Which primary tumor is most frequently diagnosed in adults?

Multiple myeloma is indeed the most frequently diagnosed primary tumor in adults. This hematologic malignancy originates from plasma cells, which are a type of white blood cell responsible for producing antibodies. It is characterized by the proliferation of monoclonal plasma cells in the bone marrow and can lead to a variety of systemic complications, including anemia, bone lesions, and renal impairment.

The prevalence of multiple myeloma is particularly relevant in older adults, as the median age at diagnosis is typically around 70 years. The tumor's association with older age, combined with its relatively common occurrence compared to other primary tumors, underscores its position as the most frequently diagnosed primary malignancy in adults.

Osteosarcoma, Ewing's sarcoma, and chondrosarcoma are all types of primary bone tumors, but they are much less common in the adult population. Osteosarcoma primarily affects adolescents and young adults, Ewing's sarcoma typically manifests in children and young adults, and chondrosarcoma, while more common in adults than the others listed, does not occur with the same frequency as multiple myeloma in the adult population. This distinction supports the conclusion that multiple myeloma holds the highest incidence among primary tumors in adults.