Hi Cobie, I also have a thick lump between my sentinel node biopsy scar, and my mastectomy scar. I found this awhile ago, and my surgeon, P.S and Oncologist and GP all think it is just scar tissue. I still worry, so have also had 2 ultrasounds 12 months apart, with emphasis on this area. Nothing has ever shown up, so they say scar tissue. It still worries me, and sometimes I am very aware of it. When I fiddle with it, it becomes tender, and so I poke it more. It is so hard to not always think the worst. When I see my surgeon at the end of the year, I will again ask him to feel it. I hope your lump is nothing sinister. Good luck. Love Chris.xx