NORMAN-05 conversion coefficients for monoenergetic neutrons below 20MeV

Neutron dosimetry in organs of an adult human phantom using linacs with multileaf collimator in radiotherapy treatments

PURPOSE

To calculate absorbed doses due to neutrons in 87 organs/tissues for anthropomorphic phantoms, irradiated in position supine (head first into the gantry) with orientations anteroposterior (AP) and right-left (RLAT) with a 18 MV accelerator. Conversion factors from monitor units to μGy per neutron in organs, equivalent doses in organs/tissues, and effective doses, which permit to quantify stochastic risks, are estimated.

METHODS

MAX06 and FAX06 phantoms were modeled with MCNPX and irradiated with a 18 MV Varian Clinac 2100C/D accelerator whose geometry included a multileaf collimator. Two actual fields of a pelvic treatment were simulated using electron-photon-neutron coupled transport. Absorbed doses due to neutrons were estimated from kerma. Equivalent doses were estimated using the radiation weighting factor corresponding to an average incident neutron energy 0.47 MeV. Statistical uncertainties associated to absorbed doses, as calculated by MCNPX, were also obtained.

RESULTS

Largest doses were absorbed in shallowest (with respect to the neutron pathway) organs. In μGyMU(-1), values of 2.66 (for penis) and 2.33 (for testes) were found in MAX06, and 1.68 (for breasts), 1.05 (for lenses of eyes), and 0.94 (for sublingual salivary glands) in FAX06, in AP orientation. In RLAT, the largest doses were found for bone tissues (leg) just at the entrance of the beam in the body (right side in our case). Values, in μGyMU(-1), of 1.09 in upper leg bone right spongiosa, for MAX06, and 0.63 in mandible spongiosa, for FAX06, were found. Except for gonads, liver, and stomach wall, equivalent doses found for FAX06 were, in both orientations, higher than for MAX06. Equivalent doses in AP are higher than in RLAT for all organs/tissues other than brain and liver. Effective doses of 12.6 and 4.1 μSvMU(-1) were found for AP and RLAT, respectively. The organs/tissues with larger relative contributions to the effective dose were testes and breasts, in AP, and breasts and red marrow, in RLAT. Equivalent and effective doses obtained for MAX06/FAX06 were smaller (between 2 and 20 times) than those quoted for the mathematical phantoms ADAM/EVA in ICRP-74.

CONCLUSIONS

The new calculations of conversion coefficients for neutron irradiation in AP and RLAT irradiation geometries show a reduction in the values of effective dose by factors 7 (AP) and 6 (RLAT) with respect to the old data obtained with mathematical phantoms. The existence of tissues or anatomical regions with maximum absorbed doses, such as penis, lens of eyes, fascia (part of connective tissue), etc., organs/tissues that classic mathematical phantoms did not include because they were not considered for the study of stochastic effects, has been revealed. Absorbed doses due to photons, obtained following the same simulation methodology, are larger than those due to neutrons, reaching values 100 times larger as the primary beam is approached. However, for organs far from the treated volume, absorbed photon doses can be up to three times smaller than neutron ones. Calculations using voxel phantoms permitted to know the organ dose conversion coefficients per MU due to secondary neutrons in the complete anatomy of a patient.

Development of an integrated couple of anthropomorphic models for dosimetric studies

Numerical models and anthropomorphic phantoms are frequently used in radiation protection studies for dose evaluation purposes. In the present paper an integrated tool consisting of a plastic phantom coupled with a voxel model is presented. The voxel model was created from the image data set of a CT scan of the plastic phantom. The model was validated through a procedure that consisted in irradiating the plastic phantom and simulating the same irradiation conditions with the voxel model. The possibility of employing such couple of models in radiation protection studies was demonstrated calculating the organ dose conversion coefficients and comparing them with those evaluated with voxel models derived from human tomographic scans.

A review of voxel model development and radiation protection applications at ENEA

The need of organ absorbed dose evaluation for radiation protection purposes stimulated, since the late 1960s, the development of anthropoid models to be used with radiation transport codes. Very significant improvements were introduced during the years, passing from stylised analytical human models to realistic voxel models based on computed tomography scans or MRI scans, and finally to advanced surface-geometry models. Besides illustrating the main contributions in this field from various international laboratories, this paper illustrates some applications of voxel models to internal (including in vivo monitoring) and external dosimetry for radiation protection.