Dosimetric Comparison of 3D Tangential Radiotherapy of Post-Lumpectomy Breast at Two Different Energies

Introduction: Radiation therapy following breast conserving surgery is one of the most common procedures performed in any radiation oncology department. A tangential parallel-opposed pair is almost always the technique of choice for this purpose. This technique is often performed based on 3D treatment planning. The aim of this study was to compare 3D treatment planning for two different energies (Cobalt 60 versus 6 MV photon beams) in tangential irradiation of breast conserving radiotherapy. In this comparison, homogeneity of isodoses within the breast volume and dose received by lungs were considered. Materials and Methods: In this study, twenty patients with breast cancer treated with conservative surgery were included. A CT scan was performed on selected patients. Three-dimensional treatment planning with 6 MV photon beams was carried out for patients on the  Eclipse 3D treatment planning system (TPS). The volumes receiving lower than 95% (Vol<95) (cold areas) and greater than 105% (Vol> 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

Keywords:

Breast Cancer , Cobalt 60 , Tangential Irradiation , 3D Treatment Planning , 6MV Photon Beam