Consideration of factors for the determination of the absorbed dose from exposure measurements in the beam of60Co teletherapy units

Most frequently the absorbed dose is determined by calculations from exposure measurements with ionization chambers. In order to check the calculations, we have examined the ratio rad/Rontgen depending on the depth in the material by direct measurement of the absorbed dose with a graphite calorimeter and exposure with an ionization chamber calibrated for60Co radiation. These measurements were performed in the beam of two special60Co teletherapy units. The difference of 9 per cent in the ratio mentioned above can be explained by the details of construction and the amount of scattering material in the vicinity of the radiation source, which changes the energy spectrum of the radiation beam to different extents. Therefore, for reasonable accuracy, the factorf exp should be determined by direct measurement for each type of60Co teletherapy unit.