Sentinal Node Disection with Margins Clear

I had a lumpectomy and have been diagnosed with breast cancer (Tubular Carcinoma).  The pathology says the margins of the removed area are clear. I do not understand why a sentinal node disection would be required.  Even with margins clear can the cancer travel to the lymph system?  

[Cleveland Clinic]

Dear sandreti, Checking the lymphnodes is part of the staging process of breast cancer care.  When there has been any note of invasive cancer - a lymphnode dissection is recommended in order to check for any evidence of cancer spreading away from the breast.  Being that there is invasive cancer despite the clear margins there can still be the possibility of cancer going to the lymph nodes.  In cases of low to moderate risk of lymph node involvement (i.e very small area of cancer)  the less extensive, type of surgery -the sentinel lymph node dissection may be recommended.  

[katarina777]

Hi, I think that 'clear margins' only refers to the suarrounding area of the particular lesion that was surgically removed. But with a lumpectomy especially, there may be cancer cells in other parts of the breast left. KAT