ESTRO 36 Abstract Book

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Results From the simulation of patient weight loss (thickness reduction), Watchdog has less sensitivity to small patient thickness reduction. From figure 1 left, it can imply that dropping by 25% c pass-rate refers to 10% patient thickness reduction or approximately 1.5 cm shrinkage. In clinical case validation, Watchdog was able to detect the significant patient anatomical changes that lead to the decision to replan all four HN IMRT patients (see figure 1 right). Based on this study, we found that Watchdog system can detect the clinically significant anatomical change in HN IMRT based on 1) at least 3 out of 7 fields of the fraction are below the SPC-based threshold, 2) the lowest c pass-rate is less than 30%, and 3) a 25% c pass- rate drop equates to approximately a 1.5 cm (-10.0%) patient thickness reduction. Conclusion The Watchdog dosimetic response to HN patient anatomical changes has been evaluated based on the simulation of patient thickness reduction/weight loss and clinical cases of HN IMRT replan. Using the SPC-based threshold, Watchdog is able to detect clinically significant anatomical changes in HN IMRT treatment.

Conclusion The implemented clinical protocol for abdominal compression is able to reduce the mean marker motion by roughly 5 mm in the initial imaging as well as in the pre- treatment imaging. Although the stand ard deviation in both imaging modalities was reduced by the abdominal compression setup, the reproducibility of the abdominal compression reflected by the decreased standard deviation in the pre-treatment imaging could only be improved slightly. PO-0868 Evaluation of Watchdog response to anatomical changes during head and neck IMRT treatment T. Fuangrod 1 , J. Simpson 1,2 , S. Bhatia 1 , S. Lim 3 , M. Lovelock 3 , P. Greer 1,2 1 Calvary Mater Newcastle, Radiation Oncology, Waratah- NSW, Australia 2 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle- NSW, Australia 3 Memorial Sloan-Kettering Cancer Center, Radiation Oncology, New York, USA Purpose or Objective Watchdog is a real-time patient treatment verification system using EPID, which has been clinically implemented as an advanced patient safety tool. However, the use of Watchdog requires an understanding of its dosimetric response to clinically significant errors. The objective of this study is to evaluate the Watchdog dosimetric response to patient anatomical changes during the treatment course in head and neck (HN) IMRT. Material and Methods Watchdog utilises a comprehensive physics-based model to generate a series of predicted transit cine EPID image as a reference data set, and compares these to measured cine-EPID images acquired during treatment. The agreement between the predicted and measured transit images is quantified using c-comparison (4%, 4mm) on a cumulative frame basis. The 71.3% c pass-rate error detection threshold in HN IMRT has been determined from our pilot study of 37 HN IMRT patients using the statistical process control (SPC) technique (1). The major source of errors was inter-fractional anatomy changes due to weight loss and/or tumour shrinkage. To evaluate the Watchdog dosimetric response to HN IMRT anatomical changes, the patient CT data was modified and used for calculating the predicted EPID images. First, soft- tissue patient thickness reduction or weight loss was progressively simulated with a range of 0%, 1%, 2.5%, 5%, 7.5%, 10%, and 12.5% based on real patient deformations using in-house software. Second, Watchdog dosimetric response was determined for four HN patients with observed weight loss during treatment who had a second CT during treatment for replanning purposes. Watchdog dosimetry was calculated using the second CT compared to the original CT. The SPC-based threshold was applied to determine the Watchdog performance for HN IMRT anatomical change detection. These simulations provide the decision rule for HN IMRT replanning based on Watchdog assessment. (1) Fuangrod (2016). Radiation Oncology, 11(1), 106

PO-0869 A population-based estimate of proton beam specific range uncertainties in the thorax Y.Z. Szeto 1 , M.G. Witte 1 , M. Van Herk 2 , J. Sonke 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiotherapy department, Amsterdam, The Netherlands 2 Institute of Cancer Sciences- University of Manchester, Molecular and Clinical Cancer Sciences, Manchester, United Kingdom Purpose or Objective Proton therapy has great potential for locally advanced lung cancer patients because of considerable reduction of intermediate and low dose to the healthy tissues. However, due to their finite beam range, proton dose distributions are more susceptible to anatomical variations. The purpose of this study was to derive a population-based map of beam specific range uncertainties due to anatomical variations. Material and Methods The planning CT (pCT) of 100 NSCLC patients treated between 2010 and 2013 with (chemo-)radiotherapy were included. To simulate realistic anatomical variations, we used a previously developed statistical model, based on principal component analysis for systematic variations in the thorax. This model generates deformation vector fields that deform the planning CT to induce systematic differences between the anatomy of planning and delivery. For each patient, we synthesized 1000 CTs (sCT) representing plausible variations in treatment anatomy. Subsequently, the water-equivalent path length differences (∆R) between the pCT and sCTs was calculated at the beam’s distal and proximal edge of the GTV for 13 equally spaced angles of 15 ⁰ through the ipsilateral lung. Undershoot and overshoot at the distal edge results in an under-coverage of the target and higher dose in normal tissues respectively, and vice versa at the proximal edge. To summarize the results, first for each scan and angle, the 95th percentile ∆R in undershoot (∆R u ) and overshoot