Most of domestic radiation therapy doses can only be traced back to air kerma of ⁶⁰Co γ-ray at present in China. The uncertainty of directly tracing radiation therapy dose to absorbed dose to water is much smaller compared to tracing radiation therapy dose to air kerma, which must be converted to absorbed dose to water.

This study aims to solve the problem of traceability and transfer of ⁶⁰Co γ-ray absorbed dose to water, as well as the problem that the dose measurement of radiotherapy cannot be directly traced through the absorbed dose to water, NIMTT (National Institute of Measurement and Testing Technology) has established an absolute measurement device of ⁶⁰Co γ-ray absorbed dose to water.

An absolute measurement device of ⁶⁰Co γ-ray absorbed dose to water was established by National Institute of Measurement and Testing Technology (NIMTT) using water calorimeterfor direct measurement of absorbed dose to water. The correction factors for water absorbed dose measurement were obtained by experimental and simulation methods. A comparison of laboratory reproducing results was made between NIMTT and NRC (National Research Council, Canada) using the method of transmitting standard to further verify the accuracy and consistency of the measurement of ⁶⁰Co γ-ray absorbed dose to water. Two PTW ionization chambers were used as the transfer standard.

The reproduced results of two laboratories for absolute measurement of ⁶⁰Co γ-ray absorbed dose to water are consistent within the relative standard uncertainty of 0.71%, and the normalized error value of E_n is -0.45.

The comparison results have verified that NIMTT laboratory has the ability of traceability and transmission of absorbed dose to water. The results of this study provide a reference for the absolute measurement of absorbed dose to water for ⁶⁰Co γ-ray.