[A Case of Secondary Rectal Cancer after Brachytherapy for Prostate Cancer].

We report a 72-year-old man who developed secondary rectal cancer 17 years after receiving brachytherapy for prostate cancer. The patient had undergone gastrectomy for gastric cancer 4 years earlier. Elevated tumor markers during follow-up prompted further investigation, revealing a rectal tumor invading the prostate. Robot-assisted total pelvic exenteration was performed after neoadjuvant chemotherapy. Histopathological examination confirmed advanced rectal adenocarcinoma invading the bladde(r ypT4bN3M0, ypStage Ⅲc). Although secondary malignancies following prostate radiotherapy are rare, our case met the established criteria for radiation-induced cancer;history of radiation exposure, latency period >4 years, histological differences from the primary tumor, and origin in previously normal tissue. Literature review indicates that, although uncommon, the incidence of secondary pelvic malignancies increases 15-20 years after radiotherapy, with a higher frequency reported following brachytherapy compared to radical prostatectomy. Our case highlights the importance of long- term follow-up in patients treated with pelvic radiotherapy because secondary cancers may emerge after decades. Ongoing surveillance may help detect such malignancies at earlier and more treatable stages, thereby improving patient outcomes.