This study was designed as a single-center retrospective analysis aimed at investigating whether COVID-19 infection increases the risk or burden of radiation-induced pneumonitis in breast cancer patients.

This study was designed as a single-center retrospective analysis aimed at investigating whether COVID-19 infection increases the risk or burden of radiation-induced pneumonitis in breast cancer patients. Breast cancer patients who underwent radiotherapy at the Cancer Center of Zhejiang Hospital between January 1, 2021, and June 30, 2024, were screened for eligibility. Patients who met the inclusion criteria were identified through the hospital information system, and their clinical records were reviewed. Follow-up was conducted via telephone interviews.

Inclusion Criteria:

1. patients with pathologically or cytologically confirmed primary breast cancer,
2. aged 18 years or older but younger than 80 years,
3. with complete and reliable medical records,
4. for those with COVID-19 infection, documented evidence of a positive test result.

Exclusion Criteria:

1. concomitant pulmonary malignancies,
2. a history of prior thoracic radiotherapy,
3. concurrent pulmonary infections other than COVID-19 during radiotherapy,
4. missing key data with loss to follow-up,
5. or any other condition considered unsuitable for enrollment by the investigators.

Breast cancer patients who underwent radiotherapy at the Cancer Center of Zhejiang Hospital between January 1, 2021, and June 30, 2024, were screened for eligibility.