ESTRO 2020 Abstract book

S1115 ESTRO 2020

sets from the recto-sigmoid junction to the anal canal by the radiotherapist. A comparison was made between the rectal volumes with each CBCT and the planning CT image- set. The change in the rectal volume was accounted as a percentage calculated as [(CBCT vol - CT vol )/CT vol ] X 100. These percentage changes in rectal volume were compared between morning (7am to 11.59am) treatments and afternoon-night (12pm to 8pm) treatments using Independent sample t test using SPSS version 21.0. Results The rectum was contoured on 1140 CBCT image sets from 38 patients treated with radical radiotherapy to prostate ± seminal vesicles. The median rectal volume in CBCT images was 67.93 cc. 480 image-sets (of 16 patients) were obtained from CBCTs done during morning treatment and 660 image sets (from 22 patients) during afternoon-night. When stratified by time, the rectal volumes showed more variation in afternoon-evening as compared to morning. The average percentage change in the rectal volumes in morning and afternoon-night cohorts over the planning CT scan volumes were- 23.44% (SD-9.61; 3.48 to 39.64) and 38.3% (SD-19.98; 8.41 to 79.38)] respectively. Conclusion There is less difference in the percentage change in rectal volumes over the planning CT volume in the morning as compared to afternoon-night. This implies treating prostate cancer patients in morning with radical radiotherapy may yield dose distribution close to intended as compared to treatments after 12pm. PO-1902 Cone Beam Computed Tomography to assess pitch, roll and rotation in Head and Neck Radiotherapy D. Thornberry 1 , R. McLauchlan 1 , D.M. Gujral 1 1 Imperial College NHS Trust, Radiotherapy, London, United Kingdom Since 2017, Cone Beam Computed Tomography (CBCT) is standard imaging practice for patients receiving radical Intensity Modulated Radiotherapy (IMRT) for head and neck cancer (HNC). The aim of this study was to review current practice by evaluating if departmental planning margins are acceptable with current imaging tolerances. Material and Methods Method Verification images of 30 patients receiving radical IMRT for HNC were retrospectively reviewed by a single, experienced observer. Patients who received concurrent Cisplatin were excluded to rule out chemotherapy changes in patient positioning. Standard imaging protocol was CBCT days 1, 2, 3 and weekly during the course of Radiotherapy (RT) with kV planer imaging on non CBCT days. Displacements were recorded in all 3 planes with addition of the Pitch, Roll and Rotation (PRR) from CBCT images into a margin recipe 2,3 . PRR values <3 degrees were accepted. Online shifts for displacements >0 were performed. Displacement values in the 6 planes were used to produce population random and systematic errors using the method as outlined in On Target 1 . Results were compared to a historical cohort who were imaged primarily with KV imaging and 2 CBCTs in a 6 week course of RT. Results With increased frequency of CBCT imaging, there was good agreement between the two cohorts in AP, SI and LR direction shown in Table 1. However, this study indicated smaller rotational shifts in patient position. This may be due to increased staff experience and confidence with head and neck imaging and improved patient positioning and set up. Purpose or Objective Objective

Conclusion Daily imaging with zero tolerance demonstrated that, if all shifts were corrected for, the minimum CTV-PTV margin would be 0.3cm. With continued staff experience and confidence, there is scope for further margin reduction. For this, future work is needed focusing on residual error/intra-fractional error by imaging at the end of each treatment References 1. On Target: Ensuring geometric accuracy in radiotherapy. The Royal College of Radiologists, 2008. Van Herk et al. Int. J. Radiation Oncology Biol. Phys., Vol. 47, pp. 1121-1135, 2000 Stroom et al. Int. J. Radiation Oncology Biol. Phys., Vol. 43, pp. 905-919, 1999 PO-1903 Inter-observer variability of CTV registration in image-guided radiotherapy of oesophageal cancer. L. Wiersema 1 , F. Voncken 1 , J.J. Sonke 1 , F. Koetsveld 1 1 Netherlands Cancer Institute, radiotherapy, Amsterdam, The Netherlands Purpose or Objective Image guided radiotherapy for oesophageal cancer is most commonly based on a vertebrae registration. Registration based on the clinical target volume (CTV) using 4D CBCT enables margin reduction(1). However a soft tissue registration (like CTV-registration) is more challenging for RTTs, due to limited image quality of the 4D CBCT and anatomical changes such as tumor regression. The purpose of the observer study is to determine the feasibility of a CTV registration using a shaped region of interest. Material and Methods Six oesophageal cancer patients were included in this retrospective study. A 4D CT scan was made, from which a mid-position (midP) scan was derived for the purpose of treatment planning. The CTV was defined on the midP planning CT (pCT) by expansion of the GTV of 35 mm in the cranial-caudal direction and 5 mm in the transverse plane, and was extended to include all pathological lymph nodes. The CTV expansion into the gastric mucosa could be limited to 20 mm. Six 4D CBCTs were selected per patient, the first three fractions and weekly for the following three weeks. A shaped region of interest was created from the CTV, excluding fiducial markers. Two groups performed CTV registration, consisting of: 1) five specialized imaging RTTs, 2) five RTTs with our standard level of training. The registration was performed using automatic grey value registration (translation only). Manual adjustment of this automatic registration was recorded. To quantify inter-observer variability, first the standard deviation (SD) of the registration results in the Left-Right (LR), Cranial-Caudal (CC) and Anterior-Posterior (AP) was calculated per scan over all the observers. Secondly, the root-mean-square (RMS) of the SD over all fractions and patients was determined. As a reference, inter-observer 2. 1.