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An emerging noninvasive approach to assess tissue proliferation uses the PET tracer 3'-deoxy-3'- 18 F-fluorothymidine ( 18 F-FLT). To evaluate the diagnostic value of this technique in myelofibrosis, 18 F-FLT PET imaging results were compared with bone marrow histology and bone marrow scintigraphy (BMS), the gold standard techniques in this clinical situation. Methods: Fifteen patients with histology-proven myelofibrosis were included consecutively in the study. Tracers' distributions were assessed using a visual grading assessment score of the uptake in the axial skeleton, proximal and distal limbs, liver, and spleen. This visual score was used to define patterns of tracer distribution and to compare the information provided either by PET or by BMS. A semiquantitative analysis with determination of SUV max in the same localizations was performed for 18 F-FLT PET. Results: The histology grade of fibrosis correlated with the SUV max in the axial skeleton (spine and iliac crests) and proximal limbs. 18 F-FLT uptake in these areas was much lower in patients with grade 3 fibrosis than in patients with grade 1 or 2 fibrosis. 18 F-FLT PET showed the same distribution of uptake as BMS in 13 of 14 patients (1 patient did not undergo BMS). In 1 patient, 18 F-FLT PET clearly showed an intense abnormal splenic uptake, whereas spleen uptake was inconclusive with BMS. Conclusion: 18 F-FLT PET appears to be a reliable and convenient technique to assess hematopoietic activity in bone marrow. It yields results close to those observed with BMS. In our study population, 18 F-FLT uptake in the axial skeleton and proximal limbs assessed by SUV max correlated with the grade of fibrosis. Thus, 18 F-FLT PET may be a useful tool to measure the severity of myelofibrosis, and to monitor noninvasively the patients' status during follow-up. Finally, 18 F-FLT PET may be foreseen as an alternative to BMS., (© 2017 by the Society of Nuclear Medicine and Molecular Imaging.)