Accuracy of MRI

I have a Grade 2 oligodendroglioma-astrocytoma tumor.  In May 2007, a neurosurgeon removed 75% of the tumor.  I've had serial MRIs since that time.  After 20 months, my oncologist noticed some slight growth based on my MRI.  He described the growth as only a couple millimeters.  

Is it possible that since my head is in slightly different positions when I have my MRIs that the growth is in fact inconsistency with the MRIs?  My head is always pointed up but never in the exact same position twice.