Medical Physicist's Notes · TruPose™
Single-isocentre multi-met VMAT-SRS is the standard of care for ≤ 4–5 brain mets, and it is exquisitely sensitive to rotational error: at 1°, 7 % of targets fall below D95 95 % (Roper 2015); at 2°, only 63 % retain that coverage. A standard radiotherapy positioning board cannot correct pitch / roll / yaw. Three upgrade paths exist — this page is the honest comparison, and the reason TruPose is the right path for any clinic on a bore-class linac.
Two independent peer-reviewed studies put hard numbers on the dose-coverage cost of rotational error in single-isocentre multi-target VMAT-SRS. The numbers below are the envelope every upgrade path has to fit inside.
all targets ≥ 95 % coverage
Roper et al. 100 lesions across 50 patients — at 0.5° rotational error, D95 ≥ 95 % was held across the whole cohort.
Source
Roper 2015
7 % of targets drop below 95 % D95; ~4 % of PTV has V95 < 95 % (min 85.1 %)
Two independent studies. Roper: targets > 4–5 cm from isocentre are the first to drop. Selvan: V95 < 95 % beyond a 6 cm radial distance from isocentre.
Source
Roper / Selvan
only 63 % of targets retain > 95 % coverage
Targets distant from the isocentre and smaller PTVs are the most exposed. Roper's regression model assigns distance roughly twice the predictive weight of PTV volume on D95.
Source
Roper 2015
~50 % of PTV has V95 < 95 %; minimum dose 48.3 %
Selvan: coverage degradation extends inwards — failures begin at ~2.5 cm radial distance instead of the 6 cm threshold seen at 1°. The single-iso plan is no longer clinically defensible.
Source
Selvan 2019
~74 % of PTV has V95 < 95 %; minimum dose 21.2 %
Out-of-envelope. Quoted here only to show the trend — a 5° residual rotation is well outside what SGRT-guided setup would leave on the table, and outside the ±4° range of any current head adjuster.
Source
Selvan 2019
Roper J et al., Int J Radiat Oncol Biol Phys 93(3):540–546 (2015), PMC4610743; Selvan KT et al., J Med Phys 44(2):84–90 (2019), PMC6580820.
Capex, QA programme weight, patient-comfort impact, multi-fraction SRT compatibility, and fit with the linac in the room.
Path A
HexaPOD-class
Pros
Trade-offs
Fit for
C-arm linac with budget headroom; high SRS volume justifying the capex and the QA programme
Verdict: A — when affordable; not applicable on Halcyon / Ethos / Radixact
Path B
Stereotactic head ring
Pros
Trade-offs
Fit for
Pure single-fraction high-dose SRS in clinics with no SGRT plan; declining indication globally
Verdict: C — narrow indication; not the SRS upgrade path for most clinics in 2026
Path C
Manual 6DoF on a standard board
Pros
Trade-offs
Fit for
Bore-class linac (Halcyon, Ethos, Radixact) with an existing thermoplastic-mask cranial workflow and SGRT in place or planned. Modest-to-moderate SRS volume.
Verdict: A — for the bore-class linac + SGRT clinic; the intended upgrade path.
The "upgrade the positioning hardware" conversation usually starts with the board, the couch or a head frame. Here's why each of those is the wrong first move.
"We need a new SRS-grade positioning board"
The standard board is fine. The missing degrees of freedom are rotational, not translational. A new board does not add pitch / roll / yaw — it adds different indexing, different head-rest cushions, maybe carbon fibre. The board upgrade does not solve the SRS rotational problem.
"We need to upgrade the couch"
On a bore-class linac, the couch upgrade path is robotic 6DoF — and that is not on offer from the linac vendor for Halcyon / Ethos / Radixact. Add the rotational correction at the head, not the couch. That is what TruPose does.
"We need to add a head frame"
Frame-based SRS is single-fraction only. Modern SRT for brain mets and high-grade gliomas runs 3–5 fractions. Han 2022 demonstrated frameless mask + SGRT keeps 94.2 % of treatment time within sub-mm tolerance — without the OR step.
TruPose's ±4.0° per axis sits over the published worst-case residual rotation.
Dhabaan 2012 measured the max residual yaw at ~1° after 6DoF correction across 63 fractions of frameless cranial SRS — well inside the dose-defensible envelope at single-iso multi-met. The TruPose range gives the RTT room to zero the worst case in one pass, no re-imaging.
Source: Dhabaan A et al., J Appl Clin Med Phys 13(6):3850 (2012). PMC5718543.
Scope of this page
TruPose is not named in Roper 2015, Selvan 2019, Han 2022 or Dhabaan 2012. Those studies define the dose-coverage and intrafraction-motion envelope that any SRS positioning upgrade has to fit inside, and we cite that envelope. Treat this page as a fit-to-evidence argument for a manual head adjuster as the SRS upgrade path on a bore-class linac — not a head-to-head device comparison.
Sources cited on this page
TruPose™ Head Adjuster
Sibling posts in the TruPose family.