Undifferentiated pelvic sarcoma 5 years post-radiochemotherapy for rectal adenocarcinoma: A case report.

Radiation-induced sarcoma (RIS) is a rare, life-threatening late complication of radiotherapy, typically diagnosed 3-5 years post-treatment. It is associated with a poor prognosis and poses diagnostic challenges due to non-specific imaging and clinical features that mimic tumor recurrence. As the long-term survival rates of patients with rectal cancer improve, recognizing late-onset treatment-related malignancies becomes increasingly paramount. The present study describes the case of a 62-year-old male patient who presented to Shaoyang Central Hospital (Shaoyang, Hunan, China) in February 2019 and underwent neoadjuvant chemoradiotherapy followed by radical resection for rectal adenocarcinoma in June 2019. After 5 years, he presented with buttock pain in December 2024. Computed tomography (CT) imaging revealed a sacrococcygeal soft tissue mass within the prior radiation field. A biopsy confirmed an undifferentiated sarcoma, histologically distinct from the primary carcinoma, fulfilling modified Cahan criteria for RIS. Despite diagnosis, the patient declined further treatment due to concerns about toxicity and exhibited disease progression. A follow-up examination in June 2025 showed that the masses had increased in number and size. The present case underscores the importance of long-term surveillance in rectal cancer survivors. A re-biopsy of suspicious masses is key to differentiate RIS from recurrence. Early recognition may improve patient outcomes, although current therapeutic options, such as standard radiotherapy and chemotherapy, remain limited. Immunotherapy and targeted therapy are considered as second-line treatment options. However, due to factors such as physical and financial conditions, this patient did not choose to continue the treatment.