The NRG Oncology template for the radiation section of a brain radiosurgery trial. Its example text asks for CT slices of 1.25 mm or less, strongly encourages MRI (contrast T1 and a 3D gradient-echo sequence) for target delineation, recommends fusing FDG-PET/CT when available, and asks that the fusion be done by a medical physicist and reviewed by the attending physician. Its plan-evaluation section defines the conformity index (CI, or R100%) as the prescription isodose volume ÷ PTV volume, R50% as the 50% prescription isodose volume ÷ target volume ("R50% defines dose falloff outside the target"), a heterogeneity index D5% ÷ D95%, and coverage as the PTV volume receiving the prescription dose ÷ PTV volume, and notes that criteria such as the CI should account for target size and treatment technology.
Conformity Index
How closely the prescription isodose matches the size, shape and position of the target — scored with the RTOG ratio (R100%), the Paddick index and others.
What is it?
A conformity index scores how well the volume receiving the prescription dose matches the size and shape of the target. Many versions have been published. This page covers the two you will meet most often in protocols and planning papers — and they behave differently.
RTOG conformity index (also written R100%). The volume enclosed by the prescription isodose divided by the target volume. It comes from the RTOG radiosurgery quality-assurance guidelines (Shaw 1993) and is the definition used in NRG's lung SBRT and brain SRS protocol templates. An ideal value is 1. Above 1, more tissue receives the prescription dose than the target contains; below 1, the prescription isodose is smaller than the target, so part of the target is missed.
Paddick conformity index. The product of two ratios: the share of the target that is covered by the prescription isodose, and the share of the prescription isodose volume that lies inside the target. Written out, TVPIV² ÷ (TV × PIV), where TV is the target volume, PIV the prescription isodose volume and TVPIV the part of the target inside it. An ideal value is also 1, but here lower is worse. It is the same quantity as van't Riet's conformation number, and ICRU Report 91 recommends it for reporting stereotactic plans.
Why does it matter?
Every cubic centimeter inside the prescription isodose but outside the target is normal tissue receiving the full dose. In stereotactic treatment, where each fraction is large, that matters a great deal. NRG's lung SBRT template makes conformity part of its second planning priority: achieve PTV coverage while minimizing conformity and gradient indices — a conformal, compact dose distribution.
The two indices exist because the RTOG ratio has a blind spot. It compares only the sizes of the two volumes, not where they sit. A prescription isodose that is exactly as large as the target but shifted off it scores a perfect 1 while missing part of the target and irradiating normal tissue. Paddick's index was designed so that this cannot happen: it can only reach 1 when the prescription isodose and the target match exactly.
Conformity does not describe what happens beyond the prescription isodose. Pair it with a fall-off measure — the Gradient Index or R50% — to judge the whole dose distribution.
How is it evaluated?
- Convert the prescription isodose to a structure and read its volume (PIV, or VRx).
- For the RTOG ratio, divide that volume by the PTV volume.
- For the Paddick index, also create the overlap of the prescription isodose structure and the PTV (a Boolean AND). Its volume is TVPIV. Square it, then divide by the PTV volume multiplied by the prescription isodose volume.
Check which definition a protocol or paper uses before comparing numbers. The RTOG ratio improves as it falls toward 1, the Paddick index as it rises toward 1, and the same plan gives different values for each.
Acceptable values are protocol-specific. The NRG SRS template notes that conformity criteria should account for target size and treatment technology, and the NRG lung SBRT template allows exceptions to its R100% recommendation for small targets. DoseLab does not reproduce those values; use the original protocol.
The Conformity Index Calculator works out both indices, and the gradient index and R50%, from the volumes you enter.
Example
CIPaddick = TVPIV² ÷ (TV × PIV)
Illustrative numbers only. A PTV of 20 cc and a prescription isodose of 25 cc overlap by 19 cc — so 19 cc of the PTV is covered.
- RTOG: 25 ÷ 20 = 1.25
- Paddick: 19² ÷ (20 × 25) = 361 ÷ 500 = 0.72
Now shift the isodose so it is still 20 cc but covers only 15 cc of the PTV. The RTOG ratio reads a perfect 20 ÷ 20 = 1.0. The Paddick index drops to 15² ÷ (20 × 20) = 225 ÷ 400 ≈ 0.56, revealing the mismatch.
Authoritative resources
The template NRG Oncology gives investigators for writing the radiation section of a lung SBRT trial. Its imaging section allows PET/CT or MRI for structure definition, asks that the images be registered accurately to the planning scan (following AAPM TG-132 for deformable registration), notes that PET may be needed to separate tumor from atelectasis or fibrosis, and suggests an extra margin when a fused image is the primary image for the GTV. Its target and planning sections (5.2.4 and 5.2.6–5.2.8) define the plan-quality measures for lung SBRT trials: the prescription as the dose covering 95% of the PTV, a D99% requirement, R100% (prescription isodose volume ÷ PTV), R50% (50% prescription isodose volume ÷ PTV) and D2cm (maximum dose at 2 cm from the PTV in any direction, as a percentage of the prescription), with a table of allowable dose spillage by PTV volume. It describes a 20 mm PTV expansion structure used to control intermediate dose spillage, asks that dose above 105% of prescription be avoided outside the PTV, describes a deliberately heterogeneous dose with the hotspot in the central GTV, defines the maximum dose on a 0.03 cc volume, and ranks planning priorities when target coverage and organ limits conflict.
RTOG task-force guidelines for radiosurgery quality assurance, setting out a standard data set and minor and major protocol deviations. The paper itself is paywalled; DoseLab checked its conformity, coverage and homogeneity definitions as quoted in Kataria 2012 and Menon 2018. Citation checked against Crossref.
Technical note reviewing earlier radiosurgery conformity measures, showing that they can give false perfect scores, and deriving a new index that does not. The abstract was read in Europe PMC; the index's formula is checked as described in Menon 2018. Citation checked against Crossref.
Retrospective evaluation of SRS/SRT plan-quality metrics for arteriovenous malformations. Its methods section defines each metric: the RTOG conformity index (prescription isodose volume ÷ target volume; ideal 1), RTOG quality of coverage and homogeneity index, and the Paddick conformity index as the product of the under-treatment ratio (share of the target covered) and over-treatment ratio (share of the prescription isodose volume inside the target), noting that ICRU Report 91 recommends the Paddick conformity and gradient indices for reporting stereotactic plans. Full text read in Europe PMC (PMC6299748); citation checked against Crossref.
ICRU report on prescribing, recording and reporting stereotactic treatments delivered with small photon beams. DoseLab has not read the report itself. Its recommendation of the Paddick conformity index and the Paddick gradient index for reporting stereotactic plans is checked as quoted in the open-access Menon 2018 paper. Citation as given in the DEGRO/DGMP statement on the report (Wilke 2019, Strahlenther Onkol).
Critical review of the conformity indices described in the literature and their fields of application. Citation and abstract checked in Europe PMC and Crossref; full text not read.
TPS guidance
Step-by-step guidance planned. Added only once verified.
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