#00066316

A 41-year-old man with a remote history of testicular seminoma presented with exercise-related chest wall discomfort. Thoracic MRI detected a large lesion in the T11 vertebra. Positron emission tomography/computed tomography studies detected fluorodeoxyglucose uptake in the T11 vertebra as a sole abnormality (standardized uptake value, 3.4). Complete blood counts were normal (white blood cell count, 8.21 × 10³/μL; hemoglobin level, 12.9 g/dL; and platelet count, 253 × 10³/μL). Targeted biopsy of the T11 vertebral lesion showed 90% to 100% cellular marrow with trilineage hematopoiesis. Megakaryocytes were markedly increased with loose clusters (panel A, original magnification ×20; panel B, original magnification ×40 [hematoxylin and eosin stain (H&E)]; and panel C, original magnification ×20 [CD61 immunohistochemistry (IHC)]). Myeloid and erythroid elements showed no overt dysplasia, and blasts were not increased. Next-generation sequencing identified a JAK2 p.V617F mutation with 50.9% variant allele frequency (VAF). A right posterior iliac crest biopsy performed 2 weeks later showed normocellular marrow (50%-60%) with maturing trilineage hematopoiesis. Megakaryocytes were not increased and did not form clusters (panel D, original magnification ×20; panel E, original magnification ×40 [H&E]; and panel F, original magnification ×20 [CD61 IHC]). No dysplasia, ring sideroblasts, fibrosis (MF-0), or increased blasts were observed. Flow cytometry detected no abnormal blast population. The same JAK2 mutation was detected with 3.5% VAF.

The marrow findings do not fulfill the criteria for myeloproliferative neoplasm (MPN) and are best classified as clonal hematopoiesis. The MPN-like morphology of the vertebral lesion and >10-fold higher VAF suggest that MPN may occasionally begin as a localized process.

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