ESTRO 2021 Abstract Book

S1627

ESTRO 2021

CPPT using FBL may make protons available to patients who currently do not have access to it. In addition, CPPT may improve treatment quality compared to both single modality proton and photon treatments. This work was funded by Krebsliga Schweiz (KFS-4528-08-2018).

PO-1906 Biologically Adaptive Dose Escalated Radiotherapy in Oropharynx Cancers: A Treatment Planning Study R. Valentine 1 , C. Paterson 2 , L. Grocutt 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics, Glasgow, United Kingdom; 2 Beatson West of Scotland Cancer Centre, Radiation Oncology, Glasgow, United Kingdom Purpose or Objective A significant proportion of patients with intermediate and high risk squamous cell cancer of the oropharynx (OPSCC) continue to relapse locoregionally despite radical (chemo)radiotherapy ((C)RT). The toxicity of the current regimen limits further uniform intensification. If a predictive biomarker for disease control can be identified during treatment then individualised and adaptive treatment strategies may be employed. The aim of this study is to assess the feasibility of adaptive and dose escalated RT to the gross tumour volume (GTV) without increasing PTV doses and still achieving acceptable organs at risk (OARs) doses. Materials and Methods Twenty representative patients with poor prognosis locally advanced OPSCC who were known to have relapsed post RT, were re-planned retrospectively using Eclipse TPS v15.5, RapidPlan® (RP) and multi-criteria optimisation (MCO). Standard dose and fractionation in our centre is 65Gy in 30# to areas of gross disease (PTV65) while areas at risk of containing microscopic disease (PTV54) is treated to 54Gy in 30#. The original clinical plans were re-optimised with a locally published RP model and MCO (Group I) to act as controls. These plans were then used to escalate the dose to the GTV to 73Gy (Group II) and 82Gy (Group III). The re-optimised clinical plans were split into two phases of 15# each. Phase 1 consisted of 1-15# set to 2.17Gy/#, while Phase 2 consisted of 16-30# set to 2.17Gy/#, 2.70Gy/# or 3.30Gy/# in order to simulate adaptive dose escalation for the remaining 15#. Plan sums were created for a total of 30#. A number of plan evaluation parameters were recorded along with assessments of plan deliverability. Results Figure 1 illustrates the dose colour wash of a representative case ranging from 51.3Gy (95% of PTV54 prescription) up to 83.9Gy, highlighting dose escalation in the GTV region. Table 1 lists percentage differences between group 1 and the escalated groups for PTV65-GTV, GTV, PTV54 and OARs. Despite the GTV being escalated by 12.3% and 26.2% to 73 and 82Gy, respectively, D98% and D50% mean values remain within 1.0% of a variation for PTV54 and PTV65-GTV. The mean D1cc for the 82Gy escalated plans was 83.5Gy. While OAR doses increased in the escalated groups and with the exception of the PRV spinal cord, these increases were not found to be statistically significant nor indeed clinically significant. MF values of 0.31, 0.35 and 0.38 and ALPO values of 2.61, 2.87 and 2.90 were recorded for group I, II and III, respectively.