Fundamentals of Dosimetry

Dosimetry is the measurement of absorbed radiation dose and the science of dose measurement. The absorbed dose is defined as the quantity of ionizing radiation imparted per unit mass of a specified material. (ASTM E170)

1. SI Unit for absorbed dose is the gray (Gy)
2. 1 Gy = 1 Joule per kilogram
3. Absorbed dose received by a material depends on the composition of the material. Therefore dose is typically expressed in a standard material as ‘dose to water’.

Dosimetry is used for process validation as well as quality control and quality assurance of the ongoing process.

The measurement of radiation dose is an essential tool for determining, validating and maintaining the radiation process outputs. Along with other controls, dosimetry helps assure that medical devices are sterile, that food is safe for consumers, and manufactured products meet quality expectations.

Doses delivered to products in industrial processes vary widely depending on the application and range from 10 Gy to 800 kGy (kilogray) or more. In some processes it can be very difficult to measure the dose due to practical reasons.

There is not a single measurement system with a capability spanning the radiation processing range or that is practical for use in every application. Different systems are used for different dose ranges, and dosimetry system selection is dependent upon the specific factors and requirements of each process and application.

1. To “characterize” the radiation processing facility; which means to validate that the installed process equipment meets specification (IQ, OQ).
2. To determine a maximum dose that will not adversely affect the product’s form, fit or function.
3. To establish the dose needed to achieve the desired change in the product (e.g. sterility).
4. To establish processing parameters for products; that is, validating the product configurations and process parameters to be used for routine production (PQ).
5. Routine Process Monitoring and Process Control (QA and QC).
6. Re-validation of the process system after maintenance or repair.

A dosimeter is a device that, when irradiated, exhibits a quantifiable change in some property of the device that can be related to absorbed dose in a given material using appropriate analytical instrumentation and techniques.(ASTM E170)

Dose is measured using a dosimetry system. A dosimetry system consists of dosimeters, measurement instruments and their associated reference standards, and procedures for the system’s use. A dosimetry system takes the name of the dosimeter used in the system and the proper function of the entire system is integral to the dose measurement.

There are two classes of dosimetry systems that are of interest to industry. (References: ASTM E-2628 Sections 6.3.1 and 6.3.2 and ICRU 80 Section 5)

• Reference Dosimetry System – used to characterize or monitor radiation sources at laboratories and used to calibrate laboratory internal dosimetry systems. Also used to ‘transfer’ dose to routine dosimetry systems at industrial facilities through a calibration method.
• Routine Dosimetry Systems – used to measure dose at radiation processing facilities.

The response of a dosimeter is the change in the signal or indication of the dosimeter, as measured by the dosimetry system. Often the normalized signal (signal change per unit mass or unit thickness) is used as the response.

Dosimetry batch calibration is the process where the response of a dosimetry system is characterized through comparison with a traceable reference (standard). The calibration method for the dosimetry system is designed to ensure the integrity of the measurements made with the user’s dosimetry system. Dosimetry Systems must be calibrated for each batch of dosimeters. In addition, a separate calibration is required for each measurement instrument.
All dosimetry systems must be calibrated traceable to a national or international standard. Dosimeter manufacturers may provide response functions on their websites or in product literature, but these are for general information only. Therefore, the end-user must calibrate the measurement system as a whole (the dosimetry system) and ensure that the calibration is suitable for the conditions of use.
Routine dosimetry systems should be calibrated in accordance with the requirements of ISO/ASTM 51261. Reference dosimetry systems should be calibrated in a manner to meet the requirements of ISO 17025.

A practice of using dosimeters and dosimetry systems to quantify absorbed-dose from sources of ionizing radiation.

A device that, when irradiated, exhibits a quantifiable change in some property of the device that can be related to absorbed dose in a given material using appropriate analytical instrumentation and techniques.
Discussion: The term “device” is used in order to describe both large dosimeters such as a calorimeter as well as small dosimeters fabricated from pieces of material such as PMMA chips, alanine films and pellets or radiochromic film pieces. The packaging materials and conditioned environment surrounding the dosimeter comprise a complete dosimeter. Since we cannot measure the actual dose absorbed the irradiated materials and materials interfaces themselves, dosimeter are used as surrogates and must be of a size and composition approximately equivalent to the materials and interfaces they represent.

System used for measuring absorbed dose, consisting of (a.) dosimeters, (b.) measurement instruments and their associated reference standards, and (c.) procedures for use of the system.

A dosimetry system generally having the highest metrological quality available at a given location or in a given organization, from which measurements made there are derived.

A dosimetry system calibrated against a reference standard dosimetry system and used for routine absorbed dose measurements, including dose mapping and process monitoring.A dosimetry system used for day-to-day monitoring and control of an industry irradiation process

A determination of performance characteristics, such as useful dose range, reproducibility and the effects of influence quantities, for a dosimeter/dosimetry system under defined conditions. Discussion: Characterization is carried out in order to determine the practices and procedures necessary to control the influence quantities or factors that effect dosimetry results such that the dosimetry system produces results within a normally expected range of performance.

Set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding values realized by (dose) standards.

Mathematical expression of the relation between dosimeter response and corresponding value of dose certified traceable to a national or international standard dose.

A property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparison, all having stated uncertainties. Discussion: The unbroken chain of comparisons is called a ‘traceability chain’. Traceability is a requirement of any measurement management system.

A parameter associated with the result of a calculation (“dose estimate”) that characterizes the spread of values that could reasonably be attributed to the derived quantity (“dose”).
Note: A statement of uncertainty is required in order to provide meaning to a dose measurement. An uncertainty statement defines the quality of a measurement in terms of calculated limits (typically ±4-7% for a routine dosimetry system) within which the “true” value of the estimated dose is expected to be found with a specified level of confidence (typically approximately 95% or =2). Refer to the ISO/ASTM 51705 uncertainties document for guidance.

Dosimetry batch calibration is the process where the response of a dosimetry system is characterized through comparison with a traceable reference (standard). The calibration method for the dosimetry system is designed to ensure the integrity of the measurements made with the user’s dosimetry system. Dosimetry Systems must be calibrated for each batch of dosimeters. In addition, a separate calibration is required for each measurement instrument.
All dosimetry systems must be calibrated traceable to a national or international standard. Dosimeter manufacturers may provide response functions on their websites or in product literature, but these are for general information only. Therefore, the end-user must calibrate the measurement system as a whole (the dosimetry system) and ensure that the calibration is suitable for the conditions of use.
Routine dosimetry systems should be calibrated in accordance with the requirements of ISO/ASTM 51261. Reference dosimetry systems should be calibrated in a manner to meet the requirements of ISO 17025.