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.
Highlights
To investigate geometric and dosimetric accuracy of frame-less image-guided radiosurgery (IG-RS) for brain metastases
Steep dose gradients of RS treatment planning combined with 0 mm safety margins explain these large dosimetric effects of small set-up errors. These results clearly demonstrate the importance and necessity of image-guidance in radiosurgery: even safety margins of 1-2 mm were insufficient to compensate missing imageguidance and none of the frame-based stereotactic mask systems described above ensures an accuracy of patient set-up, where image-guidance would become redundant
The intra-fractional errors observed in our study are in very good agreement with data in the literature [6,8,11,12,13,14,23]. Dosimetric consequences of these intrafractional errors were small on average but target coverage and dose conformity decreased by >5% in 14% and 23% of the patients, respectively, if 0 mm gross tumor volume (GTV)-to-planning target volume (PTV) safety margins were used. These results demonstrate that safety margins might be required despite highly accurate imageguided patient set-up: in our simulation study, a 1 mm GTV-to-PTV safety margin was sufficient so keep the dose to the GTV within a 5% threshold in all patients
Summary
To investigate geometric and dosimetric accuracy of frame-less image-guided radiosurgery (IG-RS) for brain metastases. Single fraction radiosurgery (RS) of intracranial malignant and benign lesions requires maximum accuracy of treatment planning and delivery to ensure that the irradiation doses are confined precisely to the target structures. For decades this accuracy of treatment delivery has been achieved by using invasive frame-based stereotactic systems: invasive fixation of the external stereotactic system to the patients’ skull and treatment on the same day without its. In-room image-guidance has become broadly available allowing frame-less image-guided radiosurgery (IG-RS) without the need for external stereotactic reference systems This image-guided approach provides a fully noninvasive treatment option and has been systematically optimized in the recent years. Intra-fractional patient motion is considered as the weakest link in frame-less IG-RS
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