Mechanisms of the Formation of Peritoneal Surface Malignancy on Omental Milky Spots from Low Grade Appendiceal Mucinous Carcinoma

Mechanisms of the Formation of Peritoneal Surface Malignancy on Omental Milky Spots from Low Grade Appendiceal Mucinous Carcinoma

Purpose: Omental milky spot (OMS) is considered to have an important role in the formation of peritoneal surface malignancy (PSM). However, human OMS and cancer metastasis has not been fully clarified. The present study demonstrates the mechanisms of the formation of metastasis on the omental milky spots (OMS) from the low grade AMC.

Methods and materials: To clarify the mechanism of the formation of peritoneal metastasis in low grade appendiceal mucinous carcinoma (AMC), 195 low grade AMC showing peritoneal cancer index (PCI) of ≤28 were studied for the distribution of peritoneal metastasis. Peritoneum was resected from 10 patients, and was prepared as whole-mount extending specimen. The specimens were studied by 5’-nucleotidase and alkaline phosphatase double stain and immunohistologic staining by D2-40, anti-CD31 and anti- Ki-67 monoclonal antibody. Furthermore, peritoneal parts were observed by scanning electron microscopy. Results: Pelvic and subdiaphragmatic peritoneum were involved in 164 (84%) and 143 (73%) patients. Greater omentum was involved in 135 (69%) patients. Under the SEM observation, no typical milky spots were observed on the peritoneum except for the greater omentum. Surface of OMS was covered with cuboidal mesothelial cells. Between the cuboidal mesothelial cells, many stomata were found. After digestion of OMS by 6N KOH, disk-like peritoneal pouch was detected at OMS. Small holes were found on the collagen plate covered on the bottom of the pouch. Below the pouch, initial lymphatic vessels closed to the pouch with stomata. Agglomerated blood capillaries distributed around the initial lymphatics. Metastasis foci were found around the stomata on OMS, but were not detected on the flat mesothelial cells on the greater omentum.. Conclusion: Peritoneal free cancer cells from low grade AMC may be adsorbed at stomata and adhere on the OMS. Then, they proliferate on OMS.