I'm going to echo what Phyl said: 1 mm of growth is statistically inconclusive. Because an MRI takes images in "slices" (cross-sections) of the brain that are spaced apart, and each MRI takes the slices at slightly different cross-sections of the brain, there is a 2mm margin of error. Most radiologists would look at your most recent MRI and conclude that your tumor is the same size, because it's impossible to tell whether it got 1 mm bigger or the MRI simply took an image at a slightly thicker part of a tumor that is nevertheless the same size as it was before.
Also, as Phyl noted, ANs are measured a little bit differently by each radiologist. Ask two radiologists what the dimensions of the same tumor are, and you might get two different measurements that are 1 to 2 mm different from each other along any axis of measurement. That's because all three axes of measurement are oblique, or measured at an angle. So, for example, the oblique transverse measurement may not be exactly along a perfectly horizontal plane but is instead measured roughly from the tumor protrusion that is most medial (toward the inside of the skull) to that which is most lateral (towards the the outside of the skull). If the medial extent of the tumor is toward the bottom of the tumor and the lateral extent toward the top, for example, the transverse measurement would be slightly angled from bottom to top. But another radiologist might measure it along a completely horizontal plane, which would exclude the protrusions at either end from the measurement, thereby arriving at a slightly smaller measurement. This is just one example of how two radiologists could look at the same tumor and come up with different measurements.
Bottom line: 1 mm of "growth" is completely inconclusive. Either the MRI took a wider slice of the same-sized tumor, or the radiologist measured it differently this time, or your tumor has indeed grown by 1 mm. Or it could've been 2 mm bigger two MRIs ago (and that MRI just happened to take a 2mm smaller "slice", missing the growth) and is now 1 mm smaller compared to then (-2 mm of overlooked growth two MRIs ago minus +1 mm larger size shown on the current MRI = -1 mm of growth, or a smaller tumor). Any one of these scenarios could be true. There's no way to know.
If your symptoms are getting worse or you just want to take action and "get it over with," those are valid reasons for seeking treatment now. But concluding that your tumor has gotten bigger is not necessarily valid. Note that some tumors can stay the same size for over 20 years (according to Dr. Derald Brackmann at House Clinic). So please don't feel like you must seek treatment now because your tumor is growing. That may not be the case. At the very least, if you're concerned, you should seek a second opinion.
Sincerely,
TW
