Microdosimetry of inhomogeneous radon progeny distributions in bronchial airways.

A Monte Carlo code, initially developed for the calculation of microdosimetric spectra for alpha particles in cylindrical airways, has been extended to allow the computation (i) of additional microdosimetric parameters and (ii) for realistic exposure conditions in human bronchial airways with respect to surface activity distribution and airway geometry. The objective of the present study was to investigate the effects of non-uniform distributions of radon progeny activities in bronchial airways on cellular energy deposition parameters. Significant variations of hit frequencies, doses and microscopic energy deposition patterns were observed for epithelial cell nuclei, depending strongly on the assumed activity distributions. Thus, epithelial cells located at different positions in a given bronchial airway may experience a wide range of biological responses. The results obtained suggest that the hit frequency may be the primary physical parameter for alpha particles, supplemented by microdosimetric single event spectra, to be related to biological effects for chronic low level exposures.

Energy deposition, cellular radiation effects and lung cancer risk by radon progeny alpha particles.

Slowing down spectra, LET spectra, hit probabilities, and radiation doses were simulated for the interaction of single 218Po and 214Po alpha particles with sensitive basal and secretory cell nuclei in the bronchial epithelium of human and rat lungs for defined exposure conditions. Probabilities per unit track length for transformation, derived from in vitro experiments with C3H 10T1/2 cells, were used to estimate transformation probabilities for randon progeny alpha particles in basal and secretory cells. Different weighting schemes were assumed to relate cellular hit probabilities, doses and transformation probabilities, obtained for different cell depths and airway generations, to lung cancer risk per unit exposure. In vitro transformation and in vivo lung cancer incidence were simulated by a state-vector model which provides a stochastic formulation of dose-rate dependent cellular transitions related to formation of double strand breaks, repair, inactivation, stimulated mitosis and promotion through loss of intercellular communication.

Modeling energy deposition and cellular radiation effects in human bronchial epithelium by radon progeny alpha particles.

Energy deposition and cellular radiation effects arising from the interaction of single 218Po and 214Po alpha particles with basal and secretory cell nuclei were simulated for different target cell depths in the bronchial epithelium of human airway generations 2, 4, 6, and 10. To relate the random chord lengths of alpha particle tracks through spherical cell nuclei to the resulting biological endpoints, probabilities per unit track length for different cellular radiation effects as functions of LET were derived from in vitro experiments. The radiobiological data employed in the present study were inactivation and mutation (mutant frequency at the HPRT gene) in V79 Chinese hamster cells and inactivation and transformation in C3H 10T1/2 cells. Based on computed LET spectra and relative frequencies of target cells, probabilities for transformation, mutation, and cell killing in basal and secretory cells were computed for a lifetime exposure of 20 WLM. While predicted transformation probabilities were about two orders of magnitude higher than mutation probabilities, they were still about two orders of magnitude lower than inactivation probabilities. Furthermore transformation probabilities for basal cells are generally higher than those for secretory cells, and 214Po alpha particles are primarily responsible for transformations in bronchial target cells.

A consistent set of neutron kerma coefficients from thermal to 150 MeV for biologically important materials.

Neutron cross sections for nonelastic and elastic reactions on a range of elements have been evaluated for incident energies up to 150 MeV. These cross sections agree well with experimental cross section data for charged-particle production as well as neutron and photon production. Therefore they can be used to determine kerma coefficients for calculations of energy deposition by neutrons in matter. Methods used to evaluate the neutron cross sections above 20 MeV, using nuclear model calculations and experimental data, are described. Below 20 MeV, the evaluated cross sections from the ENDF/B-VI library are adopted. Comparisons are shown between the evaluated charged-particle production cross sections and measured data. Kerma coefficients are derived from the neutron cross sections, for major isotopes of H, C, N, O, Al, Si, P, Ca, Fe, Cu, W, Pb, and for ICRU-muscle, A-150 tissue-equivalent plastic, and other compounds important for treatment planning and dosimetry. Numerous comparisons are made between our kerma coefficients and experimental kerma coefficient data, to validate our results, and agreement is found to be good. An important quantity in neutron dosimetry is the kerma coefficient ratio of ICRU-muscle to A-150 plastic. When this ratio is calculated from our kerma coefficient data, and averaged over the neutron energy spectra for higher-energy clinical therapy beams [three p (68) + Be beams, and a d (48.5) + Be beam], a value of 0.94 +/- 0.03 is obtained. Kerma ratios for water to A-150 plastic, and carbon to oxygen, are also compared with measurements where available.

Nuclear data for radiotherapy: presentation of a new ICRU report and IAEA initiatives.

An ICRU report entitled "Nuclear Data for Neutron and Proton Radiotherapy and for Radiation Protection" is in preparation. The present paper presents an overview of this report, along with examples of some of the results obtained for evaluated nuclear cross sections and kerma coefficients. These cross sections are evaluated using a combination of measured data and the GNASH nuclear model code for elements of importance for biological, dosimetric, beam modification and shielding purposes. In the case of hydrogen both R-matrix and phase-shift scattering theories are used. In the report neutron cross sections and kerma coefficients will be presented up to 150 MeV and proton cross sections up to 250 MeV. An IAEA Consultants' Meeting was also convened to examine the "Status of Nuclear Data needed for Radiation Therapy and Existing Data Development Activities in Member States". Recommendations were made regarding future endeavours.