Knowledge Hub · SBRT Immobilization System
"Baseplate + cushion" is the common shorthand for SBRT immobilisation. The complete AAPM TG-101 chain is closer to ten layers — and each layer locks a specific axis of setup variance. This page is the chain breakdown, the physics behind each layer and what the inspection-ready evidence trail looks like.
Why this matters
Why ten components, not three
A common simplification calls SBRT immobilisation "baseplate + cushion". The reality is closer to ten layers: indexed couch-side platform, body-conforming cushion, anterior compression (belly bridge), lower-body lock (knee bridge + cushion), upper-body posture (elevation cushion), arm support (wing-board T-grip), head support (carbon-fibre headrest), couch lock (indexing bar), motion-management signal (respiratory belt), and (for combined H&N + body workflows) thermoplastic mask. AAPM TG-101 expects every layer that is relevant to the treatment site. Omitting a layer because the workflow "usually doesn't need it" is how SBRT setup drifts into the IGRT correction budget.
Based on: AAPM TG-101 — Stereotactic Body Radiation Therapy; AAPM TG-178 — Body immobilisation device methodology.
Read source ↗Belly compression physics
A belly compression plate applied at the upper abdomen restricts diaphragmatic excursion during quiet respiration. Across a cohort of liver / kidney / upper-abdomen SBRT patients, applied compression typically reduces SI tumour motion from 15–25 mm (free-breathing) to 5–10 mm. The ITV-to-PTV margin shrinks proportionally; the high-dose volume shrinks; the dose to OAR (small bowel, kidney parenchyma, normal liver) drops. The belly bridge is not a comfort accessory — it's an active dose-reduction tool that buys real OAR-sparing benefit.
Based on: AAPM TG-101 abdominal-SBRT immobilisation guidance; published SBRT belly-compression motion-reduction literature.
Read source ↗Respiratory belt vs SGRT
A surface-guided radiotherapy (SGRT) camera tracks chest-wall position in real time and is the gold standard for DIBH gating in modern radiotherapy departments. The respiratory belt is the lower-cost, equally-reproducible alternative for centres without SGRT, AND a redundancy layer for centres with SGRT. The belt produces a single 1D respiratory waveform — sufficient for 4D-CT phase binning and for breath-hold gate triggering. Pairing belt + SGRT is the highest-confidence motion-management configuration; belt-only is the cost-effective standalone.
Based on: AAPM TG-76 — Management of respiratory motion in radiation oncology; clinical SGRT vs belt comparison studies.
Read source ↗Knee bridge dominance
Pelvic SBRT setup reproducibility is dominated by femoral rotation and pelvic tilt — both controlled by where the knees and feet land. A correctly-set knee bridge locks femoral rotation within < 2° and pelvic tilt within < 3° fraction-to-fraction; without it, the upper-body shell can be perfectly indexed but the patient still rolls 5–7 mm at the target. For oligometastatic pelvic SBRT — small nodal or bone targets — that 5–7 mm setup variance is the difference between hitting the target and missing into a critical OAR.
Based on: AAPM TG-179 — Quality assurance for IGRT; clinical pelvic-SBRT setup-uncertainty literature.
Read source ↗Indexing-bar lock
The SBRT platform indexes onto the couch-bar pitch. The lockable indexing bar is what actually locks the platform to a specific hole position — the bar engages, the platform stays put, the patient setup is reproduced. Without the indexing bar, the platform can drift in the lateral or anterior-posterior direction during transfer between rooms or during patient loading. The bar is a small mechanical part with disproportionate impact on the chain integrity — its presence is what allows the SBRT setup to claim sub-mm inter-fraction reproducibility.
Based on: AAPM TG-178 — Body immobilisation device methodology; AAPM TG-142 setup-reproducibility tolerances.
Read source ↗Quick-lock vs screw-down
SBRT fractions are typically delivered in 30–45 minutes including CBCT verification. If the immobilisation setup takes 25 minutes, the patient lies still for 25 min before any treatment — physical comfort and pose memory both degrade, which manifests as intra-fraction drift on the CBCT-to-delivery interval. Quick-lock bridges and a single-action indexing bar compress the setup time to < 10 minutes for a trained operator. The patient settles for the IGRT and treatment delivery; intra-fraction reproducibility improves; the SBRT dose lands on the target the planning system intended.
Based on: Published SBRT setup-time vs intra-fraction drift studies; AAPM TG-101 timing guidance.
Read source ↗AAPM references that anchor the SBRT immobilisation chain.
AAPM task-group report defining SBRT delivery framework and immobilisation expectations.
AAPM task-group report on body-immobilisation device characterisation and clinical-use methodology.
AAPM task-group report on respiratory motion management including belt + SGRT framework.
AAPM task-group report on dosimetric handling of immobilisation devices.
AAPM task-group framework for IGRT QA including setup-uncertainty budgets that the SBRT chain supports.
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