Prostate SBRT intra-fraction motion and margins
ForProstate SBRT 36.25 Gy/5 fx, fiducial CBCT IGRT, no intra-fraction tracking
TL;DRCurrent 6 mm/3 mm posterior PTV margins covered IFPM in 92% of fractions. BIR-guideline margins ran from 3.3 mm laterally and posteriorly to 7.0 mm inferiorly.
The 7.0 mm inferior margin is driven mainly by apex delineation error on MRI-only contours, and dropping one outlier contour gives 5.8 mm. Lateral margins of 3.3 and 3.5 mm suggest room to trim. The posterior miss (6 of 97 fractions past 3 mm) sits at the rectal edge, which argues for bladder/rectal consistency or tracking rather than a wider posterior PTV.
In fiducial-guided 36.25 Gy/5 fx prostate SBRT delivered without intra-fraction tracking, this supports existing 6 mm/3 mm posterior margins as covering most fractions; it does not inform tracked, MR-guided, or CT-only-delineated workflows.
The inferior 7.0 mm BIR margin mostly reflects apex delineation error on MRI-only contours (5.8 mm without one outlier). Lateral margins of 3.3 and 3.5 mm suggest room to trim. Posterior misses (6 of 97 past 3 mm) point to bladder/rectal consistency or tracking, not a wider rectal-side PTV.
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Retrospective single-centre audit of 97 pre/post-treatment CBCT pairs from 20 pts. Margins were calculated from IFPM alone and with the updated BIR geometric uncertainty guidelines.
36.25 Gy in 5 fractions SBRT without intra-fraction tracking, with fiducial-based daily online CBCT IGRT. Current margins were 6 mm isotropic, 3 mm posterior. Mean pre- to post-imaging interval was 9 min.
magnitude and direction of IFPM and coverage by current margins. Secondary: margins derived from IFPM alone and from the BIR guidelines.
Averaged 3D RMS IFPM was 2.5 mm (4.2 mm), mostly posterior (70%) and inferior (63%). Current margins covered 92% of fractions, and 8 of 97 exceeded them, six posteriorly.
| Direction | Current margin | BIR-derived margin |
|---|---|---|
| Right | 6 mm | 3.5 mm |
| Left | 6 mm | 3.3 mm |
| Superior | 6 mm | 3.7 mm |
| Inferior | 6 mm | 7.0 mm |
| Anterior | 6 mm | 4.4 mm |
| Posterior | 3 mm | 3.3 mm |
The authors report RL motion above comparable CBCT series, with AP and SI comparable. A larger cited margin study (n=205) found smaller systematic errors and similar random errors.
Delineation error, the dominant component, came from an MRI-only study and is probably smaller than real CT or fused MR/CT error. Deformation was set to 0, match error was assumed rather than measured, and AP and SI were not split by direction.
The guideline-derived numbers suggest lateral margins could shrink while the inferior margin grows, but the inferior increase rests heavily on one outlier contour. The authors advise checking outcomes in current trials before changing practice, given favourable control and toxicity with existing margins.
Single-centre retrospective geometric audit, n=20, no clinical outcome endpoint; proposed margin changes are untested against toxicity or biochemical control.
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Abstract
The longer read
The useful finding here is less the motion data than where the margin actually comes from. Intra-fraction motion on its own needed no more than 3.5 mm in any direction, well inside the 6 mm isotropic envelope most fiducial-guided centres use. When the full BIR recipe is applied, the margin that moves is inferior, at 7.0 mm, and the authors attribute most of that to delineation error at the prostate apex. That component came from an MRI-only contouring exercise, and one outlier case alone shifts the inferior margin from 7 mm to 5.8 mm. So the headline case for a bigger inferior margin depends on observer variation at the apex more than on prostate motion, and the remedy it points to is contouring guidance, not a larger PTV.
The posterior direction tells a different story. Six of the eight out-of-margin fractions crossed the 3 mm posterior margin, which is exactly the edge kept tight to spare the rectum. The guideline-derived posterior margin of 3.3 mm barely differs from current practice, so the calculation does not ask for more posterior expansion. The failures look like occasional large excursions rather than a systematic shift, and where a cause could be found it was a change in bladder or rectal volume. That fits the broader logic of SBRT margins the authors set out: with rapid dose fall-off and only five fractions, systematic errors carry far more weight than random ones, while a single large random excursion in one fraction is a real but bounded dose deficit. The practical levers are preparation consistency, shorter imaging-to-beam time, or intra-fraction tracking, not a wider rectal margin.
Against the other series the authors cite, AP and SI motion are comparable, and the larger RL motion is explained by two pts. A cited cohort of 205 pts had smaller systematic and similar random errors, which suggests this centre's systematic budget may be on the generous side. The weak correlation between imaging interval and motion within a 6 to 15 min window argues that trimming a minute or two from delivery will not change much here, even though shorter treatment times reduced motion in other work.
Confidence should be tempered for three reasons beyond the small cohort. Match error was assumed negligible across 31 observers without being measured, and in SBRT a poorly spread fiducial set could produce a geometric miss this design cannot see. Deformation was set to zero. And delineation error measured on MRI alone will understate what a CT-planned or fused-image department actually incurs, which would push the systematic component, and the margin, up rather than down.
Nothing here links margin to outcome. The authors say as much: incidental dose outside the prostate may contribute to control, and current margins are paired with favourable biochemical control and toxicity. A department reading this should treat the lateral reduction and inferior expansion as hypotheses for its own audit, run on its own delineation and match error, not as a margin prescription.