The effect of immunotherapy in a young patient with mismatch repair-deficient rectal cancer - a case report.

Background: The incidence of colorectal cancer (CRC) in the age group of young patients has been increasing. Furthermore, in these patients, CRC is more frequently an aggressive histological type of tumor, diagnosed in the late clinical stages of the disease. Another characteristic feature is a higher frequency of mismatch repair deficient (dMMR) tumors, in the treatment of which immunotherapy can be an effective treatment modality, used to prolong overall survival and improve quality of life. Conversely, the effect of chemotherapy may be lower.

Case: We present the case of a 35year-old patient treated with primary therapy for locally advanced dMMR rectal cancer. Neoadjuvant chemoradiotherapy was followed by rectal resection, which was accompanied by serious postoperative complications. In addition, there was an early local relapse of the disease, resistant to systemic chemotherapy treatment. After the progression of the disease, second line of the treatment with pembrolizumab was initiated.

Results: The treatment with pembrolizumab led to a partial regression of the disease and subsequently its stabilization, which has been lasting for 15 months. The improvement of the patient's quality of life, e. g. stabilization of the blood count and regression of diarrhea, is a significant benefit. In addition, the treatment has been given without the development of serious toxicity, so far.

Conclusion: Microsatellite instability testing in the management of locally advanced and metastatic colorectal cancer is of fundamental importance in setting the appropriate treatment procedures. In the future, we can expect results of several studies that will try to prove the effect of immunotherapy not only in metastatic disease, but also in the neoadjuvant regimen in resectable and potentially resectable dMMR rectal and colon cancers.