Gradient Index

Defines the gradient index (GI) as the ratio of the volume of half the prescription isodose to the volume of the prescription isodose — for a plan normalized to the 50% isodose line, the 25% isodose volume over the 50% isodose volume. The authors use it to compare plans of equal conformity, to explore prescription isodoses and to compare treatment modalities. Definition read in the abstract (Europe PMC); 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).

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.