Large pelvic mass invading into the sacrum: a case report for primary retroperitoneal P16 positive squamous cell carcinoma

Background: This case report reviews a rare form of Human Papilloma Virus (HPV)-associated squamous cell carcinoma—primary retroperitoneal disease. Case: A 69-years old female presented to Kingston General Hospital with signs and symptoms concerning for cauda equina and underwent spinal magnetic resonance imaging which demonstrated a multicystic and solid mass invading into her sacrum in addition to significant pelvic lymphadenopathy. An imaging-guided biopsy demonstrated HPV-positive squamous cell carcinoma, of unknown origin. Given the presence of symptomatic disease on initial presentation, she underwent palliative radiation. Once she was discharged in stable condition from hospital, the Gynecologic Oncology team assumed care due to the location of the mass, despite not clearly originating from the cervix. She completed 4 cycles of systemic chemotherapy with carboplatin and paclitaxel. She also received bevacizumab and immunotherapy with pembrolizumab. Over a number of subsequent visits and repeat imaging, there was a drastic reduction of tumour burden in response to this treatment regimen and no evidence of disease progression after 15 months of follow up. Conclusions: This is a novel case for retroperitoneal squamous cell carcinoma treated with systemic chemotherapy as well as immunotherapy resulting in a significant treatment response.