Purpose: To describe a novel application of HDR Ir-192 plesiotherapy for post-mastectomy chest-wall boost treatments and investigate the impact of dose heterogeneity corrections on treatment planning. As current published guidelines for skin brachytherapy suggest bolus is unnecessary, the use of bolus was investigated when considering heterogeneities.
Methods: A workflow employing custom skin moulds was used to administer HDR brachytherapy-mediated chest-wall boosts to nine post-mastectomy patients. Moulds were constructed using wax or Freiburg Flap applicators mounted on individually molded thermoplastic frames with up to 2 cm bolus (Elasto-Gel) placed on top. Plans were optimized using TG-43 dose calculation. Average target (PTV) depths were 3-5 mm. All plans were recalculated with a commercial algorithm to correct for heterogeneities (Acuros BV, Varian Medical Systems) considering thicknesses of bolus from 0-2 cm. PTV V90, V95, V100, and mean dose at average target depth, D(MTD), were evaluated on all plans.
Results: When comparing heterogeneous versus homogeneous dose calculation for shallow (3 mm) targets, PTV V90, V95, V100, and D(MTD) were reduced by up to 4.2%, 23.7%, 39.5%, and 5.8% respectively with no bolus and 1.6%, 9.0%, 22.2%, and 3.4% with 2 cm bolus. A similar comparison for deep (5 mm) targets showed maximum reductions in PTV V90, V95, V100, and D(MTD) of up to 23.8%, 34.7%, 33.7%, and 8.2% respectively with no bolus and 7.8%, 16.3%, 17.2%, and 4.2% with 2 cm bolus
Conclusion: The impact of dose heterogeneity corrections was found to be dependent on different factors. Heterogeneity corrections should be used to ensure optimal target dosimetry when treating deep targets, using no bolus, and/or benchmarking plan quality with V95 or higher. With 2 cm bolus, shallow targets, and/or robust benchmarking parameters such as V90 either homogenous or heterogeneous dose calculations may provide sufficient accuracy (within 5%) for planning and evaluating clinical plans.