Minimising contralateral breast dose in post-mastectomy intensity-modulated radiotherapy by incorporating conformal electron irradiation

Abstract

Purpose To assess the potential benefit of incorporating conformal electron irradiation in intensity-modulated radiotherapy (IMRT) for loco-regional post-mastectomy RT. Patients and methods Ten consecutive patients that underwent left-sided mastectomy were selected for this comparative planning study. Three-dimensional conformal radiotherapy (3D-CRT) photon–electron dose plans were compared to photon-only IMRT (IMRT p) and photon IMRT with conformal electron irradiation (IMRT p/e). The planning target volume (PTV) was prescribed 50 Gy and included the chest wall and the internal mammary and supra-clavicular lymph node regions. It was attempted to minimise dose delivered to heart, lungs and contralateral breast (CB), while maintaining adequate PTV coverage. Results All plans complied with objectives for PTV coverage. IMRT p/e eliminated volumes receiving ⩾70 Gy (V70) that were present in 3D-CRT at the junction of photon and electron beams. Both IMRT strategies reduced heart V30 significantly below 3D-CRT levels. Mean heart dose with IMRT p/e was the lowest and was equal to that with 3D-CRT. Minimising heart dose with IMRT p resulted in irradiated CB volumes much larger than that with 3D-CRT. With IMRT p/e, CB dose was only slightly increased when compared to 3D-CRT. Mean lung dose values were similar for IMRT and 3D-CRT. With IMRT, lung V20 was smaller, whereas V5 values for heart, lung and CB were higher than those with 3D-CRT. Conclusions Incorporation of conformal electron irradiation in post-mastectomy IMRT p/e enables a heart dose reduction which can only be obtained with IMRT p when allowing large irradiated volumes in the contralateral breast.