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R50 and D2cm — the numbers that judge SABR fall-off

Conformity says how well the prescription hugs the target. These say how fast the dose leaves, which is the part that decides whether a SABR plan is good.

Built in·updated 2026-08-04

Two numbers, both from the RTOG lung SABR protocols:

  • R50 — the volume of the 50% prescription isodose divided by the PTV volume. How wide the half-dose cloud is relative to the target.
  • D2cm — the maximum dose anywhere 2 cm or more from the PTV, in any direction, as a percentage of prescription. How much dose escaped to somewhere it has no business being.

Both limits depend on PTV volume, and the direction surprises people. A small PTV is held to a looser R50 than a large one. That is geometry, not leniency: R50 is driven by the surface-to-volume ratio, and a small sphere has proportionally far more surface. The analytic form makes it explicit —

R50≈1+SAPTVVPTV Δr[1+ΔrrPTV+13(ΔrrPTV)2]R_{50} \approx 1 + \frac{SA_{PTV}}{V_{PTV}}\,\Delta r\left[1 + \frac{\Delta r}{r_{PTV}} + \frac{1}{3}\left(\frac{\Delta r}{r_{PTV}}\right)^{2}\right]

— where Δr\Delta r is the radial thickness of the 50% shell. Halve the radius and SA/V doubles, so the same physical fall-off gives a much larger ratio. Comparing raw R50 between plans on different-sized targets tells you almost nothing.

Which protocol. The tolerance tables live in RTOG 0813 for centrally located tumours and RTOG 0915 for peripheral ones. Both give three action levels — none, minor deviation, major deviation — against PTV volume. As an anchor: a PTV of about 46 cc carries a no-deviation bound near R50 ≤ 4.1 and D2cm ≤ 61%. Read the row for your own volume out of the protocol rather than interpolating from that one.

How to move them. R50 responds to the intermediate-dose shells and to how much low-dose spray the arcs leave; D2cm is usually one direction leaking, so look at where the 2 cm ring is hottest before touching global weights. ring falloff is the mechanism; this skill is the target.

Where the numbers come from