Analysis of fiducial markers used for on-line verification in the external-beam radiotherapy of patients with cranial tumours

Evaluate the fiducial marker-based position verification in the external-beam radiotherapy of patients with cranial tumour. Thirteen patients with intracranial tumours were treated with external- beam radiotherapy using 3 gold markers implanted in the skull. Before each fraction the patient was positioned on the treatment table and 2 orthogonal portal images were performed to localise the 3 gold seeds and the target position was calculated using a commercialised computer program (ISOLOC software, MEDTEC). This program provides the couch movements required to move the target to the isocentre. When the set-up error was corrected using the coordinates of the 3 markers, the final movements were less than 2 mm in all cases: lateral, mean v., 1.21 mm; longitudinal, 1.23 mm; and anteroposterior, 1.18 mm. No serious complications related to the gold marker insertion were noted. The use of 3 implanted fiducial seeds is an optimal technique for precise set-up in patients with brain tumours treated with external radiotherapy. This commercial system is highly suitable for fractionated stereotactic irradiation.