Dose intercomparison at the radiotherapy centres in The Netherlands. 1. Photon beams under reference conditions and for prostatic cancer treatment.

In 1985, a dosimetry intercomparison was performed at all 20 radiotherapy centres in The Netherlands. Absorbed dose was determined with an ionization chamber under reference conditions in a water phantom for cobalt-60 gamma-ray and megavoltage X-ray beams. The mean difference between measured and stated dose values was 0.5% with a standard deviation of 1.9%, but up to 6% at maximum. As soon as all institutes apply a common dosimetry protocol, this maximum difference will reduce to about 2%. In addition, an anthropomorphic phantom was irradiated to simulate the treatment of a prostatic cancer. The dose, determined with an ionization chamber at the isocentre and thermoluminescent dosimeters (TLD) powder at several points situated in the target volume, the bladder and the rectum, was compared with the stated dose calculated with the local planning system. Only small differences were found between the measured and stated dose at the isocentre: on the average 1.5%, with a standard deviation of 1.5%. The difference between stated and measured dose at several points situated in the target volume was on the average 0.4%, with a standard deviation of 5.2%. Almost the same result was found for a point situated in the bladder. In the rectum, the average difference was about 4%, however, with a large standard deviation, 18%, due to the relatively steep dose gradient at these points.