How do you approach a patient with an anaplastic plasmacytoma without systemic evidence of disease in the bone marrow?

I'm not sure what an anaplastic plasmacytoma is. Is it a plasmacytoma that is made up of amorphous cells that express CD138, are clonal, and don't express EBER, ALK, or HHV8? This seems to follow along the lines of what is a plasmablastic neoplasm.

If there are no clonal plasma cells in the marrow, a PET/CT is negative, and MRI spine + pelvis show ≤ 1 focal lesion > 5 mm, then a solitary plasmacytoma still should be treated with radiation. Even if there are <10% clonal plasma cells in the marrow, radiation upfront makes sense even though the future is less sunny.

Agreed with @Dr. First Last on all fronts above. I'll specifically touch on the follow-up question (multiple biopsy-proven anaplastic plasmacytomas but BM neg).

My vote, in that case, would be to repeat the bone marrow, since I'd argue that the chances of this scenario truly unfolding are infinitesimally small. One could even consider trying to ClonoSEQ ID one of the plasmacytomas and then compare against the marrow. More for academic reasons since I'd still treat systemically, but would be good to know and would be a good way to assess and trend disease response.

Treatment-wise, again a data-free zone but totally agree with considering an auto transplant if eligible. High-dose alkylators are worth a shot against an aggressive lymphoma like...