How do you treat inoperable T1-2N0 apical lung cancers near the brachial plexus but without extension outside the lung?

This is a challenging question, and there are certainly a range of reasonable answers.

I would agree with @Dr. First Last that the Forquer/Timmerman paper establishes there is significant risk of plexopathy when exceeding 24-26 Gy in 3 fractions. On the other hand SBRT offers superior local control compared to conventionally fractionated radiation, and apical tumor in particular may become quite locally symptomatic if not controlled, so there is rationale to proceed with SBRT if at all possible/...

To update these excellent answers from 2018, there have been several helpful manuscripts on the topic of radiation-induced brachial plexopathy (RIBP) following SBRT that have been published in the past year.

Morse et al., PMID 34929402: 5 of 78 (6.4%) patients developed RIBP, 4 of whom received 50 Gy in 5 fractions. The range of doses received were very wide, with a very low threshold for NCTP giving a good fit (EQD2 with alpha/beta of 3 of 20 Gy, equivalent to 16 Gy in 5 fractions).

Milano et al...

With 3-4 fractions, Timmerman found that patients with a maximal dose of 26 Gy or less had a fairly low risk of BP (8% vs 46% for doses higher than 26 Gy; the original 3 fx protocol recommends 24 Gy/3 fx). The NCCN constraint is 32 Gy for 5 fractions. Another series with 75 patients with apical tumors showed no toxicity, with a variety of doses to the BP. Overall, with care, it is reasonable to try to meet these constraints and safely treat apical tumors.

If there is difficulty meeting these cons...

I am making an assumption that normal tissue tolerance cannot be achieved with a 3-5 fraction regimen of SBRT for the brachial plexus from the question you have written.

In that circumstance:

I have utilized a regimen from a peer-reviewed published article by Yung T et al. "Outcomes of accelerated hypofractionated radiotherapy in Stage I non-small cell lung cancer" Curr Oncol 2012 Aug 19(4): e264-9.

Typically for this location, it has been either 50 Gy in 20 fractions up to 60 Gy in 20 fractions. A...