ESTRO 37 Abstract book

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ESTRO 37

treatment, a control CT was taken and compared to the original planning CT to evaluate any anatomic and dosimetric changes, with a focus on CTV dose coverage and dose to organs at risk (OAR). Results After 3 weeks of treatment, the mean weight change was -1.8 % (range: -7.5 - 2.4 %). The volume of the CTVs tended to shrink, but the variation between the patients was large, and the median change was -1 cm 3 for the CTV68 (range: -41.3 - 9.1 cm 3 ). The volume of the parotids was also reduced, median reduction was 2 cm 3 (range: -4.3 – 4.0 cm 3 ). For 2 patients the CTV dose coverage decreased. This was caused by swelling and fixation difficulties and could not be linked to weight loss. Mean dose change to the parotids was 0.2 Gy (range: -3.3 – 4.1 Gy). The maximum dose to the medulla changed by on average 0.3 Gy (range: -0.8 – 2.0 Gy), and was still within tolerance for all patients. Conclusion VMAT plans for H&N-cancer patients at our institution were found to be robust to the most common changes observed during treatment, both regarding target dose coverage and dose to organs at risk. There is no need for adaptive radiotherapy on a routinely basis. However, identifying the few patients who actually might benefit from adaptive treatment can be challenging. EP-2066 The influence of anatomical changes on DVH parameters in dose-guided adaptive RT for lung cancer K. Surmann 1 , J.A. Baeza 1 , S. Nijsten 1 , C.M.L. Zegers 1 , F. Verhaegen 1 1 MAASTRO Clinic, Department of Radiotherapy, Maastricht, The Netherlands Purpose or Objective Anatomical changes frequently occur during radiation treatment of lung cancer patients and can lead to inacceptable target coverage or organ at risk doses. An independent Monte Carlo based dose calculation algorithm to predict the deposited dose (prediction model, PM) is part of the dose-guided radiotherapy (DGRT) workflow and can be used to automate signaling of patients with clinically relevant changes for treatment adaptation. The objectives of this study are to (1) quantify the differences in DVH parameters after anatomical changes in lung cancer patients and (2) evaluate the performance of the PM to detect these dosimetric differences. Material and Methods Forty-two treatment plans that were clinically adapted based on institutional IGRT and DGRT guidelines were evaluated in this study. For each patient, the planning CT (pCT) and repeat CT (reCT) with the corresponding contours were available. Treatment was performed with VMAT and planned in Eclipse (v11, Varian Medical Systems). For daily treatment monitoring within the DGRT workflow, the dose predicted on the daily images by the PM is compared to the dose initially calculated in the treatment planning system (TPS). CTV V95% was evaluated for the primary tumour and lymph nodes and the D0.1cc for the mediastinum. Model differences between the TPS and PM were quantified by comparing the respective DVH parameters. Anatomical changes between the pCT and reCT were quantified in the TPS and PM through the change in the DVH parameters. An adaptation criterion of change in CTV V95% > 1% was applied and the patient selection based on the TPS and PM was compared. Results There are model differences between the TPS and PM for lung cancer patients which are on average 0.8-2.8 % for the CTV V95% and on average 0.1-0.2 Gy for the mediastinum D0.1cc (see Table 1).

Fourteen patients showed change in atelectasis, 10 experienced tumour regression, 10 had pleural effusion and 8 showed other changes. Due to these anatomical changes, the V95% decreased by 9.2 ± 18.9 % (TPS) and 9.7 ± 19.7 % (PM) for the primary CTV and 3.7 ± 12.7 % and 5.3 ± 14.0 % for the lymph node CTV (see Table 1). The changes observed in the TPS are on group and patient level in agreement to the changes observed in the PM (p >> 0.05 for all DVH parameters, calculated with a paired student t-test). When applying the adaptation criterion in the PM, a sensitivity and specificity of respectively 0.93 and 1.0 was reached when compared to the patient selection in the TPS.

Conclusion (1) We quantified the changes in DVH parameters due to anatomical changes during treatment. (2) Dose distributions should be compared within the same dose engine to eliminate the effect of model differences. When applying an evaluation criterion for the CTV V95% in the TPS and PM, the same patients are signalled for adaptation. The initial prediction in the PM can therefore be used as reference in the daily DGRT workflow instead of the currently used initial dose distribution calculated by the TPS. EP-2067 B-spline deformable image registration algorithm evaluation using numerical and physical phantom. A. BADEY 1 , S. Tolsa 1 , V. Bodez 1 , C. Khamphan 1 , E. Jaegle 1 , M.E. Alayrach 1 , P. Martinez 1 , R. Garcia 1 1 Institut Sainte Catherine, Medical Physics, Avignon, France Purpose or Objective Adaptive Radiotherapy (ART) is a technique which minimizes the dosimetric impact of anatomical changes that may occur during treatment. One of the milestones of ART for dose warping is deformable image registration (DIR) between images acquired during treatment and original planning images. The aim of this study is to propose a method to evaluate the performances and characterize the use of a b-spline DIR algorithm. Material and Methods Two types of dataset were used to perform the evaluation, numerical phantoms and physical phantoms (called 'Phydeform”). A dedicated QA software provides a numerical phantoms library and the opportunity to generate deformation. For algorithm characterization, deformation scenarios on simple volumes were created and two sets of images were systematically obtained for each induced deformation: initialCT and deformedCT. The evaluation related to the comparison of deformation vectors field via error histograms and contours with metrics like DICE and MDC (Mean Distance Conformity). The contour comparison was done between distorted contours from the adaptive software and expert contours on deformedCT. Voxel’s size influence with DIR was also evaluated. Physical phantom « Phydeform » with cubic shape of a homogenous material was used to evaluate the spatial deformation with markers and dose deformation with detectors like TLD’s and MOSFET’s. Two images series were compared: no-deformedCT and deformedCT obtained following the application of uniform compression on phantom. Marker’s position was evaluated between non deformed and deformed reference. Three fixed beams were delivered. The