Radiotherapy quality assurance in the Danish breast cancer Group RT Natural trial

Partial breast irradiation (PBI) is standard for selected relatively low-risk breast cancer patients, aiming to minimise late effects while maintaining local control. The Danish Breast Cancer Group (DBCG) RT Natural trial investigated whether PBI could be safely omitted in selected patients. Here, we report the radiotherapy quality assurance in the trial, evaluating target coverage and doses to organs of interest against protocol criteria.

Eligible patients were women ≥ 60 years with T ≤ 20 mm, unilateral, unifocal, node-negative, hormone receptor-positive, HER2-normal, non-lobular invasive breast cancer. Patients were randomised 1:1 to PBI (40 Gy in 15 fractions) versus no radiotherapy. Treatment plans for all patients treated with PBI were prospectively collected in a centralised DICOM-RT database and evaluated for protocol compliance.

Data from the 238 patients receiving PBI were analysed. Dose coverage was high, with median V95% of 100% for the tumour bed and CTVp_PBI and 98% for the PTVp_PBI. Only 1% had a CTVp_PBI V95% below the intended 95%, whereas 22% had a PTVp_PBI V95% <95%. No patients exceeded 110% of the prescribed dose. Dose conformity was good with median whole-breast V40Gy of 21%.Ipsilateral median mean lung dose was 1.52 Gy, and median mean heart dose 0.25 Gy left / 0.12 Gy right with no violation of dose guidance. Respiratory gating did not significantly affect lung or heart doses.

PBI in the DBCG RT Natural trial provided robust target coverage with minimal high-dose volumes, and maintained low cardiac and pulmonary doses, consistent with protocol planning objectives.