Stereotactic Radiosurgery — Brain (SRS)

Single-fraction (or up to 5-fraction hypofractionated) high-dose radiation to intracranial targets — brain metastases, arteriovenous malformations, acoustic neuromas, meningiomas, trigeminal neuralgia, functional targets (essential tremor, Parkinson's, OCD). Millimeter targeting accuracy combined with steep dose falloff spares surrounding brain.

Clinical pathway

Immobilization — frame-based (Leksell G-frame, invasive pins) or frameless (thermoplastic mask + optical / CBCT tracking).
Imaging + target definition — high-resolution stereotactic MRI fused with planning CT; contours drawn on each lesion plus critical structures (brainstem, optic apparatus, cochlea, hippocampi).
Treatment planning — inverse planning for Gamma Knife multi-isocenter shots, MLC-based VMAT arcs on linac, or CyberKnife non-isocentric beams.
Delivery — Gamma Knife (192 Co-60 sources converging via sector collimation), linac SRS (cone or HD120-MLC-based), or CyberKnife (robotic 6 DoF tracking).
Follow-up MRI — typically 2–3 months post-SRS.

Typical systems

Dose characteristics

Single-fraction typical dose: 15–24 Gy depending on target size (smaller = higher dose, per RTOG 90-05 constraints).
12 Gy volume in normal brain is the radiation-necrosis predictor.
Steep gradients — 50% isodose at or near target edge.
Hypofractionated SRS (3–5 fractions) for larger lesions or those near critical structures.

Room + procedure characteristics

Treatment time: 20–90 min per session depending on lesion count and platform — Gamma Knife multi-target sessions can extend longer.
Team: radiation oncologist, neurosurgeon (often co-attends), medical physicist, dosimetrist, RT.
QA: TG-142-level daily + monthly + annual, plus end-to-end hidden-target tests specific to SRS commissioning. Daily Winston-Lutz on linac SRS platforms.

Equipment considerations that bite

Platform choice shapes the program — Gamma Knife is cranial-only sealed-source (different regulatory framework, source-reload cycle); linac SRS shares the rad-onc fleet; CyberKnife is robotic + frameless. Each has different commissioning, QA, and clinical workflow.
MLC class — HD120 (2.5 mm central) is the SRS-grade MLC on TrueBeam STx / Edge. Millennium 120 (5 mm) is workably for larger targets.
6 DoF couch (PerfectPitch / HexaPOD) is mandatory on linac SRS programs.
Imaging chain — kV CBCT geometric accuracy at sub-mm is the foundation; daily Winston-Lutz validates.
Treatment-planning system — Eclipse + iPlan / Elements for linac, GammaPlan for Gamma Knife, Accuray Precision for CyberKnife.

Operational reality

Multidisciplinary tumor-board integration — SRS for brain mets is increasingly the standard of care (alone or with whole-brain) and decisions are tumor-board-driven.
Frameless workflow has shifted volume; rigid-frame Gamma Knife sessions are still common but Icon's frameless option opened single-fraction frameless cranial work.
Functional SRS (Parkinson's, essential tremor, OCD) is a small but growing share, primarily on Gamma Knife.
Refurb economics — Gamma Knife refurb tracks cobalt-source remaining-life; linac SRS refurb tracks MLC class + 6 DoF couch + license tier.