Is it unusual to have a variance in tumor measurement between ultrasound and MRI? Picked up MRI report this afternoon and noticed they measured tumor as 1.8 cm vice 1.2 cm. Of course, first thought was oh crap, did it grow that much in two weeks?
The MRI can pick up much more detail. Some malignant tumors aren't evenly shaped and maybe that would be where the discrepancy is showing up. Never be afraid to call the office that did the MRI and ask to speak to a doctor. It's their job to know the answers to those types of questions.
It is also because of the way that you are positioned in the MRI machine. If you were lying on your stomach for the MRI with your breasts hanging down through the MRI table it is going to change the shape of your breast therefore making the measurements different then when you were lying on your back for the ultrasound.