May be JL (Juvenile Lympho), which differs from regular lymphosarcoma.
Something to consider.  It is most often seen below age 2.  Sorry that you
are going through this; it's hell.  Have been there.  If I had to pick a
heart breaker I'd like to see answered soon it would be JL.  They go from
being vibrant to suffering to dying so rapidly.
[Posted in FML issue 3331]