Retroperitoneal squamous cell carcinoma (SCC) is an extremely rare histological variant of retroperitoneal tumors. Fewer than 20 case reports have been published in the literature over the past few decades. In this report, we present a case of a retroperitoneal tumor of unknown primary origin in a male patient, with pathology finding highly suggestive of SCC originating from a teratoma.

A 51-year-old male presented constipation, difficult urinating, and suprapubic fullness was diagnosed with a large pelvic tumor up to 10.5cm with unclear margin with bladder, sigmoid colon and S1 spine. All tumor markers were within normal limited. After evaluation, we performed tumor excision was done with segmentectomy of left ureter with bladder base muscle and part of sigmoid colon-rectum, along with wedge bone resection of S1 vertebral body anterior portion. Reconstruction of rectum and left ureter re-implantation with Psoas hitch were also conducted.

During the surgery, a large pelvic retroperitoneal tumor up to 14 cm x 10 cm x 6 cm in size was identified, with invasion to S1 cortex, left ureter and sigmoid rectum. The surgery was performed smoothly, and all margins were free of tumor invasion. Pathological evaluation revealed differentiated, keratinizing squamous cell carcinoma arising in a cystic lesion lined by unremarkable squamous epithelium, with adjacent bronchial mucosa. After peer-reviewing with the pathologist, the diagnosis was suspected as squamous cell carcinoma arising from a teratoma. The initial discomfort resolved following the operation. Recently, follow-up abdominal CT scan conducted six months after the operation indicated no recurrence of tumor or any suspected primary site.

SCC suspected arising from a teratoma in a male patient has not been previously reported. Surgical intervention may provide favorable clinical outcomes in such cases. It is essential to conduct a comprehensive evaluation to facilitate accurate diagnosis prior to surgery.