Dosimetry in the irradiation of thin culture layers using a 60Co radiotherapy unit at 20 cm SSD

In order to reach a dose rate of ca. 20 Gy/min with a typical Co-60 radiotherapy unit, lactic acid bacteria (LAB) are irradiated at a 20 cm source-to-specimen distance (SSD). By choosing an SSD which is a fourth part of the conventional 80 cm, the dose rate in the specimen is increased by a factor of ca. 16. A Monte Carlo simulation of the CGR Alcyon II Co-60 radiotherapy unit at 20 cm SSD is performed using a BEAM-EGS4 Monte Carlo code. As a result of calculations, relative dose distributions in a solid-water phantom are shown, together with measured thermoluminescence dosimetry (TLD) data. An additional scattering component due to the sideways scattering from the beam limiting device structures of the radiotherapy unit is seen in the beginning of the BEAM data.