Of course we are not doctors, but in my understanding cerebellar peduncle is not part of the brain stem, rather connects the cerebellum to the pons (which is part of the brain stem).
Sometimes radiologists use more technical and medically accurate terminology, whereas a surgeon may oversimplify things to make them easier to understand. Also the writing style varies from radiologist to radiologist. Perhaps the tumor was mainly confined in the IAC, so your surgeon didn't bother mentioning the portion that extends in the CPA. Since you had GK, you must have had a detailed MRI back then. Can you find the radiologist report from back then and compare it?
The report states clearly that there is no change compared to the previous MRI. I would think that if there was a significant change, for sure the radiologist would have mentioned it.
Marianna