To characterize the effect of the relative motion of esophagus and tumor on radiation doses to the esophagus in patients treated with stereotactic body radiation therapy for central lung tumors.

Fifty fractions of stereotactic body radiation therapy in 10 patients with lung tumors within 2.5 cm of the esophagus were reviewed. The esophagus was delineated on each treatment's cone-beam computed tomography scan and compared to its position on the planning scan. Dose-volume histograms were calculated using the original treatment beams to determine the actual dose delivered to the esophagus for each fraction of stereotactic body radiation therapy.

Median interfraction right-left shift of the esophagus was 0.9 mm (range, -5.4 to 3.3 mm) toward the left. Median interfraction anteroposterior shift was 0.7 mm (range, -3.7 to 11.5 mm) posteriorly. The median percentage increase in dose to 1 cm3, dose to 3.5 cm3, and dose to 5 cm was 1.7%, 5.6%, and 6.6%, respectively. Two cases of significant late esophageal toxicity were observed, with change in esophageal position relative to the planning target volume resulting in significantly higher D values than anticipated.

Interfraction shifts between the internal target volume and esophagus can lead to unanticipated increases in the volume of esophagus receiving high doses when treating central lung tumors with stereotactic body radiation therapy. Certain practical steps, such as considering deep breath hold for internal target volume reduction, using a planning risk volume for esophagus, and carefully visualizing and considering esophageal position at the time of stereotactic body radiation therapy, can be taken to minimize unanticipated dose increases that could cause unexpected esophageal toxicity.