ESTRO 36 Abstract Book

S383 ESTRO 36 _______________________________________________________________________________________________

[1] T.E. Schultheiss, C. G. Orton, R. A. Peck, 'Models in radiotherapy: volume effects”. Med. Phys. 10 (1983).

11 San Raffaele Scientific Institute, Medical Physics, Milan, Italy

Purpose or Objective The main objective of this work was to fit the Normal Tissue Complication Probability (NTCP) model for the prediction of late urinary symptoms at three years after radical radiotherapy (RT) for prostate cancer. Then, to evaluate the effect of leading clinical risk factors on the NTCP. Material and Methods Patients were enrolled in a prospective, multicentre, observational trial in 2010-2014. They were treated at different prescription doses with conventional (74-80 Gy at 1.8-2 Gy/fr,) or moderately hypo-fractionated RT (65- 75.2 Gy at 2.2-2.7 Gy/fr) in 5 fractions/week. Urinary symptoms were evaluated through the International Prostate Symptom Score (IPSS) questionnaire filled in by the patients before RT, after RT, and every 6 months until 5 years of follow-up. The incidence of late toxicity at 3 years was defined as the occurrence of an IPSS>=15 at least once between 6 and 36 months after RT. Bladder dose volume histograms were collected for each patient. They were corrected to the equivalent doses in 2 Gy/fraction (EQD2), with α/β=3Gy, and then reduced to the Equivalent Uniform Doses (EUD), computed at varying n values. Several clinical factors were also collected (comorbidities, drugs, hormone therapies, previous surgeries, smoking, alcohol, age, and body mass index). Maximum likelihood estimation (MLE) was employed for calculating the best-fit NTCP parameters: n (volume parameter summarizing the organ architecture), EUD 50 (EUD causing 50% toxicity risk) and k (steepness of the NTCP) [1]. Leading clinical factors associated with urinary toxicity were evaluated with univariable logistic regression (p<0.05) and included in the NTCP model as EUD 50 modifying coefficients. Results 217 patients had complete IPSS questionnaires at 30 or 36 months, at least 3 follow-up evaluations, and did not show urinary symptoms before RT (baseline IPSS<=12). 35/217 patients (16%) showed late urinary toxicity. MLE pointed toward EUD values with very low n=0.01±0.01. Thus indicating a serial behaviour of bladder for this toxicity. The best-fit parameter for the NTCP steepness was k=12±4, while EUD 50 was estimated to be 89±5Gy. The results were confirmed in the hypofractionated population. The use of antiaggregants was the only clinical risk factor associated with toxicity (p=0.02, odds ratio = 3.6). Its inclusion in the NTCP model implied a reduction of the EUD 50 to 81±5Gy. Conclusion An NTCP model for prediction of late urinary toxicity was determined. Bladder was found to act as a serial organ, so that toxicity risk highly depends on small bladder volumes irradiated with high doses. Patients with antiaggregants exhibit enhanced radiosensitivity. The dose-response relationship points out that hypofractionation (resulting in higher EUDs) should be combined with careful optimization in the bladder-PTV overlap to reduce the risk of late urinary toxicity.

PO-0730 The independent benefit deriving from high doses and WPRT in salvage post-prostatectomy radiotherapy C. Cozzarini 1 , B. Noris Chiorda 1 , C. Fiorino 2 , M. Pasetti 1 , A. Briganti 3 , C.L. Deantoni 1 , A.M. Deli 1 , A. Fodor 1 , N. Fossati 3 , G. Gandaglia 3 , C. Sini 2 , F. Montorsi 3 , N. Di Muzio 1 1 San Raffaele Scientific Institute, Radiotherapy, Milano, Italy 2 San Raffaele Scientific Institute, Medical Physics, Milano, Italy 3 San Raffaele Vita-Salute University, Urology, Milano, Italy Purpose or Objective The success rate after salvage radiotherapy (SRT) for biochemical recurrence after radical prostatectomy is still unsatisfactory, possibly owing to inadequate irradiation doses and fields. The few retrospective studies investigating the role of whole-pelvis radiotherapy (WPRT) in this setting have pertained to patients (pts) treated with conventional (60-68 Gy) SRT doses, a factor that may have ”diluted” its true effect. The purpose of this analysis was to evaluate the possible clinical benefit of WPRT in addition to high-dose salvage radiotherapy (HDSRT), and to identify the cohort that would benefit most from it. Material and Methods From 1998 to 2009, 235 node-negative pts underwent HDSRT (median 75.6 Gy, range 64.8-77.4). 149 pts were irradiated to prostatic bed (PB) only, at a median dose of