Impact of breathing motion on whole breast radiotherapy: a dosimetric analysis using active breathing control

PURPOSE: The active breathing control (ABC) apparatus was used to quantify the effect of breathing motion on whole breast radiotherapy (RT) with standard wedges and intensity-modulated RT (IMRT). METHODS AND MATERIALS: Ten patients with early-stage breast cancer underwent routine free-breathing (FB) CT simulations for whole breast RT. An ABC apparatus was used to obtain two additional CT scans with the breath held at the end of normal inhalation and normal exhalation. The FB scan was used to develop both a standard treatment plan using wedged coplanar tangents and an IMRT plan using multiple static multileaf collimator segments. To simulate breathing, each plan was copied and applied to the normal inhalation and normal exhalation CT scans. RESULTS: The medial field border (defined by a radiopaque catheter) for the normal inhalation and normal exhalation scans moved an average of 0.6 cm anteriorly and 0.3 cm posteriorly compared with the FB position, respectively. The corresponding movement of the lateral field border was an average of 0.4 cm anteriorly and 0.2 cm posteriorly compared with the FB position. For both the wedged and the IMRT techniques, the dose delivered to breast tissue, biopsy cavity, and ipsilateral lung was similar for each of the three CT scan positions. However, the internal mammary node dose varied significantly with breathing. CONCLUSIONS: The dose delivered to breast using standard wedges or step-and-shoot IMRT is relatively insensitive to the effects of breast motion during normal breathing. However, an appreciable portion of the internal mammary nodes are irradiated during normal inhalation, contributing to the uncertainty in the analysis of the efficacy of internal mammary nodal RT in breast treatment.