Positioning deviations in frameless and frame-based intracranial stereotactic radiosurgery

Intracranial stereotactic radiosurgery (SRS) is a well established therapeutic modality for the treatment of small brain lesions. Currently, two immobilization methods for that therapeutic modality are widely used: frame-based and frameless. The literature states that both methods are very accurate, but there is no consensus on the equivalence between them. In order to add information about those methods, residual positioning deviations of frame-based and frameless SRS were evaluated and compared. We randomly selected 50 cases of SRS performed in a radiotherapy clinic. That sample represents two groups depending on the clinical indication and lesion volume: 25 frame-based SRS and 25 frameless SRS. Data were collected of each case according to the planning with 6 degrees of freedom. The conventional protocol for frame-based SRS does not make use of IGRT. Therefore, for measures relating to the frame-based SRS a reflective spheres holder was used as an extra accessory. The systematic and random errors of those methods were calculated, assuming a Gaussian distribution confirmed by the Shapiro-Wilk normality test (p < 0.01). The difference between frame-based and frameless SRS systematic errors does not exceed 0.2 mm for translational displacements and 0.2° for rotational displacements. The frameless method has smaller random errors than frame-based in the lateral and vertical translations. The difference in the random errors is smaller than 0.45 mm (translational displacements) and 0.3° (rotational displacements). The Student’s t-test did not identify significant difference for any of the samples in question (p > 0.05), therefore, for this study, the methods of immobilization can be considered equivalent.