ESTRO 37 Abstract book

Conclusion In the patients studied, PET-CT-based planning allows an accurate delineation of volumes, reducing target volumes and modifying radiotherapy cervical treatments. A large number of patients are needed to confirm our preliminary data. Therefore, recruitment continues. EP-2355 Impact of DIBH irradiation on dose reduction to the heart and left coronary artery in breast cancer P. Winczura 1 , J. Wejs-Maternik 1 , A. Blukis 1 , M. Antonowicz-Szydłowska 1 , P. Milczanowska 1 , A. Rakowiecka 1 , M. Urbanowicz 1 , A. Badzio 1 1 Radiotherapy Center Elbląg, Radiotherapy, Elbląg, Poland Purpose or Objective The aim of the study was to assess the impact of deep inspiration breath hold (DIBH) irradiation on dose reduction to the heart and left coronary artery (LCA) in left-sided early breast cancer patients treated with intact breast. Material and Methods We analyzed dose to the heart, LCA and ipsilateral lung on DIBH and free breathing (FB) plans of 30 consecutive patients treated with tangential 6MeV photon fields. 50Gy in 25 fractions to the PTV (whole breast with 5mm margin) was prescribed. Results Mean heart dose (MHD) was significantly lower in DIBH comparing to FB (2,9 vs. 5,9Gy; p<00001). Mean MHD reduction between DIBH and FB was 3,1Gy. Mean and maximum LCA doses were also reduced in DIBH (16,1 vs. 30,4Gy; p<00001 and 38,7 vs. 50Gy; p<0,0001; respectively). Mean lung dose was significantly lower in DIBH comparing to FB (9,7 vs.11Gy; p=0,002). Lower MHD dose did not correlate with lower mean LCA (p=0,14) and maximum LCA (p=0,76) dose in the whole group. Only in patients with modest MHD reduction (defined as <3Gy difference between DIBH and FB) maximum and mean LCA doses were significantly reduced in DIBH (39 vs. 49,6Gy; p=0,006 and 16,2 vs. 27,9Gy; p=0,0001; respectively). Conclusion DIBH in early left-sided breast cancer patients allows for significant MHD, LCA and left lung MLD dose reduction. Patients with modest MHD reduction in DIBH have also significantly lower doses to the LCA. EP-2356 Inter-observer variability in OAR and target volume delineation in curative prostate cancer patients K. Tiigi 1 , R. Tiigi 1 , I. Oro 1 , M. Adamson 1 , K. Kolk 1 , M. Põldveer 1 1 North-Estonian Regional Hospital Cancer Center, Department of Radiotherapy, Tallinn, Estonia Purpose or Objective In North Estonia Medical Centre prostate and OAR-s delineation is assigned to RTT-s who work in the radiotherapy department CT-SIM room. Previously the radiation oncologists did the delineation of prostate and OAR-s, but due to increasing workload the RTT-s have taken over during few previous years. The expectations for RTT-s are high – in addition to delineate easily detectable OAR-s as the lungs, spinal cord etc. they also delineate the OAR-s in the head and neck patients, SBRT and SRS cases and pelvic patients. To find out the need of precise instructions and need of training an inter- observer delineation study was done. Material and Methods 10 curative prostate patients were selected to the study and the variability of delineation within 5 observers was analyzed. 3 RTT-s and 2 radiation oncologist delineated each patient bladder, prostate, rectum and bulb of penis. From the 2 radiation oncologist one is experienced

with treating prostate patients and one with treating breast and gastrointestinal cancer patients. Delineation was done in Elekta´s Focal Contouring System. After receiving all contours, analyze was done by an independent person. Volume differences between observers were evaluated. Results Mean bladder size was 154.9 cm 3 (87.7-267.1) and mean standard deviation (SD) was 9.4 (5-19.5), mean bulbus penis size was 2.0 (1-0-3.8) and mean SD was 0.6 (0.4-1), mean prostate size was 55.3 (29.8-83.4) and mean SD was 4.1 (1.9-6.8), mean rectum size was 64.3 (41.9-85.0) and mean SD was 3.8 (1.9-7.2). There was no correlation between size of the OAR and SD. The RTT-s tend to delineate bigger volumes than radiation oncologists. Conclusion Although standard deviations in OAR volume differences were in acceptable ranges, delineation protocol was made. While delineating OAR-s in pelvic area window- level (W/L) protocol should be “Soft tissue” and check for the prostate and bladder line should be in “Brain” W/L protocol. While delineating in axial plane, check in sagittal plane should follow. Rectum contour should start from rectosigmoid junction and should stop in anal sphincter. To conclude, single center inter-observer study is a good tool to determine the differences in delineation and compendious delineation protocol helps to minimize variability among observers. EP-2357 A new perspective to evaluation criteria of IMRT plan verification A. Schwahofer 1 , F. Roden 2 , R. Kussaether 3 , M. Grimm 3 , A. Wallin 2 1 German Cancer Research Center DKFZ, Department of Medical Physics in radiation therapy, Heidelberg, Germany 2 MVZ Vivantes Neukoelln- Berlin, Radiation Therapy, Berin, Germany 3 MedCom, GmbH, Darmstadt, Germany Purpose or Objective The Monte Carlo (MC) based verification software ProSomaCore (MedCom, Darmstadt, Germany) is validated as secondary dose calculation system. Therefore, specific DVH and CT value based evaluation criteria were defined and verified in order to provide an alternative to the gamma criteria. Material and Methods The therapy planning system (TPS) Eclipse (Varian Medical Systems, Palo Alto, US) was used as reference system with the superposition algorithm AAA. 40 patients from four different entities were recalculated with ProSomaCore: Mamma, HNO, Lung and Prostate. Since the algorithms inherently show different behaviour in tissues, the gamma criteria is not reasonable anymore in the typical thresholds to compare plans. Thus, a new evaluation criterion was introduced: the relative dose and density parameter – rDD. DVH and dose distribution were separated into the three density regions air (- 1000 / - 171 HU), soft tissue (- 170 / 149 HU) and bones (> 150 HU). This was considered over the whole calculation volume in ten dose categories (see figure 1) by a self- developed script. Subsequently, a set of assessment possibilities were elaborated. Electronic Poster: RTT track: Treatment planning and dose calculation / QC and QA