INVIVO DOSIMETRY DURING PELVIC TREATMENT

High precision in vivo entrance and exit dose measurements have been performed with p-type diodes on patients during 8 MV X-ray irradiation of the pelvis, to investigate the accuracy of dose calculations in this region. Based on phantom measurements the accuracy of the p-type diode measuring system itself, i.e. the agreement with ionisation chamber dose measurements, was shown to be better than 0.7% while the reproducibility in the dose determination was 1.1%, 1.5% and 1.6% (1 S.D.) at the entrance point, isocentre and exit point, respectively, for the wedged lateral fields. Patient movement and the uncertainty in the diode position increased these values to 1.7%, 1.5% and 3.1% (1 S.D.) for dose determinations on patients. From the entrance and exit in vivo dose values the dose actually delivered to the isocentre was determined. For the anterior-posterior beams a good correspondence for most patients was observed at the entrance and exit point and at the isocentre between the in vivo and calculated dose values. For the wedged lateral beams a systematic deviation of about 3 % was observed. In addition to the in vivo dose measurements phantom dose measurements have been performed to quantify the accuracy of the dose calculation algorithms including the computation of the number of monitor units. These measurements also served to quantify the effects of the actual patient on the dose delivery. The measurements showed that accurate calculation of the dose requires a separation of the head and phantom scatter contribution of the output of the treatment machine. The dependence of the wedge factor on field size, depth and source-skin-distance has also to be considered for accurate dose calculations. The effect of the patient on the dose calculation is mainly related to the actual electron densities of fat and bone structures compared to water: neglecting these densities in the dose computation could yield deviations up to 8.5% for the exit point in wedged beams. Based on these results, improvements in the dose calculation algorithms and monitor unit calculation including the use of the actual electron densities will be implemented in the treatment planning procedure.