New VSL water calorimeter commissioned


VSL has recently developed a new transportable water calorimeter as the Dutch absorbed dose to water primary standard for Co-60 and high energy x-rays (also known as MV-photon beams). In 2014 the calorimeter was commissioned for on-site calibration of detectors in clinical photon beams and compared with the BIPM standard. The new calorimeter replaces the existing calorimeter and is designed for use in patient bores of 70 cm including MRI incorporated linear accelerators.

In the field of radiotherapy, cancer patients are irradiated with high energy x-rays to kill the tumor. In this treatment the goal is to optimize the trade-off between a high dose to the tumor and a low dose to critical organs and healthy tissue. Therefore accurate measurement of the delivered radiation dose (dosimetry) to human tissue is indispensible in this field.

In clinical dosimetry, the unit Gray (Gy = J/kg) is the unit of interest for the quantity absorbed dose to water, Dw. It can be measured absolutely using water calorimetry which is based on the radiation induced temperature change. Critical in the measurements with a water calorimeter is the determination of a small temperature rise of 0.48 mK with an uncertainty less than 1 µK for a typical dose of 2 Gy.

Since 1980, the department of Ionizing Radiation at VSL operates absorbed dose to water primary standards for high energy x-rays. Initially a graphite calorimeter was developed which was compared in 1994 with the standard at the Bureau International de Poids et Mesures, BIPM, Sèvres. In 2001, the graphite calorimeter was replaced by the first transportable water calorimeter (see picture on the left). It formed the basis for experimentally determined kQ factors in clinical MV-photon beams for the NCS-18 dosimetry Code of Practice. In 2005, a key-comparison with the water calorimeter was carried out at the BIPM.

With the advent of new radiotherapy treatment units, such as proton therapy and MRI integrated linear accelerators, and new insights to improve the performance and thermal stability of the calorimeter, VSL developed a new water calorimeter. In close cooperation with Job Kneppers Ontwerp & Realisatie B.V. and based on extensive heat transport simulations, the new calorimeter was designed in 2012 and constructed in 2013. The key-features of the new calorimeter are its compact, transportable design and measurement system, efficient on-site operation and the ability to calibrate detectors directly inside the calorimeter.

In the first half of 2014, validation of the calorimeter took place in beams of Co-60 gamma-radiation at VSL and in high energy x-rays of 6 and 10 MV at Netherlands Cancer Institute (AVL) in Amsterdam (picture, respectively in the middle and on the bottom right). In October 2014, a key-comparison with the BIPM took place for 6, 10 and 25 MV x-rays at the National Physical Laboratory, UK. The results will contribute to the acceptance of the water calorimeter as a primary standard for absorbed dose to water for on-site calibration of detectors.

In the framework of EURAMET project EMRP HLT06 'MRI-Safety', the calorimeter was optimized for operation in a magnetic field of an MRI integrated linear accelerators such as developed at UMC Utrecht. The influence of the magnetic field on the temperature measurement was investigated. In November 2014 it was mechanically fitted in the bore of the MRI linac at the UMC Utrecht (see picture on the top right).