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Field displacement during external radiotherapy in prostatic adenocarcinoma treated with radioactive 198Au implants and external irradiation.

The purpose of this work was to study displacement error and internal movements of the prostate during external beam radiotherapy. Verification films in the frontal (n = 194) and lateral (n = 64) portals were investigated in 14 patients treated with radioactive 198Au implants. Displacement errors of two implants were investigated. In seven patients, filling of the rectum and the bladder with contrast medium or isotonic saline was performed during CT investigation for planning purposes to detect movements of the prostate. Most (95%) of the displacement errors were less than 10 mm in the frontal portal and less than 15 mm in the lateral portals. No correlation to the patient's weight was found. The displacement errors were randomly distributed. The spatial relations between the implants were not altered during the treatments. Small movements of the prostate were observed. To conclude, the positioning system employed at present (laser) can be sufficient for the margins used (2 cm). In lateral portals, however, the system did not have the ability to detect a possible systematic displacement error from simulator to accelerator. The intention is to decrease the margins to 1 cm, which will necessitate a better positioning system.