Comparison of 3D-Conformal Planning Using Partially Wide Tangents and Direct Photon/electron Portals for Breast Radiotherapy with Internal Mammary Nodes Inclusion: A Dosimetric Analysis

Background: Internal mammary lymph nodes (IMNs) are a potential site of metastasis for breast cancer. Targeting IMNs as part of a comprehensive regional nodal irradiation comes at the cost of higher unwanted doses to critical nearby organs at risk. Thus, the efficacy and safety of different radiotherapy techniques for the coverage of this area remain elusive.Objectives: We present a dosimetric comparison between partially wide tangents (PWT) and direct photon/electron (P/E) portals in terms of target volume coverage and normal tissue sparing.Methods: Patients with left-sided breast cancer, who were referred to our clinic for post-lumpectomy or post-mastectomy radio-therapy, underwent computed tomography (CT) simulation. The left breast and IMNs, heart, lung, right breast, and esophagus were contoured. Dosimetric comparisons were based on dose-volume histograms (DVHs) generated for all of the aforementioned organs. A subgroup analysis was also performed based on patients' type of surgical treatment.Results: A total of 30 patients (10 with breast conserving surgery and 20 with modified radical mastectomy) were included. The P/E plan provided a higher coverage of the left breast (P-value of CTV V105%: < 0.001) and IMNs with a P-value of 0.087 regarding the mean dose received by IMNs, and also less volume of the heart (P-value of Heart V30Gy: 0.021), and lungs (P-value of Lung V20Gy: 0.003) were irradiated. However, these advantages came at the cost of a higher dose to the esophagus and right breast and more hotspots compared to the PWT technique.Conclusions: The P/E technique had advantages regarding target volume coverage and toxicity regardless of the type of surgery. Based on the results of this study, overall, the P/E portal is superior to the PWT for radiotherapy of breast cancer with internal mam-mary node inclusion. However, the appropriate treatment plan should be decided on a case-by-case basis.