Knowledge Hub · Patient Positioning Solutions
A radiotherapy patient passes through CT-sim, MR-sim (for some sites) and linac on the way to the first fraction. The planning isocentre is defined in the CT room; everything after that is a setup-transfer problem. This page is the physics behind why an indexed baseplate makes that transfer reproducible, the regulatory framework that governs it, and the device-level evidence the AAPM TG-176 process requires.
Why this matters
Indexed couch-bar pitch
A radiotherapy patient passes through three rooms — CT-sim, MR-sim (for some treatment sites) and linac — and the planning isocentre is defined in the CT room. If the indexing on the CT couch is different from the indexing on the linac couch, the setup transfer is mediated by laser alignment + skin marks; each transfer introduces a fresh setup-error contribution. An indexed baseplate locking to a standardised couch-bar pitch removes the per-room recalibration — the baseplate sits at the same hole pattern in every room. The CT-derived isocentre transfers to the linac at the indexing precision of the baseplate, which is typically below 1 mm.
Based on: AAPM TG-176 — Dosimetric effects of couch tops and immobilisation devices; ICRU Report 83 setup-reproducibility framework.
Read source ↗Knee-foot dominates pelvic reproducibility
Pelvic IMRT setup reproducibility is set by femoral rotation and pelvic tilt — both of which are dominated by where the knees and feet land. A correctly-indexed knee-foot support 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 by 5–7 mm at the prostate. CBCT or daily-IGRT verification can correct for small offsets but does not fix per-fraction internal-organ shift driven by inconsistent pelvic geometry. The knee-foot is the dominant axis for prostate, cervix and rectum IMRT setup quality.
Based on: AAPM TG-179 — Quality assurance for image-guided radiation therapy; clinical setup-uncertainty literature for pelvic IMRT.
Read source ↗Carbon-fibre attenuation
Carbon-fibre composite has an effective atomic number close to soft tissue and a density that gives roughly 1–2 % photon-beam attenuation at conventional MV energies (6 MV / 10 MV / 15 MV photon beams). For posterior or off-axis beams that pass through the baseplate, the planning-system MU calculation either explicitly models the attenuation (preferred for VMAT plans) or absorbs it inside the standard ±2 % MU tolerance. Either way, the baseplate does not introduce a clinically meaningful dose-rate shift at the target; it is functionally radiolucent for the treatment beam path.
Based on: AAPM TG-176 — Dosimetric effects of couch tops; published carbon-fibre attenuation measurements at 6–18 MV.
Read source ↗MR-compatibility physics
An MR-linac and an MR-sim need positioning devices built from non-ferromagnetic materials — ferrous metals would distort the B0 field locally and create artefact in the MR planning image. MR-compatible composite baseplates use the same carbon-fibre / glass-fibre construction as their CT-side counterparts but with non-ferromagnetic indexing pins and fasteners. The result: identical indexed pitch on the MR-sim and the MR-linac, with no MR-image artefact and no B0-field perturbation. The setup transfers without per-room recalibration; the planning isocentre is defined once.
Based on: ESTRO–ACROP guidelines for MR-linac immobilisation; MR-safety classifications (ASTM F2503).
Read source ↗Pairs with vacuum cushion for SBRT
SBRT immobilisation needs two layers — an indexed couch-side platform (the baseplate) plus a body-conforming layer (the vacuum cushion). The baseplate provides the inter-fraction reproducibility chain to the CT-sim isocentre; the vacuum cushion provides the within-fraction conformal hold that absorbs small movements without losing the gross-position reference. AAPM TG-101 SBRT immobilisation expects both. Either layer alone is insufficient: a baseplate-only setup drifts within the fraction; a cushion-only setup loses inter-fraction reproducibility.
Based on: AAPM TG-101 — Stereotactic Body Radiation Therapy; AAPM TG-178 — Methodology to determine the use of body immobilisation devices.
Read source ↗AAPM TG-176 device handling
AAPM TG-176 specifies that every immobilisation device passing into the treatment beam path is characterised dosimetrically — per-baseplate attenuation measured at the centres relevant photon energies, with the measurement filed alongside the device IQ / OQ record. The receiving radiotherapy department uses the per-device attenuation either to correct the planning-system MU calculation or to confirm the attenuation falls within the dose-tolerance budget. The characterisation is what allows the baseplate to be placed in the treatment field with confidence; without it, the planning system carries a hidden systematic error.
Based on: AAPM TG-176 — Dosimetric effects of couch tops and immobilisation devices.
Read source ↗AAPM, ICRU and ESTRO documents that anchor patient-positioning baseplate use.
AAPM task-group report covering dosimetric characterisation of couch tops and immobilisation devices including baseplate attenuation.
ICRU framework defining setup-reproducibility expectations for IMRT, SBRT and SRS treatment delivery.
AAPM task-group framework for IGRT QA including setup-uncertainty budgets that the positioning baseplate supports.
AAPM task-group report on SBRT including immobilisation expectations for body-region treatments.
AAPM task-group report on linac QA including setup-reproducibility tolerances.
ESTRO-ACROP framework for MR-linac patient immobilisation including MR-compatibility material requirements.
Where next
Product page →
Positioning baseplate specs + variant matrix
Region baseplates × construction tiers × imaging compatibility × accessories matrix.
For the medical physicist →
Per-treatment-site setup-reproducibility workflow
Per-site build sheet — H&N, brain SRS, breast DIBH, pelvic IMRT, lung SBRT, MR-linac transfer.