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.
R50%
The volume enclosed by the 50% prescription isodose divided by the PTV volume — a measure of how quickly dose falls off outside the target.
What is it?
R50% compares two volumes: the total volume of tissue enclosed by the 50% prescription isodose — everything receiving at least half the prescription dose, the PTV included — and the volume of the PTV itself. Dividing the first by the second gives a ratio with no units.
NRG's lung SBRT and brain SRS protocol templates both define it this way: the ratio of the 50% prescription isodose volume to the PTV (or target) volume.
Put simply: how big is the cloud of half-prescription dose compared with the target it surrounds?
Why does it matter?
A stereotactic plan should deliver a high dose to the target and very little just outside it. The smaller the 50% isodose cloud is relative to the PTV, the faster the dose falls away from the target. The NRG SRS template puts it plainly: R50% defines dose falloff outside the target.
The NRG lung SBRT template asks that the 50% isodose surface be made as small as possible, and tabulates recommended R50% values by PTV volume — the allowed ratio is larger for small targets and smaller for large ones, so a value only means something next to the target's size.
R50% describes the overall size of the intermediate-dose region. D2cm complements it by asking how hot the single hottest point is at a fixed distance from the target. Together they make up the dose-spillage table in the NRG lung SBRT template — its two measures of intermediate dose spill.
How is it evaluated?
- Display the isodose line at 50% of the prescription dose (not 50% of the plan maximum) and convert it to a structure.
- Read that structure's volume. Use the whole enclosed volume — the part inside the PTV counts.
- Divide by the PTV volume.
Compare the result with the recommendation for that PTV volume in the protocol you are planning to. DoseLab does not reproduce those tables; use the original protocol.
When several targets sit close together their 50% clouds merge, and a per-target ratio becomes hard to define. The NRG lung SBRT template notes that its tables were written for single targets and that compliance is more challenging with multiple targets or adjacent organs.
The Conformity Index Calculator works out R50% alongside the gradient index and the conformity indices.
Example
Illustrative numbers only — not a target value. A PTV of 25 cc whose 50% prescription isodose encloses 110 cc has R50% = 110 ÷ 25 = 4.4. A second plan for the same PTV whose 50% isodose encloses 90 cc has R50% = 3.6: its dose falls off more quickly.
Authoritative resources
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.
NRG Oncology's protocol page for RTOG 0915, linking the protocol PDF. Read the trial's normal-tissue constraints and its conformity and dose-falloff requirements in the protocol document. Results are published separately as the RTOG 0915 trial report.
TPS guidance
Step-by-step guidance planned. Added only once verified.
Step-by-step guidance planned. Added only once verified.