Variation of output with atmospheric pressure and ambient temperature for Therac-20 linear accelerator

The Therac-20 (a linear accelerator manufactured by the Atomic Energy of Canada, Ltd.) employs an unsealed monitor chamber to control the dose output. Daily fluctuations in machine output for both x rays and electron beams were observed to vary with ambient temperature and atmospheric pressure. These variations were not related to any other machine parameters. Variations as large as 3.5% were seen by monitoring 18-MV x-ray output over several months. We recommend that the manufacturers take steps to eliminate the atmospheric dependence of dose rate.