Purpose: The efficacy of a conventional, noninvasive fixation technique in combination with a commercially available system for conformal radiotherapy by intensity modulation of the treatment beam has been studied.
Methods and materials: A slice-by-slice arc-rotation approach was used to deliver a conformal dose to the target and patient fixation was performed by means of thermoplastic casts. Eleven patients have been treated, of which 9 were for tumors of the head and neck region and 2 were for intracranial lesions. A procedure for target localization and verification of patient positioning suitable for this particular treatment technique has been developed based on the superposition of digitized portals with plots generated from the treatment-planning system. A dosimetric verification of the treatment procedure was performed with an anthropomorphic phantom: both absolute dose measurements (alanine and thermoluminescent detectors) and relative dose distribution measurements (film dosimetry) have been applied. The dose delivered outside the target has also been investigated.
Results: The dose verification with the anthropomorphic phantom yielded a ratio between measured and predicted dose values of 1.0 for different treatment schedules and the calculated dose distribution agreed with the measured dose distribution. Day-to-day variations in patient setup of 0.3 cm (translations) and 2.0 degrees (rotations) were considered acceptable for this particular patient population, whereas the verification protocol allowed detection of 0.1 cm translational errors and 1.0 rotational errors.
Conclusions: The noninvasive fixation technique in combination with an adapted verification protocol proved to be acceptable for conformal treatment of the head and neck region. Dose measurements, in turn, confirmed the predicted dose values to the target and organs at risk within uncertainty. Daily monitoring becomes mandatory if an accuracy superior to 0.1 cm and 1.0 degree is required for patient setup.