My neorologist gave me some blood tests and my b12 came back with the result of 1243 and mma at 74. should i be concerned with cancer? i've had thyca

You're fine: High B12 never means anything nowadays. There are a few uncommon, obvious diseases in which it tends to be high. I am going to assume your units on methylmalonic acid suggest a high value and that also means nothing. Reference ranges are set so that a few percent of healthies fall outside on either end. Be glad for good health.

Not much: I understand that you have had Thyroid cancer in the past, but I need more details as to when? and what type of Thyroid Cancer?(There are 2-3 different types of Thyroid cancer). You do not need to be too concerned about this or any other Cancer, yet an annual wellness examination by your doctor is a good idea if you want to be free from any anxiety about Cancer. Best wishes.