15555 Background: To predict acute toxicities (tox) of the gastrointestinal syndrome (SOMA LENT) in prostate cancer (PCa) patients (pts) undergoing 3D-CRT using a tool (nomogram) with clinical as well as dosimetric variables which has proved to be significant in the AIROPROS 01–02 trial. Methods: Acute rectal tox was scored in 1,132 pts using a 10 item self assessed questionnaire (QST) describing radio-induced GI symptoms profiles for bowel frequency, tenesmus, fecal continence, rectal pain and bleeding. The correlation between hormonal therapy (HT), drug prescription, diabetes or hypertension, pelvic node or seminal vesicles irradiation, mean rectal dose, DVH constraints and rectal tox was investigated by uni- and multivariate (MVA) logistic analyses. MVA results were used the R-project software to create nomograms predicting the symptoms of the acute GI syndrome. Results: 4/10 items of the QST are reported: moderate/severe bleeding, increased bowel frequency, moderate/severe bowel urgency and severe fecal incontinence. Bleeding is related to haemorrhoids (OR=1.9), HT (protective factor, OR=0.78) and mean rectal dose (continuous variable (cv), OR=1.024). Bowel frequency is related to seminal vesicles irradiation (OR=2.8) and V60 (cv, OR=1.024), while bowel urgency is correlated to seminal vesicles irradiation (OR=3.3) and mean rectal dose (cv, OR=1.027). Fecal incontinence depends on seminal vesicles irradiation (OR=4.6) and V70 (cv, OR=1.029). MVA results were used to create a set of nomograms. Conclusions: The combined effect of several independent, prognostically valuable variables for a specific disease can be expressed using nomograms. They can evaluate clinical and technical parameters of the single pt and offer clinicians a tailored probability of a specific outcome. To our best knowledge, this work presents the first set of nomograms available in the literature specific symptoms of the GI syndrome. The prediction capability of these tools will be validated on a independent set of patients. This work was partly supported by Fondazione I. Monzino, Milan No significant financial relationships to disclose.