Rapid disease progression during adjuvant chemotherapy following concurrent chemoradiotherapy and radical resection for rectal sarcomatoid carcinoma: a case report and literature review

Background Rectal sarcomatoid carcinoma (RSC) is an exceedingly rare, aggressive malignancy characterized by a dismal prognosis, high recurrence rates, and truncated overall survival. Only 16 cases have been reported. Due to its rarity, optimal therapeutic strategies remain poorly understood. While current management typically mirrors rectal adenocarcinoma paradigms due to a lack of dedicated guidelines, clinical outcomes are frequently unsatisfactory, highlighting the challenges in managing this rare entity. Case description A 66-year-old male presented with a 3-month history of hematochezia. Colonoscopy revealed a rectal mass, and subsequent biopsy confirmed a moderately-to-poorly differentiated adenocarcinoma(~95%), with only focal areas showing sarcomatoid differentiation(~5%). Immunohistochemistry (IHC) demonstrated proficient mismatch repair (pMMR) status and low HER-2 expression (1+). Pelvic MRI clinically staged the tumor as cT4aN2Mx (MRF-). Digital rectal examination palpated a hard, cauliflower-like mass 9 cm from the anal verge. On October 16, 2025, the patient initiated neoadjuvant chemoradiotherapy (CRT) (50 Gy/25 fractions with concurrent capecitabine), followed by two cycles of XELOX. On January 9, 2026, he underwent laparoscopic radical resection, right lateral pelvic lymph node dissection, and protective ileostomy. Postoperative pathology indicated downstaging to ypT3N0 with a tumor regression grade (TRG) of 2 (~70% overall regression). Notably, the residual tumor predominantly exhibited sarcomatoid differentiation(~70%) alongside intramucosal carcinoma components(~30%), confirmed as RSC via IHC. Postoperatively, the patient received four cycles of adjuvant XELOX(interrupted temporarily for stoma reversal following MDT evaluation). During adjuvant chemotherapy (May 10, 2026), he presented with significantly aggravated abdominal distension. Follow-up contrast-enhanced CT revealed right abdominal alterations highly suspicious of abscess or metastasis, with massive ascites and pelvic effusion. Cytological and IHC analyses of the ascites cell block confirmed peritoneal metastasis originating from the sarcomatoid component, showing focal positivity for CKpan, diffuse and strong positivity for Vimentin, and positivity for CDX2 while maintaining a pMMR/MSS status. Regrettably, the patient passed away on May 18, 2026. Conclusion RSC is a rare, aggressive biphasic malignancy with a median overall survival typically under six months. It exhibits profound resistance to conventional colorectal CRT regimens, with a high propensity for recurrence and metastasis. Standard treatments are frequently inadequate, underscoring the urgent need for novel individualized therapeutic and targeted approaches.