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.
D2cm
The maximum dose at any point 2 cm or more away from the PTV, expressed as a percentage of the prescription dose.
What is it?
D2cm is the maximum dose received by any point located 2 cm or more away from the planning target volume (PTV), in any direction. It is usually reported as a percentage of the prescription dose.
Put simply: once you have moved 2 cm outside the target, how hot is the hottest point you can still find?
Why does it matter?
Stereotactic treatments deliver high doses to small targets in few fractions, so the dose needs to fall off steeply outside the target to spare surrounding tissue.
D2cm captures one aspect of that fall-off — whether meaningful dose is still reaching tissue a short distance away. A higher-than-expected D2cm points toward intermediate dose spill and is a prompt to revisit beam geometry, arc arrangement or optimization objectives.
It complements R50%: R50% describes the overall size of the 50% isodose cloud, while D2cm describes the single hottest point beyond a fixed distance.
How is it evaluated?
Conceptually, build an evaluation structure representing everything outside a 2 cm expansion of the PTV — for example, expand the PTV by 2 cm and subtract that expansion from the body contour — then read the maximum dose inside it.
Express that maximum as a percentage of the prescription dose.
Acceptable values are protocol-specific and are commonly tabulated against target size. DoseLab does not reproduce those values; use the original protocol.
Example
Authoritative resources
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.
NRG Oncology's protocol table, covering current and closed RTOG/NRG trials. A protocol's own constraint table is written for that protocol's fractionation, target definitions and contouring instructions — the numbers do not transfer to another schedule. Individual protocols are added to DoseLab as separate entries, each with its version.
AAPM task group report describing best-practice guidelines for stereotactic body radiation therapy, including a table of normal-tissue dose constraints. Bibliographic details confirmed against the journal index; table map not yet checked.
TPS guidance
Step-by-step guidance planned. Added only once verified.
Step-by-step guidance planned. Added only once verified.