Abstract Book

ESTRO 37

S531

present study (n=58). Patients were treated on Elekta linacs using 6MV VMAT. Dose plans were made in RayStation. Treatment was prescribed as 39 fractions totaling a dose of 78 Gy to the prostate, 56 Gy to elective lymph nodes, and 56 or 78 Gy to the seminal vesicles. Patients had daily pre-treatment CBCT imaging, using the Elekta XVI R4.5 system, with dual registration on bony anatomy and implanted seeds in the prostate. If deviations between the two registrations in x, y, or z, were larger than our CTV to PTV margin (5 mm LR, 8 mm CC, 7 mm AP), a flag was raised. Three flags triggered an investigation by a physicist to determine if ART was required. The number of treatment adaptations prescribed as a result of differential motion of the prostate and elective lymph node targets was determined by chart examination. From the log of the daily CBCT match, the lateral, longitudinal and vertical differences between the match on bones and prostate seeds were retrieved and analyzed. Manual evaluation of the displacement over the entire treatment course for each patient was used to determine which patients should have received ART to ensure proper target coverage. Based on the extracted data, we propose a model to predict the necessity of ART if the mean +/- one standard deviation between bone and prostate seed match is larger than our CTV to PTV margin minus 2 mm, calculated after 8 treated fractions. Results Of the 58 patients, 18 should have received ART due to either systematic or large random shifts between the prostate and bony anatomy (example provided in Figure 1). Our current practice of manually flagging deviations between bone and seed match resulted in 14/18 patients receiving ART, prescribed at a median time of 14 fractions (full range: 5-30). The proposed model predicted the need for ART in 17/18 patients after only 8 fractions. True and false positive and negative rates of the manual and predicted screening for ART are reported in Table 1.

Conclusion By calculating the mean difference between CBCT match on bone and implanted seeds on fraction 8, we were able to predict the need for ART on 17/18 patients early in the treatment course. In contrast, our current manual protocols for ART only flagged 14/18 patients, and did so almost a week later in median. The proposed model is easy to use, and suggests ART earlier in the treatment course than our current clinical practice. We are currently collecting data to validate the model in an independent patient cohort.

Poster: Physics track: CT Imaging for treatment preparation

PO-0966 Translation of a certified MR-only synthetic CT solution for prostate from photon to proton therapy C. Kurz 1,2,3,4 , M. Maspero 3,4 , G. Landry 2 , C. Belka 1,5 , K. Parodi 2 , P.R. Seevinck 4 , B.W. Raaymakers 3,4 , C.A.T. Van den Berg 3,4 1 University Hospital- LMU Munich, Department of Radiation Oncology, Munich, Germany 2 Faculty of Physics- Ludwig-Maximilians-Universität München, Department of Medical Physics, Munich, Germany 3 Universitair Medisch Centrum Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands 4 Universitair Medisch Centrum Utrecht, Center for Image Sciences, Utrecht, The Netherlands 5 DKTK, German Cancer Consortium, Munich, Germany Purpose or Objective MRI plays an increasingly important role in radiotherapy (RT). It allows for improved target and OAR delineation as well as pre-treatment and online image-guidance in adaptive RT scenarios. Still, electron density or, in the case of proton RT, stopping power ratio (SPR) information for dose calculation is typically derived from CT imaging data. An MR-only RT workflow could reduce clinical workload, radiation exposure and uncertainties from CT- to-MRI registration, but requires the generation of so- called synthetic CTs (sCT) from MRI data for dose calculation. In this work, we investigate for the first time whether a sCT generation algorithm certified for photon