Alveolar Soft Part Sarcoma

A painless, slow-growing, vascular deep thigh mass in a young adult, with an alveolar or organoid histological pattern and PAS-positive, diastase-resistant intracytoplasmic crystals, is alveolar soft part sarcoma. It is a rare soft-tissue sarcoma defined by the ASPSCR1-TFE3 gene fusion, which drives nuclear TFE3 over-expression - so I would confirm the diagnosis with nuclear TFE3 immunostaining and detection of the fusion, for example on next-generation sequencing. The critical point in management is the paradox of ASPS: despite an indolent, slow-growing primary, it has a striking propensity for early haematogenous metastasis - to the lungs, bone and characteristically the brain - and metastases can appear years later. So staging must explicitly include the chest and the brain, not just the local MRI of the primary, and the patient needs long-term surveillance. Treatment is also distinctive: ASPS is relatively resistant to conventional cytotoxic chemotherapy, so the mainstay is wide surgical resection of the primary, with metastasectomy of resectable metastases, while anti-angiogenic tyrosine-kinase inhibitors and immune checkpoint inhibitors have shown activity in advanced disease. All of this should be coordinated through a specialist sarcoma multidisciplinary team, and I would avoid an unplanned excision of the undiagnosed mass, which would compromise the oncological result.