Blastic Transformation

Blastic transformation, also called acute blast crisis or terminal phase, occurs in virtually all (nontransplanted) patients with CML, either following an accelerated phase or acutely during the course of stable phase CML.32,65 Most accept the definition of blast crisis as the development of greater than or equal to 30 percent blasts in the marrow or the peripheral blood.89 In the blast crisis stage, progressive anemia and thrombocytopenia occur with death usually due to bleeding or infection. Leukemic involvement of most organs is evident at autopsy. The median survival of those who develop a blast crisis is only about 6 months.

Extramedullary blast transformation can result in localized tumor masses, which may be found incidentally or cause serious symptoms. Deposits of immature myeloid cells, formerly called granulocytic sarcoma or chloroma, are now most appropriately termed myeloblastoma. Myeloblastomas may be localized or widespread and occur anywhere in the body. Lymph nodes are the most commonly involved, with nodal infiltration preceding marrow transformation by months or years.90 Other locations of extramedullary blasts deposition in patients with CML include skin, subcutaneous tissues, breasts, bone, and pleural cavity. The incidence of myeloblastoma may be as high as 5 percent in CML (in contrast to 2.5 percent of AML).91

Meningeal leukemia may rarely be the first manifestation of blastic transformation, although it more commonly occurs after systemic blast crisis.91,92 In an NCI series of 101 patients with CML in blastic phase, 7 subsequently developed meningeal leukemia.91 All seven patients had a cerebrospinal fluid (CSF) pleocytosis (including myeloblasts) and associated neurological features including headaches and cranial nerve palsies.

Localized blastic transformation may also occur within the marrow (intramedullary myeloblastoma), usually involving the bones of the face and cranium. While they may be nonpig-mented, green masses may adhere to the sutures, invade the dura, and fill the orbits, oral nasal sinuses, and mastoids. The green color is derived from the abundance of peroxidase enzyme in the myeloblast. Upon exposure to air, the green color rapidly fades and assumes a dirty yellow tinge. In addition to the facial bones, localized blastic transformation may occur in the sternum, ribs, vertebrae, and pelvis (Plates 3-1H-J).

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