"Collision phenomenon" of prostate and bladder cancers in lymph node metastases.

We report here the case of a 78-year-old man who, 15 months after orchiectomy for palliation of prostate adenocarcinoma, was diagnosed as having squamous cell carcinoma of the urinary bladder. The bladder cancer was treated surgically, including dissection of pelvic lymph nodes. Some of these nodes were observed at surgery to be swollen, and were found on pathologic examination to exhibit the collision phenomenon: the mixing and mingling of cancer cells representing 2 distinct topographic origins. This case suggests that the possibility of collision phenomenon should be considered whenever any metastasis (but especially one in the lymph nodes) is found in a patient diagnosed with 2 different types of cancer. Moreover, it reminds us that diagnosis of one type of cancer does not rule out the possibility of another.