In vivo dosimetry during conformal therapy of prostatic cancer

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In vivo dose measurements were performed during the simultaneous boost technique for prostatic cancer to check the accuracy of dose calculations by a monitor unit calculation program and a three-dimensional planning system. The dose of the large field and the boost field are given simultaneously using customized 10 mm thick Roses-metal blocks in which the boost field is cut out. Following the procedure of the quality assurance protocol for this technique, the dose at the specification point has been determined by in vivo dosimetry. The measured dose was initially too high for 5 out of 16 patients, due to unexpected differences in two beams with the same nominal beam quality and a different density correction for the femoral heads; the monitor unit calculation program was therefore checked and improved. The dose at the specification point was also compared with calculations performed by a CT-based three-dimensional (3-D) planning system, The average deviation of the 3-D planning system from the measurements is 0.1% ± 1.2%. Entrance, midline and exit dose values in the central axial plane, in a cranial plane and in a plane under the transmission block have also been compared with calculations performed by the 3-D treatment planning system. The measured entrance dose is, on average, 3.4% higher than the calculated dose for the AP beam and up to 5.5% for the lateral beams. Phantom measurements were performed and showed that these differences were not related to patient set-up errors. For the various midline points the difference between the measurements and the calculations is about 0.5%. No systematic deviations were found for the exit points. The deviations resulted mainly from a change in the percentage depth dose due to the tray supporting the transmission block. Our results demonstrated that for every change in the treatment procedure the validity of the dose calculation has to be rechecked. Using the CT-based 3-D treatment planning system we are able to calculate the dose very accurately for this high dose-high precision technique.