A Rare Presentation of Prostate Cancer: Psoas Muscle Metastasis Without Bone Involvement

Prostate cancer is one of the most common cancers worldwide. We present a unique case of metastatic prostate cancer manifesting initially with a psoas muscle mass causing abdominal and hip pain. Computed tomography (CT) imaging revealed a large mass within the left psoas muscle with prominent pelvic and retroperitoneal lymphadenopathy, initially concerning for rhabdomyosarcoma. However, a biopsy of the psoas mass confirmed metastatic prostate adenocarcinoma. Interestingly, a nuclear whole-body bone scan and spinal MRI were negative for bone involvement or other metastases. The patient was treated with anti-androgen therapy along with abiraterone and docetaxel and had a positive serologic and radiologic response. He was also offered radiation as per the phase III Prostate Cancer Consortium in Europe-1 (PEACE-1) trial. This case underscores the importance of recognizing atypical metastatic sites, such as skeletal muscle, in prostate cancer, as they can complicate initial diagnosis and delay management. Understanding the mechanisms of rare metastatic patterns and the diagnostic approaches required is crucial for improving outcomes in prostate cancer patients.