Selection criteria for high-dose-rate surface brachytherapy and electron beam therapy in cutaneous oncology

Introduction
High-dose-rate (HDR) brachytherapy is an alternative treatment to electron external beam radiation therapy (EBRT) of superficial skin lesions. The purpose of this study was to establish the selection criteria for HDR brachytherapy technique (HDR-BT) and EBRT in cutaneous oncology for various clinical scenarios.

Material and methods
The study consists of two parts: a) EBRT and HDR-BT treatment plans comparison analyzing clinical target volumes (CTVs) with different geometries, field sizes, and topologies, and b) development of a prediction model capable of characterization of dose distributions in HDR surface brachytherapy for various geometries of treatment sites.

Results
A loss of CTV coverage for the electron plans (D90, D95) was recorded up to 45%, when curvature of the applicator increased over 30. Values for D2 cm3 for both plans were comparable, and they were in range of ±8% of prescription dose. An increase in higher doses (D0.5 cm3 and D0.1 cm3) was observed in HDR-BT plans, and it was greater for larger lesions. The average increase was 3.8% for D0.5 cm3 and 12.3% for D0.1 cm3. When CTV was approximately flat, electron plans were comparable with HDR-BT plans, having lower average D2 cm3, D0.5 cm3, and D0.1 cm3 of 7.7%. Degradation of quality of electron plans was found to be more dependent on target curvature than on CTV size.

Conclusions
Both EBRT and HDR-BT could be used in treatments of superficial lesions. HDR-BT revealed superior CTV coverage when the surface was very large, complex, curvy, or rounded, and when the topology was complicated. The prediction model can be used for an approximate calculation and quick assessment of radiation dose to organs-at-risk (OARs), at a depth or at a lateral distance from CTV.