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 (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.