Hi @bunr I am a Radiation Therapist so We are the 1s who do your “simulation” when we decide on the best position for you particular body shape and take a CT scan, we then use that CT to design a “plan” for you and then treat you daily using that plan.
For left sided breast to protect your heart there is a technique called DIBH (deep inspiration breath hold) this is where you take a deep breath thus expanding your lungs and moving your heart further away from your breast tissue. Please note every department uses different techniques, this may not be available at your department.
We follow strict guidelines when it’s comes to how much radiation dose each particular organ can receive, this is based on EVIQ (you can google it I can’t remember if it’s .org or .com sorry).
Hope that helps anything else please ask :)