Definitely ask your Rad Onc about the 'prone' position @aj456 .... but it totally depends on the site and type of your surgery & how extensive it was - and your breast shape. If your tumour was closer to your chest, I am guessing you'd have to be face up.
Is it your left breast or right breast? If left breast, there may be some 'breathing controls' done during the treatments.
The machines are very good at doing targeted treatment these days .... and they are all very aware of not wanting to damage other organs.
take care and have faith in your Rad Onc xx