How would you manage a patient with history of follicular lymphoma treated with Bendamustine-Rituximab who now has a new FDG avid lesion that is biopsy proven CD30 positive CD20 negative DLBCL?

Certainly an unusual and challenging situation, and one that raises multiple questions. How long ago was the rituximab - is the CD20- status "true" or might it be a false-negative in the presence of blocking antibodies? Is it really totally negative or rare/dim?

But taking at face value that the disease is truly transformed CD20-CD30+ tFL->DLBCL, what might the optimal approach be? In a young fit patient, one could entertain a standard approach with anthracycline-based chemotherapy (CHOP14, CHOEP...

I agree with Dr. @Dr. First Last. If the patient is young/fit and the duration of remission was < 2 years, I would use CHOP x 3-4 cycles and consolidation with autologous stem cell transplant. Also agree that brentuximab vedotin is known to have clinical activity here as a single agent (Jacobsen et al., PMID 25573987) but would not be considered standard of care and it may be difficult to obtain insurance approval for this.