Target And Threshold

In most studies, 20 X 104 CFU-GM/kg or 2 X 106CD34+ cells/kg is generally accepted as the minimum threshold below which rapid hematopoietic reconstitution may not occur. In a study reported by Bensinger et al.,49 patients receiving more than 2.5 X 106 CD34+cells/kg had more rapid neutrophil and platelet recovery than patients who received less than that. However, comparing patients who received more than 5 X 106 CD34+ cells/kg to those receiving 2.5-5 X 106 CD34+ cells/kg, there was no difference in neutrophil engraftment but there was more rapid recovery of platelets. They also found that very high level of CD34+ cells (>10 X 106 CD34+ cells/kg) did not result in a significant faster recovery of either neutrophils or platelets. Kiss et al.50 showed that patients who received more or equal to 5 X 106 CD34+ cells/kg were found to have statistically significantly faster neutrophil recovery (median 10 days vs 11 days; P = 0.002) and platelet recovery (median 9 days vs 21 days; P = 0.004) than those receiving less than that. Based on these studies, a minimum cell dose of 2.5 X 106 CD34+ cells/kg may be sufficient to ensure rapid recovery of neutrophils and platelet; the optimal transplant cell dose of 5 X 106CD34+ cells/kg is recommended.

Previous chemotherapy is also an important factor in determining a threshold of CD34+ cells for rapid engraftment. Tricot et al.51 analyzed 225 patients receiving autologous PBSC transplants for multiple myeloma and found that the minimum CD34+ cell threshold was more or equal to 2 X 106 cells/kg for patients receiving less than 6 months of melphalan, but more than 5 X 106 CD34+ cells/kg were required for patients who received more than 12 months of melphalan.

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