Case Report: A rare solitary peroneus longus metastasis from rectal cancer managed with multimodal therapy

Solitary skeletal muscle metastasis from rectal cancer is exceptionally rare and carries a generally poor prognosis, with no established treatment guidelines currently available. We present a case of a patient with advanced rectal cancer (T3cN2bM0) who refused recommended neoadjuvant chemoradiotherapy and underwent laparoscopic radical resection alone. Subsequently, the patient developed an isolated metastasis to the left peroneus longus muscle merely three months postoperatively. The metastasis presented as a painful mass, and ¹⁸F-Fluorodeoxyglucose positron emission tomography/ computed tomography (¹⁸F-FDG PET/CT) revealed its hypermetabolic nature, suggesting malignancy. This suspicion was confirmed histopathological, establishing the diagnosis of skeletal muscle metastasis from rectal adenocarcinoma. Following a combined treatment of surgical resection and systemic chemotherapy, the patient remarkably remained free of local recurrence or further metastasis throughout a 30-month follow-up period. This case underscores the importance of clinical alertness for rare skeletal muscle metastases and highlights that individualized treatment is essential for improving patient prognosis.