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Dosimetric and spatial impact of high-resolution SPACE MRI on target delineation for glomus tumor radiosurgery: a
Arun Thimmarayappa1, Manish Beniwal2, Jitender Saini3
1Department of Neurosurgery, National Institute of Mental Health and Neurosciences (NIMHANS), Bengaluru, 560029, India. 47arunt@gmail.com.
Purpose:
To evaluate sequence-dependent volumetric, spatial, and dosimetric discrepancies between standard T1c gradient-recalled echo and high-resolution T1c turbo spin-echo (SPACE) sequences for stereotactic radiosurgery of glomus jugulare tumors.
Methods:
We retrospectively analyzed 47 patients undergoing Gamma Knife radiosurgery for glomus tumors. Pre-treatment 1.5T MRI included T1c MPRAGE and T1c SPACE sequences acquired during a single session. Gross tumor volumes (GTV) were delineated by multidisciplinary consensus. Geometric variances were quantified using the Dice Similarity Coefficient (DSC) and 95% Hausdorff Distance (HD95). A three-phase dosimetric analysis evaluated the clinical T1c-optimized plan, the hypothetical target coverage (V100%) variance when applying the unchanged T1c plan to the SPACE target, and a corrective SPACE-optimized research replan.
Results:
Median SPACE-derived GTVs (10.67 cm³) were significantly larger than T1c-derived GTVs (9.51 cm³, p < 0.001). Volumetric expansion occurred predominantly at the peripheral tumor-vessel interface. The median DSC was 0.917 and HD95 was 1.89 mm. Applying clinical T1c plans to SPACE targets produced a sequence-dependent geographic discrepancy, reducing hypothetical V100% coverage from 95.6% to 91.0% (p < 0.001). Corrective replanning with SPACE targets restored coverage while significantly lowering brainstem and cochlear organ-at-risk doses (p < 0.001), though this may partially reflect retrospective optimization bias.
Conclusion:
MRI sequence selection fundamentally alters GTV delineation and dosimetric parameters in glomus radiosurgery. Flow-suppressed SPACE sequences consistently yield larger target volumes at the complex tumor-vessel interface, mathematically reducing target coverage against conventional T1c planning. Lacking a definitive pathological benchmark, the true clinical significance of these specific contour discrepancies requires prospective oncological validation.
