HDR vaginal brachytherapy cylinder dosimetry
ForPost-hysterectomy endometrial cancer, adjuvant HDR vaginal cuff brachytherapy
TL;DRSurface SD 6% vs 18% with surface vs 0.5-cm depth prescription (2-cm cylinder). Omitting dome optimization points gave hot spots >150% and cold spots <60%.
The actionable physics: the dome needs optimization points. Without them the apex runs cold (50-65%) or hot (140-170%) depending on tip space, and the 0.5-cm depth prescription hides surface SD up to 18%. If you switch to a surface prescription, M = 1.00 + 0.64/L + 1.23/S gives the surface dose that keeps the 0.5-cm dose unchanged.
For post-hysterectomy endometrial pts on single-channel cylinder VBT, this supports checking that apex and dome points are in the optimization and reporting both surface and 0.5-cm doses. It gives no outcome evidence for changing an established 0.5-cm depth prescription.
The actionable physics: the dome needs optimization points. Without them the apex runs cold (50-65%) or hot (140-170%) depending on tip space, and the 0.5-cm depth prescription hides surface SD up to 18%. M = 1.00 + 0.64/L + 1.23/S converts a depth dose to its surface equivalent if you switch conventions.
12 details
A treatment-planning study with no patients. Plato BPS 14.2.4 plans were built on Nucletron radiographic cylinders (tip space ~0.5 cm) and CT/MR-compatible cylinders (tip space 0.6-0.8 cm). Dose uniformity at each depth was measured as the mean dose (MD) and SD across dose points spaced evenly around the dome and along the cylinder.
Cylinders were 2.0, 2.5, 3.0, 3.5 and 4.0 cm in diameter, with active lengths of 3 to 8 cm (upper third to the whole vagina). Prescription was to the surface (P0) or to 0.5-cm depth (P5), with or without optimization points on the curved end (NEP). The worked illustration used 7 Gy × 3 fractions.
Primary measures: MD and SD at the surface and at 0.5-cm depth. Dose fall-off was the ratio of MD at 0.75 cm to MD at 0.5 cm.
Surface prescription gave surface SD 6% vs 18% with P5 (2-cm cylinder, 0.8-cm tip), with almost the same fall-off beyond 0.5 cm. NEP plans produced hot spots >150% and cold spots <60%. A shorter tip space raised dosimetric uncertainty >30%.
The paper frames its result against the ABS 2000 guidance (dome optimization points, report doses at both surface and 0.5 cm) and an ABS survey in which >90% prescribe to 0.5 cm. It also cites Sorbe's randomized 2.5 vs 5.0 Gy × 6 trial, in which higher dose increased atrophy, bleeding and shortening.
The M regression was fit on only two lengths × two diameters, using one vendor's applicators and TPS. The claim that a lower surface dose reduces atrophy and shortening is extrapolated from other trials, not measured.
A 0.5-cm depth prescription delivers its intended depth dose only when dome points are included, and the surface dose it gives varies with cylinder size and tip space. Prescribing to the surface with a geometry-scaled dose fixes the surface number. Whether that changes toxicity remains open.
Planning-system dosimetry with no patients or outcomes. 'Surface prescription is optimal' rests on mucosal dose uniformity, a surrogate never tied to toxicity or vaginal control here.
- Does surface prescription reduce vaginal atrophy or shortening clinically?
- Does the M factor hold on modern 3D image-guided planning?
📚 Sources · 📄 1 paper
The longer read
This paper is best read as a planning-hygiene argument rather than evidence for a particular prescription convention. Its most defensible finding is the negative one. When the optimizer is given no points on the cylinder dome, the apex dose stops being predictable, and the direction of the error depends on hardware. With the 0.8-cm tip space of the CT/MR-compatible tube, the apex ran cold (50-65%) and the depth mean fell about 10%. With the 0.5-cm tip space of the thin metal tube, the apex surface ran hot (140-170%). The cuff apex is where recurrence risk concentrates, so an omission that silently underdoses it in one applicator and overdoses it in another is a real QA problem. The survey the authors cite says more than one tenth of users were making that omission.
The headline recommendation, prescribing to the surface at a higher dose, is weaker. The authors show that surface prescription flattens mucosal dose (surface SD 6% vs 18% in the 2-cm case) while preserving the 0.5-cm mean and fall-off. They then reason that a lower and more uniform surface dose should reduce atrophy, bleeding and shortening. That last step borrows its clinical footing from Sorbe's randomized dose comparison, which tested dose per fraction rather than prescription point. Nothing in this dataset links surface SD to a toxicity outcome. The recommendation may be right, but here it is a hypothesis built on a surrogate.
The magnification factor M = 1.00 + 0.64/L + 1.23/S is useful arithmetic for anyone converting between conventions, and the paper's own example (7 Gy at 0.5 cm becoming 10.8 Gy at the surface for a 3-cm cylinder and 5-cm length) shows how far the two prescriptions sit apart. It also shows how easily a surface prescription copied from the literature can underdose depth when the cylinder is large. The regression was fit on a small geometric grid with a single planning system. A reader should check it against their own TPS before trusting it outside 3-5 cm lengths.
Two things would have to be true for the practical conclusion to fail. First, modern volumetric or library-based plans might already have closed the dome-point gap. Second, surface inhomogeneity at these magnitudes might not drive late vaginal toxicity once dose per fraction is held constant. Neither question is addressed here. The durable takeaway is procedural: put points on the dome, know your tip space, and report surface and 0.5-cm doses together, whatever point you prescribe to.