Shell ladder for conformity and fall-off

A single ring is the beginner's version. The published method uses five shells at 2, 5, 10, 15 and 25 mm from the target margin, and the distances are not arbitrary — each band controls a different part of the distribution:

• 2 mm — conformity. This is the one that shapes the prescription isodose to the target.
• 5 and 10 mm — the fall-off. These are what buy you gradient index.
• 15 and 25 mm — spillage. Low-dose control, out where the isodose has already dropped to 10–30%.

Start with target and shell weights at 1.0 and an MU penalty of 1.0, then move one shell at a time.

The alternative. Precision also has a Normal Tissue Objective, which does the same job with three reference points instead of five structures — 100% at the margin, 50% at 5 mm, 20% at 100 mm. In a 2025 comparison the NTO produced a 7% better gradient index on meningiomas (3.19 vs 3.44), 11% less V12Gy (1.99 vs 2.25 cc) and 10% shorter treatment (48.8 vs 54.1 min), with no MU difference. Conformity was within 2%.

So the shell ladder is worth knowing because it explains what you are controlling; the NTO is often the faster way to control it.