Fluence-to effective dose conversion coefficients

In computing effective dose an hermaphrodite mathematical model has been used. It was derived from the male phantom developed for MCNP by GSF-Forschungszentrum für Umwelt und Gesundheit (Germany) [21]. The MCNP phantom was translated in terms of bodies and regions appropriate for the combinatorial geometry of FLUKA. Then the female organs (breast, ovaries and uterus) were added. The various organs and tissues of human body have been represented by 68 regions. Internal organs have been considered to be homogenous in composition and density. The composition was limited to the 14 elements: H, C, N, O, Na, Mg, P, S, Cl, K, Ca, Fe, Zr and Pb. Different densities have been used for the lungs (0.296 $g \cdot cm^{-3}$), bone (1.486 $g \cdot cm^{-3}$), red marrow (1.028 $g \cdot cm^{-3}$), soft tissue (0.987 $g \cdot cm^{-3}$) and skin (1.105 $g \cdot cm^{-3}$).

Calculations of effective dose have been carried out on the basis of equation 3 using the radiation weighting factors shown in table 1. For charged pions and kaons the approximation given by eq. 2 was used. If one is willing to adopt other $w_T$ values, the calculated data ($f_E$ calculated following eq. 4) should be scaled with respect to the $w_R$ used.

Concerning the tissue weighting factors $w_T$, values shown in table 3 were used and the so called ``remainder dose'' has been evaluated from the doses to nine additional individual organs and tissues as arithmetic mean. In the present calculations, the dose to a given organ or tissue spread throughout the whole body and represented in the mathematical model by several regions (for instance skin, bone, red bone marrow, muscle ...). has been determined as arithmetic mean of the doses received in the single constituent regions. According to ICRP Publication 67 the higher value of doses to the ovaries and testes was applied to the gonad weighting factor. The dose to muscles has been assumed as the arithmetic mean of the doses received by the part of the body volume which is not attributed to any other organ or tissue.

Calculations were performed for fully isotropic radiation incidence (obtained by the use of an inward-directed, biased cosine source on a spherical surface), from semi-isotropic (from the top) radiation source and with broad parallel beams with the following directions of incidence: antero-posterior, postero-anterior, right lateral, from the top and from the bottom. The medium between the source and the phantom was assumed to be vacuum.

The energy per primary incident particle deposited in the regions representing the various organs and tissues has been determined by use of MonteCarlo simulations.

Once the effective dose ($E(\epsilon)$) as a function of particle energy for various kinds of radiation was computed, the fluence-to-effective dose conversion coefficients ($f_E(\epsilon)$) were calculated in terms of effective dose per unit of fluence ($Sv \cdot cm^2$):

$$f_E(\epsilon) = \frac{E(\epsilon)}{\Phi(\epsilon)}$$ (4)

where $\Phi(\epsilon)$ is the fluence of primary particle of energy $\epsilon$.

Calculation results are presented in section 4.