Soaking Up Some Rays

The first two sessions of SBRT are complete. From my perspective it's the easiest thing in the world. I walk into a room, lay down on a table, slide into a machine, and get my dose of radiation. Due to the location of the target I don't even have to take my clothes off, I just pull up my shirt. It's absolutely painless and the machine barely makes any noise at all. Fifteen minutes later I'm on my way home. 

But then there's this...  I know what that machine does and a little bit about how it works. I know that when it's running there are high energy photons passing through my body and damaging everything they hit. The target is like the axle of a wheel. The beam moves around the target, blasting it from multiple positions so that the non-target areas never get the full dose of photons while the target does. The plan is to kill off the bad cells and spare the good cells. But if I move even a fraction of an inch while I'm in that machine, does any part of the invisible beam miss the target and hit healthy cells? How good was the targeting in the first place? How can you not think of this thing as a "death ray?" If you let your mind run away with itself some creepiness can sneak in. Cold. Industrial. Remote. Ominous. Best not to linger on these thoughts. Gotta have some faith in the system. 

There is one small technical thing that bothers me. In the treatment room there are warning lights, one red and one green. When the machine is off (not emitting radiation) the red light is lit and says "beam not on." When everyone leaves the room and you're on the table, the green light comes on and says "beam on." Why? I'm the guy on the table. It doesn't mean anything to me. I expect to get blasted and I don't move until someone comes into the room and says the session is over. But if I worked in that room all day I'd want to know that the beam is off when I'm in there. A red light universally signals danger. I brought this up to one the radiation techs. She said that the warning lights outside of the door are the opposite. If there is a red light they don't go into the room (I'm going to guess that there are safety interlocks on the doors anyway). Is there any logic to explain this? Is it possible that the people who installed the lights mixed them up? Stay tuned...