Abstract Book

ESTRO 37

S430

the PTV was 0.13 ± 0.96 Gy higher than prescribed, while both the D 2 (mean 0.33 ± 1.04 Gy) and the D 98 (mean 11.47 ± 8.13 Gy) were lower. The bladder volume showed the biggest interfractional volumetrical variability. On average, the bladder volume ranged at 286 ± 168 ml, resulting in a dose deviation of 2.84 ± 6.17 Gy. The average rectal volume was 76 ± 33 ml, leading to an increase in the median delivered dose of 0.49 ± 3.50 Gy. The γ-analysis to a tolerance level of 3 mm and 3 % dose difference resulted in 95 ± 1.4%.

Roundness, SphericalRadius, EquivSphericalPerimeter, Flatness) to detect pass/fail on the treated and ground truth contours using ten-fold cross validation. Results The TRAC tool was tested on 74 individual trial patients, the number of cases for training varied from 38 to 80. For patients treated with the Rectafix TM , the TRAC tool demonstrated accuracy (standard deviation) results of 44(15)%, 56(10)%, 52(15)% and 89(8)% for the Rectafix TM , bladder, rectum and prostate respectively. For patients treated with the SpaceOAR® the TRAC tool demonstrated accuracy (standard deviation) results of 58(24)%, 56(18)% 78(21)%, and 92(12)% for the SpaceOAR®, bladder, rectum, and prostate respectively. Figure 1 exhibits Dice similarity scores for each cohort demonstrating that Dice alone is not sufficient to discern passing and failing delineations.

Figure 1. Comparison of Pass/Fail Dice Similarity Coefficient values for two different rectal stabilisation devices a) SpaceOAR® and b) Rectafix TM Conclusion The TRAC tool provides an automated and efficient means of conducting individual case review of delineation accuracy for the PROMETHEUS trial. Results reported here are for limited training data and could be improved with more data and refinement. Further testing is required for other treatment sites and imaging modalities. PO-0824 Dose effects by interfractional variability of tumor and OAR on the example of prostate-Ca-patients M. Splinter 1 , T. Bostel 2 , P. Häring 1 , C. Lang 1 , P. Huber 1 , E. Meyerhof 2 , J. Debus 2 , O. Jäkel 1 , N.H. Nicolay 2 1 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology- E040, Heidelberg, Germany 2 Universitätsklinikum Heidelberg, Radioonkologie und Strahlentherapie, Heidelberg, Germany Purpose or Objective During radiotherapy for prostate cancer, interfractional variability results from changes in bladder and rectal position and filling. As a result, the delivered dose may deviate from the planned dose. However, the effects of interfractional variability on dose deviations are largely unknown. Material and Methods Ten consecutive patients receiving intensity-modulated radiotherapy for low- or intermediate-risk prostate carcinoma to a total dose of 76.5 Gy in 2.25 Gy fractions were included in this analysis. Patients were instructed to present to treatment with an empty rectum and a comfortably filled bladder; all patients received daily in- room CT imaging (fxCT) in treatment position for position verification. For a total of 340 applied treatment fractions, GTV, CTV, PTV and bladder, rectum, sigma, femoral heads were contoured, and volumes and positions were compared to those on the respective planning CT scans. By using deformable registration between planning- and fxCT, the delivered dose was tracked and compared to the planned dose using dose volume histograms and γ-Analysis. Results The mean CTV volume was 74 ± 41 ml; the mean volume for the PTV was 140 ± 90 ml. The delivered dose D 50 to

Conclusion The observed anatomical variabilities of bladder, rectum and target volumes resulted in significant dose deviations to the bladder, while both the PTV and the rectum only exhibited small dose deviations. Thus, regular adaptive re-planning may result in lower doses to the organs at risk, especially the bladder and more conformal doses to the target volumes, thereby potentially impacting treatment-associated side effects. PO-0825 Urinary calprotectin as a promising bio- marker of RT-induced urinary toxicity. Preliminary results. G. Rossi 1 , D. Cante 2 , S. Beretta 1 , C. Piva 2 , C. Bianconi 1 , R. Ciccocioppo 3 , M. De Amici 4 , C. Klersy 5 , L. Di Cintio 6 , C. Fiorino 7 , V. Pellis 6 , N. Di Muzio 1 , C. Cozzarini 1 1 San Raffaele Scientific Institue, Radiotherapy, Milan, Italy 2 Ospedale di Ivrea- A.S.L. TO4, Radiotherapy, Ivrea, Italy 3 AOUI Borgo Roma- University of Verona, Gastroenterology Unit- Department of Medicine, Verona, Italy 4 Fondazione IRCCS Policlinico San Matteo, Immuno- Allergy Laboratory of Padiatrics, Pavia, Italy 5 Fondazione IRCCS Policlinico San Matteo, Servizio di Biometria e Statistica - Direzione Scientifica, Pavia, Italy 6 Eurospital, Diagnostic Division, Trieste, Italy 7 San Raffaele Scientific Institue, Medical Physics, Milan, Italy Purpose or Objective Urinary toxicity (UT) still represents a crucial issue in the radiation treatment (RT) of prostate cancer (PCa), owing both to the unchanged need over time (despite the widespread availability of modern IMRT techniques) to include the bladder neck/vesico-urethral anastomosis in the so-called 'high-dose” region and, more importantly, to the unpredictability of late events. Stool calprotectin