Use the tabs below to calculate a conformity index. The “Solve Paddick CI” tab lets you enter any 3 values (TV, PIV, TVPIV, or CI) to calculate the missing variable.

Conformity Index Calculator

Enter volumes in cm³ to get CI in 10 seconds.

Paddick CI
RTOG CI
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Conformity Index Formula

The following formula is used to calculate the Paddick Conformity Index (a commonly used conformity metric in radiation therapy). 

CI_{Paddick} = (TV_{PIV}^2) / (TV * PIV)

Variables:

  • CIPaddick is the Paddick Conformity Index (dimensionless)
  • TV is the target volume (commonly in cm³)
  • PIV is the prescription isodose volume (commonly in cm³)
  • TVPIV is the target volume covered by the prescription isodose (commonly in cm³)

To calculate the Paddick Conformity Index, square the target volume covered by the prescription isodose (TVPIV) and divide by the product of the target volume (TV) and prescription isodose volume (PIV). Use consistent volume units.

What is a Conformity Index?

A Conformity Index is a quantitative measure used in radiation therapy to assess how well the prescription isodose conforms to a target volume (such as a tumor) while limiting dose to surrounding healthy tissues. Multiple definitions exist; two commonly used examples are the RTOG conformity index (CIRTOG = PIV/TV) and the Paddick conformity index (CIPaddick = TVPIV2/(TV×PIV)). For these indices, an ideal value is 1 (best conformity).

How to Calculate Conformity Index?

The following steps outline how to calculate the Paddick Conformity Index (CIPaddick).

  1. First, determine the target volume (TV) in cm³ (or any consistent volume unit).
  2. Next, determine the prescription isodose volume (PIV) in the same units.
  3. Next, determine the target volume covered by the prescription isodose (TVPIV) in the same units.
  4. Finally, use the formula CIPaddick = TVPIV2 / (TV × PIV) to calculate the Conformity Index.
  5. After inserting the values of TV, PIV, and TVPIV into the formula, calculate the result.

Example Problem:

Use the following variables as an example problem to test your knowledge:

Target volume (TV) = 50 cm³

Prescription isodose volume (PIV) = 60 cm³

Target volume covered by the prescription isodose (TVPIV) = 45 cm³ → CIPaddick = 45² / (50 × 60) = 0.675