Dosimetric advantage of ipsilateral lung and cardiac sparing of left breast cancer prone position compared with supine free breathing in the COVID-19 era and personalized medicine.

Background: As coronavirus disease 19 (COVID-19) run unabated across the globe, its potential survival detrimental effects on lung function may be potentiated by systemic therapy and/or radiotherapy. Limiting lung exposure to high radiation dose in addition to sparing the heart may be critical for left-sided breast cancer patients. Deep inspiration breath-hold allows heart sparing. However, a minority of patients cannot hold breath for radiotherapy. We aim to evaluate whether a prone setup can be advantageous in these patients.

Materials and methods: Left breast cancer patients who had dual supine and prone planning, both in free-breathing, were retrospectively identified. A multiple-structures penalty score was computed from the mean absolute dose deviation (MADD) to heart, lungs, breasts, and tumor bed for each supine and prone plan. Dosimetric advantage of prone was assessed by the reduction of penalty score compared with supine. Patients' characteristics effect on the reduction of penalty was analyzed using robust linear regression.

Results: The prone vs. supine MADD for 27 patients demonstrated significant sparing for the ipsilateral lung and was 0.6 vs. 3 Gy, respectively, without differences regarding heart and target volumes. The average penalty ± standard deviation was 0.90 ± 0.28 Gy prone, vs. 1.13 ± 0.38 Gy supine, p = 0.024. Overall, 70.4% (19/27) patients had a reduction of penalty with prone setup, as compared with 29.6% (8/27) supine, p = 0.0065. Pre-dosimetry characteristics could not predict the reduction of penalty.

Conclusion: Prone conferred substantial lung sparing without dose-deterioration to other structures, providing a significant advantage as compared with supine free-breathing radiotherapy in left-breast cancer patients.