SRS Techinique

X- Knife technique involves application of the BRW stereotactic headframe with an attempt to center the lesion as much as possible in stereotactic space. We routinely utilize a fused data set, so prior to frame application, patients are usually scanned in the MRI unit, then sedated with Ativan and the scalp is anesthetized with 0.5% Sensorcaine (9 parts Sensorcaine: 1 part sodium bicarbonate to minimize sting) for BRW frame application. After CT, patients are discharged to a room where they rest in the frame during treatment planning.

Treatment planning proceeds as described above and when completed, the LINAC is prepared and each isocenter qualified prior to patient treatment according to the printed treatment plan protocol. Qualification of the LINAC begins as a routine prior to any patient treatments with verification of mechanical isocenter. Since we utilize a couch-mount system, a device referred to as a mechanical isocenter standard (MIS) is placed in a floor housing and allows assessment of the alignment of four independent aiming lasers (Figure 3a). Any drift which may have occurred over a 24 hour period is corrected and the MIS is then removed. The appropriate collimator is installed and the couch mount microdrives are adjusted, guided by the aiming lasers, to the first isocenter position utilizing the rectilinear phantom pointer (RLPP, the same device utilized as a phantom in CRW-based stereotactic surgery, Figure 3b&c), which is inserted in the couch mount and calibrated to isocenter coordinates by one technologist. To verify isocenter colinearity and stability, we replace the RLPP target pointer with a spherical tungsten ball and, utilizing a film holder attached to the collimator housing unit, expose a film strip at three different gantry positions (Figure 3c). Each developed film strip therefore represents a survey of gantry rotational stability at a particular isocenter, and a variation less than 0.8mm between the ball center and radiation field center in all three exposures is considered acceptable. As a redundant check of isocenter coordinates, a second device called the laser target localizing frame (LTLF, Figure 3d) which is calibrated by a second technologist is placed on the RLPP. This device has engraved brass burnishments and an arcing steel burnishment all of which align with the aiming lasers when at isocenter. When placed on the RLPP, correct alignment at isocenter is once again confirmed and the couch mount is cleared of these devices.

When these quality assurance checks are complete, the patient is brought to the LINAC suite and placed on the couch with the BRW headring immobilized in the couch mount apparatus calibrated at isocenter. Isocenter is once again verified with the patient in position utilizing the LTLF which is now applied to BRW headring. We also verify that the BRW headring has not moved from initial frame application by reassessing scalp marks alignment made at frame application using a device called the depth confirmation helmut which is discussed below. The patient is now ready for treatment.