NIM has developed a primary standard for absorbed dose to water at radiotherapy dose levels. The standard is based on a water calorimeter and is designed to operate both in Co-60 gamma rays and megavoltage photon beams from the linac. The water calorimeter operated at 4 ℃ to eliminate the problems associated with convection in water phantoms at room temperature. A low-noise temperature measurement circuit is able to resolve temperature differences at the microkelvin level. Using open and sealed vessels, the calorimeter has the capability to investigate different water purities and determine the heat defect due to dissolved gases.

The set-up operates with the horizontal photon beams from the linac. The absorbed dose to water measured by water calorimeter for photon of 10 MV with the uncertainty of 0.35% (k=1). The BIPM.RI(I)-K6 comparison was completed in 2016, and the report was published in Metrologia.

Partnering with the Medical Physics Society of China, NIM has trained medical physicists to support quality control for more than 2000 linacs nationswide.

Linac primary standard for radiotherapy dosimetry