How would you manage a patient with history of clear cell RCC >4 years ago who has a solitary oligometastatic brain lesion that is asymptomatic and too small for stereotactic radiation?

I've not encountered a CNS lesion too small for SRS. Sometimes, very small abnormalities are uncertain and are watched, but usually, they are big enough to be diagnostic and treated. If that's the only site of disease, I would do SRS then observe that patient.