Dose to organs in the supraclavicular region when covering the Internal Mammary Nodes (IMNs) in breast cancer patients: A comparison of Volumetric Modulated Arc Therapy (VMAT) versus 3D and VMAT

During breast/chest wall and regional nodal irradiation (RNI), standard 3D conformal techniques can fail to meet the dosimetric constraints for the heart and ipsilateral lung. VMAT can improve the dosimetric sparing of the heart and lungs. However the unnecessary increase in dose to the organs in the supraclavicular region as a result of using VMAT can be avoided. In this work we investigate potential dosimetric advantages of combining 3D with VMAT to improve sparing of these organs. Ten breast cancer patients requiring radiation therapy to the breast/chest wall and RNI including the IMNs, and who did not have a viable 3D conformal plan were chosen for the study. Each patient was planned with VMAT and with a combination of 3D for the supraclavicular region and VMAT for the breast/chest wall followed by a dosimetric comparison. Prescription dose was 50.4 Gy in 28 fractions. For similar coverage to the PTV and IMNs, doses to the esophagus and cord were reduced by 17.8 Gy and 15.5 Gy while mean dose to the thyroid and larynx were also reduced by 16.5 Gy and 11.7 Gy respectively. Maximum brachial plexus dose was the same in both techniques. The ipsilateral lung V(20Gy) increased by 3.1% but was still < 30%. No significant differences were noted in doses to the heart, total lung and contralateral breast. However V(5Gy) to the contralateral lung was reduced by 8.5% with the combined plan. Using 3D conformal planning for the supraclavicular region and VMAT over the breast/chest wall improves sparing of the esophagus, cord, thyroid and larynx while reducing low dose exposure to the contralateral lung and does not compromise doses to the heart, ipsilateral lung and total lung.