Seeing radiation: Applications of an imaging survey meter in radiation oncology

Radiation shielding surveys require location and evaluation of radiation hotspots in shielding barriers. This process is normally done by using a conventional survey meter and slowly moving the survey meter manually across the face of the barrier. It would be advantageous to have a radiation image of the barrier showing hot spots. This would hasten the process, potentially reveal hidden hot spots and provide clear and unprecedented documentation for a variety of radiation sources, including HDR afterloaders, linacs, Gamma Knife and CT simulators. The purpose of this study is to evaluate a new survey instrument that can superimpose radiation "heat" maps on optical images, allowing the user to "see" the radiation distribution.

A new survey instrument, the RAVIN (manufactured by M3D, Inc. Ann Arbor MI) has been evaluated for potential use in shielding surveys of linac vaults, HDR suites, Gamma Knife, CT simulators, and the search for lost radioactive seeds. This instrument also provides a NIST traceable calibrated measurement of the exposure rate or the air-kerma rate. In addition, the instrument can show the spectrum of the measured radiation and can identify a wide range of commonly used isotopes. Comparisons have been made with conventional survey meters.

Heat maps have been acquired for the open door of a linac vault, linac head activation, an HDR afterloader, a Gamma Knife, the entrance to a CT simulator and an I-125 seed. The camera provides some interesting information about the spectrum of radiation encountered in different shielding circumstances in radiation oncology. An example is the spectrum of radiation emerging from a thick linac 18 MV concrete primary barrier. The instrument shows a pronounced peak between 100 and 150 keV, with a long high energy, but low amplitude tail. Comparison with ionization chamber survey meters shows that this instrument can accurately measure air kerma rates from continuous high energy sources of radiation but cannot be used to measure air kerma rates from pulsed linac beams due to its relatively long dead time of 30 µs.

The RAVIN camera is an innovative instrument that allows visualization of radiation under some circumstances while simultaneously accumulating an energy spectrum. The ability to "see" radiation is remarkable. This instrument is, however, probably not useful for routine radiation surveys of barriers in radiation oncology. This is due to the inability to reliably image heat maps for low levels of radiation (as are generally encountered outside shielding barriers) and to accurately localize the source.