Extramedullary accumulation of myeloblasts or immature myeloid cells form tumors called myeloid sarcoma in the WHO classification.

Such tumors develop in lymphoid organs, bone (skull, orbit, etc.), skin, soft tissue, various mucosae and organs, and the CNS.

They may precede or occur concurrently with acute myeloid leukemia, or reveal blastic transformation of chronic myeloproliferative disorders or myelodysplastic syndromes. They may also reveal relapses in treated patients.

Microscopical synopsis

 diffuse infiltrate made up of medium-to-large cells

Immunohistochemistry

 granulocytic myeloperoxidase (MPO+, CD 68+ [KP1+/-, PGM1-] lysozyme+, CD 34+/-)
 monoblastic (MPO-, CD 68+, [KP1+, PGM1+] lysozyme+, CD 34-)
 myelomonoblastic (MPO-, CD 68+, [KP1+, PGM1+] lysozyme+, CD 34-)
 megakaryoblastic (positivity for factor VIII, CD 61, CD 31).

 CD 43
 CD 7
 CD 79a
 CD 56 ( t[8;21])

 CD 99
 CD 117

References

 Audouin J, Comperat E, Le Tourneau A, Camilleri-Broet S, Adida C, Molina T, Diebold J. Myeloid sarcoma: clinical and morphologic criteria useful for diagnosis. Int J Surg Pathol. 2003 Oct;11(4):271-82. PMID: #14615822#