Lung SBRT Plan Built on a Single Free-Breathing Scan With 11 mm Motion
A 74-year-old former smoker with stage I non-small cell lung cancer in the right upper lobe (2.6 cm, peripheral, abutting the pleura) is scheduled for 54 Gy in 3 fractions with VMAT. His FEV1 is 38% predicted and he cannot hold his breath for more than 8 seconds without coughing. At simulation, abdominal compression was applied and a 4DCT was acquired. The residual craniocaudal tumor motion on the 4DCT is 11 mm, with 3 mm in the anteroposterior direction. The plan in the record was generated on the single free-breathing axial scan, and the PTV equals the GTV plus a 5 mm expansion. The 4D phases and the ITV were never imported into the planning system. The plan report shows PTV D99 = 94.8%, spinal cord maximum 9.2 Gy, and lung V20 = 8%. The first fraction is scheduled for tomorrow morning.
1. What is wrong with the plan as described, and what is the risk to the patient if it is treated as written?
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The geometry and the motion data do not match. An 11 mm craniocaudal excursion means the GTV occupies a volume that extends roughly 11 mm beyond its position on any single scan. A 5 mm expansion around the GTV on one free-breathing phase cannot cover that envelope; at end-inspiration the tumor can sit outside the high-dose region entirely. That is a geographic miss, and the consequences are the ones that matter in SBRT: underdosing part of a small, high-dose-per-fraction target and shifting dose into lung, chest wall, or mediastinum.
There is a second problem. The plan was optimized on one phase, so the dose distribution on the other phases is unknown. Even if the PTV covered the static tumor, the actual delivered dose during the breathing cycle has never been calculated. The cord maximum of 9.2 Gy in 3 fractions and the lung V20 of 8% look acceptable on paper, but they describe a plan that was never evaluated against the anatomy the patient will actually have during treatment.
The correct conclusion is that this plan is not treatable. It needs to be rebuilt from the 4D dataset, with the ITV or gating window defined and the dose recalculated across the motion envelope before the first fraction.
2. The patient cannot tolerate deep inspiration breath hold. Compare the available motion-management options and recommend one, with your reasoning and its practical limitations.
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Option 1: gating at end expiration. A 30-50% phase window around end expiration can reduce the treated excursion substantially and is often tolerated by patients who cannot breath-hold, because they only need to breathe regularly, not hold. It requires a coaching session, a gating system, and a repeat 4DCT acquired in the treatment position with the gating signal. Limitation: irregular breathing, coughing, and a longer treatment time in a patient with poor lung function.
Option 2: abdominal compression with an ITV. This was already attempted and left 11 mm of residual motion, which is too much for a 5 mm margin. More compression may not be tolerated and can worsen setup reproducibility.
Option 3: free-breathing ITV with a larger margin. Build the ITV from all phases and add a margin derived from the measured motion plus setup uncertainty. This covers the tumor reliably but irradiates more lung and chest wall, which matters at V20 and in a patient with an FEV1 of 38%.
Recommendation: attempt a gating session at end expiration, because it gives the best combination of target coverage and normal-tissue sparing in a patient with poor pulmonary reserve. If he cannot breathe regularly enough to gate, fall back to the ITV-based plan with a justified margin, and accept the higher lung dose rather than accept a geographic miss. In either case the current plan is discarded.
3. Assuming a gated or ITV plan is produced, what should be verified at the first fraction, and how?
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Image guidance must match the tumor, not bone. A cone-beam CT with soft-tissue review by the radiation oncologist confirms that the target is in the planned position; a 4D CBCT or a slow scan confirms the motion amplitude and the phase window. Bony alignment alone is inadequate because the tumor moves relative to the spine and ribs with respiration.
Check three things explicitly. First, that the gating window in use matches the window on which the plan was built; a changed threshold silently changes the treated volume. Second, that the residual motion on the day matches the motion measured at simulation; if it is larger, the plan is no longer valid and the physicist is consulted before the beam is turned on. Third, that the ITV or gating envelope covers the tumor on the CBCT, not just the centroid.
Document the verification, the window used, and any discrepancy. If the first fraction cannot be verified to the same geometry the plan assumed, the right move is to stop and re-plan, not to proceed on the grounds that the schedule is tight.