Validating Superficial Radiotherapy Dosimetry: Beyond Manual Dose Calculations
Abstract
Purpose
To provide guidance regarding the selection of superficial kilovoltage (kV) photon radiotherapy as a treatment option for skin cancer patients, by developing a programme of Monte Carlo dose calculations and 3D-printed verification phantom production.
Methods
Patient dose calculation models were created using patient CT imaging or by combining patient 3D optical surface scanning with registered CT scans from a library of pre-existing images, to obtain an accurate indication of surface geometry in combination with an approximate indication of the locations of underlying bones and air cavities. A commissioned model of the WOmed T300 kV unit was used to perform EGSnrc Monte Carlo dose calculations with treatment geometries defined using the OrthoPlan user code (Nikandrov et al., doi: 10.1002/acm2.13832). Physical patient models were 3D-printed to enable film measurements to be performed in and around treatment areas.
Results
Fifteen sample cases have now been calculated with full Monte Carlo simulations of patient anatomy and treatment geometries, with three calculations so far validated using film in patient-specific 3D-printed measurement phantoms. For example, a 20% decrease in dose from the treated side of the nose to the contralateral side of the nose was retrospectively calculated with Monte Carlo and measured with film, for one case where a simple manual calculation had predicted a 34% decrease in dose and yet an anomalous skin reaction had been observed. Where available, the superficial dose calculations were also compared with corresponding VMAT treatment plans, for use as a familiarisation tool for radiation oncologists considering kV therapy as a treatment option.
Conclusion
The simulation of superficial dose distributions has allowed the production of an educational handbook as a reference for radiation oncologists, when considering which modality to use for treatments.