Would you consider treating a patient who has an unresectable slowly growing PET avid lung mass with concurrent chemo RT when multiple biopsy attempts have only resulted in pathology "suspicious" for adenocarcinoma of the lung?

I would consider treating the patient if they have a highly suspicious biopsy with the proper clinical context and after speaking with the pathologist to understand why the report indicates only "suspicion" rather than "confirmation". The patient in question had "multiple biopsy attempts" and demonstrated growth on CT imaging that was positive on PET scan. In a similar circumstance, if the biopsy indicated "atypical cells", I would want more tissue to confirm this is cancer. Of course, a transpa...

No. We need to protect against all slippery slopes that may allure us to treat without a cancer diagnosis. The use of empiric SBRT without path confirmation for stage I lung cancer is a unique situation as it parallels empiric Surgery without path confirmation. Extrapolating such a paradigm to deliver chemorads without tissue is unwise. I agree with Dr. @Dr. First Last to follow-up in a few months and revisit a different biopsy approach.

I would not treat with chemotherapy or targeted therapy until a definitive diagnosis is made. Blastomycosis and other conditions have been mistaken for malignancy. If you treat based on suspicion of... sooner or later a rather serious error will be made.

If the patient is not overly enthusiastic to receive therapy and asymptomatic, I don't think it would be wrong to observe very closely with follow-up imaging in 2-3 months to assess the rate of growth.

It sounds like this is stage I disease. Chemotherapy is never indicated for stage I disease. The therapeutic choices are surgery, SBRT, or observation. If fit, surgery is preferable. If life expectancy is less than 5 years, then observation is appropriate for a slowly growing lesion. For the rare patient who is unfit for surgery but with a life expectancy of > 5 years, TTF positive atypical cells are enough evidence to justify SBRT.