) into simpler sub-volumes. Treatment plan quality was compared to that provided by a standard VMAT approach. Material and Methods The new technique investigated in this work relies on a partitioning of the PTV that is dedicated to spinal anatomy. The spine PTV is segmented into multiple sub- volumes using a k-means algorithm, such that each sub- volume minimizes concavity. Each sub-volume is then associated with a separate arc segment for VMAT delivery. The rationale of this approach is that the delivery of dose to multiple, mainly convex target volumes provides flexibility to the VMAT optimizer in prioritizing spinal cord sparing. Treatment plans were established with the novel algorithm using the Spine SRS Element (Brainlab, AG, ver 1.0 beta) and compared to clinical treatment plans generated using standard VMAT planning approach in our centre (Rapidarc, Varian Medical Systems). Test cases included a range of spinal target volumes, including the vertebral body only, vertebral body and pedicles, or spinous process only. Plan quality was compared with regard to PTV coverage, PTV dose homogeneity, dose conformity, dose gradient, sparing of spinal cord PRV and MU efficiency. Results PTV coverage and dose homogeneity were equivalent, however improved high-dose (90%) conformity was observed for the new approach (p=0.002). Sharper dose gradient was produced in 75% of cases but did not reach statistical significance. The percent volume of the PRV spinal cord receiving 10 Gy was reduced (p=0.05). Despite the fact that the new method involves delivery of dose to PTV sub-volumes with separate arc segments, MU efficiency was approximately equivalent to the status-quo