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QA of dosimetry equipment

Introduction

Quality Assurance of dosimetry equipment serves to guarantee its correct performance during normal usage, spotting problems at an early stage, and removing suspect equipment from service as quickly as possible for recalibration, repair or replacement. QA checks are generally more a matter of continuous observation of equipment performance rather than tracking deviations in equipment characteristics.

Important Principles

Dosimeters used in hospitals for absolute dose measurements must have a calibration coefficient traceable (directly or indirectly) to a primary standard. Primary standards are not used for routine calibrations, but instead secondary standard dosimeters are cross-calibrated against the primary standard in Standards Laboratories, that in turn are used for calibrating the reference dosimeters. The reference dosimeter is used in a hospital to calibrate field instruments under reference conditions in a water phantom, and is recalibrated regularly, e.g. every two years, by an accredited calibration laboratory. QA checks of field instruments can be performed in plastic phantoms, although these are not recommended for calibration purposes.

Introduction to References

A general discussion of the various types of dosimeters is presented in Chapter 3 of the IAEA Radiation Oncology Physics Handbook. Calibration of dosimeters and their use in high-energy external beams is discussed in Chapter 9 of the IAEA Handbook, while details of absolute and relative dose measurements can be found in IAEA Report TRS 398. The Canadian CPQR document provides an example of a QA program for dosimetry equipment.