Stereotactic Radiosurgery · Technical Foundations

Imaging, Registration & Immobilization

Putting the dose where the target actually is

Radiosurgery's precision is only as good as its localization chain: the imaging that defines the target, the registration that places it in treatment coordinates, and the immobilization and image guidance that keep it there during delivery. Each link has a characteristic failure mode, and the most consequential one — image mis-registration or MRI geometric distortion — is invisible on the treatment console. This page walks the chain and where it breaks.

Timoteo Almeida, MD, PhD | Department of Neurological Surgery

Orientation

Three things must align to a fraction of a millimeter: where the target is on imaging, where the planning system thinks it is in space, and where the beam actually goes. The planning CT provides the geometry and electron density for dose calculation; MRI provides the soft-tissue definition; their fusion ties the two together; immobilization fixes the patient; and image guidance verifies position at treatment. Errors at the imaging/fusion step are the most dangerous because, unlike a setup error, they are baked into the plan and reproduced at every fraction.

Part I

Imaging & Registration

1.The CT-MRI partnership and indication-specific sequences

A thin-slice planning CT remains the geometric and dosimetric reference (it carries electron density and is geometrically faithful), while MRI supplies the target definition for most cranial work. The two are rigidly co-registered. Sequence selection is indication-specific — thin contrast-enhanced T1 (and high-resolution volumetric sequences) for metastases and most tumors; heavily T2-weighted CISS/FIESTA for cranial-nerve and cisternal anatomy (vestibular schwannoma, trigeminal nerve); TOF-MRA plus catheter angiography for AVM nidus definition; and dynamic sequences for pituitary microadenomas — and is summarized for quick reference on the hub's quick-reference page.

2.MRI geometric distortion — the quiet hazard

MRI is geometrically imperfect. Gradient nonlinearity, main-field inhomogeneity, and susceptibility and chemical-shift effects can displace anatomy by clinically meaningful distances, worsening toward the periphery of the field of view and at air/bone interfaces (skull base). Because radiosurgery uses essentially no margin, an uncorrected distortion places the contour — and the dose — off target. Mitigation includes vendor distortion-correction algorithms, distortion-optimized sequences, sequence and phase-encode choices, QA phantoms to characterize distortion, and — critically — using the geometrically faithful CT as the spatial reference and verifying the fusion on multiple planes and landmarks rather than trusting an automated result.

Verify the fusion; respect MR distortion The commonest serious radiosurgery error is geometric, not mechanical: a mis-registration or uncorrected MRI distortion that moves the target. Confirm CT-MRI fusion against bony and vascular landmarks on multiple planes, apply distortion correction, and be especially wary at the periphery of the MR field of view and near the skull base. A perfectly delivered plan onto a mislocalized target is precisely wrong.

Part II

Immobilization

3.Frame, mask, and body systems

Immobilization trades invasiveness for repeatability:

Invasive stereotactic frame (e.g., Leksell coordinate frame) — pinned to the skull, it provides a rigid coordinate system and the highest single-session accuracy (sub-millimeter), the historical standard for single-fraction cranial SRS.
Thermoplastic mask (frameless) — non-invasive and necessary for fractionation, achieving roughly ~1–2 mm accuracy when combined with image guidance and intrafraction monitoring; the basis of mask-based Gamma Knife Icon and most linac cranial SRS/SRT.
Body immobilization — vacuum bags and stereotactic body frames for spine/trunk, with abdominal compression, breath-hold, or gating to manage respiratory motion for thoracic/abdominal SBRT.

The key point is that frameless accuracy is not a property of the mask alone — it depends on the image-guidance and motion-monitoring system that accompanies it.

Part III

Image Guidance at Treatment

4.Seeing and correcting position before and during delivery

Modern delivery is image-guided. Cone-beam CT verifies volumetric position; stereoscopic kV imaging (e.g., ExacTrac) localizes bony anatomy and supports six-degree-of-freedom (6DOF) couch correction; optical surface monitoring (e.g., surface-guided systems) tracks the patient surface for setup and intrafraction motion, triggering a beam hold if the patient moves. Spine SBRT relies on bony (vertebral) tracking; mobile body targets use implanted fiducials and respiratory tracking (e.g., the robotic Synchrony approach). The radiation-isocenter accuracy underlying all of this is verified by the Winston-Lutz test and the broader QA program on the planning and QA page.

Immobilization and guidance approaches (representative; accuracy depends on the full guidance chain).
Approach	Setting	Representative accuracy	Notes
Invasive stereotactic frame	Single-fraction cranial	Sub-millimeter	Rigid coordinate system; historical SRS standard
Thermoplastic mask + IGRT	Cranial SRS/SRT, fractionated	~1–2 mm	Requires image guidance + intrafraction monitoring
Body frame / vac-bag (+ motion mgmt)	Spine/body SBRT	~1–2 mm (target-dependent)	Compression/gating/breath-hold for respiratory motion

Key points

The localization chain is CT (geometry/density) + MRI (soft tissue) + fusion + immobilization + image guidance; errors at imaging/fusion are baked into every fraction.
Use indication-specific MR sequences: contrast T1/volumetric for tumors, CISS/FIESTA for cranial nerves, TOF-MRA + DSA for AVM, dynamic for pituitary.
MRI geometric distortion (gradient nonlinearity, susceptibility, chemical shift) can move targets, worse peripherally and at skull base — correct it and use CT as the spatial reference.
Frame = sub-mm single-session standard; mask = ~1–2 mm but only with image guidance and intrafraction monitoring; body needs motion management.
Delivery guidance: CBCT, stereoscopic kV (ExacTrac), surface monitoring, 6DOF couch; spine uses bony tracking, mobile body uses fiducials/respiratory tracking; isocenter verified by Winston-Lutz.
The most dangerous error is geometric (fusion/distortion), not mechanical — verify the fusion.

References

Winston KR, Lutz W. Linear accelerator as a neurosurgical tool for stereotactic radiosurgery. Neurosurgery. 1988;22(3):454–464. PubMed
Klein EE, Hanley J, Bayouth J, et al. Task Group 142 report: quality assurance of medical accelerators. Med Phys. 2009;36(9):4197–4212. PubMed
Seibert TM, White NS, Kim GY, et al. Distortion inherent to magnetic resonance imaging can lead to geometric miss in radiosurgery planning. Pract Radiat Oncol. 2016;6(6):e319–e328. PubMed
Benedict SH, Yenice KM, Followill D, et al. Stereotactic body radiation therapy: the report of AAPM Task Group 101. Med Phys. 2010;37(8):4078–4101. PubMed

Educational synthesis for neurosurgery and radiation-oncology trainees; immobilization accuracy figures are representative and depend on the complete guidance chain. Imaging, distortion, and QA references verified against PubMed during review.