A novel constancy check technique for a well type ionisation chamber using a 90Sr plane-parallel check source.

Accurate determination of source strength in brachytherapy is vital to achieving safe and effective radiotherapy treatments. The recommended detectors to be used for determining source strength are well type ionisation chambers and it is essential to the process that they retain a stable response to radiation, thus maintaining a valid calibration. A new Standard Imaging Inc. HDR-1000 Plus well chamber was acquired by GenesisCare Western Australia and commissioned for use clinically. As part of this process, a novel technique was developed to allow constancy checks of the well chamber to be performed using a Sr-90 check source designed for use with plane-parallel chambers (PTW model T48010). The proposed methodology is simple, robust and sufficiently accurate to measure well chamber constancy within ± 0.5%. Most radiotherapy departments will already possess a Sr-90 check source as they are required for external beam dosimetry equipment constancy checks. Therefore, no new additional equipment need be purchased making this method particularly cost-effective.