In 1984, a major advance in the treatment of HCL was achieved with the evidence that alpha interferon was cytotoxic to hairy cells and could induce complete remissions in some patients.9 The ability to eradicate the hairy cell population from the bone marrow led to more stringent criteria for response, culminating in the consensus resolution for the response published in 198 78 (Table 32.1). A CR now required a hemoglobin level >12 g/dL, a neutrophil count >1500/^L, and

Criteria for response assessment in hairy cell leukemia8

Complete response leukemia8

Criteria for response assessment in hairy cell

Complete response

Partial response*

Neutrophils >1500|ola Platelets >100,000/^1 Hemoglobin >12 g/dLa No hairy cells in bone marrow by H&Ea Regression of spleen to normal by physical exama

No circulating hairy cellsa

Reduction in organomegaly >50% <5% circulating hairy cells Decrease by 50% hairy cells in bone marrow

Hemoglobin >12 g/dL

Neutrophils a 1500/^L Platelets a 100,000/^L

aCatovsky6 definition allowed 1000 neutrophils and hemoglobin of 11 for complete remission and does not include clearance of marrow or peripheral blood for hairy cells. Partial remission just required improvement of two of three blood valus to the CR definition or improvement in all three values.

clearance of hairy cells from blood and bone marrow, with resolution of organomegaly on physical exam. A PR required a 50% decrease in organomegaly and less than 5% circulating hairy cells, with normalization of the blood counts as described for a CR.

Despite its obvious activity, interferon rarely completely eliminated the hairy cells from the bone marrow, and the majority of patients were left with significant bone marrow infiltration.1011 The amount of remaining hairy cell involvement in the marrow after treatment had prognostic significance, with early relapse occurring in patients with greater than 30% hairy cell infiltration.

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