Assessment of the accuracy of a conventional simulation for radiotherapy of head and skull base tumors.

In this prospective study we investigated the absolute accuracy of the conventional simulation in head and skull base tumors. 41 isocenters in 40 consecutive patients with tumors of the head and skull base were included. In all cases a rigid stereotactic mask system was used for non-invasive fixation. The stereotactic ("calculated") coordinates of the isocenter were defined by the treatment planning computer. Each patient underwent a physical simulation using exclusively anatomical reference points to define the "preliminary" isocenter. The displacement between its coordinates and those of the stereotactic target point was recorded in X-, Y- and Z-direction with help of the targeting device, and the spatial error was calculated. Additionally, the patients were stratified by basal or calvarial tumor site to estimate the importance of the basal bone structures in the simulation accuracy. The influence of the learning effect on simulation accuracy was also determined. The results showed an accuracy of set-up at the linac within 1 mm in all three directions as calculated from orthogonal portal films. Mean shift of the isocenter coordinates obtained from physical simulation compared to the calculated stereotactic coordinates was 2.15 mm, 2.54 mm, and 2.69 mm for X-, Y-, and Z-direction, respectively. Mean spatial displacement amounted 5.06 mm, and the median was 4.50 mm. No significant difference could be noted between basal and calvarial location of the isocenter. A significant "learning effect" was observed with a decrease in spatial shift with increasing patient numbers. This effect was stronger in basal lesions, whereas calvarial lesions showed only a minor, insignificant effect. In conclusion, a physical simulation requires a safety margin of 5 mm in PTV definition in addition to other factors, e.g. organ movement.

[Precision of conventional simulation: which extent of accuracy is reachable in head and neck region?]. / Präzision der konventionellen Simulation: Welche Genauigkeit ist im Kopf-Hals-Bereich möglich?

PURPOSE: 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. PATIENTS AND METHODS: 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. RESULTS: 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. CONCLUSION: 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.