Irradiating cell samples in an x-ray radiation cabinet--the effect of tube filtration.

The cell irradiation cabinet described is used for creating DNA damage in cell samples in order to study tumourigenesis. The medical research laboratory involved was using the manufacturer's quoted dose rate (32.2 mGy s(-1)) to determine the required exposure time to impart a dose of 10 Gy. The x-ray output was investigated when the exposure failed to produce cell cycle arrest. The x-ray tube was fitted with only a 0.76 mm beryllium filter, and the spectrum therefore contained a high proportion of low energy photons which were being removed by the polystyrene sample flask as demonstrated by dose measurements in air and through the sample flask. Incorporation of a 0.5 mm aluminium filter removed most of these low energy photons but greatly reduced the dose rate to 3.8 mGy s(-1). The manufacturer's quoted dose rates from a lightly filtered tube are misleading: the contribution of the very low energy component of the spectrum to the dose is not relevant since it will be removed by a sample flask.