DOSIMETRIC CONTROL OF CONFORMAL TREATMENT OF PAROTID-GLAND TUMORS

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 t umours are generally situated near the patient's skin. Entrance in viv o dose measurements with diodes were obtained for 20 patients during 5 sessions per patient, in the anterior-oblique and posterior-oblique w edged fields, on the central beam axis as well as in points situated i n a cranial plane 2 or 3 cm off-axis. Phantom measurements were perfor med in order to determine the actual position of the 95% isodose surfa ce. The measurements were compared with calculations performed with ou r three-dimensional treatment planning system. The reproducibility of the diode measurement of patients was found to be 1.4% (1 SD). The tot al accuracy in the entrance dose determination for the average of 2 me asurements was 1.8% (1 SD). The central axis entrance dose for the ant erior field was on average 1.5% +/- 3.2% (1 SD) higher than the calcul ated 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 t he measured entrance dose values, differed 0.3% +/- 2.1% from the calc ulations. The accuracy in dose determination at the isocentre was 2% ( 1 SD). After correction for the difference in prescribed and actual so urce-to-skin distance, the results showed good agreement with phantom measurements on a polystyrene phantom without inhomogeneities, perform ed both with diodes and an ionization chamber. The measured 95% isodos e 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- 15 mm shift towards the block edges was observed. These results formed the basis of a disometric quality assurance protocol. The entrance do se on the central beam axis is measured twice at the beginning of the treatment. The total dose at the isocentre is calculated using the per centage depth dose values. If the difference between the average measu red and calculated dose at the isocentre is larger than 4% (2 SD of th e measurement accuracy at the isocentre), the reason for the discrepan cy will be investigated and, if necessary, the number of monitor units will be adjusted.