Dosimetric comparison of two different three-dimensional conformal external beam accelerated partial breast irradiation techniques.

PURPOSE: We compare the dosimetry of two techniques for three-dimensional, conformal, external beam, accelerated partial breast irradiation (3D-CPBI) in the supine position. METHODS AND MATERIALS: Sixteen patients with Stage I breast cancer had PBI treatment plans generated using the multiple, noncoplanar photon field technique and the three-field, mixed-modality technique. Planning target volumes (PTVs; lumpectomy site plus 1.5-2.0 cm margin) and total dose (32 Gy) were held constant to facilitate dosimetric comparisons. Plans were optimized for conformality and PTV coverage.
RESULTS: Mixed-modality plans employed fewer fields than multiple, noncoplanar photon field plans (mean 3.2 vs. 4.1). Both techniques provided comparable PTV coverage and in all cases, 95% of the PTV received 90% of the prescribed dose. Volumes of ipsilateral breast receiving greater than 16 Gy were similar; however, the mean volume of ipsilateral breast receiving 8 Gy was significantly lower for mixed-modality plans (58% vs. 66%). No differences in the volumes of ipsilateral lung or heart receiving greater than 5 Gy were observed, however, the mixed-modality technique delivered 2.5 Gy to larger volumes of these organs.
CONCLUSIONS: Both techniques for supine position, 3D-CPBI provides excellent normal tissue sparing with adequate PTV coverage. The multiple, noncoplanar photon field technique exposes smaller volumes of ipsilateral lung and heart to low dose radiation at the expense of increased plan complexity and larger irradiated breast volumes.