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Dosimetry system calibration

Introduction

Maintenance and calibration of measurement equipment is a critical responsibility of the medical physicist. Calibration applies to ionization chamber dosimetry systems, thermoluminescent dosimeters (TLDs), optically stimulated luminescent (OSL) dosimeters, diode dosimeters, film, and computed radiography (CR) plates, and kVp meters, and other electronic measurement equipment.

Important Principles

In order to make accurate and repeatable measurements the measuring equipment must be properly maintained and calibrated.  In addition, good practice requires that, for some equipment, traceability to a national standard be maintained.  This requires not only good oversight of measurement equipment but also clear and concise records.
Calibrations are usually carried out on an annual or biennial (every two years) basis.  However, cross checks of equipment accuracy between calibrations is considered good practice, along with proper documentation of such checks.

The medical physicist must be able to determine the HVL of the beam used for calibration purposes and understand the impact changes in the HVL and beam kilovoltage will have on measurements and calibrations.  It is also important to be able to calibrate kerma-air product chambers, field check the calibration of CT pencil chambers, and to calibrate TDL and OSL for use in patient and occupational dose measurements.

Film, both silver-halide and radiochromic films, are in used in radiology.  Consequently, calibration and the limitations of these two forms of dosimeters should be understood by the medical physicist.  Likewise, CR plates can be used for dosimetry purposes and require the same calibration and care as films.

Kilovoltage (kVp) meters and current clamps must be cross calibrated periodically, and calibrated by a standards laboratory at least annually.  This is the responsibility of the Medical Physicist.

Introduction to References

The key reference for dosimetry system calibration is the IAEA International Code of Practice.  The AAPM Instrumentation Requirements document provides insight into the accuracy and precision of equipment required for medical physics measurements.  AAPM Report 31 provides information regarding the equivalency of different types of phantoms for diagnostic dosimetry..