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 (R2) therapy. He developed respiratory distress on day 11 of R2 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 R2 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 R2 therapy beyond TFR.