01 Therapy

Homogeneity index ICRU 83

HI = (D2% − D98%) / D50%. Zero is perfectly homogeneous.

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Simulation

Homogeneity index ICRU 83 — Change the numbers; the scene follows.

Where it works

Linear accelerator

Isocenter

At isocenter, on the central axis through the patient (or a phantom in the same place).

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Formula

\mathrm{HI}=\frac{D_{2\%}-D_{98\%}}{D_{50\%}}

Variables

D_{2%} · Near-max doseGyD_{98%} · Near-min doseGyD_{50%} · Median doseGy

Results

HI
Homogeneity index
0.07
D2−D98
Dose span
4.9Gy

Explanation

\mathrm{HI}=\frac{D_{2\%}-D_{98\%}}{D_{50\%}}

What it means

ICRU 83 replaced the old ±5% of ICRU 50 with a DVH-based spread between near-max (D2%) and near-min (D98%), normalised to median dose. IMRT/VMAT PTVs typically sit at HI ≈ 0.05–0.15; a simultaneous integrated boost is allowed to be hotter. SBRT PTVs are deliberately inhomogeneous (HI 0.2–0.4) because the prescription isodose is 70–80%. This is a working relation in Radiotherapy.

Where it is used

Clinically it sits on the Linear accelerator — Isocenter. At isocenter, on the central axis through the patient (or a phantom in the same place). Radiotherapy equations sit at the console and in the bunker: output, depth dose, equivalent square, and the monitor units that treat the patient. Hand-calc them beside the TPS, never instead of a commissioned plan.

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How to use it

Enter D2%, D98% and D50% in the same unit (Gy or %). A 70 Gy PTV with D2%=73.5, D98%=68.6, D50%=70 gives HI = 0.07. Change one input and watch the curve and the simulation follow.

Symbols

D_{2%}Near-max dose73.5 Gy
D_{98%}Near-min dose68.6 Gy
D_{50%}Median dose70 Gy

Worked example

A typical case from the default values: D_{2%} = 73.5 Gy (Near-max dose); D_{98%} = 68.6 Gy (Near-min dose); D_{50%} = 70 Gy (Median dose). Substituting into the relation gives HI = 0.07; D2−D98 = 4.9 Gy. These are teaching numbers — align them with your machine.

Typical values give

HI = 0.07
D2−D98 = 4.9Gy

Where it comes from

The displayed formula is the working relation. HI = (D2% − D98%) / D50%. Zero is perfectly homogeneous. Usual reference: ICRU 83. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: ICRU 83

Assumptions & limits

Requires a DVH of the PTV (or CTV, as specified). D2%/D98% sampling is sensitive to voxel size on small structures. Does not replace a look at the actual hot-spot location.

Pitfalls

Never mix PDD from one SSD with TMR from another without converting. Field size at the surface is not the size at isocentre. A hand MU is a check, not a treatment. Requires a DVH of the PTV (or CTV, as specified). D2%/D98% sampling is sensitive to voxel size on small structures. Does not replace a look at the actual hot-spot location.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. Requires a DVH of the PTV (or CTV, as specified). D2%/D98% sampling is sensitive to voxel size on small structures. Does not replace a look at the actual hot-spot location.

In this specialty

Radiotherapy