Is Head Frame Still the Gold Standard of Fixation for Linac Stereotactic Radiosurgery?

Abstract

Frameless stereotactic radiosurgery (SRS) has been introduced into the field as an alternative to frame based SRS for brain. However, the potential head movement within the mask and its impact on position variation are not fully studied. This study compared the difference of intra-treatment position variation between frameless and frame based SRS. Twenty-eight frame based patients (43 targets) and 19 frameless patients (37 targets) treated at our institution since last October were used in this study. Head frame and mask system were used as the fixation device, respectively. Patient was initially setup using stereotactic localization box. Patient positioning image guidance procedure was then performed and correction was made if initial setup error exceeded tolerance, which was 0.75 mm for translation and 1 degree for rotation. Three additional sets of verification x-rays were obtained pre, mid, and post treatment for all patients. The patient positioning software calculated residue shifts and rotations were recorded. The translational variation along each direction, and 3D variation among pre-, mid-, and post-treatment were calculated and compared between frame based and frameless groups. For frame based SRS, the mean and standard deviation of the variations along each direction are 0.18±0.16 mm anterior-posterior (AP), 0.22±0.18 mm superior-inferior (SI), and 0.13±0.08 mm left-right (LR), respectively. For frameless SRS, the mean and standard deviation of the variations are 0.27±0.19 mm AP, 0.32±0.19 mm SI, and 0.19±0.14 mm LR. The 3D variation of frame based SRS is 0.37±0.15 mm. It is less than that of frameless SRS, 0.51±0.19 mm (t-test p<0.001). The probability for a target being within 0.5 mm and 0.7 mm 3D variation is 81% and 100% respectively for the frame based SRS, and 46% and 86% respectively for the frameless SRS. Frame based SRS shows statistically significant less intra-treatment variation than frameless SRS. When selecting fixation method, this should be taken into consideration when targets are near critical structures or very small. However, the absolute variations are very small for both methods. For most SRS applications, the difference is not considered clinical significant. The largest variation is on the SI direction for frameless SRS, which may reflect the current clam shell mask design that leaves an opening on top of head gives the patient less restriction on this direction. The variation may be caused by the potential head movement within the mask or frame, mechanical uncertainty caused by couch rotation, and calculation uncertainty of patient positioning software. However, the latter two uncertainties can be factored out when comparing the results, since both groups are facing the same uncertainties and the only difference is the fixation method.