ESTRO 36 Abstract Book

S557 ESTRO 36 _______________________________________________________________________________________________

Institute of Oncology, Department of Radiotherapy, Warsaw, Poland

Purpose or Objective On-board kilovoltage (kV) Cone Beam Computed Tomography (CBCT) imaging is being used predominantly for patients' positioning to improve the precision and accuracy of treatment delivery. Moreover, volumetric CBCT images can be used to evaluate anatomy changes like tumour shrinking or weight loss as well as changes in organ volume, deformation or position. Also reproducibility of accessories used in radiotherapy (the vacuum bag, bolus position, etc.) can be monitored. Additionally, the CBCT data set can be used for calculation of dose distribution. The aim of our study was to analyze reproducibility of bolus’s position based on kV CBCT imaging and compare planned and delivered dose distribution in Clinical Target Volume (CTV). Material and Methods For 10 post-mastectomy patients, 35 sarcoma patients and 5 patients with vulva cancer the treatment CT based plans with bolus were prepared (Eclipse, Varian). For the post- mastectomy patients the planning CT was acquired with bolus. For the two other groups the planning CT was made without bolus. Bolus was drawn in the treatment planning system. For each patient CBCTs were acquired in the first and mid fraction and also at the end of treatment. CBCTs were co-registered offline (automatic rigid match) to the planning CT. The correctness of boluses positions were evaluated. Also dose distributions were calculated with CBCT images and compared with dose distributions obtained with planning CT. For each patient we took a photo to document the bolus’s position. To compare dose distribution calculated on CT and CBCT, a new HU- density calibration curve was measured and introduced into treatment planning system. CatPhan®503 phantom was used. Results Fusions of CTs and CBCTs showed that there are several different problems with reproducibility of bolus position. First of all, bolus generated in TPS will never adhere to skin like it is presented in TPS, especially when a irregularity of patient surface is high (Fig. 1a). Moreover, usually air gaps occur even when there is a smooth surface of the patients’ body (Fig. 1b). Another problem is the compatibility of bolus position relative to a field edge (Fig. 1c) which is difficult to reproduce despite it is accurately described in patient folder. The last two discrepancies appear regardless if bolus was placed on patient’s skin during CT scanning or generated in TPS. Preliminary calculations for 10 soft tissue patients treated with 3D- CRT plan show, that there is 5,9% ±2,0% a discrepancy between D98% calculated on CT and CBCT. More advanced data analysis will be presented for each location separately, for treatment planning techniques and information about taken/not taken photo before CT scanning.

Conclusion CBCT is a very useful method for accuracy of treatment planning verification. It allows not only patient position verification but also for evaluation of bolus positioning accuracy. Based on CBCT several mistakes influencing dose deposited to CTV were revealed, what was not noticeable during a routine verification based on two- dimensional orthogonal images. PO-1011 Is it safe to omit a setup correction validation scan for central lung lesions treated with SBRT? M.M.G. Rossi 1 , H.M.U. Peulen 1 , J.S.A. Belderbos 1 , J.J. Sonke 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Our standard IGRT protocol for SBRT of pulmonary lesions consists of a pre correction CBCT to determine the couch shift that aligns the tumor, post correction (PCorr) scan for verification and two intra-arc CBCT scans to monitor position stability. The intra-arc scans are acquired simultaneously during VMAT delivery. To limit the number of scans for patients with centrally located lesions treated with 8x7.5 Gy, the pCorr CBCT is omitted and the 1 st intra- arc CBCT is used as the verification scan. In this study we evaluate the positional accuracy of this protocol for the 8x7.5Gy patient cohort and compare this to a 3x18Gy cohort. Material and Methods All 16 patients treated since the implementation of a setup protocol in April 2016 without a validation scan for a single centrally located tumor with 8x7.5Gy(Gp1), were included. Fifty patients were randomly selected from our