L. A. Braby and J. R. Ford
Texas A&M University, College Station TX
In recent years studies with low doses of high LET radiation (primarily alpha particles), delivered at random or targeted on specific cells and cellular components by microbeam irradiation systems, have shown that individual cells often do not respond solely to the energy deposited in them, but also to energy deposited in their neighbors or their ancestors. Although low LET radiations are responsible for the majority of the dose received by most individuals, it is extremely difficult to tell if low LET radiations can induce similar responses in cells which are not directly irradiated. At the doses where bystander effects can be observed in random alpha particle irradiation, experiments with low LET irradiation will produce nearly identical doses in every cell. Microbeam irradiation approaches to low LET irradiation are just beginning to be used, and are limited by the scattering properties of electrons. We have recently assembled a microbeam irradiation system which delivers monoenergetic electrons at selected energy between a few keV and 100 keV to cells attached to the surface of a thin plastic film making up the bottom of a special petri dish. Since some bystander effect experiments may benefit from macrobeam irradiations which cover all cells in a defined area, beam diameters from 5 micrometers up to over 300 micrometers have been made available. We are beginning studies of bystander cell effects in human bronchial epithelial primary cells and in a human skin fibroblast cell line which has shown bystander effects in our high LET irradiation experiments. In these experiments we are irradiating approximately 10% of the cells in a 5 mm diameter cluster near the center of the 50 mm diameter dish, and are comparing levels of p21 and PCNA in targeted and unirradiated cells.