PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s): By assessing the accuracy of patient repositioning using CBCT to quantify the inter-fractional setup accuracy and intra-fractional motion the study aims to determine which immobilisation is better for frameless intracranial image-guided radiosurgery - a head only double reinforced thermoplastic mask-based system (TM) or thermoplastic mask and mouthbyte system (TMMB)
Materials/Methods: 42 consecutive patients with intracranial benign lesions (n=16) & solitary brain metastases (n=24) who underwent treatment with multi-fraction non-coplanar volumetric Modulated Radiosurgery on 6DOF C-arm 5mm MLC linac were analysed. The lesions either were ranging in volumes 11-14cm3 (n=20), were close to critical normal structures (n=10) or were multiple in number (12). All patients received 27-30 Gy in 3 fractions. Patients were randomly assigned to either forms of commercially available Fraxion™ immobilisation devices for frameless radiosurgery with customised head cushion – TM (20 pts) or TMMP immobilisation (22 pts). Before each treatment, a KV- CBCT of the whole brain was obtained and images were auto-registered based on bony anatomy match with planning CT to generate the translational and rotational adjustments that were necessary to re-position the patient with respect to isocenter position established on planning scan. Using the pre-treatment, intra-fraction and post-treatment CBCT images the comparative metrics computed were average translational & rotational shifts magnitude inter- and random intra-fraction positioning error. The results were compared for statistical significance using an analysis of variance test.
Results: Overall Mean and standard deviation of the intra-fractional motions were -0.3 ± 0.4mm (lateral), 0.1±0.6mm (vertical), -0.2±0.4mm (longitudinal), -0.06 ± 0.180 (pitch), -0.1±0.230 (yaw), and -0.03±0.170 (roll) for both the systems. Inter-fractional rotation errors were -0.10±0.250 (pitch), -0.20±0.410 (yaw), and -0.08±0.160 (roll) for TM system versus 0.50±0.690 (pitch), 0.6±0.820 (yaw), 0.7±0.560 (roll) for frameless TMMB system. In a 3D vector space, a tumor located 5cm from the center of image fusion would require a 0.8 mm margin with the TM system and a 1.9 mm margin with TMMB system.
Conclusion: Both TM & TMMB immobilisation were effective immobilisation systems for intracranial Radiosurgery & were associated with high degree of positioning accuracy. The inter-fractional rotational setup errors using TMMB was larger than TM immobilization.Extra care need to exercised while defining the clip-box while using TMMP system. For single lesion far from the center of image registration or for multiple lesions, additional margin (upto 2mm) may be needed to account for the uncorrectable rotational setup errors when a TMMB immobilisation system is used. A double reinforced mask immobilisation alone with 6-DOF C-arm linac would be ideal and comfortable for all patients, when indicated, for frameless intracranial radiosurgery.
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