ESTRO 2020 Abstract book

S941 ESTRO 2020

While the volume changes between 3.4% and 59.7%, the maximum electron density changes between 3.8% and 9.2%. In the profiles obtained, there was no difference in the electron density profile between the fractions for ipsilateral lung, whereas there was a difference between the fractions in the profiles obtained for the tumor. Changes in the maximum dose and maximum dose volume in the tumor volume has been demonstrated in the dose distributions obtained on the CBCT images transferred to the planning system. The maximum dose in the tumor volume can increase 2.7%, while the volume of the maximum dose can increase from 3.6% to 6.4%.

Conclusion Detection of inter-dose and dose-volume changes in lung cancer irradiation is important for predicting adaptive treatment planning for the tumor because changes in these parameters affect the dose distribution. PO-1628 Deconvolution of different range error sources using proton radiography and neural networks C. Seller Oria 1 , G. Guterres Marmitt 1 , S. Brandenburg 2 , S. Both 1 , J.A. Langendijk 1 , A. Knopf 1 , A. Meijers 1 1 University Medical Center Groningen- University of Groningen, Radiation Oncology, Groningen, The Netherlands ; 2 KVI – Center for Advanced Radiation Technology- University of Groningen, Medical Physics, Groningen, The Netherlands Purpose or Objective This study assesses the feasibility to detect different sources of error affecting the proton range, individually and in combination, using proton radiography. Material and Methods A square range probing field was simulated in 16 CTs of head and neck cancer patients (see Figure 1(a)). The depth dose profile of each probe in a discretized detector was extracted. Range shift maps were obtained by comparing an unperturbed reference radiogram against a radiogram in which one or more errors affecting range measurements were introduced. Over and under estimations in the conversion from CT numbers to density were simulated in the soft (S+, S-), bone (B+, B-) and adipose (F+, F-) tissue regions. Setup errors in the anterior – posterior (AP+, AP-) and inferior – superior (IS+, IS-) directions were simulated (see Figure 1(a)). Furthermore, the combined impact of calibration curve and setup errors was investigated. First, an analytical method employing mean range shifts, standard deviations and mean gradients in the AP and IS directions as evaluation metrics was used to interpret the range shift maps. Secondly, a convolutional neural network (CNN) was trained to classify the range shift maps into various categories according to each source of error. The CNN was first trained with range shift maps arising from individual sources of error (case I in Table 1), and afterwards including also combinations of errors (cases II and III in Table 1). The performance of the CNN was evaluated for 3 testing sets of maps (I, II and II in Table 1). For each case, the percentage of maps that were correctly, partly and wrongly classified (exact, partial and wrong match) were obtained, comparing the predictions

Conclusion ART solutions for cervix EBRT reduce OAR doses. Daily online re-planning delivers the largest OAR dose reductions. Clinical studies combining MRI-guided ART and daily online re-planning are now required to assess clinical benefit. PO-1627 Evaluation of Tumor Volume and Electron Density Changes on CBCT Images in Lung Cancer Patients E. Kara 1 , O. Ozdemir 2 , A. Hicsonmez 1 , F. Zorlu 3 1 Onko Ankara oncology center, Oncology Department, Ankara, Turkey ; 2 Koru Ankara Hospital, Radiation Oncology, Ankara, Turkey ; 3 Hacettepe University, Radiation Oncology, Ankara, Turkey Purpose or Objective The aim of this study is to evaluate the volumetric changes on gross tumor volume (GTV) during the treatment and to determine the changes in the irradiated tumor volume and the electron density values of the normal ipsilateral lung and GTV by using kilo-voltage cone beam (kV-CBCT) in lung cancer patients. Material and Methods In this study, 10 patients with lung cancer, whose tumor location was different from each other and the volume changes could be observed by CBCT during the treatment were selected for this study. CBCT images obtained in fractions 1, 15 and 30 were examined to detect changes in volume and electron density for GTV. The electron density profiles of the tumor volume and ipsilateral lung in the 1st, 15th and 30th fractions were obtained. CBCT images were transferred to the treatment planning system and the plans were re-examined on these images to determine the effect of inter-fraction electron density change on dose distribution. Results As a result of our study, GTV volume and maximum electron density decreased throughout the fractions.