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.