ESTRO 2020 Abstract book

S57 ESTRO 2020

not given equal time and weight by all planners during optimization. Number of fields had no influence.

Proffered Papers: Proffered papers 7: Evaluating and predicting toxicity in RT

OC-0108 DW MRI analysis of salivary glands during head and neck radiotherapy to predict late xerostomia. A. Duffton 1 , O. Kemp 2 , P. McLoone 3 , L. Devlin 1 , C. Paterson 4 1 The Beatson West of Scotland Cancer Center, Department of Radiotherapy, Glasgow, United Kingdom ; 2 University of Glasgow, Medicine- veterinary and life science, Glasgow, United Kingdom ; 3 University of Glasgow- Glasgow- United Kingdom, Institute of Health & Wellbeing, Glasgow, United Kingdom ; 4 The Beatson West of Scotland Cancer Center, Department of Clinical Oncology, Glasgow, United Kingdom Purpose or Objective Xerostomia is a common and permanent side effect following radiotherapy (RT) for head and neck (H&N) cancer. There is no validated method of predicting, during treatment, the severity of subsequent xerostomia. Serial diffusion weighted MRI (DW MRI) allows evaluation of changes in the salivary glands during RT. The aim of this study was to investigate association between change in apparent diffusion coefficient (ADC) during RT and severity of xerostomia at 12 months. Material and Methods Patients recruited to the MeRInO study, who had completed scanning protocol and follow up were included. Parotid (PG), submandibular (SMG) and sublingual glands (SLG) were delineated on a baseline dataset acquired prior to fraction 1 (MR_1) and on a repeat dataset acquired before fraction 11, dose delivered 21.6Gy (MR_2). All scans were acquired on a GE Signa 1.5T HDxt (GE, Crawley, UK). Structures were delineated on the T1 post contrast FS sequence, modified to account for visual assessment, motion and artefact on b=0, and registered to the ADC map on Eclipse v 15.5 (Varian medical systems, Palo Alto). Mean ADC measurements and volumes on ADC map were recorded for each gland. Radiation Therapy Oncology Group (RTOG) scoring criteria was used to record toxicity at 12 months. Results 31 intermediate-high risk patients with SCC of the oropharynx (OPSCC) were included in the analysis. 30 patients had completed a course of VMAT RT 65Gy/30 fractions, 1 patient received 62.8Gy/29 fractions. Median age (range) was 57 years (37-70). Majority of patients were male (87%), 15 (48%) patients had HPV negative disease. At 12 months toxicity was recorded for 25 (81%) patients. Toxicity was unavailable at 12 months for 6 patients, 4 of whom died and 1 who had recurrence. Number of patients experiencing each toxicity score are shown in Table 1. The only parameters meeting statistical significance when correlated with 12 month xerostomia scores were left SLG volume which indicated a larger increase in patients with G2+ toxicity compared to G0-1 (94.7% versus 38.9%, p=0.016), and a smaller increase in mean % ADC in left PG (8.3% versus 19.8%, p=0.007) in those with G2+ compared to G0-1 (Table 2). However, patients with more severe toxicity at 12 months (G2+) appeared to demonstrate a smaller increase in ADC for all PGs and SMGs.

Conclusion The national guidelines generally resulted in high plan quality, meeting all clinical constraints. Using the additional quantitative metrics, we saw notable inter- planner variation. . Although plans were not made under strict audit-conditions, this study demonstrates the potential for interplanner variations within clinical acceptability and under a guideline, and methods for quantifying these variations. Systematic application of quantitative metrics during planning should be investigated as a method of supporting dissemination of best practices in treatment planning.