Transient atelectasis due to hilar lymph node swelling affected by lenalidomide-induced tumor flare reaction.

Tumor flare reaction (TFR) is a unique immune-mediated tumor recognition phenomenon presenting as rapid enlargement of the tumor, which mimics disease progression, developing in the early stage of treatment using immunomodulatory drugs or immune checkpoint inhibitors. A 59-year-old man with follicular lymphoma had residual tumor burden in the left hilar lymph nodes after R-CHOP therapy, and received lenalidomide and rituximab (R) therapy. He developed respiratory distress on day 11 of R therapy. Chest X-ray and CT demonstrated left lung atelectasis due to left hilar lymph node swelling. We performed transbronchial lung biopsy on day 20 of R therapy. The biopsied left bronchus tissue exhibited extensive necrosis, which had a B-cell phenotype consistent with that of follicular lymphoma. Neither NK cells nor cytotoxic T cells were detected. It was unclear whether the immune effector cells disappeared at the time of transbronchial lung biopsy. Atelectasis in our patient improved by continuing R therapy beyond TFR.