Objectives
Anaplastic thyroid carcinoma (ATC) is one of the rarest and most aggressive malignant tumors. In the last few years, the treatment paradigm for anaplastic carcinoma has changed dramatically with the introduction of targeted and immunotherapy. Recently, protocols have emerged for the treatment of BRAF-negative ATC with a combination of lenvatinib and pebrolizumab. Pembrolizumab has a lower incidence of diarrhea and colitis compared to other immune checkpoint inhibitors (ICI). And we were unable to find clinical cases describing the relationship between the use of pembrolizumab and Clostridium difficile infection (CDI).

Methods
We presents the case of a 54-year-old male patient on pembrolizumab with ATC stage IVB who presented with diarrhea (up to 35 times a day).

Results
Stool sample was positive for toxin A and B of CDI and was negative for over pathogens. The level of fecal calprotectin was above 1800 μ/g. A colonoscopy with biopsy showed pancolitis. Pembrolizumab was  discontinued, and the patient was managed with Vancomicin per os 500 mg 4 times a day, IV fluid. With the proposed therapy, improvement in the condition occurred only on the 5th day, when the number of defecations decreased to 4 per day. Therapy was continued for up to 10 days. After stabilization of the patient's condition therapy with Pembolizumab was resumed. CDI relapsed rapidly, but this time clinical improvement occurred earlier after initiation of similar therapy.

Conclusions

Previously, it was believed that CDI was transmitted from a sick person only in a hospital setting. However community reservoirs have recently been recognized as the the most likely source of colonization. This clinical observation is important for determining the need for testing for CDI in patients with ICI therapy and colitis signs. CDI it can persist for a long time with periodic exacerbations in the presence of provoking factors.