Stereotactic body radiation therapy (SBRT, also called SABR) delivers high doses in a small number of fractions to small, well-defined targets. For lung, that puts particular weight on three things: accounting for respiratory motion, shaping dose tightly around the target with a steep falloff, and respecting nearby organs — which ones depend heavily on whether the tumor is peripheral or central.
This page organizes the concepts and resources behind each step, from simulation to plan evaluation. It explains why each step matters; the numbers — margins, prescriptions, constraints — belong to the protocol you’re following.
Lung tumors move with breathing. Everything downstream — target volumes, margins, delivery — depends on how that motion is measured and managed. AAPM TG-76 is the standard reference for this section.
A CT acquired across the breathing cycle and sorted into phases, so the tumor's position can be viewed at each point of the cycle.
A single free-breathing CT is a snapshot: it can catch the tumor anywhere in its path and even distort its shape. 4DCT shows how far and in which direction the tumor actually travels, which drives the choice of motion-management strategy and the target volumes built from it.
Review every phase, not just the derived images. Irregular breathing during the scan can create artifacts that look like changes in tumor shape. Derived datasets — maximum intensity projection (MIP), average intensity projection (AIP) and individual phases — each have a job; which one is used for contouring and which for dose calculation is set by your protocol and physics team.
Movement of the tumor and surrounding anatomy caused by breathing, usually largest in the superior–inferior direction.
Motion varies a lot between patients and between tumor locations; tumors in the lower lobes near the diaphragm tend to move more than apical tumors. Motion that isn't accounted for becomes a geometric miss; motion that is over-accounted for irradiates extra lung.
TG-76 recommends measuring tumor motion for each patient when significant motion is expected — see the report for its thresholds. Breathing at simulation may not match breathing on treatment days; image guidance at treatment is how that difference is caught.
A target volume that encompasses the tumor's positions over the breathing cycle. Concept page →
The ITV is the most common way to account for motion in lung SBRT without gating or breath hold: rather than chasing the tumor, the plan covers its whole path.
An ITV is commonly built from the GTV on every phase, or from a MIP. A MIP can under-represent the tumor where it moves next to other high-density tissue (diaphragm, chest wall, mediastinum), so check it against the individual phases.
Delivering the beam only during a chosen portion of the breathing cycle.
Treating in a narrow window reduces the motion the plan has to cover, which can shrink the target volume for tumors that move a lot.
Gating depends on a breathing signal (an external surrogate or internal markers) that has to correlate with where the tumor really is. Treatment takes longer, and the simulation needs to be acquired and reconstructed for the same gating window.
Treating while the patient holds their breath at a reproducible level.
Holding the breath nearly freezes tumor motion for the length of the hold.
Works only for patients who can hold reproducibly — often a challenge for patients with poor lung function. Coaching and a monitoring system help; image guidance verifies the tumor position during the hold.
Applying pressure to the upper abdomen to limit diaphragm excursion (a “forced shallow breathing” technique in TG-76 terms).
It can reduce the amplitude of motion, especially for tumors near the diaphragm, so a smaller ITV covers the tumor's path.
The device position and pressure need to be recorded and reproduced exactly each day, since they change the anatomy and the motion. Patient comfort matters: SBRT sessions can be long.
Image fusion aligns an outside scan with the planning CT so the target can be drawn using information the CT doesn't show. For lung SBRT this is usually the staging FDG-PET/CT; MRI can also be used. The GTV is still drawn mainly on the planning CT — in lung windows, checked in soft-tissue windows.
Tumor can sit against atelectasis or fibrosis that looks much the same on CT. PET helps tell them apart, and one template says it may be necessary when there is a lot of either; IV contrast on the planning CT also helps, because collapsed lung does not enhance. The PET/CT is a separate scan, so check the registration around the tumor — and if a fused image becomes the main image for the GTV, the template suggests an extra margin for registration uncertainty.
Devices and positioning that keep setup reproducible across treatment — commonly arms raised above the head for beam access, with an indexed body cushion or frame.
SBRT uses tight margins and steep dose falloff, so small setup differences matter more than in conventional treatment.
A position the patient can hold comfortably for the whole session is more reproducible than an ideal position they can't. Image guidance before each fraction checks setup; the immobilization keeps it stable during delivery.
How target volumes build on one another in lung SBRT. Margins and expansion rules come only from a protocol or institutional policy.
The visible, palpable or demonstrable extent of tumor.
Every other volume is built from it. In lung, the appearance of the tumor depends on the CT window and level used, so protocols often specify the window for delineation. PET is frequently used alongside CT to distinguish tumor from collapsed lung.
A volume accounting for internal target motion — in lung, typically derived from 4DCT. Concept page →
Represents the space the tumor occupies over the breathing cycle, so the plan covers the tumor wherever it is during delivery.
The volume containing gross tumor plus suspected microscopic disease. Whether a separate CTV expansion is used in lung SBRT depends on the protocol.
Many SBRT protocols rely on the high dose around the tumor edge to treat microscopic extension rather than a separate CTV; others add one. Check which approach your protocol takes before building volumes.
A geometric expansion that accounts for setup and delivery uncertainty so the intended dose reaches the target.
In SBRT the PTV is also the volume most plan-quality metrics are measured against — coverage, conformity, R50% and D2cm are all defined relative to it — so its size directly shapes how a plan is judged.
Where the tumor sits changes which structures limit the plan.
Protocols define “central” using distance from the proximal bronchial tree and other mediastinal structures, and the definitions differ. Some groups also describe an “ultracentral” category. Before planning, confirm which definition your protocol uses and which structures it asks you to contour.
Lung is one of the hardest places to calculate dose accurately: a small, dense tumor surrounded by low-density tissue.
Accounting for the different densities of tissue (lung, soft tissue, bone) when calculating dose.
Lung tissue is far less dense than water. Ignoring that difference misrepresents the dose actually delivered to the tumor and surrounding lung.
Heterogeneity-corrected calculation is the norm for lung SBRT; AAPM TG-101 discusses it. Your protocol will state what is required.
The method the planning system uses to compute dose — for example pencil-beam, convolution/superposition (e.g. AAA, Collapsed Cone), Acuros XB or Monte Carlo.
In low-density lung, lateral electron transport increases and the dose at the tumor edge changes. Simpler algorithms model this poorly and can overestimate dose to small targets surrounded by lung.
Know which algorithm your plan was calculated with and whether the protocol restricts it. When comparing plans or reading outcomes data, note the algorithm used — numbers from different algorithms aren't directly comparable.
SBRT plans typically use many beam directions — multiple static fields, non-coplanar beams or VMAT arcs — to spread entrance dose and concentrate dose at the target.
Many beam angles produce the steep dose falloff that metrics like R50% and D2cm measure. Techniques and angle choices also affect skin and chest-wall dose for peripheral tumors.
Collision checks and couch clearance matter for non-coplanar beams. With VMAT and IMRT, small targets with a moving tumor raise the interplay question: how modulated delivery and tumor motion interact. Your physics team's policy governs how much modulation is acceptable.
Imaging before (and sometimes during) each fraction to confirm the target's position — most often cone-beam CT.
With tight margins, daily verification that the tumor sits where the plan expects is central to safe SBRT delivery.
Matching can be on the tumor itself or on bony anatomy; know which your department uses and why. A CBCT of a moving tumor shows a motion-blurred tumor, and its extent should be consistent with the ITV drawn at simulation.
Click an organ for its anatomy, why it matters and contouring resources. Superscripts link to the study behind each value.
| Organ | 30–34 Gy / 1 fxPeripheral | 54 Gy / 3 fxPeripheral | 50 Gy / 5 fxCentral |
|---|---|---|---|
| Spinal Cord |
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| Esophagus |
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| Heart / Pericardium |
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| Great Vessels |
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| Trachea & Proximal Bronchial Tree |
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| Brachial Plexus |
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| Chest Wall / RibsNever at the expense of PTV coverage. |
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| Skin | |||
| LungsBoth lungs minus iGTV. |
Values quoted from eviQ 3389 (last reviewed 16 March 2023; review due 30 June 2027) and eviQ 4018 (last reviewed 16 March 2023; review due 30 June 2027), checked by DoseLab on 2026-09-22. Verify against your protocol, institutional policy and physician directive before clinical use.
Trials and reports every lung SBRT planner should know exist. Bibliographic details marked Under Review are awaiting DoseLab's check against the original.
NRG Oncology's protocol page for RTOG 0813, with the protocol document and supporting forms. Read the trial's own dose constraints, its definition of a central tumor and its plan-evaluation requirements in the protocol document. Results are published separately as the RTOG 0813 trial report.
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.
Phase 2 multicenter trial of SBRT for medically inoperable, early-stage, peripheral non-small cell lung cancer. The original normal-tissue limits written for this trial are reproduced and discussed in Timmerman 2022 (Table 1).
Randomized phase 2 comparison of a single-fraction and a multi-fraction SBRT schedule for peripheral stage I non-small cell lung cancer. An erratum and a long-term follow-up report were also published.
Seamless phase 1/2 dose-escalation trial of five-fraction SBRT for centrally located early-stage non-small cell lung cancer. The protocol's definition of a “central” tumor is widely referenced.
UK consensus on dose–volume constraints for SABR of oligometastases, primary lung cancer and hepatocellular carcinoma. Citation checked against Crossref. DoseLab read the tables in the authors' accepted manuscript (UCL Discovery and White Rose Research Online): Table 6 (intracranial, 1, 3 and 5 fractions) for the brain pages, and Tables 2, 3 and 5 (thoracic, abdominal and spinal) for the liver SBRT page. Each table gives optimal and mandatory limits by fraction count; the liver limits apply to patients with, at worst, Child-Pugh A6 liver disease. The kidney limit is written as a "cold" constraint — at least 200 cc must receive no more than the stated dose.
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.
AAPM task group report on respiratory motion in thoracic, abdominal and pelvic treatment. It groups management approaches into motion-encompassing methods, respiratory gating, breath-hold techniques, forced shallow breathing (such as abdominal compression) and respiration-synchronized (tracking) techniques. Read in the AAPM report PDF (Report No. 91, July 2006): using ICRU Report 62 nomenclature, the PTV adds margins to the CTV for respiratory (intrafraction) motion, interfraction motion and set-up error; without a respiratory management device the entire range of motion should be considered in the internal margin; and for lung tumors a maximum intensity projection (MIP) can give the motion-encompassing volume, provided there is no mediastinal involvement.
A thoracic organ-at-risk contouring atlas with discussion of dose limits, covering the lung, proximal bronchial tree, esophagus, spinal cord, ribs and brachial plexus. A published letter in response discusses esophageal delineation.
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.