In a previous work we reported that the fraction of the electron energy absorbed in the basal cell layer of the anterior nasal passages was not very sensitive to changes in the surface area or radius of the cylindrical model adopted in Publication 66 of the International Commission on Radiological Protection. These absorbed fraction data are used in calculation of the dose to a 10-microm-thick basal cell layer located at a depth of 40 microm in the epithelial cell layer of the extrathoracic (ET1) region. However, these data may only be applicable to the assumed cylindrical geometry and may not be valid for more realistic ET1 geometries. The nose differs in size and shape from one person to another, its shape is not cylindrical but closer to a truncated elliptical cone, and in most humans the nostrils are elliptical in shape. We propose herein a more realistic geometry model, the frustum of a cone, for the anterior nose region (ET1) as an alternative to the cylinder model provided in ICRP 66. The results of absorbed fraction calculations using MCNP4B with the new model are reported. These absorbed fractions are compared to the values previously obtained using the MCNP4B code and a cylindrical model (10 cm2 surface area). We also investigate the effects of changing the size of the truncated cone to represent variations due to sex and age.

The Human Respiratory Tract Model of ICRP Publication 66 is used for calculations of dose in the extrathorathic (ET1) airways. Scaling for age and body size is included in determining the mass of the target tissue (basal cells) in ET1 but is not included in deriving the absorbed fraction for particulate radiation. For dose calculations, it has been assumed that all absorbed fractions for particulate radiation published in ICRP Publication 66 are independent of age and body size. Therefore, these absorbed fractions are applied to calculate specific effective energy values not only for the Reference Worker but also for non-adults with noses of different sizes. In this note changes to the size of the cylinder model of the anterior nose in ICRP 66 are made by varying the surface area, the cylinder radius, and the thickness of tissue beyond the target region (basal cell). The energy deposition (absorbed fractions) in the target region (basal cells) is calculated using the MCNP4B (Monte Carlo) code to study the effects of these changes on the predicted absorbed fractions within the cylinder model.

Dose calculations using the respiratory tract model presented in Publication 66 of the International Commission on Radiological Protection (ICRP) frequently predict that the basal cells of the anterior portion of the nose, the extrathoracic region ET1 of the model, are the most highly irradiated tissue of the body. The dose to the basal cells is averaged over a layer of tissue 10 microm thick located at a depth of 40 microm into the airway. Reported here are the results of a series of absorbed fraction calculations undertaken to compare with values tabulated in ICRP Publication 66. The Monte Carlo code MCNP4B and the geometric model of the ET1 region specified in Publication 66 were used in the calculations. Although some calculated differences are evident between the two sets of absorbed fractions, typically less than 20%, the calculations confirm that the electron absorbed fractions tabulated in Publication 66 are not responsible for the high estimates of the ET1 dose.