See review article and MRI scans:  here, here, here, here, here, here, here
untreated these lesions can grow rapidly in the brain (see picture here)

Primary central nervous system (CNS) lymphoma is defined as lymphoma limited to the cranial-spinal axis without systemic disease. An increasing incidence of this disease has been seen among patients with acquired immunodeficiency syndrome (AIDS) and among other immunocompromised persons. The natural history of this disorder differs between patients with AIDS and those without AIDS. Computed tomographic (CT) scans may show ring enhancement in one half of the AIDS patients while patients without AIDS almost always show only homogeneous enhancement. Both groups do equally poorly without therapy (1-3 month mean survival), but the overall survival for treated patients is much better for patients without AIDS (18.9 months) than for those with AIDS (2.6 months).

In non-AIDS patients we generally treat with chemotherapy first followed by radiation see the NCCN guidelines and NCCN discussion  (as noted below the risk of brain complications, e.g. dementia can be high and the use and proper dose or timing of radiation is under investigation see Fine , recent consensus statements , RTOG trial and new RTOG Trial . There are more trials using doses as low as 23Gy (go here). In the RTOG 0227 trial it starts with chemoRx (MTX, Rituximab, TMZ) then XRT (whole brain 120cGy bid to 36Gy) then more TMZ

For more information on treatment go here and here