Knowledge Hub · Vacuum Cushions
AAPM TG-101 SBRT delivery specifies a two-layer immobilisation chain: indexed baseplate for inter-fraction reproducibility plus conforming cushion for intra- fraction hold. Neither layer alone is enough. This page is the physics of why, plus the per-treatment-workflow build sheet and the AAPM TG-176 dosimetric- characterisation process the cushion goes through.
Why this matters
Baseplate + cushion is the SBRT chain
AAPM TG-101 specifies a two-layer immobilisation chain for SBRT body treatments: an indexed couch-side platform (the baseplate) plus a body-conforming layer (the vacuum cushion). The baseplate provides inter-fraction reproducibility — the same indexed couch position every fraction. The cushion provides intra-fraction conformal hold — the patient does not drift within the fraction because the bead bag has locked the body shape. Either layer alone is insufficient. A baseplate-only setup drifts during the fraction; a cushion-only setup loses inter-fraction reproducibility. The combination is what AAPM TG-101 expects.
Based on: AAPM TG-101 — Stereotactic Body Radiation Therapy; AAPM TG-178 — Methodology to determine the use of body immobilisation devices.
Read source ↗Conformal-hold physics
A polystyrene-bead fill at atmospheric pressure flows freely between the beads — the cushion is soft and conforms to the patient. When the vacuum pump draws air out of the bead bag, the atmospheric pressure on the outside compresses the beads against each other. The friction between adjacent beads locks them in place. The cushion stiffens to a rigid cradle in the shape the patient was simulated in. The patient sits in that exact cradle every fraction — internal anatomy reproduces because external posture reproduces.
Based on: AAPM TG-178 — Body immobilisation device methodology; clinical setup-uncertainty literature for vacuum-bag immobilisation.
Read source ↗Reusability economics
A vacuum-bag cushion is reusable across many patients with standard wipe-down decontamination. The valve releases the vacuum, the bead-fill returns to free-flow, the cushion is cleaned and stored. For the next patient, the cushion conforms to the new body shape and locks into the new cradle. A high-throughput radiotherapy department reusing a single cushion across 50+ patients over its service life brings the per-patient cushion cost to a small fraction of the single-use alpha-cradle alternative — without any compromise on setup reproducibility.
Based on: AAPM TG-178 — Body immobilisation device methodology; clinical economic analyses of reusable vs single-use immobilisation.
Read source ↗MR-compatible variant
An MR-linac SBRT workflow needs immobilisation devices built from non-ferromagnetic materials — ferrous metals would distort the B0 field locally and create artefact in the MR planning image. The MR-compatible vacuum cushion uses non-ferromagnetic valve assembly, MR-safe bead fill and MR-compatible outer fabric. The result: identical conformal hold to the conventional cushion but with no B0-field perturbation and no MR-image artefact. The patient setup transfers from MR-sim to MR-linac at the same reproducibility as CT-sim to conventional linac.
Based on: ESTRO–ACROP MR-linac immobilisation guidelines; ASTM F2503 MR-safety classifications.
Read source ↗Indexed to the baseplate
A vacuum cushion that floats freely on the baseplate gives a perfect conformal hold to the patient but no fraction-to-fraction reproducibility of the cushion-to-baseplate position. An indexing strap locks the cushion to a specific position on the indexed baseplate platform. The chain becomes: indexed baseplate → indexed cushion position → conformal patient hold. Every link in the chain has a defined position; setup reproducibility now flows end-to-end from the planning isocentre to the treatment beam.
Based on: AAPM TG-178 — Body immobilisation device methodology; AAPM TG-176 dosimetric handling of immobilisation devices.
Read source ↗Dosimetric handling per TG-176
AAPM TG-176 specifies that immobilisation devices passing into the treatment beam path are characterised dosimetrically — typical attenuation 1–3 % for a polystyrene-bead vacuum cushion at conventional MV photon energies, depending on bead density and cushion thickness in the beam path. The planning system either explicitly models the attenuation (preferred for VMAT plans) or absorbs it inside the standard ±2 % MU tolerance. The per-device characterisation is filed alongside the cushion IQ / OQ record and referenced for any beam that traverses the cushion.
Based on: AAPM TG-176 — Dosimetric effects of couch tops and immobilisation devices.
Read source ↗AAPM, ICRU and ESTRO documents that anchor vacuum-cushion immobilisation in SBRT and IMRT workflows.
AAPM task-group report on SBRT delivery including immobilisation expectations for body-region treatments.
AAPM task-group report on body-immobilisation device characterisation and clinical-use methodology.
AAPM task-group report on dosimetric characterisation of immobilisation devices including vacuum-cushion attenuation.
ICRU framework defining setup-reproducibility expectations for SBRT delivery.
ESTRO-ACROP framework for MR-linac patient immobilisation including MR-compatibility material requirements.
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