105) (hot areas) of the reference dose, and the volume of lung receiving ≥30Gy (Vol≥30Gy) were derived from dose volume histograms (DVHs). Dose homogeneity index was calculated as: DHI = 100 – (Vol>105 + Vol<95). In the second stage, each patient was replanned with Cobalt 60 and the quantities of interest were obtained from the DVHs. Results: Comparing 3D treatment planning by 6 MV and Cobalt 60 photons, cold areas were reduced (p<0.001), hot areas were decreased (p<0.7), DHI was improved (P<0.001) and Vol≥30Gy was reduced (P<0.017) with 6 MV photons. Discussion and Conclusion: Our results indicate that treatment planning with Cobalt 60 causes more cold areas in the treated volume as compared with 6 MV photons. Cold areas within the target volume of the breast result in reduced tumor control probability. The plan can be optimized to limit dose to lung volume by using 6 MV photons. So lung fibrosis is more likely to occur with Cobalt 60. In addition, DHI was improved with 6 MV photon beams. Improvement of dose homogeneity can lead to higher tumor control probability and cosmetic results, and less skin and lung side effects. Hence, 3D treatment planning with 6 MV photon beams is a more suitable option for patients with breast cancer treated with conserving surgery]]>
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