Alveolar soft part sarcoma mimics prostate cancer metastasis.

A 61-year-old man presented to the oncology clinic with Gleason 9 (4 + 5) prostate cancer. Staging CT showed multiple nodules in both lungs. Since the lung lesions were too small for biopsy, he was started on anti-androgen therapy for suspected metastatic, hormone-sensitive prostate cancer. While his prostate-specific antigen decreased from 32 to <0.1 ng/ml, the multiple lung lesions showed no response on subsequent imaging. The patient presented during follow-up with severe right leg pain, at which time magnetic resonance imaging revealed a large, hyperintense mass in the femur. The mass was resected along with two lung nodules, with pathology demonstrating metastatic alveolar soft part sarcoma. This serves as an important reminder that lesions suspicious for metastases may be due to cancers of multiple primary origins, particularly if the pattern of metastasis is atypical or there is varied response to therapy.