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The current UK code of practice for high-energy photon therapy dosimetry (Lillicrap et al 1990 Phys. Med. Biol. 35 1355-60) gives instructions for measuring absorbed dose to water under reference conditions for megavoltage photons. The reference conditions and the index used to specify beam quality require that a machine be able to set a 10 cm × 10 cm field at the point of measurement. TomoTherapy machines have a maximum collimator setting of 5 cm × 40 cm at a source to axis distance of 85 cm, making it impossible for users of these machines to follow the code. This addendum addresses the specification of reference irradiation geometries, the choice of ionization chambers and the determination of dosimetry corrections, the derivation of absorbed dose to water calibration factors and choice of appropriate chamber correction factors, for carrying out reference dosimetry measurements on TomoTherapy machines. The preferred secondary standard chamber remains the NE2611 chamber, which with its associated secondary standard electrometer, is calibrated at the NPL through the standard calibration service for MV photon beams produced on linear accelerators with conventional flattening filters. Procedures are given for the derivation of a beam quality index specific to the TomoTherapy beam that can be used in the determination of a calibration coefficient for the secondary standard chamber from its calibration certificate provided by the NPL. The recommended method of transfer from secondary standard to field instrument is in a static beam, at a depth of 5 cm, by sequential substitution or by simultaneous side by side irradiation in either a water phantom or a water-equivalent solid phantom. Guidance is given on the use of a field instrument in reference fields.

ConclusionTreatment decision-making for localised prostate cancer can be informed by these 6-year functional and QoL outcomes.

Diamond detectors have become an increasingly popular dosimetric method where either high spatial resolution is required or where photon or electron spectra are likely to change with depth or field size. However, little work has been previously reported for superficial energies. This paper reports the response of a commercially available diamond detector (PTW Freiburg/IPTB Dubna) at 45 kVp (0.55 mm Al first HVL) and 100 kVp (2.3 mm Al first HVL) including dose and dose-rate linearity, percentage depth-dose and output factors as a function of applicator size. Comparisons are made with Br J. Radiol. supplement 25 data, measurements using a PTW parallel-plate chamber and Monte Carlo simulations based on spectra determined from transmission measurements in aluminium. Excellent agreement was obtained for percentage depth-dose curves between Monte Carlo and diamond after correcting for sublinearity of the dose-rate response and energy dependence of the diamond detector. However, significant differences were noted between diamond/Monte Carlo and the parallel-plate chamber, which is attributed to the perturbation caused by the polyethylene base of the chamber

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