Quality assurance and independent dosimetry for an intraoperative x-ray device

Quality assurance is an essential component of accurate and safe radiotherapy delivery, and should include measurements which are independent of manufacturer-provided calibration. However, the physical and dosimetric properties of the INTRABEAM compact mobile 50 kV x-ray source are different from conventional kilovoltage therapy units and few reports describe methods for independent checks, frequencies, or tolerances for quality assurance tests.

Based on the available evidence and local experience, methods are described for determination of the key dosimetric parameters: beam quality, output, isotropy, and depth doses. Internal system checks are also described, along with measurements of long-term stability.

A small volume parallel plate ionization chamber in a liquid water tank is the gold standard for measurements with this unit, but solid water-equivalent materials, thermoluminescent dosimeters and radiochromic film can all be used as practical alternatives with an accuracy of 5%-10%. The main cause of measurement uncertainty is positioning of the detector in the steep dose gradient, but energy dependence should also be considered.

A quality assurance schedule with suggested tolerances is proposed, which includes both internal tests, before each treatment and on a monthly basis, and independent tests every year or after servicing or recalibration.