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

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 (IMRTp) and photon IMRT with conformal electron irradiation (IMRTp/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. IMRTp/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 IMRTp/e was the lowest and was equal to that with 3D-CRT. Minimising heart dose with IMRTp resulted in irradiated CB volumes much larger than that with 3D-CRT. With IMRTp/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 IMRTp/e enables a heart dose reduction which can only be obtained with IMRTp when allowing large irradiated volumes in the contralateral breast.