ESTRO 2021 Abstract Book

S613

ESTRO 2021

interface (Elekta AB, Sweden). An empty (cold) phantom at room temperature and a warm water filled phantom were used. The difference between both positioning methods (ΔD POSITION = ǀ D X-RAY -D S+T ǀ ) was calculated and an independent t-test was done to investigate the significance of the differences for a cold and a warm phantom. Results Fig. 1 shows the temperature curves, where a stabilisation is seen 10 min after filling the phantom and during the next 55 min. In 6 out of 7 locations values measured within nearly 1h were between 36°C and 32°C. The 4°C difference is not expected to impact the measurements as the surface/thermal imaging reference is updated and zeroed after X-rays are acquired.

Tab. 1 shows the couch translations and the difference between both positioning methods. No statistical significance between the positioning of the cold and warm phantom was found ( p ˃ 0.05).

Conclusion Considering the surface temperature stability this phantom demonstrated to be suitable for measurements with ExacTrac Dynamic for at least 1h. The measurements with a warm phantom showed good agreement between surface/temperature and X-ray imaging. No deviations larger than 0.1 mm were found, leaving open the possibility of monitoring the patient using more surface guidance and less ionising radiation for cranial treatments. Measurements with a cold phantom showed slightly higher deviations. However, as the differences were not statistically significant, a larger set of measurements is planned to determine whether a heated phantom is essential for SRS QA.

Poster discussions: Poster discussion 10: The interface of physics and radiobiology

PD-0782 Predicting bRFS after salvage post-prostatectomy RT with a “one-size-fits-all” TCP-based formula M. Olivieri 1 , A. Magli 2 , D. Cante 3 , B. Noris Chiorda 4 , F. Munoz 5 , A. Faiella 6 , E. Olivetta 7 , M.A. Signor 2 , C. Piva 3 , B. Avuzzi 4 , L. Ferella 5 , A. Pastorino 7 , S. Broggi 1 , A. Fodor 8 , C. Deantoni 8 , T. Rancati 9 , G. Sanguineti 10 , R. Valdagni 4 , N. Di Muzio 8 , C. Cozzarini 8 , C. Fiorino 1 1 IRCCS San Raffaele Scientific Institute, Medical Physics, Milan, Italy; 2 Azienda Ospedaliero Universitaria S. Maria della Misericordia, Radiotherapy, Udine, Italy; 3 Ospedale di Ivrea, Radiotherapy, Ivrea, Italy; 4 Fondazione IRCCS Istituto Nazionale dei Tumori, Radiotherapy, Milan, Italy; 5 Ospedale Regionale Parini-AUSL Valle d’Aosta, Radiotherapy, Aosta, Italy; 6 IRCCS Istituto Nazionale dei Tumori “Regina Elena", Radiotherapy, Roma, Italy; 7 A.O.SS. Antonio e Biagio, Radiotherapy, Alessandria, Italy; 8 IRCCS San Raffaele Scientific Institute, Radiotherapy, Milan, Italy; 9 Fondazione IRCCS Istituto Nazionale dei Tumori, Medical Physics, Milan, Italy; 10 IRCCS Istituto Nazionale dei Tumori “Regina Elena”, Radiotherapy, Roma, Italy

Purpose or Objective Despite many studies identified predictors of long-term biochemical-relapse-free survival (bRFS) after salvage