Radiation Pneumonitis, Really? A Case of Pulmonary Toxicity from CDK4/6 Inhibitor

Background/Aim: Radiation pneumonitis is a known complication of radiotherapy. It is also a rare complication of CDK4/6 inhibitors, and it can be difficult to differentiate the two. This is a report of a case of pulmonary toxicity from a CDK4/6 inhibitor, which was initially ascribed to radiation pneumonitis. Case Report: A 77-year-old female was diagnosed with pneumonitis after receiving radiation to the thoracic spine. She had also been treated with abemaciclib. Upon review, the patient's lung mean dose was 11.54 Gy with a V20 of 17.02%, and the area of pneumonitis was largely outside of the treatment field. Abemaciclib was ceased. The patient was started on supportive oxygen as well as steroids. She no longer required oxygen and she was discharged from the hospital. Radiation pneumonitis is largely correlated with the volume of lung radiated and dose of radiation to the lung. CDK4/6 inhibitor pulmonary toxicity, while rare, is possible and will likely become more frequent with increasing use of these agents. Conclusion: Patients receiving CDK4/6 inhibitors are at an increased risk for pneumonitis. It can be confused with radiation pneumonitis and must be included in the differential diagnosis.