Calculate the Paddick conformity index from target, prescription isodose and overlap volumes, with the RTOG index and an optional gradient index.

Optional settings
V50 for the Paddick gradient index (V50 ÷ PIV). Leave blank to skip it.

Conformity Index Formula

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

CIPaddick = (TVPIV²) / (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)). Paddick CI = 1 requires complete geometric coincidence. RTOG/PITV = 1 means equal volumes and does not prove target coverage; neither metric alone establishes clinical treatment quality.

Homogeneity index versus conformity index

Conformity describes how a prescription isodose fits the target. Homogeneity describes dose variation within that target. The optional Homogeneity index mode uses the ICRU 83 definition: HI = (D2% − D98%) / D50%. D2%, D98% and D50% are doses received by at least 2%, 98% and 50% of the same target volume. D50% is the median dose; it is not automatically the prescribed dose. This definition is used in the AAPM multi-institutional ICRU 83 study.

Enter the three doses in Gy or cGy. For example, D2% = 63 Gy, D98% = 57 Gy and D50% = 60 Gy give HI = (63 − 57) / 60 = 0.1. The result is dimensionless. Zero means the entered near-maximum and near-minimum dose percentiles coincide.

Entering both optional maximum and prescribed doses also gives the RTOG-style ratio Dmax / prescribed dose, described in Stanley et al.’s radiosurgery study. These are different definitions, so their numbers are not interchangeable. Neither index alone establishes clinical plan acceptability or accounts for every target-coverage and organ-at-risk constraint.

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