Dosimetric control of conformal treatment of parotid gland tumours

Abstract

The purpose of this study was to determine the dosimetric accuracy of the treatment of parotid gland tumours using 8 MV X-ray beams. These tumours are generally situated near the patient's skin. Entrance in vivo dose measurements with diodes were obtained for 20 patients during 5 sessions per patient, in the anterior-oblique and posterior-oblique wedged fields, on the central beam axis as well as in points situated in a cranial plane 2 or 3 cm off- axis. Phantom measurements were performed in order to determine the actual position of the 95% isodose surface. The measurements were compared with calculations performed with our three-dimensional treatment planning system. The reproducibility of the diode measurements on patients was found to be 1.4% (1 SD). The total accuracy in the entrance dose determination for the average of 2 measurements was 1.8% (1 SD). The central axis entrance dose for the anterior field was on average 1.5% ± 3.2% (1 SD) higher than the calculated value. For the posterior field, the difference was 0.9% ± 3.1% (1 SD). The deviations for the off-axis points were somewhat smaller, mainly due to overestimation of the block effect in the calculations. The value of the dose at the isocentre, obtained by extrapolation of the measured entrance dose values, differed 0.3% ± 2.1% from the calculations. The accuracy in dose determination at the isocentre was 2% (1 SD). After correction for the difference in prescribed and actual source-to-skin distance, the results showed good agreement with phantom measurements on a polystyrene phantom without inhomogeneities, performed both with diodes and an ionization chamber. The measured 95% isodose surface was situated 10.6 mm under the phantom surface, which meant a shift of 0.5 mm towards the surface compared with calculations. A 1–1.5 mm shift towards the block edges was observed. These results formed the basis of a dosimetric quality assurance protocol. The entrance dose on the central beam axis is measured twice at the beginning of the treatment. The total dose at the isocentre is calculated using the percentage depth dose values. If the difference between the average measured and calculated dose at the isocentre is larger than 4% (2 SD of the measurement accuracy at the isocentre), the reason for the discrepancy will be investigated and, if necessary, the number of monitor units will be adjusted.