Microbeam

What is a Microbeam?

A microbeam is a micrometer or sub-micrometer diameter beam of radiation, that allows damage to be precisely deposited at specific locations within a biological target.

The use of micro-irradiation techniques in radiation biology dates back to the 1950s to the work of Zirkle (1957) and Munro (1961). However, we are now able to take advantage of recent developments in particle delivery, focusing and detection, image processing and recognition and computer control, coupled with the benefits of new assays of individual cellular response.

The main components of the microbeam are:

• A source of radiation
• A focusing system
• An ion detector for determining delivered dose
• A beam shutter
• An imaging system for detecting the target of interest
• A stage for moving the target of interest into the beam and/or a beam deflector for moving the beam to the target.

In recent years, several groups have developed charged-particle microbeams, in which cells on a dish are individually irradiated by a predefined exact number of alpha particles, allowing the effects of exactly one (or more) alpha particle traversals to be investigated. Biological interest in the microbeam stems from the potential to define the ionizing energy absorbed by a cell, in terms of space, time, and number:

• The microbeam allows irradiation of many cells, each in a highly localized spatial region, such as part of the nucleus, the cytoplasm, or through the immediate neighbor cells of a given cell.
• The microbeam also allows particles to be passed through a cell with a known temporal separation, to investigate, for example, the dynamics of cellular repair.
• Microbeam techniques can deliver exactly one or more particles per cell.

Irradiation of Cells in Precise Spatial or Temporal Locations

Whilst it is generally true that radiation is an excellent and commonly used probe of cellular damage and repair mechanisms, such studies normally lack the ability to target specific components of a cell. This is however achievable with a microbeam. A controlled amount of energy can be placed, via a charged particle, through a sub-cellular component, rather than via spatially random depositions. In this way, the effect on gene expression, cell cycle signaling, DNA damage and repair, lesion interaction, chromosomal changes, and on later mutagenic and carcinogenic change can be evaluated. Spatially, particle traversal locations can be placed either in the nucleus, through the cytoplasm (intentionally missing the nucleus), a specific target in the cell (either nuclear or cytoplasmic), or as bystander irradiation where one cell is targeted a neighboring cell is guaranteed not to be irradiated.

The microbeam provides the spatial resolution lacking from all other sources of ionizing radiation which is particularly pertinent for examining mechanisms of chromosome aberration formation, and early gene responses to spatially precise energy deposition. In addition to evaluating the consequences of precise numbers of events with or without precise placement, the timing between energy deposition events can be altered such that the kinetics of interactional repair processes can be probed. Also given the capacity of the microbeam to record and return to individual cellular positions, the timing between particle irradiations can range from microseconds to periods within one phase of the cell cycle, between phases of the cell cycle, and even between parent-progeny over cellular generations.

Irradiation with an Exact Number of Particles

Microbeam techniques are necessary to elucidate the biological effects of exactly one particle because, due to the random (Poisson) distribution of tracks, this cannot practically be simulated in the laboratory using conventional broad-field exposures. Microbeam techniques can overcome this limitation by delivering exactly one (or more) particle per cell nucleus. This is important since, at the low doses of relevance to environmental radiation exposure, individual cells only rarely experience traversals by an ionizing particle, and almost never experience more than one traversal.

For example, in the case of radon, which dominates the radiation exposure of the general public, radon risk estimation involves epidemiological studies of uranium miners. The average lifetime radon exposure of these miners is sufficiently high that risk estimates are driven by data on miners whose target bronchial cells are subject to multiple alpha particle traversals. On the other hand, for an average house occupant, about 1 in 2,500 target bronchial cells will be exposed per year to a single alpha particle, but less than 1 in 107 cells to traversals by more than one particle. Therefore, in order to extrapolate from miner to environmental exposures, it is necessary to be able to extrapolate from the effects of multiple traversals to the effects of single traversals of particles.

To see how one might predict the effects of single particles consider an experiment designed to measure the effects of single a particles traversing, say, C3H10T½ cells. Assuming an LET of 150 keV/µm, at a low practical dose of about 0.1 Gy, on average each cell nucleus will be traversed by a single alpha particle. However, as the number of traversals of a given cell is Poisson distributed, about 26% of the cells will be traversed by more than one particle, about 8% by more than two, and about 2% by more than three particles. But we are interested in the effects of exactly one alpha particle.

The approximate number of cells (~103) exposed to different numbers of a particle traversals in the bronchial epithelium of a) a Colorado uranium miner (averaged), b) an average environmentally-exposed person, and c) in a 0.1 Gy in vitro experiment (from Brenner, 1989).

The most direct solution to this problem is the use of a microbeam which can deliver exactly one particle to a cell. True single-particle irradiations should thus allow measurement of the effects of exactly one alpha particle traversal, relative to multiple traversals. The application of such systems to low-frequency processes such as oncogenic transformation depends very much on the technology involved. With an irradiation rate of at least 5,000 cells per hour, experiments with yields of the order of 10-4 can practically be accomplished.

Learn more about how to access the Microbeam on our Ion Beam and Neutron Core Facility page.