01 Therapy

Paddick conformity & gradient

CI = (TVPIV)² / (TV × PIV) and GI = PIV₅₀ / PIV.

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Simulation

Paddick conformity & gradient — 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{CI}=\frac{\mathrm{TVPIV}^2}{\mathrm{TV}\cdot\mathrm{PIV}},\quad \mathrm{GI}=\frac{\mathrm{PIV}_{50}}{\mathrm{PIV}}

Variables

TV · Target volumecm³PIV · Prescription isodose volumecm³TVPIV · Target in prescription isodosecm³PIV50 · 50% isodose volumecm³

Results

CI
Paddick CI
0.7872
PIV/TV
RTOG conformity
1.1667
GI
Gradient index
3.4286

Explanation

\mathrm{CI}=\frac{\mathrm{TVPIV}^2}{\mathrm{TV}\cdot\mathrm{PIV}},\quad \mathrm{GI}=\frac{\mathrm{PIV}_{50}}{\mathrm{PIV}}

What it means

Paddick’s conformity index rewards a prescription isodose that covers the target without spilling outside: CI = (TVPIV)²/(TV×PIV). Perfect conformal coverage scores 1. The gradient index GI = PIV₅₀/PIV describes how fast dose falls outside the target — crucial in SRS/SBRT. 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.

Linear accelerator · Open this machine

How to use it

Take volumes from the TPS DVH: TV (target), PIV (volume inside the prescription isodose), TVPIV (overlap), and PIV50 (50% isodose volume). SRS goals often CI > 0.8 and GI < 3. Change one input and watch the curve and the simulation follow.

Symbols

TVTarget volume12 cm³
PIVPrescription isodose volume14 cm³
TVPIVTarget in prescription isodose11.5 cm³
PIV5050% isodose volume48 cm³

Worked example

A typical case from the default values: TV = 12 cm³ (Target volume); PIV = 14 cm³ (Prescription isodose volume); TVPIV = 11.5 cm³ (Target in prescription isodose); PIV50 = 48 cm³ (50% isodose volume). Substituting into the relation gives CI = 0.7872; PIV/TV = 1.1667; GI = 3.4286. These are teaching numbers — align them with your machine.

Typical values give

CI = 0.7872
PIV/TV = 1.1667
GI = 3.4286

Where it comes from

The displayed formula is the working relation. CI = (TVPIV)² / (TV × PIV) and GI = PIV₅₀ / PIV. Usual reference: Paddick 2000 / RTOG. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: Paddick 2000 / RTOG

Assumptions & limits

Geometric indices only — they ignore location of spill (brainstem vs fat). RTOG CI = PIV/TV can exceed 1 and does not penalise under-coverage; Paddick does.

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. Geometric indices only — they ignore location of spill (brainstem vs fat). RTOG CI = PIV/TV can exceed 1 and does not penalise under-coverage; Paddick does.

Keep this

Name the SSD, energy, and field size with every PDD or TMR you quote. Geometric indices only — they ignore location of spill (brainstem vs fat). RTOG CI = PIV/TV can exceed 1 and does not penalise under-coverage; Paddick does.

In this specialty

Radiotherapy