Utility of Three-dimensional Planning for Axillary Node Coverage with Breast-conserving Radiation Therapy: Early Experience

Abstract

PURPOSE: To examine the dosimetric axillary nodal coverage with standard tangential breast radiation fields and determine the utility of three-dimensional treatment planning for such coverage. MATERIALS AND METHODS: Six consecutive patients who were to undergo whole-breast irradiation underwent computed tomographic scanning with 5-mm sections at the time of treatment simulation. Contours were made with a commercial workstation for the lower axillary tissues, lungs, and heart. Axillary coverage was examined with three-dimensional isodose visualization and dose-volume histograms for four plans for each patient: (a) standard tangential radiation fields designed to cover only the breast, with clinical setup; (b) standard tangential fields with beam's-eye-view optimization of collimator angles for axillary and breast coverage; (c) standard tangential fields with adjustment of field width and collimator angles; and (d) customized fields, by adjusting width, collimator angle, and gantry angle and by using customized blocks. RESULTS: With plan a, only one patient had a simulated mean axillary dose greater than 90% of that prescribed. Underdosing occurred primarily in the posterior-superior axillary nodal region. Plan b improved axillary coverage; five patients had a simulated mean axillary dose of 89% or more of the prescribed dose, with adequate whole-breast coverage and no increased pulmonary or cardiac doses. Adjusting the field width and gantry angle further improved simulated mean axillary doses; however, customized blocking was then required to avoid increased mean pulmonary and cardiac doses and unacceptable contralateral breast doses. CONCLUSION: When coverage of lower axillary nodal tissue is desired at breast irradiation, three-dimensional planning with beam's-eye-view adjustment of tangential fields should be considered.