Comparison of wedge versus segmented techniques in whole breast irradiation: effects on dose exposure outside the treatment volume.

PURPOSE: To compare two irradiation techniques for whole breast irradiation: tangential wedged beams (WT) versus "open" fields (without wedges) with forward planned segments (ST). PATIENTS AND METHODS: For 20 patients two comparative 3-D plans were defined using Pinnacle P3D and analyzed with respect to dose, dose homogeneity in the target volume, and scattered dose to organs at risk. The plans of six patients were reproduced in an Alderson phantom. Measurements were performed in the planning target volume (PTV), contralateral breast, lungs, heart, thyroid gland and in mid-pelvis.
RESULTS: Dose distribution in the PTV was nearly identical for WT and ST with the exception of D(1). Scattered doses were significantly smaller for ST. In the contralateral breast the doses per 2-Gy fraction were 7.3 cGy +/- 2.1 cGy (WT), and 4.7 cGy +/- 1.9 cGy (ST; p < 0.01). Similar doses were measured for lung and heart. In mid pelvis the largest difference was observed (WT: 1.0 cGy +/- 0.2 cGy, ST: 0.2 cGy +/- 0.1 cGy; p < 0.01).
CONCLUSION: Partial volume segments can replace wedges for improved dose coverage and homogeneity in the PTV. The ST causes significantly less scattered dose to extra-target organs. This may have implications for long-term risks after exposure to low radiation doses.