Graphite calorimeters are the primary standards for absorbed dose at NPL.
Radiotherapy is one of the two most effective treatments for cancer. The success of radiotherapy in curing cancer depends critically on accurate targeting and delivery of the correct radiation dose. If the dose delivered to a patient is too low then cancerous cells may survive leading to a recurrence of the cancer. If the dose delivered is too high then surrounding healthy tissue is more likely to be damaged. For example, optimal treatment of some head and neck tumours requires that the dose delivered should be within only a few percent of that prescribed. Uncertainty in patient positioning means that it is crucial for all other errors to be as small as is possible. Accurate dosimetry is essential to maintain and improve patient survival rates.
The fundamental technique for measuring absorbed dose is through the heating effect of the radiation on the absorber. Measurement of that effect through calorimetry has been for many years the basis of the UK primary standards for calibration of absorbed dose measurements for high-energy photon and high-energy electron beam treatments, and now calorimetry will also be at the foundation of dosimetry for HDR brachytherapy calibrations and for high-energy proton and light ion beams. A calorimeter for use in the small fields characteristic of beams used in tomotherapy or image-guided radiotherapy is also being built. The NPL calorimeters are constructed from graphite, and the measured absorbed dose to graphite is converted into the quantity of interest, absorbed dose to water. Each calorimeter has a small core of mass up to a few grams, surrounded by vacuum to limit the heat flow, inside 'jacket' pieces which are designed to reduce the effect of perturbations on the uncertainty of the measured dose and to control the heat flow.
In graphite calorimetry, NPL has had for many years the world's first disseminated primary standards for absorbed dose at therapy-level dose rates in both high-energy photon beams (1988) and electron beams (1997). Scientists at NPL also designed and produced the first operational portable graphite calorimeter to be used for therapy-level dose measurements off site, including work in proton beams (2003). Recently in 2011, the first measurements of absorbed dose in a graphite calorimeter from an HDR brachytherapy seed were successfully carried out.
Graphite calorimeters may be operated in quasi-adiabatic or isothermal modes. Temperature is sensed using small thermistors connected into a Wheatstone bridge. In quasi-adiabatic operation, the temperature rise in the graphite core is measured and the dose calculated through knowledge of the specific heat capacity. In isothermal operation, the components are maintained at constant temperature through electrical heating, and the energy absorbed from radiation is measured by electrical substitution. This energy is combined with the graphite absorber mass to obtain the absorbed dose in the calorimeter core.
These calorimetric absorbed dose standards enable the provision of services for direct calibration in terms of absorbed dose to water in many different radiotherapy treatment modalities. NPL has considerable expertise in the design, modelling and operation of calorimeters. Through its research, NPL remains a world-leading NMI in absorbed-dose graphite calorimetry.