Intra- and interfraction breathing variations during curative radiotherapy for lung cancer

Background and purposeThis study aimed at quantifying the breathing variations among lung cancer patients over full courses of fractionated radiotherapy. The intention was to relate these variations to the margins assigned to lung tumours, to account for respiratory motion, in fractionated radiotherapy.Materials and methodsEleven lung cancer patients were included in the study. The patients’ chest wall motions were monitored as a surrogate measure for breathing motion during each fraction of radiotherapy by use of an external optical marker. The exhale level variations were evaluated with respect to exhale points and fraction-baseline, defined for intra- and interfraction variations respectively. The breathing amplitude was evaluated as breathing cycle amplitudes and fraction-max-amplitudes defined for intra- and interfraction breathing, respectively.ResultsThe breathing variations over a full treatment course, including both intra- and interfraction variations, were 15.2 mm (median over the patient population), range 5.5–26.7 mm, with the variations in exhale level as the major contributing factor. The median interfraction span in exhale level was 14.8 mm, whereas the median fraction-max-amplitude was 6.1 mm (median of patient individual SD 1.4). The median intrafraction span in exhale level was 1.6 mm, and the median breathing cycle amplitude was 4.0 mm (median of patient individual SD 1.4).ConclusionsThe variations in externally measured exhale levels are larger than variations in breathing amplitude. The interfraction variations in exhale level are in general are up to 10 times larger than intrafraction variations. Margins to account for respiratory motion cannot safely be based on one planning session, especially not if relying on measuring external marker motion. Margins for lung tumours should include interfraction variations in breathing.