prone versus supine

Thats a good question. I had cancer on the right and a bmx with expanders placed. I wasnt given the option of prone. They did mine supine. I always assumed it was because I had cancer on right side.

I understand that if you have positive nodes, you are out of the running as a candidate for prone (I looked into this extenstively myself).  They need to be able to get to the clavicular area, the chest wall, the armpit, etc.  I do not have positive nodes and I would be able to do prone however the only facility that does it (for Kaiser), is across the bay in Oakland.  It would make my life a living hell far more than just down to south of San Francisco every day so I had to weigh my quality of life against a likely only 0.5% reduction in possible side effects, plus, and this kind of made my mind up as well, my chest wall would NOT be getting scatter and I want full coverage thank you very much!  Sucker be gone, damnit!  It should also be noted that I have left side cancer, so I do understand the risks are a little higher for me.  But I really like and trust my radiation oncologist and he was very open and clear about the statistics and I felt far less anxious about it than I had before going in to talk to him.  Also, I think you need to have, uhm, how was it put "large, pendulous breasts", or at least that makes prone (or as I affectionally called "Boob-In-A-Board".  Which I "did" also have.  Well, still have on one side!

I, too, had prone rads. following my left-side lumpectomy. I am a big fan of having left rads this way if possible to minimize heart exposure, but I did have to ask because it wasn't standard protocol. Although my hunch is that a mx would make this position unlikely, DEFINITELY ASK THE HEAD OF YOUR RAD.ONC.FACILITY.

Prone Procedure

I researched (not many across the US)
before interviewing facilities.  I went
in knowing what I wanted and who I wanted. 
I was fortunate as the facility I selected had recently purchased a
second new rad machine (due to volume of patients needing treatments), 75% of
all patients treated at this location are treated in prone.  Left side with enough tissue to properly hang
in the allocated chamber and custom chest cavity form (stable the body to reduce possibility
of movement) for target area to be treated, including node range and ribs.  Patients are tested for flexibility of neck (facing
away from radiation and arms to be in same position (over your head) each day.  If flexibility is not compliant and painful,
prone will not be considered.

http://igrt.com/breast-therapy.html

Chiropractic adjustments and massage once a week or every other week during rads incorporating proper lymph flow of soft tissue.  It really helps with the pain and flexibility issues.  Oncology rehab can provide instruction sheets on correct technique. 

I waited two months after surgery to allow immune system and surgery site to heal before starting rads.  My tumor bed is one inch from my heart.

Ask for paper copies of the suggested computer model they are preparing for treatment so you know and understand the beam range for daily and boost treatments.  You can take a camera in the treatment room and photograph the computer screen details prior to your daily individual treatment (if this is something you are interested in documenting and keeping).