Stereotactic Body Radiotherapy (SBRT)
Hypofractionated ablative radiation therapy — delivers very high dose per fraction (typically 6–20 Gy) in a small number of fractions (1–5) to well-localized extracranial tumors. Principal applications: early-stage non-small-cell lung cancer, liver metastases, prostate cancer, spine metastases, oligometastatic disease, pancreatic cancer (emerging on MR-Linac), adrenal metastases.
SBRT's clinical success depends on sub-millimeter spatial accuracy + motion management. A 3 mm error at 10 Gy × 5 fractions is meaningfully different from the same error at 2 Gy × 30 fractions — the dose gradient around an SBRT target is steep.
Workflow
- Simulation — CT simulation with 4DCT for moving targets. Contrast-enhanced when indicated; fusion with MRI / PET for contouring.
- Contouring — physician + physicist define GTV, CTV, ITV (for motion), PTV margins + OARs (organs at risk).
- Planning — VMAT or IMRT plan optimization; dose constraints per AAPM TG-101 or institutional guidelines.
- Plan QA — physicist-signed; patient-specific QA (ArcCHECK, MapCheck, portal dosimetry, ion-chamber).
- Delivery — daily CBCT IGRT before each fraction; motion management (gating, breath-hold, tracking) as appropriate.
Typical systems
- Varian TrueBeam + RapidArc + FFF.
- Varian TrueBeam STx + HD120 MLC + 6 DoF couch.
- Varian Edge (SRS / SBRT-dedicated).
- Elekta Versa HD + HexaPOD + Symmetry / Response.
- Elekta Infinity.
- CyberKnife — tracking-based, different geometry.
- Elekta Unity MR-Linac — for moving / deformable targets where MRI soft-tissue visualization matters (pancreas, liver, gyne, prostate).
- ViewRay MRIdian — 0.35T MR-linac alternative.
Motion management options
- ITV (internal target volume) — plan for the full motion envelope; simplest, highest normal-tissue dose.
- Breath-hold — patient + surface-guided RT (SGRT — AlignRT, Catalyst); intermediate complexity.
- Gating — beam-on only during specific respiratory phase (Varian RPM, Elekta Active Breathing Coordinator).
- Real-time tracking — CyberKnife Synchrony (orthogonal kV imaging + fiducial / fiducial-free) or Varian Calypso (electromagnetic beacons).
- MR-cine on table — Unity / MRIdian; on-table re-planning against the MR-of-the-day.
Clinical characteristics
- Fractions: 1–5.
- Dose per fraction: 6–20+ Gy.
- Room time per fraction: 30–60 min (longer than conventional IMRT due to IGRT + motion verification).
- Physicist time: significant — SBRT plans are not "set and forget."
Equipment considerations that bite
- MLC class — HD120 / Agility preferred for sub-cm conformity. Millennium 120 is workably for larger targets but sub-optimal for spine SBRT near critical structures.
- 6 DoF couch is mandatory for sub-mm setup. PerfectPitch (Varian) / HexaPOD (Elekta).
- FFF beams — high dose-rate FFF (1200–2400 MU/min) keeps single-fraction delivery time clinically reasonable.
- Imaging chain — daily kV CBCT geometric accuracy is the foundation; weekly Winston-Lutz validates.
- Motion-management hardware — gating / tracking / SGRT systems each are licensed add-ons with their own commissioning.
- MR-linac is its own program — months-long commissioning, distinct QA framework, distinct planning system.
Operational reality
- SBRT is concentrated at programs with appropriate platforms — community 3D-CRT-only programs refer SBRT cases to tertiary centers.
- MR-linac value is clearest in moving / deformable targets — pancreas SBRT is the canonical clinical narrative; lung / liver / oligometastatic also benefit.
- Refurb economics — TrueBeam STx and Edge with HD120 + PerfectPitch carry meaningful refurb premium for SBRT-grade work.