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A method to calibrate the RS 2000 x-ray biological irradiator for radiobiological flank irradiation of mice

This work investigated a method to calibrate the irradiation by an x-ray cabinet of tumours inserted on the flanks of mice. For the x-ray energies used by such devices, the radiotherapy treatment metric of absorbed dose to water (DW) requires knowledge of the half value layer (HVL) of the beam spectrum. This is usually measured in a scatter free environment, but such a measurement inside a cabinet with our specific exposure geometry is impractical. Therefore, a novel way of evaluating HVL was devised and used to calibrate an RS 2000 cabinet in terms of DW. This calibration used the relevant IPEMB code of practice and was traceable to the UK primary standard for air kerma. This code is used by every radiotherapy department in the UK. To facilitate convenient calibration of different types of dosimeter in the hospital radiotherapy department a cabinet was produced in-house and used with a clinical superficial x-ray device to mimic the spectral properties in the university based RS 2000. To measure the HVL inside the cabinet, the response of thermoluminescent dosimeters (TLDs) to that of a secondary-standard ionisation chamber (MTLD/MS) was derived across an appropriate range of HVLs free-in-air. Measurements were then taken in the in-house cabinet with TLDs and ion chamber under conditions that simulated the position of the animal flank implanted tumour (i.e. with our specific experimental geometry) to derive an average MTLD/MS from which the HVL of the incident beam was calculated. Chamber readings were converted to DW (without the need to use modified backscatter factors) and used to calibrate TLDs which were subsequently employed in the RS 2000 cabinet to allow conversion of exposure time to dose. The uncertainty in calculated dose was ±3.8%.