Partial response to posterior line immunotherapy for more than 15 months in a pMMR patient with cutaneous metastasis of rectal carcinoma: a case report.

The vast majority of colorectal cancers (CRCs) are proficient mismatch repair (pMMR) and microsatellite stable, and their immune microenvironment appears as a "cold tumor," which is not sensitive to single immunotherapy based on immune checkpoint inhibitors (ICIs). The utilization of ICIs in pMMR advanced CRC is still in the exploratory phase. Cutaneous metastasis from colorectal carcinoma is extremely rare, presenting with diverse clinical manifestations, and there is a lack of standard treatment options for such cases. Patients with skin metastasis from CRC usually progress rapidly and are associated with a dismal prognosis. Herein, we report the case of a 66-year-old woman with extensive cutaneous metastasis of pMMR advanced rectal carcinoma. The patient presented to the abdominal oncology clinic with a complaint of erythema on the right lower limb, perineum, and abdominal skin. The patient underwent radical surgery for rectal carcinoma 3 years before the presentation. The histologic examination revealed low-grade squamous cell subepithelial adenocarcinoma. The patient was treated with sintilizumab in combination with fruquintinib, which exhibited remarkable efficacy and improved the patient's quality of life significantly. Previous cases of cutaneous metastasis of colorectal carcinoma were retrieved to characterize the clinicopathological features. For the rare subset of patients with skin metastasis from CRC, immunotherapy combined with anti-angiogenic targeted therapy may be considered.