A water calorimeter for on-site absorbed dose to water calibrations in 60Co and MV-photon beams including MRI incorporated treatment equipment

In reference dosimetry the aim is to establish the absorbed dose to water, Dw, under reference conditions. However, existing dosimetry protocols are not always applicable for rapidly emerging new treatment modalities. For primary standard dosimetry laboratories it is generally not feasible to acquire such modalities. Therefore it is strongly desired that D-w measurements with primary standards can be performed on-site in clinical beams for the new treatment modalities in order to characterize and calibrate detectors. To serve this need, VSL has developed a new transportable water calorimeter serving as a primary D-w standard for Co-60 and MV-photons including MRI incorporated treatment equipment. Special attention was paid to its operation in different beam geometries and beam modalities including the application in magnetic fields. The new calorimeter was validated in the VSL Co-60 beam and on-site in clinical MV-photon beams. Excellent agreement of 0.1% was achieved with previous Co-60 field calibrations, i.e. well within the uncertainty of the previous calorimeter, and with measurements performed in horizontal and vertical MV-photon beams. k(Q) factors, determined for two PTW 30013 ionization chambers, agreed very well with available literature data. The relative combined standard uncertainty (k = 1) for Dw measurements in Co-60 and MV-photons is 0.37%. Calibrations are carried out with a standard uncertainty of 0.42% and kQ-factors are determined with a relative standard uncertainty of 0.40%.