spine involving partitioning of the Planning Target Volume. (PTV) into simpler sub-volumes. Treatment plan quality was compared to that provided by a standard ...
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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. Robar1 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 subvolumes using a k-means algorithm, such that each subvolume 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 technique.
Conclusion The novel target volume splitting technique offers an efficacious new approach to VMAT optimization, producing high dose gradients in the vicinity of the spinal cord and allowing prioritization of spinal cord sparing.
EP-1521 Non-coplanar beam orientation and fluence map optimization based on group sparsity K. Sheng1 1 David Geffen School of Medicine at UCLA, Radiation Oncology, Los Angeles- CA, USA Purpose or Objective With the increasing availability of non-coplanar radiotherapy systems in clinical set-tings, it is essential to develop effective and efficient algorithms for integrated non-coplanar beam orientation and fluence map optimization. To achieve this goal, we investigate the novel group sparsity approach for non-coplanar beam orientation optimization. Material and Methods The beam orientation and fluence map optimization problem is formulated as a large scale convex fluence map optimization problem with an additional group sparsity term that encourages most candidate beams to be inactive. The optimization problem is solved using an accelerated proximal gradient method, the Fast Iterative Shrinkage-Thresholding Algorithm (FISTA).We derive a closed-form expression for a relevant proximal operator which enables the application of FISTA. The beam orientation and fluence map optimization algorithm is used to create non-coplanar treatment plans for six cases (including two head and neck, two lung, and two prostatecases) involving 500 - 800 candidate beams. The resulting treatment plans are compared with 4treatment plans created using a column generation algorithm, whose beam orientation and fluence map optimization steps are interleaved rather than integrated. Results In our experiments the treatment plans created using the group sparsity method meet or exceed the dosimetric quality of plans created using the column generation algorithm, which was shown superior to that of clinical plans (Figure shows a head and neck case). Moreover, the group sparsity approach converges in about 5 minutes in these cases, as compared with runtimes of more than an hour for the column generation method. Table shows the PTV dose statistics and runtime comparison. Conclusion This work demonstrates that the group sparsity approach to beam orientation optimization, when combined with an accelerated proximal gradient method such as FISTA, works effectively for non-coplanar cases with a large number of candidate beams.In this paper we obtain an orders of magnitude improvement in runtime for the \group sparsity"approach to beam orientation optimization by using an accelerated proximal gradient method to solve the ℓ2;1-norm penalized problem. Furthermore, the dosimetric quality of our group sparsity plans meets or exceeds the quality of treatment plans created using a column generation approach to beam angle selection, which has been demonstrated in recent literature to create high quality treatment plans. EP-1522 Quantifying the operator variability reduction driven by knowledge-based planning in VMAT treatments A. Scaggion1, M. Fusella1, S. Bacco1, N. Pivato1, A. Roggio1, M. Rossato1, R. Zandonà1, M. Paiusco1 1 Istituto Oncologico Veneto IOV-IRCCS, Medical Physics, Padova, Italy Purpose or Objective The purpose of this study is to evaluate the potential of a commercial knowledge-based planning (KBP) algorithm to standardize and improve the quality of the radiotherapy treatment. This study evaluates if the predicted DVH constraints generated by the KBP algorithm can reduce the
