Bilateral pleural effusion associated with atezolizumab in a patient with Lynch syndrome-related urothelial carcinoma: a case report.

Lynch syndrome (LS) is an inherited syndrome associated with an increased risk of cancer caused by abnormalities in DNA mismatch repair (MMR) genes. Immune checkpoint inhibitors (ICIs) have been reported to lead to a good response in cancers accompanied by LS. However, ICI therapy can cause immune-related adverse events (irAEs). In addition, post ICI treatment, some patients can show a falsely aggravated response, called pseudoprogression, causing difficulties in initial drug response evaluation. A 61-year-old man presented with back and pelvic bone pain. He had a history of surgery for stomach and colon cancer, and his daughter was treated for endometrial cancer. The patient was diagnosed with primary urothelial carcinoma (UC) in the left ureter with adrenal gland and multiple bone metastases. Through next-generation sequencing (NGS), mutations in MLH1 and MSH2 were identified, and diagnosis of LS was confirmed. On the 11th day from the start of atezolizumab, left pleural effusion occurred with exacerbation of the rib metastasis; the amount of effusion increased, and percutaneous catheter drainage (PCD) was performed. On the 27th day, right pleural effusion developed, and drainage was initiated. After the third cycle of atezolizumab, the bilateral pleural fluid decreased, and the drainage tube was removed. Positron emission tomography/computed tomography (PET-CT) revealed improvement in the cancer lesions, including metastatic bone lesions. This is a rare case of bilateral pleural effusion due to pseudoprogression of rib lesions after atezolizumab treatment in a patient with ureter cancer accompanied by LS. UC associated with LS is expected to show a good response to ICI therapy. For proper identification of pseudoprogression, appropriate response evaluation and close monitoring of the side effects are necessary.