Assalamoalaikum!
We frequently come across children with enlarging thymus on follow-up scans with ongoing chemotherapy; its a well known diagnostic dilemma especially in lymphoma cases where lymphomatous infiltration vs thymic hyperplasia is the query.
I just read a nice article (radiology.rsna.org/content/243/3/869.full) that suggests use of MRI CSI to diagnose thymic hyperplasa

So, will it be follow-up CT's / PET-CT / MRI CSI or biopsy? I have a feeling MRI CSI has a potential to provide a non-invasive solution better than the other modalities based on the article linked above!

P.S : Does this thread belong to 'Cardiac' or 'Pulmonary' ? lol

I personally believe its a dilemma at the end of the Rx (EOT) and PETCT is usually done at this point. This is our practice, to rely on CT plus pet.