Abstract P4-16-13: Relationship between coverage of axillary lymph nodes, with tangential breast irradiation, and body mass index

Abstract

Abstract Background: The ACOSOG Z0011 trial confirmed axillary clearance is not necessary in select, clinically lymph node-negative women with positive sentinel lymph node biopsies (SLNBs) who undergo breast-conserving surgery(BCS) or receive whole-breast radiotherapy(RT) and systemic therapy. The results suggested that the receipt of adjuvant breast RT and systemic therapy is adequate for loco-regional control. But it was unknown whether the low regional recurrence rates in these patients were caused by the treatment of a portion of the axilla with the tangent fields. In any case, axillary lymph node dose with tangential breast irradiation differs for each patient. It could depend on the proportions or body mass index(BMI) of each patient. If the difference of BMI impacts axillary coverage in breast RT, it could also influence loco-regional control. The purpose of this study was the dosimetric analyses of the axillary coverage with standard breast tangential irradiation, by BMI categories. Methods and materials: Between April 2007 and September 2011, 256 breast cancer patients were treated with breast tangential irradiation after BCS. They were classified using BMI categories as follows: underweight (<18.5), normal weight (18.5–24.9), overweight (25.0–29.9) and obese (≥30). From all the patients, 35 patients (10 in each category, but only 5 in obese due to the small-number) were randomly selected. In these 35 cases, after contouring the level of the axillary lymph nodes (Level Ior II) on CT images, the irradiation mean dose to the axillary level I/II and the percentage of axillary volumes that received at least 95% of the prescribed dose (VD95) were analyzed using the dose-volume histograms for each patient. Results: The median patient age at diagnosis was 54 years (range, 32–77 years). All the patients had undergone BCS without axillary clearance. Mean doses to level I were 39 Gy (77% of prescribed dose) for obese, 37 Gy (74%) for overweight, 32 Gy (65%) for normal-weight, and 29 Gy (58%) for underweight, respectively. Whereas the mean VD95% of axillary level I were 55% for obese, 47% for overweight, 31% for normal-weight, and 19% for underweight, respectively. No patient had adequate coverage of level II regardless of BMI category. Although the irradiation doses to axillary lymph node were relatively high for the high BMI groups, the tangential field designed to treat only the breast did not adequately cover the axillary region and, therefore, cannot be relied upon for prophylactic therapy of the axilla. In this study, BMI among 256 Japanese women ranged from 15.8 to 34.2 (mean, 22.3). Only 19% of the women were overweight or obese, with only 3% classified as obese. Whereas, in the NSABP B-14 trial, BMI among American women ranged from 13.8 to 55.2 (mean, 26.2). Approximately 50% of the women were overweight or obese, with 20% classified as obese. Conclusion: Standard tangential breast irradiation, especially for patient of low BMI, results in substantially reduced dose to the level I axilla. Further specific treatment planning and beam arrangement are required to irradiate axillary lymph node with the therapeutic dose. The difference in BMI by regional physical characteristics may impact on axillary coverage in breast RT. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-16-13.