Estimating the volume of lung irradiated during tangential breast irradiation using the central lung distance

The aim of this study was to evaluate the central lung distance (CLD) as a surrogate for the volume of lung irradiated during tangential breast radiotherapy. 20 women underwent a planning computed tomography (CT) scan and three-dimensional (3D) treatment planning for both breasts. The field size was perturbed in a systematic manner to give a number of plans with the CLD increasing from 0 to 30 mm. The volume of lung irradiated was determined directly using dose-volume histograms of the lung and correlated with the known CLD. The results indicate that absolute and percentage lung volumes increase with increasing CLD according to a quadratic relationship which is different for both left and right lungs. It is concluded that while there is no substitute for a 3D plan and a lung dose-volume histogram calculation, CLD may be used as a guide to the volume of lung included within the tangential fields used for breast radiotherapy.