4.1 The assurance that blood and blood components have been properly irradiated is of crucial importance for patient health. The irradiator operator must demonstrate by means of accurate absorbed-dose measurements on the product, or in simulated product, that the specified absorbed dose has been achieved throughout the product.

4.2 Blood and blood components are irradiated at predetermined doses to inactivate viable lymphocytes to help prevent transfusion-induced graft-versus-host disease (GVHD) in certain immunocompromised patients and those receiving related-donor products (1, 2).6

4.3 Blood and blood components may be treated with ionizing radiation, such as gamma radiation from 137Cs or 60Co sources, and from self-contained X-ray (bremsstrahlung) units and medical linear X-ray (bremsstrahlung) and electron accelerators used primarily for radiotherapy.

4.3.1 The terms "gamma rays" and "gamma radiation" are used interchangeably, as are the terms "X-ray" and "Xradiation."

4.4 Blood irradiation specifications include a lower limit of absorbed dose, and may include an upper limit or central target dose. For a given application, any of these values may be prescribed by regulations that have been established on the basis of available scientific data. See 2.4.

4.5 For each blood irradiator, an absorbed-dose rate at a reference position within the canister is measured by the manufacturer as part of acceptance testing using a reference-standard dosimetry system. That reference-standard measurement is used to calculate the timer setting required to deliver the specified absorbed dose to the center of the canister with blood and blood components, or other reference position. Either relative or absolute absorbed-dose measurements are performed within the blood- or blood-equivalent volume for determining the absorbed-dose distribution. Accurate radiation dosimetry at a reference position which could be the position of the maximum absorbed dose (Dmax) or minimum absorbed dose (Dmin) offers a quantitative, independent method to monitor the radiation process.

4.6 Dosimetry is part of a measurement quality assurance program that is applied to ensure that the radiation process meets predetermined specifications (3).

4.7 Absorbed-dose mapping is often performed using simulated product (for example, polystyrene is considered blood equivalent for 137Cs photon energies).

4.8 Blood and blood components are usually chilled or frozen. Care should be taken, therefore, to ensure that the dosimeters and radiation-sensitive indicators can be used under such temperature conditions.

4.9 Proper documentation and record keeping are critical components of radiation processing. This standard does not address this issue since the pertinent governing bodies set minimum requirements.

4.10 Most dosimeters have significant energy dependence at photon and electron energies less than 100 keV, so great care must be exercised when measuring absorbed dose in that energy range.