How to scare off a door-to-door salesman

I am not one to try to use my misfortune of having cancer to my advantage, but I have found an occasional benefit to my condition.

A couple of weeks ago, I was home after a chemotherapy session when the doorbell rang. It was some guy selling home security systems. He started in on his spiel, but I stopped him mid-sentence:

"Sorry, man. I had chemotherapy yesterday. I really can't stand here and talk to you."

Seeing as I have no hair on my head and I looked beat, he took it as more than an excuse. He backed away from the door as quickly as possible, apologizing to me profusely all the way back to his car.

Didn't mean to make him feel bad, but I really was in no mood.