ESTRO 2020 Abstract book

S811 ESTRO 2020

the influence of radiation beam parameters (beam entry direction, field size, MU) on the dose level outside the irradiated volume were checked. Next, 3 different type of plans for prostate region have been prepared: (A) standard, and with blocked beam direction (B) from head side and (C) legs side. The plans were verified and passed with gamma criteria (L3%/2mm). Results The measurements show that the values of the measured out-of-field doses were influenced by the parameters used: beam entry direction, field size, MU. The results of relative doses outside the irradiated volume for different beam directions (0°, 45° - head side, 315° - legs side) were presented in the figure 1. Out-of-field doses relative to this prescribed in target were [%]: (A) 0.173, 0.160, 0.151, 0.706; (B) 0.171, 0.164, 0.140, 0.636; (C) 0.145, 0.141, 0.142, 0.723 in head, thyroid, right lung and intestine, respectively. The plans were verified and passed with gamma criteria (L3%/2mm): (A) 99,7%, (B) 98,8%, (C) 95,2%.

Material and Methods To assess the ASC fuctioning ten prostate and ten oropharynx clinical VMAT plans were newly optimized without human interaction using different levels of ASC. The reduction in plan quality was assessed computing 18 different modulation metrics, among those proposed in literature and performing a textural analysis of the reconstructed fluence maps. To compare ASC with other competing strategies the same twenty plans were optimized from scratch by an experienced user using three different strategies to limit plan complexity: a) setting the ASC to its highest level, b) imposing a limit on the maximum deliverable MUs and c) coupling them. The change in plan complexity was newly computed, the plan deliverability was measured through 3D dosimetric measurements, while the overall plan quality was assessed through a dedicated Plan Quality Metric (PQM). Results The plans optimized exploiting the highest level of ASC generally show a reduced number of total Monitor Units, a more constant gantry rotation and a MLC modulation characterized by larger and less complicated shapes with shorter leaves travelling overall lengths. ASC reduces the plan complexity limiting the number of subfields and leaves approximately unchanged the BEV area but reduces number of disjointed BEV components. The complexity reduction is more pronounced for oropharynx plans than for prostate and it is proportional to the ASC level. The ASC results more effective than MU limitation in oropharynx plans while they are comparable for prostate. Nevertheless, dosimetric verifications show that the ASC grants a significantly better plan deliverability than the limit on the total MU. Moreover, for oropharynx treatments, coupling MU limit and ASC does not grant better deliverability even if plans are characterized by a lower plan complexity. The results reported herein suggest that the use of ASC and the MU limit might somehow induce complementary changes in the plan complexity. The lower plan complexity and the ameliorated plan deliverability does not come at significant detriment of the overall plan quality. Conclusion The use of ASC in Eclipse optimization leads to a reduction in plan complexity comparable or superior to a 15% reduction in the total delivered monitor unit. On average, the reduced complexity does not compromise the overall clinical plan quality but ensures an ameliorated plan deliverability. This study suggests that the Aperture Shape Controller is an effective tool to reduce the unnecessary complexity of a plan. PO-1435 Large treatment plan quality enhancement in robotic radiotherapy M. Giżyńska 1 , L. Rossi 1 , W. Den Toom 1 , M. Milder 1 , L. Inrocci 1 , K. De Vries 1 , J. Nuyttens 1 , B. Heijmen 1 1 Erasmus MC Cancer Institute, Department of Radiation Oncology, Rotterdam, The Netherlands Purpose or Objective The new VOLO optimizer was recently implemented in the Precision TPS (Accuray Inc, Sunnyvale, USA) for CyberKnife treatment planning, next to the existing Sequential Optimizer (SO). The underlying optimization approaches are very different. The purpose of this study was to compare VOLO and SO for two tumor sites. Material and Methods Fifty prostate and lung cancer patients (twenty-five each) treated with SBRT (4 x 9.5 Gy and 3 x 17 Gy respectively) were included in the study. For each tumor site, five arbitrarily selected patients, previously planned with SO, served as a training set for getting acquainted with VOLO planning. Goal of the training was to explore opportunities in VOLO for generation of acceptable plans with clinically favorable dose distributions, guided by the SO dose

Conclusion Based on phantom measurements, it has been shown that in radiotherapy for the prostate using CK, out-of-field doses are affected by radiation beam parameters. For special cases, the doses can be reduced at a large distance from the irradiated volume, limiting the number of beams from the side that should be minimized. However, by blocking some beam angles for CK, the dose distribution near the field edge could be less satisfactory or the plan may be more difficult to implement.

Poster: Physics track: Treatment plan optimisation: algorithms

PO-1434 Thorough assessment of a new commercial tool to reduce plan complexity A. Scaggion 1 , M. Fusella 1 , A. Bettinelli 1 , F. Busato 2 , N. Pivato 1 , A. Roggio 1 , M.A. Rossato 1 , M. Sepulcri 2 , R. Zandonà 1 , M. Paiusco 1 1 Istituto Oncologico Veneto, Medical Physics, Padova, Italy ; 2 Istituto Oncologico Veneto, Radiation Oncology, Padova, Italy Purpose or Objective The Aperture Shape Controller (ASC) is a new component in the Photon Optimizer (PO) algorithm of Eclipse TPS (Varian Medical Systems, Palo Alto, CA) that tends to decrease the MLC aperture complexity increasing its size. The aim of this work is to investigate the capabilty of ASC in reducing plan complexity increasing plan deliverability without compromising the overall plan quality. The ASC effectiveness has also been compared to other complexity-limiting strategies.