A portable graphite calorimeter for onsite reference dosimetry and beam quality correction factor determination

To present a portable, reliable, and fast setup graphite calorimeter for onsite absolute dose measurements at hospitals.

We have designed and built a portable graphite calorimeter optimized for fast setup and accurate measurements at clinical centers. Our system has minimum readout and auxiliary electronics and is inserted in a water fillable phantom in which chamber holders can be positioned in order to perform chamber calibrations at user's beam quality. System operation parameters were adjusted to obtain thermal equilibrium in minimum time. Our system accuracy was validated by inter-comparison against the National Physical Laboratory (NPL) Primary Standard. Also, its portability and reliability have been tested during a measurement campaign in hospitals.

Optimization of system operation parameters together with an appropriate design yielded a fully portable, fast and easy-to-set-up system with only 3 h treatment room occupation time. The system exhibits thermal stability of ± 100 µK and a maximum temperature drift of 60 µK/min, allowing for precise measurements with few repetitions. Mean relative local deviation between chambers calibrated by NPL calorimeter and our calorimeter was 0.1% demonstrating our system accuracy. We measured beam quality correction factors for four chamber types and performed a successful measurement campaign in five hospitals.

We have developed an accurate absolute dosimeter that can be used for the determination of beam quality correction factors at clinical sites with minimal disruption of hospital workload.