Session Item

Electronic brachytherapy (eBT) is a cost-effective radiotherapeutic modality for the treatment of skin lesions or intraoperative partial breast irradiation among others. The present calibration of these sources relies on indirect calibration methods not traceable to standards laboratories and with uncertainties potentially larger than clinically acceptable. A EURAMET project has been started in Europe to deliver a harmonised, simplified and traceable dosimetry for eBT and measurement devices, to ensure optimal clinical use. The present work is specifically dedicated to the translation of the calibration for clinical users for skin applicators. This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.

The work with skin applicators is performed in a collaboration between UHasselt (BE), Maastro Clinic (NL), VSL (NL) and PTB (GE). The available source is the Xoft Axxent (iCAD, inc, USA) which was modelled using the Topas Monte Carlo (MC) code, including the complexly shaped conical skin applicator with a diameter of 3.5 cm (fig1). Simulations were used to obtain the photon spectrum in air at 1.5 m, from which HVL was calculated and compared to measurements. VSL created a matching beam quality by combining x-ray tube kV with added filtration and to simulate the spectrum of the source+skin applicator. The VSL matched beam was used to calibrate an Exradin A20 ion chamber in terms of air-kerma free in air. Subsequently this ion chamber was used in air to obtain a clinical calibration of the source using the special U-shaped holder provided by Xoft (fig 2). These measurements were converted to dose-to-water at the surface of the phantom using 1) the AAPM TG-61 protocol and 2) the protocol proposed by Fulkerson et al¹. These results were compared to the absorbed dose-to-water measured at the surface of a Plastic Water (PW) LR (CIRS, USA) phantom with the A20 chamber, taking into account the position of the effective point of measurement.

¹  Med. Phys. 41 (2), Feb 2014

An absorbed dose-to-water rate at the surface of a water phantom of 166.2 cGy/min for a nominal air-kerma of 10000 μGy/min was obtained using the TG61 formalism. Using the Fulkerson protocol, the conversion leads to 158.1 cGy/min. The same measurements carried out for the cone of 1 cm, 2 cm and 5 cm give respectively ratios Fulkerson/TG-61 of 0.944, 0.946 and 0.949. These results were then compared to the measurements of the dose at the surface of the PW phantom for which we obtained 151 cGy/min for the 3.5cm cone. Additional comparisons with measurements in the PW phantom using the A20 chamber and EBT3 radiochromic films are in progress.

The differences between the different protocols to obtain the dose at the surface of a water phantom could be significant and confirm the need for further research to obtain a correct calibration in the clinic for those eBT sources.