The mandate for this Task Group - Mesh-type Reference Computational Phantoms (MRCP) - will be focused on converting the current voxel-type reference computational phantoms into a high-quality mesh format to address the limitations of the voxel-type phantoms in some dose coefficient calculations.

Specific work will include:

• The Task Group will develop the adult, paediatric and pregnant-female MRCPs. As a first step, the adult MRCPs will be developed by converting the existing voxel phantoms in Publication 110 into a high-quality mesh format. Then, the paediatric MRCPs will be developed from the paediatric voxel phantoms in Publication 143.

• Finally, the pregnant-female MRCPs will be newly developed. All the development processes will be discussed among TG members and the developed phantoms will be comprehensively reviewed by anatomists.

Work completed to date: 

• The adult MRCPs were completely developed and released in Publication 145 - Adult Mesh-type Reference Computational Phantoms.

• The paediatric MRCPs were completely developed and the relevant report will be released soon as Publication 156 - Paediatric Mesh-type Reference Computational Phantoms.

• Recently, the pregnant-female MRCPs were completely developed and the report is currently under preparation. Public consultation is expected in 2025.

The primary dosimetric data on exposures in X-ray imaging procedures are measurements of entrance air kerma (for radiography), air kerma-area products (for diagnostic fluoroscopy), and CTDIvol and DLP (for computed tomography, CT). Such dose metrics are used to set Diagnostic Reference Levels that allow comparisons of doses received from the same procedure in different hospitals and help ensure that exposures are the minimum required to produce appropriate quality of images. However, effective dose is also used extensively in diagnostic x-ray imaging to provide a detriment-related dose quantity to inform clinical judgements, including the comparison of different x-ray procedures, and comparisons of imaging practice across different hospitals and medical facilities. For many years, the ICRP has produced, through its joint C2/C3 Task Group 36, reference dose coefficients for common diagnostic nuclear medicine procedures. However, ICRP has not provided reference dose coefficients for X-ray imaging procedures and consequently different methodologies are used to convert measurements to estimates of effective dose or some surrogate of effective dose. These calculations necessarily rely on disparate published data based on the use of older stylized hermaphrodite phantoms that are not in alignment with the most recent ICRP reference phantoms. In addition, different computational methods for radiation transport have been used to report organ doses from which the effective dose is computed. Those responsible for such calculations and their interpretation would welcome the availability of ICRP reference organ and effective dose coefficients. 

In large part, the current System of Radiological Protection for humans is based on population averaged approaches for practical and ethical reasons, for example in the age- and sex-averaged values of Effective dose. There are nonetheless some areas where stratification is applied; detriment for example is calculated for the entire population and for a working age population in ICRP Publication 103, also an additional restriction to occupational dose limits is applied to pregnant workers. ICRP Publication 147 sets out an approach whereby Effective dose can be considered as an approximate indicator of possible risk, recognizing the lifetime risks vary with age at exposure, sex, and population groups (for example, Western European, Asian, North American etc). Principally, within the medical imaging community there is an increasing interest in having better indicators of risk from procedures for the individual patient. Furthermore, recent developments in dosimetry, particularly in the libraries of phantoms that encompass age, sex, height, weight and posture make it more feasible to provide more precise organ/tissue-specific doses accounting for these factors. Regarding cancer risk, a review of the method of calculation of the radiation-related detriment has been recently performed. As a result, a revision of the method has been launched, and proposes to include an update of cancer risk models and reference rates, better consideration of variations with region, sex and age, and improvement of severity weighting and interpretability.