A custom three-dimensional electron bolus technique for optimization of postmastectomy irradiation

Postmastectomy irradiation (PMI) is a technically complex treatment requiring consideration of the primary tumor location, possible risk of internal mammary node involvement, varying chest wall thicknesses secondary to surgical defects or body habitus, and risk of damaging normal underlying structures. In this report, we describe the application of a customized three-dimensional (3D) electron bolus technique for delivering PMI.

A customized electron bolus was designed using a 3D planning system. Computed tomography (CT) images of each patient were obtained in treatment position and the volume to be treated was identified. The distal surface of the wax bolus matched the skin surface, and the proximal surface was designed to conform to the 90% isodose surface to the distal surface of the planning target volume (PTV). Dose was calculated with a pencil-beam algorithm correcting for patient heterogeneity. The bolus was then fabricated from modeling wax using a computer-controlled milling device. To aid in quality assurance, CT images with the bolus in place were generated and the dose distribution was computed using these images.

This technique optimized the dose distribution while minimizing irradiation of normal tissues. The use of a single anterior field eliminated field junction sites. Two patients who benefited from this option are described: one with altered chest wall geometry (congenital pectus excavatum), and one with recurrent disease in the medial chest wall and internal mammary chain (IMC) area.

The use of custom 3D electron bolus for PMI is an effective method for optimizing dose delivery. The radiation dose distribution is highly conformal, dose heterogeneity is reduced compared to standard techniques in certain suboptimal settings, and excellent immediate outcome is obtained.