[Precision of conventional simulation: which extent of accuracy is reachable in head and neck region?]

In this prospective investigation we tested the absolute accuracy of a conventional localization of the isocenter obtained from a three-dimensional treatment plan in conformal radiotherapy of head and neck tumors.

41 isocenters in 41 consecutive patients with histologically proven tumors of the brain or head and neck region were included into this investigation. In all cases a stereotactic mask (Stryker-Leibinger) was made for fixation and positioning of the patients. The stereotactically guided fractionated radiotherapy was carried out on the base of CT and MRI. The stereotactic coordinates were defined by an external localization system. Afterwards each patient underwent a conventional simulation using exclusively anatomical reference points. Additionally, the patients were adjusted to the linac isocenter using a stereotactic targeting device. Deviations between the simulated and the external adjusted target point coordinates were recorded in X-, Y- and Z-direction and spatial error was calculated.

Mean deviation was 2.15, 2.54, and 2.69 mm for X-, Y-, and Z-direction, respectively. The largest deviation was found in Z-direction with a maximum of over 11 mm. The spatial deviation per patient amounted 1.3-12.2 mm with a mean of 5.1 and a median value of 4.5 mm. That means that in half of the cases a deviation of 5 up to over 12 mm occurs in conventional simulation. Only in a quarter of the sample the deviation was 4 mm or below.

The planning target volume definition requires a consideration of the inaccuracy of the conventional simulation. A reduction of the safety margin in the planning target volume assumes the use of the stereotactic target positioner. This is to postulate especially for the treatment of benign lesions or lesions or lesions adjacent to critical structures.