Background/Aims: Differential diagnosis of RCTB; Ewing sarcoma, small cell osteosarcoma, mesenchymal chondrosarcoma, osteoblastoma, chondroblastom, primary bone lymphoma and multiple myeloma is still challenging. Given the significant differences in treatment, accurate diagnosis is imperative. This study aimed to evaluate some histochemical and immunohistochemical criteria of RCTB.

Subjects and Methods: PAS, CD99, CD138, osteocalcin, and LCA were evaluated in 113 cases of RCTB.

Results: PAS was positive in neoplastic cells of all Ewing sarcomas, 27% of osteosarcomas, 92% of chondrosarcomas, all osteoblastomas and chondroblastomas and the osteoid tissue of all osteosarcomas and osteoblastomas. CD99 was positive in all Ewing sarcomas, in 11%, 4% and 11% of osteoblastomas, multiple myelomas and bone lymphomas respectively. CD99 was higher in Ewing sarcoma than in other RCTB (p<0.0001). Osteocalcin was positive in neoplastic cells of all osteosarcomas, osteoblastomas and 20% of chondroblastomas, 84% and 78% of osteoid of osteosarcomas and osteoblastomas respectively.  CD138 was positive in all multiple myelomas, 12% of Ewing sarcomas, 20% of osteosarcomas, 44% of osteoblastomas, 8% of chondrosarcomas and 40% of chondroblastomas. CD138 was higher in multiple myeloma (p<0.0001) than in other RCTB.  LCA positivity was higher (p<0.01) in bone lymphomas (100%) than in multiple myelomas (73%).

Conclusion: PAS negativity excludes multiple myeloma and bone lymphoma from other RCTB that could be differentiated by LCA and CD138. CD99 positivity confirms diagnosis of Ewing sarcoma. PAS could detect areas of osteoid in osteosarcoma and osteoblastoma. Osteocalcin suggests osteogenic tumor origin; osteosarcoma/osteoblastoma. Double negativity of CD99 and osteocalcin suggests chondrogenic tumor origin; chondrosarcoma/ chondroblastoma