In my (very limited) experience with rad onc attendings, I have found them generally approachable and very willing to help. But: I would never ask anyone, in any field, no matter how approachable, to help me in a professional area unless they knew me professionally. That being said, I don't think it would be wrong to contact this individual and ask for advice on what to do in your current predicament (e.g. should you wait longer? seek out back-up positions? etc. etc.) Who knows, he/she might even offer to help! But unless he does, don't ask.