In a patient with history of pneumonectomy would you consider SBRT for treatment of a new tumor in the remaining lung?

I've been pulled into this a few times and it is tricky. I think I've posted a few of my misadventures about this before (central tumor in a patient with pneumonectomy that died from SOB related to mechanical changes). However, I have done it successfully several other times and so I believe it's doable with careful attention to detail and a strongly worded consent that contains "death" a few times. Here's how I think about it...

1. What is it? Are you 100% or close that this is a malignant process and is life-threatening? For tumors closer to the mediastinum, have you biopsied it via bronch? Bronch is also important here to rule out mediastinal disease and I would do it regardless if the PET is negative, assuming the patient can tolerate it. Is liquid biopsy an option?
2. Where is it? Peripheral is so much easier, but if it's a lower lobe nodule in a pneumonectomy patient it's harder that it appears b/c the tumor motion will be outrageous (see below).
3. Systemic options available? If so, do that. Try a round of immunotherapy and buy time prior to RT. Sometimes med onc doesn't realize the risks of SBRT in patients with 1 lung so they have something they are meh about but it's worth trying that and watching closely.
4. Do you have to do this? It's a malignant process, with no good systemic options, but is it growing slowly or in the context of another more immediately lethal process (bone mets, end stage COPD, etc). I have also watched some in patients when the growth rate is mm/year. In that setting, you should work out with the patient when you will pull the trigger so there are no surprises, but with 1 lung, COVID, seasonal flu...sometimes you can watch till they are > 1 cm or the growth rate suggests a more malignant transformation. Lastly, I STRONGLY DISLIKE protons for lung SBRT, but I do entertain it for this 1 group of patients as the alveoli get less. I use protons for lung SBRT in pneumonectomy patients and IPF patients only really.

If you work out that this is a tumor with no systemic options and you have to do this, and you gave the patient the "this could kill you" talk, AND you're getting to do this ready consider the following:

• Sim with motion management. Breath hold is rarely feasible, but abdominal compression and gating are usually good options. I like to be at the sim for these and assess motion and patient tolerance for the different modalities.
• Plan with the idea that you're conserving alveoli as a priority. Run the center of your rapid arc plan HOT. The system isn't used to doing that with VMAT/rapid ARC so it takes intention but is doable. My physicist explained it to me once but I glazed over about 10 min in. Something about taking off the top constraint and forcing a normalization, similar to what we did in the 3D era of SBRT. Alternatively, you could do 6+ beam 3D, which works great.

Follow them closely and pull the trigger on steroids earlier than you would have otherwise. Tell the patients to be doubly careful to call you if they feel SOB. Warn the patients how high a risk they are of COVID and swab them if they call. I would consider these patients candidates for Paxlovid (but I haven't had this come up since it became available). See them at 6 weeks and 3, 6, and 9 months and see how they are faring. Once they are past the year and the risk of pneumonitis fades, you can swap them to however you follow your patients.

Remember, progressive tumor in a pneumonectomy patient is lethal nearly 100% of the time. It's okay to take a risk, together with the patient, to give them a chance at avoiding that.

Live Long and Prosper.