Volumetric modulated arc therapy ( VMAT )

New calorimeters Simon Duane, NPL Calorimetry remains the most fundamental method for the measurement of absorbed dose and is the basis of almost all absorbed dose primary standards worldwide. Absorbed dose to water is realised either directly, by using water as the absorbing medium, or indirectly, by using another medium (usually graphite) and determining a factor to convert dose from that medium to water. NPL maintains separate primary standards of absorbed dose for high-energy electron and photon beams. These graphite calorimeters are the devices to which dosimetry for most external beam radiotherapy in the UK is traceable and, having been developed between 15 and 25 years ago, these devices are now due for replacement. Several projects in calorimetry at NPL are underway, including the construction of one new primary standard for measurements in both electron and photon beams and another for use in particle therapy beams. Two further calorimeters are in development , one for the measurement of absorbed dose in HDR brachytherapy another for use in small field/complex dose distributions such as IMRT. Their key features will be outlined in this presentation. Heat flows relatively quickly through graphite and the sensitive volume, or core, in a calorimeter is defined by the thermal barriers (preferably evacuated gaps) which surround it. These gaps perturb the dose distribution and so are kept as narrow as possible. The optimal geometry of the core is determined by the application. For measurements in a single external beam, whether electrons, photons or other particles, the point of interest is on the central axis at a defined depth from the front face of a phantom. The effect of dose gradients at the point of measurement is minimised by choosing a flat cylindrical core, of minimal thickness in the direction of the beam. For measurements in brachytherapy, the core is an annulus surrounding the source, 25 mm in radius, 2 mm thick and 5 mm high. For measurements in IMRT, the core is a squat cylinder, no more than 5 mm in diameter and 5 mm high. The temperature of the core changes in response to radiation, by 0.0014 oC/Gy in graphite, but is also affected by heat transferred to/from its immediate environment. The correction due to heat transfer should be minimised, and is estimated by also measuring the temperature of the surrounding jacket. Self-heating within a brachytherapy source means that the source is always warmer than its environment: additional heat transfers due to this warm source present a significant perturbation to the radiation measurement. Likewise, the complex variation of dose rate with position and time significantly increases the effect of heat transfers in calorimetry for IMRT. In each case, it appears that isothermal calorimeter operation offers the possibility to largely eliminate these problems. Volumetric modulated arc therapy (VMAT) James L Bedford, Karen E Rosser and Alan P Warrington Royal Marsden NHS Foundation Trust