This case is highly concerning for peritoneal malignancy such as carcinomatosis, mesothelioma, or lymphoma. The omentum was biopsied with ultrasound revealing a very unusual manifestation of extramedullary hematopoiesis (EMH). 

EMH is characterized by hematopoietic cells outside of bone marrow and occurs in hemoglobinopathies, myeloproliferative, bone marrow disorders (such as myelofibrosis, as in this case). It can involve the liver, spleen, kidneys, posterior mediastinum, and peritoneum. It likely arises from subpopulations of cells which retain pluripotent potential.