Breast surgery

Hi, 

Last Friday I went to hospital to have a surgical procedure for removal of breast lump, lump was a B3 lesion, they said it was being removed for further testing. On my discharge letter it says wide excision and sentinel node biopsy.  I only recall the surgeon telling me they would remove lump and take a margin around it for testing. What I don't understand is why has my sentinel node been biopsied when they don't know for sure if the lump is cancerous!

Has anyone else experienced this?