Dosimetric Evaluation of Chest Wall Superficial Dose With or Without Bolus in Postmastectomy Radiotherapy

Abstract

Chest wall bolus use has been standard practice in postmastectomy radiotherapy (PMRT) for patients without skin involvement, resulting in increased toxicity without strong data to show improved outcomes. Centers are moving away from use of bolus in PMRT, but the dosimetric effects have not been quantified. We aim to compare differences in dose between non-bolus and bolus plans to the superficial chest wall. We conducted a retrospective analysis on 30 patients treated with PMRT from December 2018 to June 2019 at our center. Patients received 4256 cGy in 16 fractions to chest wall and regional nodes using 4-field 3D conformal technique, 6 and/or 15 MV photons, and 0.5 cm bolus. The clinically planned datasets were used to generate non-bolus and bolus plans for each patient using the anisotropic analytical algorithm (AAA). The first 10 patients also had non-bolus and bolus plans generated using Acuros XB (AXB). Evaluation volumes were created by extracting surface layers from the clinical chest wall: 3 mm surface, 5 mm surface, and the layer between 3-5 mm depth. The 3 mm and 5 mm surface layers were chosen to match the most common definitions of skin in current breast studies, while the 3-5 mm layer represents the subcutaneous tissue immediately deep to the skin. Mean age, mean chest wall thickness, dermatitis grade level per CTCAE v5.0, and the mean V80%, V95%, and D2cc were reported for all plans. Mean patient age was 56 years (range 36 to 85 years). Mean chest wall thickness was 2.3 cm (1.1 to 6.5 cm). The most common beam energies were 6 and 15 MV photons (70%), followed by 15 MV alone (20%) and 6 MV alone (10%). Most had grade 1 (63%) and 2 (33%) dermatitis to the chest wall and axilla, with one patient developing grade 3 dermatitis. Mean V80%, V95%, and D2cc for the non-bolus and bolus plans were calculated using AAA (n = 30) and AXB (n = 10), as reported in the below table. While large differences exist in superficial layers, the 3-5 mm layer demonstrates consistent coverage for V80% with or without bolus. When AXB calculation is used, the differences are even more minimal in the 3-5 mm layer for both V80% and V95%. This result is expected based on AXB’s superior superficial dose modelling. Our findings support the reduced use of bolus for PMRT patients without risk factors for dermal or dermal-lymphatic involvement.Tabled 1Abstract 2770; Table3 mm surface5 mm surface3-5 mm layerD2cc (%)Non-BolusBolusNon-BolusBolusNon-BolusBolusNon-BolusBolusAAA V80% (%)38.497.662.397.796.498.1107107AAA V95% (%)4.788.322.390.049.594.7AXB V80% (%)60.099.676.999.799.9100109109AXB V95% (%)23.796.249.397.286.298.7 Open table in a new tab