What lung constraint do you use when delivering SBRT for local failure of early stage NSCLC following surgery?

We treat these patients in very similar fashion as a primary early stage NSCLC.

Provided that the target volume is well defined, does not directly involve the proximal bronchial tree, and in these post-operative cases does not involve the pulmonary vascular hilum we would typically treat with SBRT. 

If the lesion is poorly defined or has direct involvement of these structures we would recommend conventionally fractionated chemoradiation (60 Gy in 30 fractions) or hypofractionated radiation alone (45-60 Gy in 15 fractions), depending upon the patients fitness for chemotherapy (presumably fit for chemo if tolerated surgery). 

In most cases however the GTV can typically be defined reasonably well between CT and PET, allowing for consideration of SBRT.  GTV is then expanded to an ITV based on 4DCT.  The standard 5 mm ITV to PTV expansion is reasonable, though in many cases we have elected to include an additional 3-5 mm of the staple line adjacent to the lesion as a "CTV" prior to PTV expansion as there presumably may be more risk of microscopic seeding than in a primary non-manipulated lesion. 

Dose selection for SBRT follows the typical paradigm for stage I NSCLC using a risk adapted approach combining the size and location of the target with respect to central versus peripheral location, and using standard accepted fractionation regimens as if treating primary stage I disease for that particular target volume (54-60 Gy/3 fx, or 48 Gy/4 fx for peripheral targets; 50 Gy/5 fx for central targets by RTOG 0813 definition). 

With regards to lung constraints we have not altered these from the setting of primary SBRT (whether the patient underwent wedge resection or lobectomy), basing the lung constraints rather on their residual pulmonary function (combination of FEV1/DLCO & functional status + stability) at the time of treatment.  The same would be true for an SBRT patient having undergone prior pulmonary resection for a previous unrelated malignancy or benign cause. 

This is due to the assumption that (nearly) all patients undergoing SBRT have some element of lung dysfunction, whether that comes from the classic medically inoperable patient with COPD/emphysema (making up the majority of patients we would treat), a patient with good lungs but having cardiac dysfunction, a previously operated patient with reduced reserve, or even an operated patient with adequate remaining reserve who is now technically un-resectable.  As such prior lung resection simply becomes their cause for medical inoperability, and I would not necessarily treat them differently than another patient with other causes of lung dysfunction undergoing SBRT, with the decision to treat being based predominately on their medical stability in the same way one would make decisions for other early stage lung cancer patients undergoing SBRT.  If one believes in the lung as a parallel organ structure this makes sense as the patient just needs to have adequate functional reserve following SBRT (regardless of the cause of reduction in initial functional capacity). 

I might have additional concern of cause to be conservative in a patient with greater than expected functional decline after surgery (just as in considering SBRT for a metachronous primary in a patient with poor tolerance to initial SBRT), however typically this is not the case. 

Sarah Sittenfeld recently presented our institutional results for SBRT in 50 patients with local-only recurrence after primary surgical resection for early stage NSCLC at the recent IASLC World Conference on Lung Cancer (https://library.iaslc.org/conference-program?product_id=10) demonstrating a disease control and toxicity profile very much in keeping with primary SBRT for stage I NSCLC.