ESTRO 38 Abstract book

S203 ESTRO 38

RO4= 1 (years). In 2018, all 4 ROs had 2 years of experience in TRUS. A volume comparison between both modalities and between ROs was performed. A rigid registration between CT and TRUS images was completed for 16 patients using gold fiducials markers. The Jaccard Index (JI) was computed to evaluate the inter-observer variability. In addition, the intra-observer variability was evaluated by comparing the volumes contoured in 2016 and 2018. Paired t-tests were used to evaluate the volume and JI differences. Results The mean CT and TRUS volumes showed a significant correlation for the prostate of 0.95 (p<0.001). The volume ratio of TRUS/CT was 0.82 (95% interval 0.79-0.87), meaning the volume was 18% bigger on CT. The mean JI was 87% for CT and 92% for TRUS when comparing all 4 ROs; CT and TRUS JIs were significantly different (p<0.001). The mean JI for the prostate on CT was significantly better (p<0.05) when comparing RO1, 2 and 3 together (RO1-2, RO1-3, RO2-3, mean=89%) than when comparing RO4 to the others (RO1-4, RO2-4, RO3-4, mean=85%). For TRUS planning, the mean JI was not significantly different (p>0.05) when comparing all ROs (91-93%). The intra-observer volume variability between 2016 and 2018 for CT and TRUS was evaluated and revealed a bigger mean difference for CT than for TRUS, 6.9 cc vs 4.5 cc respectively (p<0.001). The average ROs intra-observer variability was not significant in TRUS (p>0.05).

15.5Gy), D98=18Gy (14.2-19.1Gy), D95=19.5Gy (15.8- 21.5Gy), and D90=21.5Gy (17.8-24.2Gy). In BT 5Gy was administered to 87.7ccm (19.28-570.75ccm) of the liver, 10Gy (EQD2 26Gy, α/β3) to 35.92ccm (8.38- 254.42ccm). Correspondingly in SBRT normal liver tissue received 15Gy median 135.53 (56.88-827.2ccm), 15.9Gy (EQD2 26.4Gy, α/β3) 120.93 ccm (51.18-734.91ccm), and 20Gy to 73.11ccm (16.59-390.73ccm) of normal liver tissue.

Conclusion Compared with SBRT, high-dose interstitial brachytherapy in patients with hepatocellular carcinoma allows for a good dose coverage while sparing normal liver tissue. OC-0400 Are prostate contours affected by the RO's clinical experience in prostate HDR brachytherapy? H. Lavoiegagnon 1 , E. Poulin 1 , A. Martin 1 , L. Pilote 1 , E. Vigneault 1 , W. Foster 1 , L. Archambault 1 , F. Lacroix 1 1 CHU de Quebec, Radiation Oncology, Quebec, Canada Purpose or Objective The aim of this study was to compare the prostate contours for two imaging modalities, computed tomography (CT) and trans-rectal ultrasound (TRUS), used in HDR brachytherapy. The authors hypothesized that the contour variability was affected by the radiation oncologists (ROs) clinical experience. Material and Methods CHU de Québec is a high volume prostate brachytherapy center (>250 patients/year) employing multiple ROs that have performed CT-based HDR brachytherapy since 1999. A TRUS-based HDR procedure was introduced in 2016. The first t hirty patients to undergo the TRUS procedure were given a 15 Gy HDR boost. While still under general anesthesia, a CT on rails located in the brachytherapy suite was used to image the pelvis after the treatment. The treating RO (RO1, 2 or 3 in 2016) then contoured the prostate on both modalities. Two years later, four ROs (RO4 started in 2017) re-contoured the prostate on the CT and TRUS images for each patient. The RO’s CT clinical experience was as follow: RO1=13, RO2= 18 RO3=7 and

Conclusion Contoured prostate volumes were 18% bigger on CT compared to TRUS. In CT, the inter-observer variability is significant even for ROs with more than 7 years' experience. Interestingly, the inter-observer and intra- observer variability were significantly smaller in TRUS