IMRT

Invasive frame-based: targeting

g Couch mounted positioning rely on:

Positioning overlays

Figure 1: The invasive head frame fixation (frame-based) (left) and the mask immobilization (frameless approach) (right) Invasive fixation

CT localization Isocenter localization

Ø Wall mounted lasers / cross-hair Ø Visually aligning Ø Alignment accuracy § 0.5-1.5 mm Maciunas 2004 Lamba 2009

Finally, even with the technological advancements that enable a good dosimetrical plan with sub-millimetric positioning of the target and immobilization of the patient, some uncertainties in target localization and positioning are still present and must be taken into account during a SRS treatment (9).

1.2.1.

Evaluation of a stereotactic radiosurgery planning

7 The large number of parameters involved in radiosurgery means that there are many degrees of freedom so that many possibilities can be explored to achieve a desired objective, but this also makes the task of finding the optimum choice very difficult. The planner is faced with the often conflicting aims of making the radiation dose conform as closely as possible with the treated volume, hereby minimizing the dose to critical structures and brain (steep dose gradient), and maintaining dose uniformity in the target (10). Therefore, treatment quality indices were made to analyze whether or