Clinicoradiological course of abemaciclib-induced pneumonitis with histology findings.

A woman in her late 40s presented with multiple abnormal shadows on high-resolution CT (HRCT), was treated with abemaciclib for recurrent right breast cancer post-surgery and chemoradiation therapy. During the 10-month chemotherapy, HRCT revealed a recurrent pattern of a partly appearing and disappearing organising pneumonia pattern without clinical symptoms. Bronchoalveolar lavage analysis revealed lymphocytosis, while transbronchial lung biopsy revealed alveolitis with epithelial cell injury. Based on the diagnosis of drug-induced pneumonitis due to abemaciclib, the discontinuation of abemaciclib and administration of prednisolone were effective. Abnormal shadow on HRCT disappeared gradually, while elevated Krebs von den Lungen (KL)-6 and surfactant protein (SP)-D levels were restored to normal range. This is the first case report of abemaciclib-induced pneumonitis with histology findings. Since the severity of abemaciclib-induced pneumonitis ranges from mild to fatal, regular monitoring of pneumonitis with radiography, HRCT, and measurement of KL-6 and SP-D levels should be considered.