01 Therapy
RTOG conformity index
CI = V_RI / TV. Ideal = 1. Paddick CI also accounts for overlap.
Listen
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Simulation
RTOG conformity index — 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).
Open this machineFormula
Variables
Results
CI_RTOG
RTOG conformity
1.2
CI_Paddick
Paddick conformity
0.7521
Explanation
What it means
Conformity asks how tightly the prescription isodose hugs the target. RTOG’s simple ratio V_RI / TV is 1 when they match in volume, but a sphere of prescription dose sitting next to the target still scores 1. Paddick’s index (already in the library) multiplies by the overlap squared and is the SRS/SBRT reporting standard. 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 machineHow to use it
Enter prescription-isodose volume V_RI (cm³) and target volume TV. Optionally the overlapping volume TV_PIV to also get Paddick CI. RTOG: <1.2 is often per-protocol for SRS; 1.2–1.5 is acceptable variation. Change one input and watch the curve and the simulation follow.
Symbols
- V_RIPrescription isodose volume12 cm³
- TVTarget volume10 cm³
- TV_PIVTarget ∩ PIV9.5 cm³
Worked example
A typical case from the default values: V_RI = 12 cm³ (Prescription isodose volume); TV = 10 cm³ (Target volume); TV_PIV = 9.5 cm³ (Target ∩ PIV). Substituting into the relation gives CI_RTOG = 1.2; CI_Paddick = 0.7521. These are teaching numbers — align them with your machine.
Typical values give
- CI_RTOG = 1.2
- CI_Paddick = 0.7521
Where it comes from
The displayed formula is the working relation. CI = V_RI / TV. Ideal = 1. Paddick CI also accounts for overlap. Usual reference: RTOG 90-05 / Paddick 2000. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: RTOG 90-05 / Paddick 2000
Assumptions & limits
Volumes from a TPS DVH; interpolation of the isodose surface matters at small targets. Does not describe gradient or homogeneity — report GI and HI alongside.
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. Volumes from a TPS DVH; interpolation of the isodose surface matters at small targets. Does not describe gradient or homogeneity — report GI and HI alongside.
Keep this
Name the SSD, energy, and field size with every PDD or TMR you quote. Volumes from a TPS DVH; interpolation of the isodose surface matters at small targets. Does not describe gradient or homogeneity — report GI and HI alongside.
In this specialty