Axillary Lymph Node Coverage in Breast Cancer Patients Treated with Adjuvant Radiation Using High Tangent Fields Technique: A Single Institution’s Experience

Abstract

Background: The high tangent field (HTF) technique is used to provide radiation coverage of the inferior axillary nodal levels for breast cancer patients when the lower axilla is at risk for micrometastatic disease. Despite its use in clinical practice, there is concern about whether HTF provide sufficient coverage of level I and II axillary nodal regions. The purpose of this study is to quantify and evaluate the coverage of HTF at our institution. Methods: Patients diagnosed with early invasive breast carcinoma (pT1-2 pN0-1a) who received HTF radiation between January 1st, 2012 and December 31st, 2016 were retrospectively reviewed. Level I and II axillary nodal regions were contoured on each patient’s simulation CT. Dosimetric parameters were re-calculated to evaluate coverage. Statistical analysis was conducted using Mann-Whitney-U method.  Results: Thirty-seven patients with low-risk breast adenocarcinoma were included. For level I and II, the mean V90% was 94.63% ± 7.60% and 73.33% ± 21.83% respectively. Twenty-nine patients received adequate V90% coverage of level I and had a mean level II V90% of 76% ± 18.71% while eight patients who did not receive adequate V90% level I coverage had a mean level II V90% of 63.64% ± 30.22%. The median level II V90% of patients receiving adequate and inadequate level I V90% was 77.74% and 71.11% respectively; the difference was not statistically significant (P = 0.33). Conclusion: HTF provides adequate coverage for level I nodes, but inadequate coverage for level II. Contouring nodal volumes may assist field placement and improve nodal volume coverage.