PET/CT in Evaluation Bone Marrow Infiltration in Malignant Lymphoma. Badr, S1. Abdelwahab, M.A1 and Moustafa, H2. 1NCI and 2NEMROCK Centre, Cairo University, Egypt.

The bone marrow is found within the central cavities of axial and long bones. It consists of hematopoietic tissue islands and adipose cells surrounded by vascular sinuses interspersed within a meshwork of trabecular bone. It accounts for approximately 5% in humans (1). Bone marrow consists of a hematopoietic component (parenchyma) and a vascular component (stroma). The parenchyma includes hematopoietic stem cells (HSCs) and hematopoietic progenitor cells, which are not randomly distributed in the bone marrow but rather are localized close to the endosteum of the bone and more around blood vessels (1). Bone marrow stroma contains multi-potential non-hematopoietic progenitor cells capable of differentiating into various tissues of mesenchymal origin, including osteoblasts, endothelial cells, reticular cells, fibroblasts and adipocytes. The bone marrow's microvasculature with a single layer endothelium forms sinusoid, which radically distributes around the draining central sinus. The vasculature provides the barrier between the bone marrow compartment as a functional and spatial entity from extra lymphoid organ and the peripheral circulation (2). Bone marrow displays structural and functional features resembling a secondary lymphoid organ, and contains follicle-like structures similar to lymph nodes or spleen, although it lacks the organized T- and B-cell areas .Bone marrow microenvironment provides appropriate support for T cells to develop in the absence of the thymus (3). Approximately 8%–20% of bone marrow mononuclear cells are lymphocytes, with a T cell/B cell ratio of 5∶1 (4, 5) Bone marrow lymphocytes are distributed throughout stroma and parenchyma, and condensed in lymphoid follicles. Approximately 1% of the bone marrow mononuclear population represents plasma cells, which can produce antibodies (4, 5).