This study aimed to assess the applicability of photon-based NTCP models to rectum morbidity outcomes following PT for prostate cancer patients. The data of gastrointestinal morbidities (grade >=2) reported by 1151 prostate cancer patients treated with passive scattering PT and 159 patients treated with conventional 3DCRT were analyzed.

This study found that photon-based rectal NTCP models either over- or underestimated the clinically observed gastrointestinal morbidity when used on the proton cohort, but four of the six photon-based NTCP models showed a good fit to the photon outcome data. This study concluded that large differences in morbidity predictions between cohorts and modalities, therefore NTCP models should be carefully investigated prior to clinical application.