How would you treat an early-stage gastric adenocarcinoma in a patient who is medically inoperable and not fit enough for chemotherapy?

In my n=1 experience, I treated a medically inoperable patient with a T1N0M0 gastric body adenocarcinoma with "definitive" RT with mild Xeloda. I had the gastroenterologist place clips around the tumor. I treated the whole empty stomach and lymphatics to 45Gy and the gross disease to 54Gy. The patient was cautioned that he was at risk for ulceration, symptomatic gastritis and death. The data from the 60s and 80s demonstrated a benefit with the addition of chemo to RT but with an increase in death. My patient is cNED 2 yrs out without toxicity.

https://www.ncbi.nlm.nih.gov/pubmed/4186452

https://www.ncbi.nlm.nih.gov/pubmed/6176313

I agree in general with @Dr. First Last. I have treated a small number of patients with chemoradiation alone. While I would hesitate to call this definitive therapy, there are good prospective data, (e.g. Ajani et al. JCO 2005) that demonstrate a significant (20-30%) path CR rate in the neoadjuvant setting after induction chemotherapy and chemoradiation. Certainly, there are additional patients with a PR who derive clinical benefit from this treatment.

However, the question does ask about radiotherapy without chemotherapy. While this is not preferable, I have treated the rare patient with radiation alone. Again, this is not expected to be definitive therapy, but would really be (hopefully durable) palliation. I have not gone over 50 Gy, and ...

One would need to make a judgment about whether a patient who cannot tolerate even an endoscopic resection can tolerate combined chemoradiation. The pCR rate for locally advanced gastric cancer with chemoRT is in the 20-30%. It is presumably higher with a T1a tumor, though I'm not aware of literature reporting the outcome of chemoRT in such early stage disease.

If it is felt the patient would not tolerate chemoRT, one could consider a short regimen of radiation alone.

Observation is another option, particularly if the comorbidities that make the patient ineligible for surgery or chemoRT lead to competing risks of mortality from other causes within a relatively short time frame.