PTL-Post Transplant Lymphomas - in the brain

my husband (40 years young) was told he has 2 PTLs one on the left hemisphere and one on the brain stem. 4 years ago he had a kidney transplant. we are at the end of chemo (just this week) and in 3 weeks he will have another MRI. The previous MRIs showed no growth but also no shrinkage. We have an awesome medical team...they are like family to us. PTLs of the brain we are told is very rare. If this is the end of chemo...no growth, no shrinkage...what happens from here? This has been asked to our Doctor and he said lets see what MRI shows then we can work from there. Has anyone been through any of this?