SRT Techinique

Our SRT techniques involved fractionation protocols based on radiobiology of the lesion and associated dose-limiting structures. Historically, prior to the installation of a dedicated Varian 600SR LINAC, we designed hypofractionation schemes involving 4 Gy fractions delivered twice a week, typically Monday / Thursday or Tuesday/ Friday, over five weeks for a total dose of 36 Gy. This fractionation method was designed to maximize tumor control and minimize cranial neuropathies considered high with single fraction techniques reported at that time. Without the logistical constraints of a retrofitted LINAC, we additionally created a conventional fraction treatment scheme designed to preserve special sensory cranial nerves in patients with tumors involving or near these critical structures12. This paradigm involved the use of 1.8 Gy fractions delivered daily to a cumulative dose of 50 to 54 Gy over five weeks. Imaging data included both CT and MRI data sets which are fused for treatment planning and treatment and involved the use of the Gill-Thomas-Cosman relocatable frame utilizing Reprosil®. Single isocenters, infrequently two, and rarely three, are utilized and high conformality is once again established by utilizing non-coplanar arc beam-shaping and differential beam weighting.

For SRT treatment, we have developed a quality assurance program as previously described12. The accurate reproduceablility of GTC frame application is verified by assessing the relationship of the frame to the skull contour utilizing a device called a depth conformation helmut. This device, designed by the Radionics Corporation, is a lucite spherical helmut which attaches to the GTC frame. Its design includes 26 tube portals arrayed equidistantly at right angles to the sphere over its outer surface which allow the technologist to assess the distance from the top of each tube to the scalp with a probe which is calibrated in millimeters (Figure 3e). Each time the frame is applied, serial measurements for each tube portal should agree within a millimeter of previous measurements obtained prior to CT data acquisition. We have successfully treated over 600 patients with accuracy and precision utilizing this technique on the Varian 600SR.

All other quality assurance procedures for SRT treatments follow the same protocols as outlined under SRS.