Introduction: Rectal and prostate cancers are common cancers occurring globally, and both can metastasize to the pelvic lateral lymph nodes (LLNs). Presentation of case: A 69-year-old man, presenting with blood in stool, was diagnosed with rectal cancer. Computed tomography revealed a 7-mm LLN in the right internal iliac artery region, leading to the suspicion of metastasis. The patient underwent laparoscopic low anterior resection and LLN dissection. Histopathological findings of the metastatic tissue in the LLN were different than that of rectal cancer, and endocrine tumor was suspected. Immunostaining performed based on high serum prostate-specific antigen (PSA) level revealed positivity for PSA and α-methylacyl-CoA racemase in the dissected LLN. Thus, he was diagnosed with synchronous rectal and prostate cancers and received hormonal therapy for stage IV prostate cancer, which led to a dramatic reduction in PSA level after three months. He was followed regularly and did not relapse or experienced disease progression for either cancer for approximately four years after the initial diagnosis. Discussion: Few studies reported synchronous rectal and prostate cancers, both of which can metastasize to pelvic LLNs. However, preoperative diagnosis of the primary cancer metastasizing to the LLNs is challenging. Treatment of synchronous rectal and prostate cancers requires a strategy to diagnose each tumor stage and corresponding degree of progression because lymph node metastases affect staging in both cancers. Conclusion: Lymph node dissection may be useful in determining progression and treatment plan in cases of concurrent rectal and prostate cancers with suspected LLN metastasis.
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