Biological equivalent dose is associated with radiological toxicity after lung stereotactic ablative radiation therapy.

Introduction: Stereotactic ablative radiation therapy (SABR) is the standard of care for inoperable early-stage non-small-cell lung cancer. Although the probability of grade ≥ II toxicities is low, many patients present radiological subclinical toxicities usually associated with long-term patient management challenges. We evaluated radiological changes and correlated them with the received Biological Equivalent Dose (BED).

Methods: We retrospectively analyzed chest CT scans of 102 patients treated with SABR. An experienced radiologist evaluated the radiation-related changes 6 months and 2 years after SABR. The presence of consolidation, ground-glass opacities, organizing pneumonia pattern, atelectasis and the extent of affected lung were recorded. Dose-volume histograms of the lung healthy tissue were transformed to BED. Clinical parameters such as age, smoking habits, and previous pathologies were registered and correlations between BED and radiological toxicities were drawn.

Results: We observed a positive and statistically significant correlation between lung BED over 300 Gy and the presence of organizing pneumonia pattern, the degree of lung affectation and the 2-year prevalence and/or increase of these radiological changes. Radiological changes in patients receiving BED > 300 Gy to a healthy lung volume ≥ 30 cc increased or remained in the 2 years follow-up scan. We found no correlation between radiological changes and the analyzed clinical parameters.

Conclusions: There seems to be a clear correlation between BEDs higher than 300 Gy and radiological changes both short and long term. If confirmed in an independent patient cohort, these findings could lead to the first radiotherapy dose constraints for grade I pulmonary toxicity.