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BIPM on-site comparison for radiation therapy dosimetry successfully uses new technique

The BIPM carried out a joint comparison with the NPL, UK, and VSL, the Netherlands, in the NPL’s medical accelerator facility from 23 September to 20 October 2014. The BIPM used its transportable graphite calorimeter system to make a direct measurement of absorbed dose to water - a key quantity for traceability in radiation therapy - in three of the high-energy photon beams from 6 MV to 25 MV. Through this comparison, the NPL has robustly verified the present UK primary standard to realize absorbed dose to water in accelerator beams; a graphite calorimeter of Domen design. The VSL used their newly-fabricated water calorimeter, with its unique design, and made a successful determination of absorbed dose to water in the same beams.

The BIPM used a remote-controlled motorized monitoring and shutter system for the first time to reliably track and correct for intrinsic intensity variations in the beams. The new design is compact and demonstrated a high reproducibility in the positioning of the shutter. Being controlled remotely, the system avoids staff having to enter the radiation area between irradiations, which not only saves time but also improves radiation protection for the operator.

These parallel comparisons were the 7th and 8th in the ongoing BIPM comparison BIPM.RI(I)-K6.

The new compact and remote-controlled BIPM monitor and shutter system, shown fixed in front of the accelerator head.
From left to right: Ileana Silvestre Patallo (NPL), Leon de Prez (VSL), Bartel Jansen (VSL) and David Burns (BIPM).
From left to right: Simon Duane (NPL), Philippe Roger (BIPM), Graham Bass (NPL), Susanne Picard (BIPM) and James Manning (NPL).
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