Patient information: An 86-year-old male presented with a progressively enlarging back mass for the past two months. He was a non-smoker. His past medical history was significant for hypertension (HTN), ischemic heart disease (IHD), and percutaneous coronary intervention (PCI). Additionally, he had undergone femur surgery in the past.

Laboratory investigations: Thyroid function tests revealed a mildly suppressed thyroid-stimulating hormone (TSH) level of 0.108 µIU/mL, while free thyroxine (FT4) was within normal limits at 19.8 pmol/L.

Chest CT with IV contrast: A contrast-enhanced CT scan of the chest was performed to evaluate the posterior back mass. Imaging revealed a well-defined soft tissue density mass located in the right upper back, measuring 8 × 4.5 cm. The lesion was situated in the subcutaneous fat, superficial to and inseparable from the right trapezius muscle. No internal fat or calcification was noted. The mass exhibited mild heterogeneous post-contrast enhancement on the venous phase. The underlying scapula appeared normal. The radiological impression was suggestive of a soft tissue sarcoma, and tissue diagnosis was recommended.

Core needle biopsy: Under local anesthesia and ultrasound guidance, seven Tru-Cut core biopsies were obtained from the large posterior chest wall mass. Histopathological examination identified the lesion as a spindle cell sarcoma, and further immunohistochemical staining was advised.

The biopsy revealed a spindle cell sarcoma with a mitotic rate of 19 per 2 mm², indicating a high proliferative index. No necrosis or lymphovascular invasion was identified. Immunohistochemical analysis was recommended to confirm the diagnosis, including markers such as SS18-SSX, CD99, MUC4, S100, SMA, Desmin, STAT6, and CD34, with the possibility of additional markers if needed. The primary suspicion was monophasic synovial sarcoma, pending further immunostaining for definitive characterization.