A comparison of calculated and measured data for total body irradiation by 10 MV x-rays

This paper describes the rationale for using computed rather than measured data as a reference in the dosimetry of total body irradiation. The proof of this statement rests on a comparison of measured dosimetric values for large fields at extended distances with those derived by simple recalculation from the data measured at the isocentre. The depth doses and dose rates were experimentally obtained for 10 MV x-rays at distances of 100, 200 and 300 cm for collimated square fields with sides ranging from 5 to 30 cm. Phantoms of different volumes and shapes, including a RANDO phantom, and a large (50 cm x 50 cm x 50 cm), intermediate (25 cm x 25 cm x 25 cm), and 'mini-phantom' (build-up cap, 4.6 cm diameter) were used. Comparison of the measured and computed data for the largest collimated field shows that (i) calculated dose rates do not differ by more than +1% from the measured data, phantom size and shape having no effect on the results, (ii) depth doses measured in a patient-like RANDO phantom are a maximum of 2% higher than the calculated data but are also 2% lower than the depth doses measured in a standard water tank. For the radiation fields and treatment distances commonly employed in total body irradiation, we conclude that the calculated data can serve as reference values for dosimetry because they have the same inherent uncertainty as the data measured in non-patient-like phantoms.