SU‐FF‐T‐46: Dosimetric Comparison of Superficial X‐Rays and a Custom HDR Surface Applicator for the Treatment of Superficial Cancers

Purpose: The purpose of this study was to design and characterize a custom HDR applicator for the treatment of cancers in the region 0–5 mm, comparing it to the dosimetric properties of the Maxima R‐100 superficial x‐ray unit. Accelerator produced electrons of energy 4–6 MeV with the addition of a buildup bolus are in competition with superficial X‐rays and are very well suited for the treatment of shallow lesions as the maximum dose, D_max, is brought to the surface. With this applicator, HDR may be a modality suited for treatment of Basal and squamous cell carcinomas, Kaposi's sarcoma, and Merkel cell carcinomas. Method and Materials: The superficial machine was characterized in terms of percent depth dose, beam profiles, and dose rate. Similar measurements were taken from a Varian Varisource HDR unit applying a newly designed copper HDR applicator. Both data sets were compared in terms of dosimetric properties and clinical efficacy. Results: The applicator alone had an unacceptably large dose inhomogeneity across the the HDR beam so a primary filter, modified filter, and secondary lead collimator were designed and manufactured to help shape the dose into a profile similar to the superficial applicator. Beam flatness and width were nearly replicated but beam divergence was more pronounced for the HDR applicators. Conclusions: Percent depth dose measurements for the HDR applicator showed a dose fall‐off nearly identical to superficial x‐rays rather than the assumed inverse square dose fall‐off. Flatenning the HDR beam through this filtered applicator provides means to deliver uniform dose to surface lesions with superior treatment delivery while implementing the HDR billing codes.