Dosimetric evaluation of incidental irradiation to the axilla during whole breast radiotherapy for patients with left-sided early breast cancer in the IMRT era

The purpose of this study was to compare the dosimetric parameters for incidental irradiation to the axilla during whole breast radiotherapy (WBRT) with 3-dimensional conformal radiotherapy (3D-CRT) and intensity-modulated radiotherapy (IMRT). Twenty left breast cancer patients treated with WBRT after breast-conserving surgery (BCS) were enrolled in this study. Remnant breast tissue, 3 levels of the axilla, heart, and lung were delineated. We used 2 different radiotherapy methods: 3D-CRT with field-in-field technique and 7-field fixed-beam IMRT. The target coverage of IMRT was significantly better than that of 3D-CRT ( D mean : 49.72 ± 0.64 Gy vs 50.24 ± 0.66 Gy, P  < 0.001; V 45 : 93.19 ± 1.40% vs 98.59 ± 0.30%, P  < 0.001; V 47.5 : 86.43 ± 2.72% vs 95.00 ± 0.02%, P  < 0.001, for 3D-CRT and IMRT, respectively). In the IMRT plan, a lower dose was delivered to a wider region of the heart and lung. Significantly lower axillary irradiation was shown throughout each level of axilla by IMRT compared to 3D-CRT ( D mean for level I: 42.58 ± 5.31 Gy vs 14.49 ± 6.91 Gy, P  < 0.001; D mean for level II: 26.25 ± 10.43 Gy vs 3.41 ± 3.11 Gy, P  < 0.001; D mean for level III: 6.26 ± 4.69 Gy vs 1.16 ± 0.51 Gy, P  < 0.001; D mean for total axilla: 33.9 ± 6.89 Gy vs 9.96 ± 5.21 Gy, P  < 0.001, for 3D-CRT and IMRT, respectively). In conclusion, the incidental dose delivered to the axilla was significantly lower for IMRT compared to 3D-CRT. Therefore, IMRT, which only includes the breast parenchyma, should be cautiously used in patients with limited positive sentinel lymph nodes and who do not undergo complete axillary lymph node dissection.