ESTRO 36 Abstract Book

S936 ESTRO 36 2017 _______________________________________________________________________________________________

The use of different coils at greater distance from the organ of interest and the use of a flat carbon fibre tabletop produced a reduction of the SNR. Even safe under the protocol we used, we believe that the use of more appropriated materials for MRI should be recommended. Nevertheless, this setup could be a low-cost first step for departments who want to start to integrate MRI images into their RT workflow. EP-1728 Inter-observer contouring similarity metrics, correlation with treatment outcome for prostate cancer D. Roach 1,2 , M. Jameson 1,2 , J. Dowling 3 , M. Ebert 4,5,6 , P. Greer 7,8 , S. Watt 2 , L. Holloway 1,2,5,9 1 University of New South Wales, South Western Clinical School, Sydney, Australia 2 Sydney South West Area Health Service, Ingham Institute and Liverpool and Macarthur Cancer Therapy Centres, Sydney, Australia 3 CSIRO, Australian e-Health Research Centre, Brisbane, Australia 4 Sir Charles Gairdner Hospital, Radiation Oncology, Perth, Australia 5 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia 6 University of Western Australia, School of Physics, Perth, Australia 7 Calvary Mater Hospital Newcastle, Radiation Oncology, Newcastle, Australia 8 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle, Australia 9 University of Sydney, Institute of Medical Physics, Sydney, Australia Purpose or Objective To determine the geometric and statistical metrics quantifying inter-observer contouring variation displaying the strongest correlation with simulated treatment outcome for prostate cancer. Material and Methods Data was available for 39 patients with localised prostate cancer, each having undergone CT and MRI scanning prior to radiotherapy. Three observers independently contoured CTV, bladder, and rectum on T2 MRI. A 7mm margin was applied to each observer’s CTV to create observer PTVs. An estimate of the true volume of each structure was generated using the STAPLE algorithm. Geometric and statistical metrics spanning the literature for inter-observer contouring variation studies were calculated for each observer’s contours with respect to the STAPLE volume. VMAT treatment plans (78 Gy to PTV) were simulated for each observer’s contoured structures, as well as for the STAPLE volumes, for all patients. Radiobiological metrics assessing treatment outcome (TCP, EUD, NTCP, etc.) were calculated for STAPLE CTV, PTV, bladder, and rectum for all treatment plans. Correlations between contouring variation metrics and radiobiological metrics were assessed using Spearman’s rank correlation coefficient ρ Results In total 117 observer treatment plans were simulated, resulting in a study with power to detect statistically significant (p < 0.05) correlations of ρ ≥ 0.3. No statistically significant correlations were found between contouring variation and radiobiological metrics for CTV and bladder. Figures 1 and 2 observed correlations for PTV and rectum respectively. For both structures volume similarity, sensitivity, and specificity showed moderate levels of correlation with a range of radiobiological metrics, although no correlations were observed between contouring variation and maximum dose within the rectum. Dice Similarity Coefficient (DSC) and Jaccard Index were found to have no significant correlation with simulated outcome for either structure, despite their prevalence within the literature. Centre-of-mass variations in the coronal and sagittal planes for PTV and

rectum respectively were the only distance metrics displaying significant correlations to simulated treatment outcome. Euclidean centre-of-mass variations, Hausdorff Distance, and Mean Absolute Surface Distance showed no correlation with any radiobiological metric. Conclusion Results indicate that volume similarity, sensitivity, specificity, and centre-of-mass significantly correlate with simulated treatment outcome within the rectum and PTV for prostate cancer radiotherapy. This information could inform future automated registration and atlas methods, allowing them to be guided on metrics based on clinical significance.

Electronic Poster: Physics track: Implementation of new technology, techniques, clinical protocols or trials (including QA & audit)

EP-1729 Air pockets in the urinary bladder during hyperthermia treatment reduce thermal dose G. Schooneveldt 1 , H.P. Kok 1 , E.D. Geijsen 1 , A. Bakker 1 , J.J.M.C.H. De la Rosette 2 , M.C.C.M. Hulshof 1 , T.M. De Reijke 2 , J. Crezee 1 1 Academic Medical Center, Radiotherapy, Amsterdam, The Netherlands 2 Academic Medical Center, Urology, Amsterdam, The Netherlands Purpose or Objective Hyperthermia is a (neo)adjuvant treatment modality that increases the effectiveness of radiotherapy or chemotherapy by heating the tumour area to 41–43 °C. This has been shown to improve treatment outcome for a number of tumour sites, including the urinary bladder. Hyperthermia may be given both for muscle-invasive bladder cancer, where it is combined with radiotherapy, and for non-muscle invasive disease (NMIBC), where it is combined with chemotherapy. However, some air may be present in the bladder during treatment, which effectively blocks the microwave radiation used to warm the bladder. This may lead to a lower thermal dose to the bladder wall, which is associated with a lower treatment response. This study investigates the size of that effect. Material and Methods We analysed thirteen NMIBC patients treated at our institute with mitomycin C (40 mg in 50 ml) plus hyperthermia (60 min). Hyperthermia was delivered using our hyperthermia device with four 70 MHz antennas around the pelvis. A CT scan was made after treatment and a physician delineated the bladder on the CT scan. On the same scan, the amount of air present in the bladder was delineated. Using our in-house developed hyperthermia treatment planning system, we simulated the treatment using the clinically applied device settings. We did this with the air pocket delineated on the CT scan, and alternatively with the same volume filled with fluid The patients had on average 4.2 ml (range 0.8 – 10.1 ml) air in the bladder. The bladder volume delineated by the physician (including air pocket and bladder wall), was on average 253 ml (range 93 – 452 ml). The average bladder volume in which changes exceeded 0.25 °C was 22 ml (range 0 – 108 ml), with the bladder being up to 2 °C cooler when an air pocket was present. There was no evident relation between the quantity of air and the difference in temperature. Although in particular the part of the bladder close to the air pocket absorbs less energy, the temperature in the entire bladder is typically lower because of convective mixing in the bladder contents. (urine). Results