Hello dani1985 and thanks for getting back in touch.

When a thyroid cancer is removed my understanding is that the surgeon has to cut through a small amount of adjoining additional tissue. This is to be able to separate out and detach half or all of the gland. This additional tissue is the surgical margin, that is a border/rim of adjoining tissue surrounding the cancer and removed along with it. 

Within this adjoining tissue border (surgical margin), the pathologist routinely looks to see if any cancer cells are present. 

A R1 resection in thyroid cancer cancer means that all the cancer that can be seen has been removed, but a small number of cancer cells have been detected in the edge of the additional tissue that was cut away (the margin). There may or may not be more cancer cells in the remaining tissue that joined the outer edge of that margin. I don't know if this has any bearing on the likely success of treatment - it might not because the idea with the radioactive iodine is that it can target and destroy any remaining thyroid cancer cells wherever they are.  Our website has more information about it at this link 

I hope this helps. Please give us a ring if you want to talk about anything over. The number to call is Freephone 0808 800 4040 and we are around from 9-5, weekdays. 

Best wishes,

Julia