Sarcoma of unknown primary: myth or reality?

Sarcoma of unknown primary (SUP) designates an enigmatic entity with histologic confirmation of a metastatic tumor without an identifiable primary after a thorough diagnostic workup. The term "unknown primary" is heavily debatable given that sarcomas can arise from any tissue that harbors its histological structure. In this review, we discuss the validity of SUP as a distinct entity. Medline/PubMed and Google Scholar were searched from 1990 until April 2020 for publications in the English language reporting on SUP. We excluded articles reporting on cases with sarcomas from known organ sites such as lung or uterine sarcomas as well as synovial sarcomas. The Kaplan-Meier method was used to compute the median overall survival. A total of 26 patients with SUP were identified. The median age at diagnosis was 17.5 years with a similar prevalence among men and women. The tumors most commonly reported were alveolar rhabdomyosarcoma and rhabdomyosarcoma not otherwise specified. Almost two-thirds of the patients were reported to have more than one metastatic site. Among the 13 patients with survival data, the median overall survival was 10.0 months. Two patients underwent autopsy and had their primary culprit identified in the chest wall and paravertebral.

This review showed that SUP shares with sarcomas of known primary similar clinical features including an aggressive clinical course, generally poor response to chemotherapy, and dismal patient outcomes. Thus, SUP does not appear to display a different natural history and biological properties that would allude to a distinct entity.