Measurements of equivalent dose rates from non-primary beam x rays in therapy rooms of medical electron accelerators

Equivalent dose rates from non-primary beam x rays in therapy rooms of 10 medical electron accelerators producing x rays at 4,6,8,10,15 and 18 MV, have been measured with FARMER 257O/1A dosemeter and FJ347A x, γ dosemeter. Survey results indicate that the equivalent dose rates from nonprimary beam x rays at measurement points in patient plane are increased with increasing primary beam doses at the isocenter and with enlarging field areas on the patient or a phantom, but the dose rates are gradually lowered with increasing the distance from the measurement points to the isocenter and are insensitive to the variation of the energy of the primary photons. The ratio of the equivalent doses from non-primary beam x rays to the doses from the isocenter, the radiation ratio, may be calculated with the simple relation of: f = 4.6 x 10-5 F1/2Z-3/2. The values estimated with this equation were in good agreement with the survey results. Both evaluated and measured values generally agree within 30%