Dosimetric consequences of translational and rotational errors in frame-less image-guided radiosurgery

Abstract

BackgroundTo investigate geometric and dosimetric accuracy of frame-less image-guided radiosurgery (IG-RS) for brain metastases.Methods and materialsSingle fraction IG-RS was practiced in 72 patients with 98 brain metastases. Patient positioning and immobilization used either double- (n = 71) or single-layer (n = 27) thermoplastic masks. Pre-treatment set-up errors (n = 98) were evaluated with cone-beam CT (CBCT) based image-guidance (IG) and were corrected in six degrees of freedom without an action level. CBCT imaging after treatment measured intra-fractional errors (n = 64). Pre- and post-treatment errors were simulated in the treatment planning system and target coverage and dose conformity were evaluated. Three scenarios of 0 mm, 1 mm and 2 mm GTV-to-PTV (gross tumor volume, planning target volume) safety margins (SM) were simulated.ResultsErrors prior to IG were 3.9 mm ± 1.7 mm (3D vector) and the maximum rotational error was 1.7° ± 0.8° on average. The post-treatment 3D error was 0.9 mm ± 0.6 mm. No differences between double- and single-layer masks were observed. Intra-fractional errors were significantly correlated with the total treatment time with 0.7mm±0.5mm and 1.2mm±0.7mm for treatment times ≤23 minutes and >23 minutes (p<0.01), respectively. Simulation of RS without image-guidance reduced target coverage and conformity to 75% ± 19% and 60% ± 25% of planned values. Each 3D set-up error of 1 mm decreased target coverage and dose conformity by 6% and 10% on average, respectively, with a large inter-patient variability. Pre-treatment correction of translations only but not rotations did not affect target coverage and conformity. Post-treatment errors reduced target coverage by >5% in 14% of the patients. A 1 mm safety margin fully compensated intra-fractional patient motion.ConclusionsIG-RS with online correction of translational errors achieves high geometric and dosimetric accuracy. Intra-fractional errors decrease target coverage and conformity unless compensated with appropriate safety margins.

A comprehensive evaluation of treatment accuracy, including end-to-end tests and clinical data, applied to intracranial stereotactic radiotherapy

“…Ramakrishna et al [8], Guckenberger et al [9] and Tryggestad et al [29] reported an intrafraction error in the order of 1 mm.…”

“…Weltens et al [6] assessed the impact of the addition of MR imaging on the interobserver variability of brain tumour delineations. Patient setup and immobilization device accuracy has been studied by for example Meyer et al [7], Ramakrishna et al [8] and Guckenberger et al [9]. http End-to-end tests are used to measure the overall geometrical and dosimetric accuracy of the treatment chain for an ideal patient.…”

Geometric Verification of Dynamic Wave Arc Delivery With the Vero System Using Orthogonal X-ray Fluoroscopic Imaging

“…Influence of rotational setup errors, which can be identified with systematic G/R deviations, were extensively reported in literature (23)(24)(25). VMAT has been successfully implemented for different treatment sites with tolerance of gantry angle to agreement of 0.5 (26).…”

Single-Isocenter Frameless Volumetric Modulated Arc Radiosurgery for Multiple Intracranial Metastases

“…Alternatively, a patient may not require repositioning with singleisocenter technique, but accurate setup is critical since both translational and rotational error degrade conformality. 29, 30 At our institution, intrafraction motion after initial setup is minimized by continuous monitoring in real-time with SIG. 20 …”