In vivo dosimetry for tangential breast irradiation: role of the equipment in the accuracy of dose delivery.

Entrance dose measurements (920) have been performed on both tangential treatment fields for 105 breast cancer patients. About half of them (52) were treated on a modern unit (Mevatron, Siemens), supplied with an automatic verification system, the other half (53) were treated on an old therapy unit (cobalt-60). A team of three radiographers worked on the modern unit, while on the old unit only one radiographer was responsible for the treatment set-up. A small systematic error (+ 1.4%) has been detected on the old unit: a small discrepancy existed in the region of the maximum dose between the measured and the published percentage depth-dose values (Br. J. Radiol., Suppl. 17), the latter being used for dose calculation. A striking difference has been observed for the rate of large deviations (deviations of 5% and more from the mean dose): 2.3% (10/430) for the modern unit versus 15% (75/490) for the old one. This shows clearly that the precision in dose delivery is strongly dependent on the characteristics and staffing of the therapy unit, the modern unit being supplied with a check-and-confirm system and with the beam accessory devices in a fixed position in the beam and a team of radiographers being responsible for the set-up.