Hi Mary,
You sound like a person after my own heart, with volunteering etc so I'm going to suggest that, if it's still something that you want to do, enjoy doing and feel up to it, chat to your Onc/Breastcare Nurse/Dr to get a second opinion/change dates etc but otherwise go and enjoy yourself. I don't know if you've ended up with the dreaded fatigue [I didn't until months afterwards] but, if so, explain to the organisers that you might need 'rest time' or a desk job etc. Everything I've volunteered at have been fine with it - they need the help!
Take care, have a great weekend & healing thoughts going your way.
Go and enjoy yourself :)