Omental Tumors

Excerpt

The omentum is an intraabdominal organ connected to the spleen, stomach, pancreas, and colon, served by the left and right gastroepiploic arteries. The omentum is visceral adipose comprising a single layer of mesothelial cells sandwiched by connective tissue containing fibroblasts, adipocytes, mesothelium, endothelium, mast cells, lymphocytes, granulocytes, macrophages, blood vessels, connective tissue, and angiogenic and immunogenic pluripotent cells lines. The omentum plays a role in peritoneal homeostasis, including fluid and solute transport, tissue and cellular repair, angiogenesis, infection control, and lipid storage, and serves as a reservoir for stem cells.

Immune cells move among omental regions via fenestrations. Milky spots are areas of immune cells arranged around blood vessels near fenestrations, allowing inflammatory cell egress into the peritoneal cavity; these areas are upregulated during active infection. Milky spots also aid perfusion, absorb fluid, and synthesize and release neurotransmitters associated with sympathetic nerves, including dopamine, epinephrine, norepinephrine, and choline acetyltransferase. The lymphatics from milky spots drain to the subpyloric and splenic lymph nodes.

Stem cells within the omentum promote angiogenesis by producing vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). Inflammation and hypoxia can drive VEGF expression; these cytokines promote the migration of neural stem cells to the injury site. Laminin within the omental stroma produces vasoactive growth factor and neurotrophic factor. Mechanisms have been proposed for the role of the omentum in stroke recovery, treatment for Alzheimer Disease, and increasing perfusion to ischemic limbs.

The omentum is a frequent site of malignant transformation and metastatic disease. Immune cells within the milky spots are thought to mitigate inflammation and contribute to the spread of malignancy. Milky spots likely behave as secondary lymphoid organs, able to mount responses to peritoneal antigens and generate and differentiate peritoneal macrophages. Still, the omental milieu, including milky spots, mesenchymal cells, and adipocytes, is favorable for promoting metastatic disease. Metastates to the omentum often confer a worse prognosis compared to metastases at other sites, as the omentum contributes to chemoresistance and propagation of tumor.

Malignancies can arise from omental cell lines, leading to a primary omental tumor. Primary omental tumors are rare and frequently presented as case reports. Primary omental malignancies include liposarcoma, hemangiopericytoma, fibrosarcoma, leiomyosarcoma, mesothelioma, and malignant fibrous histiocytoma; indolent processes such as intraabdominal desmoid tumor have been reported. More commonly, the omentum is the site of a variety of metastatic processes, most commonly uterine, ovarian, colorectal, and gastric cancers. Of note, a gastrointestinal stromal tumor may originate primarily from the omentum or be located within the omentum as metastatic disease. The omentum is also a frequent site for inflammatory processes that may be difficult to distinguish from malignancies.