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.
Intermediate Dose Spill
Dose spreading into the tissue around the target at levels below the prescription. In lung SBRT it is judged with R50% and D2cm.
What is it?
Every plan deposits some dose outside the target. Intermediate dose spill is the dose reaching the tissue around the target at levels below the prescription but still a substantial fraction of it — the region that R50% and D2cm describe.
The term comes up mainly in stereotactic planning. The NRG lung SBRT protocol template groups two measures under "allowable dose spillage":
- R50% — the volume of the 50% prescription isodose divided by the PTV volume: how big the intermediate-dose cloud is.
- D2cm — the maximum dose at 2 cm from the PTV in any direction, as a percentage of the prescription: how much dose still reaches tissue a short distance away.
Dose at or above the prescription outside the target is assessed separately, through conformity and hotspots.
Why does it matter?
Stereotactic treatment gives a large dose per fraction, so even a fraction of it can be significant for nearby tissue. A compact dose distribution — one where the dose falls off quickly outside the target — keeps that intermediate dose close in. NRG's lung SBRT template makes this part of its second planning priority: achieve target coverage with conformity and compactness of the dose distribution.
Intermediate dose spill is shaped by planning choices: beam and arc geometry, the use of non-coplanar beams or arcs, and how the optimizer is told to treat the tissue around the PTV. A plan that meets every organ limit can still have more spill than necessary.
How is it evaluated?
- Calculate R50%: convert the 50% prescription isodose to a structure, read its volume and divide by the PTV volume.
- Calculate D2cm: build a structure for everything 2 cm or more beyond the PTV (for example, the body minus a 2 cm PTV expansion) and read its maximum dose as a percentage of the prescription.
- Compare both with the protocol's table for the PTV volume. The NRG lung SBRT template tabulates both measures against PTV volume, with "per protocol" and "variation acceptable" levels.
The same 2 cm expansion can be used to control spill while planning: the NRG lung SBRT template lists a 20 mm PTV expansion structure (PTV_5400+20 in its example) "defined to control intermediate dose spillage."
In radiosurgery, fall-off is also reported with the Gradient Index, which ICRU Report 91 recommends; the NRG brain SRS template uses R50%. DoseLab does not reproduce protocol limits; use the original protocol.
Example
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.
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.
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).
TPS guidance
Step-by-step guidance planned. Added only once verified.
Step-by-step guidance planned. Added only once verified.