Metastatic Carcinoma of Prostate as a Mimicker of SAPHO Syndrome.

Paraneoplastic arthritides are a group of immune-mediated inflammatory arthropathies associated with occult or manifest malignancy. Musculoskeletal spread of an underlying malignancy may also mimic many rheumatologic conditions. Distinguishing primary rheumatologic condition from paraneoplastic arthritides versus direct musculoskeletal spread of malignancy can be challenging especially in individuals with prior history of cancer and new musculoskeletal complaints. SAPHO (synovitis, acne, pustulosis, hyperostosis and osteitis) syndrome is an uncommon, although under recognized autoimmune disorder. Two musculoskeletal manifestations, namely inflammatory osteitis and hyperostosis of anterior chest wall with or without dermatologic manifestations, constitute a unifying feature of SAPHO syndrome. However, diagnosis of SAPHO syndrome is one of exclusion, and a wide variety of disorders including infections, malignancy (chondrosarcoma/osteosarcoma/metastasis), metabolic bone disorders (Paget's disease), osteoarthritis, seronegative spondyloarthropathy (spA) and osteonecrosis form part of a broad differential diagnosis. We present the case of a man, aged 72 years, with signs and symptoms of SAPHO syndrome and skin findings. Detailed history, radiological imaging, dermatology appearance, and role of immunohistochemical markers, especially staining for NKX3.1 protein with a novel antibody, led to a diagnosis of metastatic prostate adenocarcinoma. To our knowledge, this is the first case of metastatic adenocarcinoma of the prostate manifesting as SAPHO syndrome and cutaneous metastasis.