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.
Maximum Dose
The highest dose in a structure. A single-point maximum depends on the calculation grid, so protocols often define the maximum on a small volume instead.
What is it?
The maximum dose is the highest dose found anywhere in a structure — the far right-hand end of its cumulative DVH.
In practice "maximum" can mean two different things:
- A point maximum — the dose in the single hottest calculation point (voxel).
- A small-volume maximum — the lowest dose received by the hottest small volume of the structure. NRG protocol templates define their maximum this way, on 0.03 cc; the UT Southwestern SBRT tables (Timmerman 2022) define a "point" as 0.035 cc or less. See D0.03cc / Near-Max Dose.
Why does it matter?
Organs treated as serial tissues are limited by their maximum and small-volume doses. The NRG lung SBRT template's serial-tissue list — spinal canal, esophagus, brachial plexus, heart, great vessels, trachea and bronchus, rib, skin and stomach — gives each a 0.03 cc maximum alongside a small-volume limit.
A true point maximum is fragile. It depends on the calculation grid size and where the grid points fall, so the same plan can report a slightly different maximum after a small change in calculation settings. Kataria and colleagues give this as the reason dose to a small volume, such as D2%, is used instead of a point — and it is why protocols specify the volume.
How is it evaluated?
- Find out which definition your protocol uses — a point, 0.03 cc, 0.035 cc, or another volume — and read that metric.
- Read the maximum from the structure's DVH statistics, or as Dx at the protocol's volume.
- Find where the maximum sits by looking at the dose slice by slice. A maximum in a PRV margin is a different situation from one inside the organ itself.
A limit written only as "Max" in a table or older protocol may not say which definition it uses. Check the source before assuming it means a single point. Limits themselves are protocol-specific — use the Constraint Finder.
Example
The schematic zooms in on the high-dose tail of a cumulative DVH. The single hottest voxel sits at the very end of the tail; the dose to the hottest 0.03 cc is a little lower and much less sensitive to the calculation grid.
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.
An editorial history of the UT Southwestern SABR constraint tables — often called the “Timmerman tables” — covering how they were developed and revised, and their relationship to AAPM TG-101 and NRG/RTOG protocols. Tables 2–11 separate serial tissues (listed with a volume, a volume max and a max point dose) from parallel tissues (listed with a critical volume and a critical-volume dose max). In Tables 2–7 a footnote defines “point” as ≤0.035 cm³; Tables 8–11 add a contouring-instructions column for each structure.
Retrospective study of 99 plans comparing five homogeneity index formulas. Its introduction summarizes the RTOG index (maximum isodose in the target ÷ reference isodose), D5%/D95%, and (D2% − D98%) ÷ prescribed dose, and explains that D2% and D98% are used because true point maximum and minimum doses are sensitive to calculation grid size and placement. Full text read in Europe PMC (PMC3532749).
Proposes a new homogeneity index. Its introduction lists the indices in use — Dmax/Dp, D5/D95, (D2 − D98)/Dp and the ICRU Report 83 version (D2 − D98)/D50 — notes that lower values of the last two mean a more homogeneous dose, and that each is based on only two or three points of the DVH. Full text read in Europe PMC (PMC6839365).
Recommendations for standardized naming of targets, organs at risk and dose-volume metrics. Section 9 sets out the DVH-metric nomenclature DoseLab follows: Vx (volume receiving at least dose x), Dx (minimum dose to the hottest sub-volume x), CVx and DCx (the "cold" complements), output units in square brackets (Gy or % of prescription; cc or % of the structure), and an EQD2Gy unit with the α/β written in parentheses — for example D90%(10)[EQD2Gy]. Section 10 asks that DVH metrics say whether a lower-dose PTV is segmented (excludes the higher-dose PTV) or not. Read in the AAPM report PDF.
TPS guidance
Step-by-step guidance planned. Added only once verified.
Step-by-step guidance planned. Added only once verified.