Pure Small Cell Recurrent Prostate Cancer Developing Syndrome of Inappropriate Antidiuretic Hormone Secretion

Abstract

Purpose Pure small cell neuroendocrine carcinoma of the prostate is a rare entity characterized by a poor prognosis due to early metastatic spread as well as resistance to treatment. Considering its increasing occurrence, clinicians should be aware of its aggressive behavior, the relevance of an early diagnosis, and proper management. Methods A 71-year-old man treated with brachytherapy for localized low-risk prostate cancer developed widespread disease 7 years later with a prostate-specific antigen-negative neuroendocrine small cell phenotype. He was also diagnosed with syndrome of inappropriate antidiuretic hormone secretion (SIADH) soon after starting chemotherapy. Results A substantial radiologic and clinical response to chemotherapy was observed and the paraneoplastic SIADH was successfully treated with oral vaptan therapy. Conclusions Secondary small cell prostate carcinoma is an underestimated entity with high sensitivity to chemotherapy, although a standard treatment has not yet been defined. Moreover, oral vaptans demonstrated prompt efficacy and simple management in correcting SIADH-related hyponatremia.