EP672 - Multifocal Soft Tissue Sarcomas in a Patient After Neoadjuvant Chemoradiation for Rectal Cancer

Purpose/Background: Sarcoma development is a rare complication of radiation therapy that can manifest years after treatment. We present a case of a 47-year-old male who presented with two separate adenocarcinomas of the rectum and sigmoid colon. He was treated with total neoadjuvant therapy followed by surgical resection, and subsequently developed two separate, synchronous sarcomas within the radiation field.

Methods/Interventions: The patient initially presented with fatigue and hematochezia. A diagnostic colonoscopy demonstrated two separate adenocarcinomas of the sigmoid colon and rectum. Initial staging revealed locally advanced disease. He was started on neoadjuvant chemotherapy with FOLFOX, followed by consolidation chemoradiation (including low pelvic and inguinal regions). The patient underwent an uncomplicated robotic low anterior resection and a loop ileostomy creation. Final pathology returned with two invasive moderately differentiated adenocarcinomas of the rectum and sigmoid colon, both with a partial response. All surgical margins were negative, and 5 out of 40 lymph nodes were positive for metastatic disease. The patient was started on surveillance with cross sectional imaging every 6 months. Three months later, he underwent an uncomplicated loop ileostomy reversal.

Results/Outcomes: Follow-up imaging demonstrated persistent presacral soft tissue thickening, which was initially attributed to post-surgical change. A follow-up CT scan two years after the index operation revealed a new soft tissue mass in the left perirectal region, contacting the colorectal anastomosis and concerning for local recurrence. This mass demonstrated increased metabolic activity on a PET/CT. MRI of the rectum showed a tumor contiguous with the serosal margin of the left posterior rectum, extending into the left ischiorectal fossa and measuring 4.5×4.9 cm. The MRI also showed an enhancing soft tissue mass on the left anterior pelvic abdominal wall measuring 6.0×2.5 cm. Endoscopic ultrasound was performed with a transrectal fine needle aspiration (FNA) of the perirectal mass. Pathology showed atypical spindle cell proliferation with no evidence of adenocarcinoma. He was started on systemic chemotherapy, and after follow-up discussion at our multidisciplinary tumor board, the decision was made for synchronous surgical excision. At the time of surgery, both masses were excised via a trans-gluteal incision and left lower quadrant incision. Final surgical pathology of both masses confirmed desmoid type fibromatosis. His recovery was again uneventful. Follow-up treatment with systemic chemotherapy was resumed and he is currently without evidence of recurrence.

Conclusion/Discussion: Radiation-induced sarcoma development after treatment for rectal cancer is extremely rare, with almost no available published literature to help guide treatment decisions. Improved understanding of risk factors and treatment options are needed to help guide patients and providers in development of an appropriate treatment plan. For this patient, recurrence of his original adenocarcinoma was initially thought to be the most likely etiology of his recurrent tumors. While biopsy ultimately confirmed the diagnosis, post-radiation sarcoma development should be considered in this patient population.

MRI Rectum
T1 weighted MRI, transverse image of soft tissue thickening with homogenous enhancement of the lower rectus abdominis muscle.
MRI Rectum
T1 weighted MRI, transverse image of well-circumscribed, low signal tumor along the undersurface of the posterior levator ani muscle.