A prototype low-cost secondary standard calorimeter for reference dosimetry with ultra-high pulse dose rates.

OBJECTIVES: Ultra-high pulse dose rate modalities present significant dosimetry challenges for ionisation chambers due to significant ion recombination. Conversely, calorimeters are ideally suited to measure high dose, short duration dose deliveries and this work describes a simple calorimeter as an alternative dosemeter for use in the clinic. METHODS: Calorimeters were constructed featuring a disc-shaped core and single sensing thermistor encased in a 3D-printed body shaped like a Roos ionisation chamber. The thermistor forms one arm of a DC Wheatstone bridge, connected to a standard DMM. The bridge-out-of-balance voltage was calibrated in terms of temperature. A graphite-core calorimeter was calibrated in terms of absorbed dose to water (J/kg) in Co-60 and conventional 6, 10 and 15 MV X-rays. Similarly, an aluminium-core calorimeter was calibrated in a conventional 20 MeV electron beam and tested in a research high dose per pulse 6 MeV electron beam. RESULTS: Calorimeters were successfully calibrated in terms of absorbed dose to water in conventional radiotherapy beams at approximately 5 Gy/min with an estimated uncertainty of ±2-2.5% (k = 2), and performed similarly in a 6 MeV electron beam delivering approximately 180 Gy/s. CONCLUSIONS: A simple, low-cost calorimeter traceably calibrated to existing primary standards of absorbed dose could be used as a secondary standard for dosimetry for ultra-high pulse dose rates in the clinic. ADVANCES IN KNOWLEDGE: Secondary standard calorimeters for routine measurements are not available commercially; this work presents the basis of a simple, low-cost solution for reference dosimetry for ultra-high pulse dose rate beams.

IPEM code of practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.

The 1990 code of practice (COP), produced by the IPSM (now the Institute of Physics and Engineering in Medicine, IPEM) and the UK National Physical Laboratory (NPL), gave instructions for determining absorbed dose to water for megavoltage photon (MV) radiotherapy beams (Lillicrap et al 1990). The simplicity and clarity of the 1990 COP led to widespread uptake and high levels of consistency in external dosimetry audits. An addendum was published in 2014 to include the non-conventional conditions in Tomotherapy units. However, the 1990 COP lacked detailed recommendations for calibration conditions, and the corresponding nomenclature, to account for modern treatment units with different reference fields, including small fields as described in IAEA TRS483 (International Atomic Energy Agency (IAEA) 2017, Vienna). This updated COP recommends the irradiation geometries, the choice of ionisation chambers, appropriate correction factors and the derivation of absorbed dose to water calibration coefficients, for carrying out reference dosimetry measurements on MV external beam radiotherapy machines. It also includes worked examples of application to different conditions. The strengths of the 1990 COP are retained: recommending the NPL2611 chamber type as secondary standard; the use of tissue phantom ratio (TPR) as the beam quality specifier; and NPL-provided direct calibration coefficients for the user's chamber in a range of beam qualities similar to those in clinical use. In addition, the formalism is now extended to units that cannot achieve the standard reference field size of 10 cm × 10 cm, and recommendations are given for measuring dose in non-reference conditions. This COP is designed around the service that NPL provides and thus it does not require the range of different options presented in TRS483, such as generic correction factors for beam quality. This approach results in a significantly simpler, more concise and easier to follow protocol.

A multi-institutional dosimetry audit of rotational intensity-modulated radiotherapy.

BACKGROUND: Rotational IMRT (VMAT and Tomotherapy) has now been implemented in many radiotherapy centres. An audit to verify treatment planning system modelling and treatment delivery has been undertaken to ensure accurate clinical implementation. MATERIAL AND METHODS: 34 institutions with 43 treatment delivery systems took part in the audit. A virtual phantom planning exercise (3DTPS test) and a clinical trial planning exercise were planned and independently measured in each institution using a phantom and array combination. Point dose differences and global gamma index (γ) were calculated in regions corresponding to PTVs and OARs. RESULTS: Point dose differences gave a mean (±sd) of 0.1±2.6% and 0.2±2.0% for the 3DTPS test and clinical trial plans, respectively. 34/43 planning and delivery combinations achieved all measured planes with >95% pixels passing γ<1 at 3%/3mm and rose to 42/43 for clinical trial plans. A statistically significant difference in γ pass rates (p<0.01) was seen between planning systems where rotational IMRT modelling had been designed for the manufacturer's own treatment delivery system and those designed independently of rotational IMRT delivery. CONCLUSIONS: A dosimetry audit of rotational radiotherapy has shown that TPS modelling and delivery for rotational IMRT can achieve high accuracy of plan delivery.

The calibration of parallel-plate electron ionization chambers at NPL for use with the IPEM 2003 code of practice: summary data.

The most recent electron dosimetry code of practice for radiotherapy written by the Institute of Physics and Engineering in Medicine was published in 2003 and is based on the NPL electron absorbed dose to water calibration service. NPL has calibrated many Scanditronix type NACP-02 and PTW Roos type 34001 parallel plate ionization chambers in terms of absorbed dose to water, for use with the code of practice. The results of the calibrations of these chamber types summarized here include the absorbed dose to water sensitivity, where the mean calibration factor standard deviations are 5.8% for NACP-02 chambers and 1.1% for PTW Roos chambers. The correction for the polarity effect is shown to be small (less than 0.2% for all beam qualities) but with a discernible beam quality dependence. The correction for recombination is shown to be consistent and reproducible, and an analysis of these results suggests that the plate separation of the NACP-02 chambers is more variable from chamber to chamber than with the PTW Roos chambers. The calibration of these chambers is shown to be repeatable within +/-0.2% over 2-3 years. It is also shown that check source measurements can be repeated within +/-0.3% over several years. The results justify the use of NACP-02 and PTW 34001 chambers as secondary standards, but also indicate that the PTW 34001 chambers show less variation from chamber to chamber.