Impact of a large breast separation on radiation dose delivery to the ipsilateral lung as result of respiratory motion quantified using free breathing and 4D CT-based planning in patients with locally advanced breast cancers: a potential for adverse clinical implications.

Purpose: We examined the effects of large breast separation (BS) on dosimetric and positional differences of radiation treatment plans of locally advanced breast cancers during a free-breathing respiratory cycle. Materials and Methods: Computed tomography (CT) datasets of 18 patients were acquired using 3D, 4D-0% (end-inspiration), and 4D-50% (end-exhalation). BS was examined in relation to the positional and dosimetric changes to organs-at-risk (OAR). Based on dosimetric analysis of all three plans, we compared 4D-0% and 4D-50% for V-5, V-10, and V-20 to 3D for heart and ipsilateral lung. Using 4D-0% and 4D-50% CTs, we recorded positional variations of the organs' centroid (centers of mass) and their effects on dosimetry. Results: Median BS was 23.95 cm (range: 16.86-29.48 cm). There was a strong positive correlation between BS and dose to the ipsilateral lung for V-5, V-10, and V-20, with the greatest linearity observed at V-20 (R-2 = 0.23). At BS >= 27 cm, the dose increased during inspiration at V-5, V-10, and V-20 (P < 0.05). When comparing 4D and 3D for the heart, V-5 and V-10, decreased by average of 0.94% and 0.96% (P < 0.008), respectively. Conclusions: This study offers the first evidence of the impact of a large BS on radiation dose to the ipsilateral lung.