Synchronous Nasopharyngeal Carcinoma and Rectal Adenocarcinoma Treated With Integrated Multimodal Therapy: A Case Report and Literature Review.

Multiple primary malignancies are rare events, but when they happen, especially for curable cancers, they can cause management challenges. Locally advanced nasopharyngeal carcinoma is currently treated with induction chemotherapy followed by concurrent chemoradiotherapy, while locally advanced rectal cancer is currently treated with total neoadjuvant therapy, including chemotherapy and radiotherapy, followed by surgery. To integrate both treatments is really challenging and requires meticulous planning and monitoring to have a successful outcome with manageable toxicity. We present the case of a synchronous locally advanced nasopharyngeal carcinoma and locally advanced rectal adenocarcinoma in a 63-year-old Saudi Arabian man who was treated with induction chemoimmunotherapy including capecitabine and oxaliplatin (CAPEOX) with pembrolizumab for four cycles, starting in November 2023. This was followed by short-course radiotherapy to the pelvis, two additional cycles of CAPEOX plus pembrolizumab, definitive concurrent chemoradiotherapy to the nasopharynx, and finally abdominoperineal resection of the rectal tumor. He tolerated the procedure very well, and he is still in complete remission till his last follow-up in April 2026. The present report may possibly be the first to report a case of synchronous locally advanced nasopharyngeal carcinoma and locally advanced rectal adenocarcinoma that has been treated successfully by integrating immunotherapy into the standard treatment for both.