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General principles

Introduction

In radiotherapy, dose information is required at all points of interest in the target volume and in organs at risk. For this purpose dosimeters are required for absolute dose determinations under reference conditions, and for relative dose measurements under non-reference conditions. Depending on the specific application, e.g., in low-/high-energy photon or electron beams, specific dosimeters are chosen. Detailed knowledge of the properties of a dosimeter is therefore a prerequisite for obtaining accurate measurements.

Important Principles

A useful dosimeter exhibits the following properties: high accuracy and precision, linearity of signal with dose over a wide range, small dose and dose rate dependence, flat energy response, small directional dependence, high spatial resolution and large dynamic range. Convenience of use and the stability of its calibration factor are also important properties of a dosimeter. These characteristics should be determined for each type of dosimeter before using them clinically. The most commonly used radiation dosimeters are ionization chambers, radiographic films, TLDs, diodes and MOSFETs, each having advantages and disadvantages that determine their specific application. Other dosimeters used for clinical dosimetry are alanine, plastic scintillators, diamond detectors, optically stimulated luminescence (OSL) and gel dosimetry systems.

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 high-energy external beams is discussed in Chapter 9 of the Handbook. Details of the absorbed dose calibration procedure are given in reports TRS 398, TRS 469 and in the Handbook.

A training module (online version or downloadable) has been developed to assist medical physicists in the implementation of IAEA Technical Report Series No. 398 for high energy linear accelerator photon beams (chapter 6).