I was also unfortunate enough to be counselling a mother with breast cancer a few years ago. This mother was breastfeeding her 9mo (2nd baby) when she developed a mass in her right breast. I saw her a couple of weeks down the track when the mass measured 5cms x 7cms (I was amazed at the size of it, it had grown in 2 weeks). She had had an ultrasound and it was suspicious looking ie, didn't look like mastitis. It was not very painful and the baby was feeding well from that side.
After much counselling she weaned and had a radical mastectomy. I lost touch with her after discharge, but have heard she is doing well.
This was a good lesson for me, I felt when I saw her that it wasn't mastitis and was happy that she had to have surgery, but the way she was treated with regard to her baby and breastfeeding wasn't done in the most empathic way. She obviously had to wean, but they wanted it done in 24hrs and she had to deal with the shock of cancer and weaning all at the same time. It was pretty devastating for her (and me at the time)

Jackie Cesnik IBCLC
De Novo Lactation Consultants 
Vice President ALCA Vic
NMAA Breastfeeding Counsellor
Vic Branch Information Service Co ordinator