Most codes of practice for dosimetry of proton beams do not give a clear recommendation on the determination of recombination correction factors for ionization chambers. In this work, recombination corrections were measured in the low-energy clinical proton beam of the Clatterbridge Centre of Oncology (CCO) using data collected at different dose rates and different polarizing voltages. This approach allows the separation of contributions from initial and volume recombination and was compared with results from extrapolation and two-voltage methods. A modified formulation of the method is presented for a modulated beam in which the ionization current is time dependent. Using a set-up with two identical chambers placed face-to-face yielded highly accurate data for plane-parallel ionization chambers. This method may also be used for high-energy photon and electron beam dosimetry. At typical dose rates of 26 Gy min(-1) used clinically at the CCO, the recombination correction is 0.8% and thus is of importance for reference dosimetry. The proton beam should be treated as purely continuous given the high pulse repetition frequency of the cyclotron beam. The results show that the volume recombination parameter for protons is consistent with values measured for photon beams. Initial recombination was found to be independent of beam quality, except for a tendency to increase at the distal edge of the Bragg peak; this is only relevant for depth dose measurements. Using a general equation for recombination and generic values for the initial and volume recombination parameters (A = 0.25 V and m2 = 3.97 x 10(3) s cm(-1) nC(-1) V2), the experimental results are reproduced within 0.1% for all conditions met in this work. For the CCO beam and similar proton beams used for treating optical targets operating at high dose rates, the recombination correction factor can be overestimated by up to 2%, resulting in an overestimation of dose to water by the same amount, if the recommendation from IAEA TRS-398, which is only valid for pulsed beams, is followed without consideration.

Flattening filter free (FFF) beams are now commonly available with new standard linear accelerators. These beams have recognised clinical advantages in certain circumstances, most notably the reduced beam-on times for high dose per fraction stereotactic treatments. Therefore FFF techniques are quickly being introduced into clinical use. The purpose of this report is to provide practical implementation advice and references for centres implementing FFF beams clinically. In particular UK-specific guidance is given for reference dosimetry and radiation protection.

Calorimetry has been recommended and performed in proton beams for some time, but never has graphite calorimetry been used as a reference dosimeter in clinical proton beams. Furthermore, only a few calorimetry measurements have been reported in ocular proton beams. In this paper we describe the construction and performance of a small-body portable graphite calorimeter for clinical low-energy proton beams. Perturbation correction factors for the gap effect, volume averaging effect, heat transfer phenomena and impurity effect are calculated and applied in a comparison with ionization chamber dosimetry following IAEA TRS-398. The ratio of absorbed dose to water obtained from the calorimeter measurements and from the ionization measurements varied between 0.983 and 1.019, depending on the beam type and the ionization chamber calibration modality. Standard uncertainties on these values varied between 1.9% and 2.5% including a substantial contribution from the kQ values in IAEA TRS-398. The (Wair/e)p values inferred from these measurements varied between 33.6 J C(-1) and 34.9 J C(-1) with similar standard uncertainties. A number of improvements for the small-body portable graphite calorimeter and the experimental set-up are suggested for potential reduction of the uncertainties.

Dosimetry audit plays an important role in the development and safety of radiotherapy. National and large scale audits are able to set, maintain and improve standards, as well as having the potential to identify issues which may cause harm to patients. They can support implementation of complex techniques and can facilitate awareness and understanding of any issues which may exist by benchmarking centres with similar equipment. This review examines the development of dosimetry audit in the UK over the past 30 years, including the involvement of the UK in international audits. A summary of audit results is given, with an overview of methodologies employed and lessons learnt. Recent and forthcoming more complex audits are considered, with a focus on future needs including the arrival of proton therapy in the UK and other advanced techniques such as four-dimensional radiotherapy delivery and verification, stereotactic radiotherapy and MR linear accelerators. The work of the main quality assurance and auditing bodies is discussed, including how they are working together to streamline audit and to ensure that all radiotherapy centres are involved. Undertaking regular external audit motivates centres to modernize and develop techniques and provides assurance, not only that radiotherapy is planned and delivered accurately but also that the patient dose delivered is as prescribed.

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