# Cyclin-Dependent Kinase 4/6 Inhibitors Combined with Radiotherapy in Curative Breast Cancer Patients Induced Pneumonitis: A Case Report

## TL;DR

A breast cancer patient developed lung inflammation after receiving radiation therapy and a CDK4/6 inhibitor, highlighting a potential safety concern.

## Contribution

This case report highlights pneumonitis as a potential adverse effect of combining CDK4/6 inhibitors with radiotherapy in curative breast cancer treatment.

## Key findings

- A patient developed grade 2 pneumonitis five months after receiving radiation therapy and abemaciclib.
- Pulmonary toxicity is a concern when combining RT and CDK4/6 inhibitors in curative breast cancer treatment.
- Asian populations may be particularly susceptible to this adverse effect.

## Abstract

Background: The role of CDK4/6 inhibitors (CDK4/6i) has expanded from the treatment of advanced breast cancer to early-stage disease, as recent studies have demonstrated their therapeutic benefits. However, evidence regarding the safety of combining CDK4/6i with adjuvant radiation therapy (RT) in a curative setting remains limited. This study aims to present clinical experiences of pulmonary toxicity following the combined use of adjuvant RT and CDK4/6i. Case presentation: We report a case of an Asian female with left breast cancer who underwent a modified radical mastectomy followed by adjuvant chemotherapy, RT, endocrine therapy, and CDK4/6i (abemaciclib) treatment. Cancer therapy-induced grade 2 pneumonitis was impressed by clinical signs and image findings. A 57-year-old postmenopausal woman was diagnosed with left breast invasive lobular carcinoma, hormone receptor–positive, human epidermal growth factor receptor 2–negative (HR+/HER2−), K67 index of 5–10%, and classified as pT3N3aM0 (stage IIIC). She received adjuvant chemotherapy with FEC followed by docetaxel, endocrine therapy with letrozole, and adjuvant RT of 50.4 Gy in 28 fractions to the left chest wall and regional nodal irradiation. Abemaciclib was initiated after completing RT. Treatment-related pneumonitis developed five months after RT and abemaciclib use. Conclusions: In breast cancer patients receiving a combination of RT and CDK4/6i as curative adjuvant treatment, pulmonary toxicity is a concern and requires careful monitoring, particularly in Asian populations.