Underlying synovial sarcoma undiagnosed for more than 20 years in a patient with regional pain: a case report.

Synovial sarcoma (SS) is a malignant tumor comprising 5-10% of all soft tissue sarcomas. SS has distinct characteristics, such as a predilection for young adults and relatively slow growth compared to other soft tissue sarcomas. Some patients with SS experience long-standing pain at the tumor site before the development of a palpable mass. Herein, we report the case of a 39-year-old woman with SS in the upper arm who presented with pain for > 20 years. The tumor detected on magnetic resonance imaging at 17 years was an SS. To the best of our knowledge, no English-language reports on imaging study-based identification of SS, which was undiagnosed for > 20 years, are known in the literature. This report discusses the imaging features of this latent lesion and the volume-doubling time of this unusual tumor.