ESTRO 2020 Abstract book

S859 ESTRO 2020

compared with advanced photon planning technology or when robustness is applied to proton planning. The aim of this study was to compare the dosimetric benefit of robust optimised Intensity Modulated Proton Therapy (IMPT) with photon Volumetric Arc Radiation Therapy (VMAT) for HNC patients. Material and Methods 11 patients with locally advanced oropharyngeal cancer were replanned using eclipse TPS 15.5. 5 plans were made for each patient: (MCO) c)IMPT- (no robust optimisation parameters included) d)IMPT+3mm (3mm perturbations included in the optimisation process) e)IMPT+5mm (5mm perturbations included in the optimisation process) Prescribed dose and fractionation for each patient was 65Gy in 30 fractions for the high-risk PTV (defined as gross disease with a margin and entirety of involved nodal level) and 54Gy in 30 fractions for the low-risk PTV (defined as areas considered at risk of containing microscopic disease). The PTV margins for all patients were 3mm added isotropically to CTV. A PRV of 3mm was applied to critical organs at risk (OARs). ‘Larynx’ was used to describe the midline mucosa from hyoid to cricoid. Plans were normalised such that the mean target dose (PTVs for photons, CTVs for protons) was 100% of the prescribed dose. Standard planning constraints derived from the PARSPORT trial were followed. Target volume dosimetry and dose to standard OARs were evaluated. Results Target volume constraints were met for both CTVHR and CTVLR with all planning modalities, see table 1. The PRV brainstem constraint of D1% <48 Gy was met comfortably across all planning techniques. This dose did increase with increasing IMPT perturbations, but the increase was not clinically significant. PRV Spinal Cord constraint of D1% <44 Gy was also met across all techniques. Mean dose to larynx was around 40Gy with RP, MCO and IMPT+5mm but IMPT- and IMPT+3mm achieved a mean dose of 31.0±12.7Gy and 33.7±15.4Gy respectively. Lowest contralateral mean parotid dose was achieved with IMPT- (16.0±7.8Gy) and the highest with IMPT+5mm (24.0±10.0Gy). A similar trend was observed for the ipsilateral parotid however no modality met the constraint of mean dose < 24Gy. a)Rapidplan TM (RP) b)RP+Multi-Criteria Optimisation

Conclusion Target volume coverage and dose to critical OARs were clinically acceptable across all modalities. While IMPT- produced improved dosimetry for contralateral parotid gland and larynx, this benefit was lost with the addition of any robustness for parotids and larynx although IMPT+3 did maintain improved sparing of the larynx compared to RP, MCO and IMPT+5. Overal for contralateral parotid sparing and CTV coverage MCO performed as well or better than protons with robustness. PO-1503 Evaluation of a generalized knowledge-based planning for VMAT irradiation of breast and lymph nodes M. Rago 1 , L. Azario 1 , M. De Spirito 1 , L. Placidi 1 1 Fondazione Policlinico Universitario A. Gemelli IRCCS, Medical Physics, Roma, Italy Purpose or Objective To evaluate the performance of two Knowledge-Based (KB) models for planning optimization (RapidPlan, Varian) of Volumetric Modulated Arc Therapy (VMAT) applied to whole-breast irradiation comprehensive of nodal station, internal mammary and/or supraclavicular regions. Material and Methods Two different KB models have been generated and trained based on 120 VMAT plans data-set (62 with Simultaneous Integrated Boost) where dose is calculated with AcurosXB Algorithm. Model A (120 VMAT plans) includes plans with whole right and left breast irradiation with lymph nodes. Model B is a subgroup of model A (100 VMAT plans) without plans with only whole breast irradiation. Only two optimization structures, generated by the planner in order to achieve a clinical acceptable standard plan, have been included in the models: a ring structure (defined around the target volumes to ensure a high dose gradient) and an optimized PTV, enlarged with a margin of 1 mm. To evaluate dosimetrically the models, a set of 18 plans, 4 of them without lymph nodes, clinically delivered in our radiotherapy department, were selected. These plans were manually performed by an expert planner to obtain the desired dose-volume objectives after several iterations. Results