Immunotherapy as a promising treatment strategy for dMMR colorectal cancer with brain metastasis: a case report

Brain metastasis in colorectal cancer is a rare occurrence with poor prognosis and limited treatment options. This case report presents a unique and previously unreported case of brain metastasis in a patient with dMMR (DNA mismatch repair-deficient) colorectal cancer. The patient, a 70-year-old male, initially presented with abdominal pain and was diagnosed with moderately differentiated adenocarcinoma of the right colon. Following surgical resection and adjuvant chemotherapy, the patient developed cognitive decline and was found to have a metastatic lesion in the left temporal lobe. Immunohistochemical analysis revealed MSH2 positivity and MSH6, MLH1, and PMS2 negativity, indicating dMMR status. Further genetic testing showed wild-type Kras, Nras, and Braf, and high tumor mutational burden (TMB). The patient was subsequently treated with pembrolizumab immunotherapy, resulting in a significant improvement of symptoms and a reduction in the size of brain metastasis. This case highlights the rarity and challenging management of brain metastasis in colorectal cancer, particularly in the context of dMMR tumors. The successful use of immunotherapy in this case provides valuable insights into the potential efficacy of immune-based treatments for dMMR colorectal cancer with brain metastasis, underscoring the need for further research in this field.