Breast-Conserving Therapy: Radiotherapy Margins for Breast Tumor Bed Boost

Purpose: To quantify the interfraction position variability of the excision cavity (EC) and to compare the rib and breast surface as surrogates for the cavity. Additionally, we sought to determine the required margin for on-line, off-line and no correction protocols in external beam radiotherapy. Methods and Materials: A total of 20 patients were studied who had been treated in the supine position for 28 daily fractions. Cone-beam computed tomography scans were regularly acquired according to a shrinking action level setup correction protocol based on bony anatomy registration of the ribs and sternum. The position of the excision area was retrospectively analyzed by gray value cone-beam computed tomography-to-computed tomography registration. Subsequently, three setup correction strategies (on-line, off-line, and no corrections) were applied, according to the rib and breast surface registrations, to estimate the residual setup errors (systematic [Σ] and random [σ]) of the excision area. The required margins were calculated using a margin recipe. Results: The image quality of the cone-beam computed tomography scans was sufficient for localization of the EC. The margins required for the investigated setup correction protocols and the setup errors for the left-right, craniocaudal and anteroposterior directions were 8.3 mm (Σ = 3.0, σ = 2.6), 10.6 mm (Σ = 3.8, σ = 3.2), and 7.7 mm (Σ = 2.7, σ = 2.9) for the no correction strategy; 5.6 mm (Σ = 2.0, Σ = 1.8), 6.5 mm (Σ = 2.3, σ = 2.3), and 4.5 mm (Σ = 1.5, σ = 1.9) for the on-line rib strategy; and 5.1 mm (Σ = 1.8, σ = 1.7), 4.8 mm (Σ = 1.7, σ = 1.6), and 3.3 mm (Σ = 1.1, σ = 1.6) for the on-line surface strategy, respectively. Conclusion: Considerable geometric uncertainties in the position of the EC relative to the bony anatomy and breast surface have been observed. By using registration of the breast surface, instead of the rib, the uncertainties in the position of the EC area were reduced