Dosimetric Comparison of Four Different External Beam Partial Breast Irradiations: Proton, Tomotherapy, Intensity Modulated and 3D Conformal Radiation Therapy

To compare the dosimetry of four different external beam partial breast irradiation (PBI) plans of proton, tomotherapy, intensity-modulated radiation therapy (IMRT) and 3D conformal radiation therapy (3D-CRT). Ten breast cancer patients were included and the plans were developed for each patient using each methods of PBI. The PTV was constructed as a uniform expansion of 1.5 cm margin from lumpectomy site marked with surgical clips. To facilitate dosimetric comparisons, total dose were held constant with 30 Gy. Proton and tomotherapy plans provided better homogeneity indices (0.21, 0.18, respectively) compared with IMRT and 3D-CRT (0.44, 0.29, respectively). Conformity index of tomotherapy (1.14) was better than others (proton 1.31, IMRT 1.30, 3D-CRT 1.26). Coverage index was acceptable in all plans of ≥0.95. The average percentage of the PTV receiving 95% of the prescribed dose (PD) was 100%, 99%, 100%, and 98% for proton, tomotherapy, IMRT and 3D-CRT, respectively. The total volume of ipsilateral breast receiving 75% and 50% of PD was highest in the 3D-CRT (41% and 53%, respectively) compared with others, among which proton plan was best (32% and 36%, respectively). Proton also reduced the volume of non-PTV breast tissue receiving 50% of PD significantly by an average of tomotherapy (7%), IMRT (10%) and 3D-CRT (16%). In lung and heart dosimetry, proton was best and tomotherapy was worst. The ipsilateral lung volume percentage receiving 5 and 10 Gy (V5 Gy and V10 Gy) was significantly lower in proton (1.7%, 0.9%) compared with tomotherapy (20.7%, 4.5%), IMRT (4.3%, 1.1%) and 3D-CRT (8.1%, 3.5%). The heart V5 Gy and V10 Gy of proton was negligible (0.15%, 0.04%), but tomotherapy plan showed high V5 Gy and V10 Gy of 26.3% and 7.2%, respectively. Though four techniques showed acceptable coverage of PTV, proton provides excellent normal tissue sparing. Tomotherapy does not seem to be suitable for PBI with unacceptably high dose exposure of heart and lung.