Poorly differentiated carcinoma unknown primary (CUP)

idontwanttobeLONG, I can see why the answer might be nonexistent. I suspect that "CUP" is a misnomer in your chart, but the waiting would be for the appearance of the tumor that would indicate the time to treat. The primary with myeloma is the bone marrow. This video is excellent but won't answer any CUP question, though it explains the watch and wait. https://www.youtube.com/watch
All of your serum labs contain important information about progression or latency.
I don't have any experience, but admire the thoroughness of your search and your conclusion that there is no scientific evidence of this type of CUP.
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@gently
My conundrum is that the CUP tumor is a solid tumor, not blood cancer. I’m told that myeloma can’t be a primary for a “poorly differentiated squamous or urothelial cancer.”
Because the tumor was located in the inguinal lymph node it is, by definition, metastatic and either class 3 or 4.

oh, synchronous but unrelated. It's good that the tumor remained inside the node. Do you know if there was somatic testing of the removed tumor https://www.foundationmedicine.com/test/foundationone-cdx When the tumor metastasizes it mutates further from the original cancer mutation, but retains clues regarding its origin. Somatic (also called biologic or somatic) is more common now. Knowing the origin wouldn't trigger treatment, but I'd want to know where to watch. By law they hold the biopsy material so you'd only have to request testing. You probably already know that the origins for inguinal metastasis are usually genital urological or anal rectal.

I had a similar experience, except that the CUP started in a lymph node & invaded the ureter, because of a bullseye presentation in the ureter, they decided to aggressively treat me. Eighteen months later, a tumor showed up in the anus with the same “unspecified” characteristics. It was then determined that my primary is squamous cell of the anus. It was in multiple lymph nodes in my body, & have metastasized to the pelvic muscles. My prognosis was not good, but here I am, still kicking. Best of luck to you. Unknowns are difficult to deal with!

@idontwanttobelong, I understand your username. Belonging to this online community is not something you wanted. But searching for answers has led you here. Welcome.

I can imagine that you're concerned about the unknowns of "carcinoma of an unknown primary" and next treatment is watch and wait after the poorly differentiated tumor in left inguinal area was surgically removed. I bet that feels like a non answer and as if you're waiting for the other shoe to drop.

"Inguinal lymph nodes are the frequent sites of metastasis for malignant lymphoma, squamous cell carcinoma of anal canal, vulva and penis, malignant melanoma and squamous cell carcinoma of skin ..." https://pmc.ncbi.nlm.nih.gov/articles/PMC5312497/

From what you've written, the tumor board has determined that the inguinal tumor is not metastatic from the smoldering myeloma. But rather they believe it is metastasis from another cancer type, as yet not determined. Do I have that right so far?

What further testing will you have done to understand the characteristics of the tumor to determine next steps? Have you considered getting a second opinion?

Pathology believes it’s most likely squamous

Or, perhaps, cutaneous squamous cell carcinoma