DCIS and Isolated Tumor

I wasn't sure where to post this so I apologize if it's in the wrong section. 

Here's my question... what exactly is Isolated Tumor Cell?  My first pathology of my left breast said DCIS and micrometastatic sentinel lymph node.  My oncologist wasn't satisfied or confused about that so she sent the breast tissue for a 2nd opinion. The 2nd pathology report came back, DCIS and LCIS  isolated tumor cell sentinel lymph node.  Oncologist said that basically means no lymph node involvement but I'm confused about that. Does that mean that there was a cancer cell in my lymph node but not large enough to travel to other lymph nodes? Why wasn't I offered radiation if there is involvement?  I had 4 TC and now Tamoxifen. 

My mind will not stop going to the what ifs and when.  I keep thinking I'll have a reocurrence and metastasis. I can't stop my mind from going there.  My right breast had 3cm IDC and DCIS and clear sentinel lymph nodes.  I'm just confused.  Thanks for your help.