Appendiceal mucinous neoplasms: a clinicopathologic analysis of 107 cases

The classification of appendiceal mucinous tumors is controversial and terminology used for them inconsistent, particularly when they lack overtly malignant features but are associated with extra-appendiceal spread. We reviewed 107 appendiceal mucinous neoplasms and classified them as low-grade appendiceal mucinous neoplasm (LAMN) (n = 88), mucinous adenocarcinomas (MACAs) (n = 16), or discordant (n = 3) based on architectural and cytologic features. LAMNs were characterized by a villous or flat proliferation of mucinous epithelium with low-grade atypia. Thirty-nine tumors were confined to the appendix, but 49 had extra-appendiceal tumor spread, including 39 with peritoneal tumor characterized by mucin pools harboring low-grade mucinous epithelium, usually dissecting in a hyalinized stroma. Eight of the 16 MACAs lacked destructive invasion of the appendiceal wall and eight showed an infiltrative pattern of invasion. Extra-appendiceal tumor spread was present in 12 MACAs (four peritoneum, seven peritoneum and ovaries; one ovaries only). In MACAs with an infiltrative pattern, peritoneal tumor consisted of glands and single cells in a desmoplastic stroma. The peritoneal tumor in the remaining cases consisted of mucin pools that contained mucinous epithelium with high-grade atypia and, in some cases, increased cellularity compared with that seen in peritoneal spread in cases of LAMN. Three cases were classified as discordant because the appendiceal tumors were LAMNs but the peritoneal tumors were high-grade. Follow-up was available for 49 LAMNs, 15 MACAs, and 2 discordant cases. None of the patients with LAMNs confined to the appendix experienced recurrence (median follow-up 6 years). LAMNs with extra-appendiceal spread were associated with 3-, 5-, and 10-year survival rates of 100%, 86%, and 45%, respectively. Patients with MACA had 3- and 5-year survival rates of 90% and 44%, respectively (p = 0.04). The bulk of peritoneal disease correlated with prognosis among patients with MACA (p = 0.04) and, to a lesser degree, among patients with LAMNs (p = 0.07). We conclude that: 1) appendiceal mucinous neoplasms can be classified as either low-grade mucinous neoplasms or mucinous adenocarcinoma based on architectural and cytologic features; 2) tumors that can be confidently placed in the low-grade group (which requires rigorous pathologic evaluation of the appendix) and are confined to the appendix are clinically benign in our experience to date; 3) low-grade tumors confined to the appendix are morphologically identical to those with extra-appendiceal spread (except for the usual identification of breach of the wall in the latter cases) and the same designation is appropriate for the appendiceal neoplasia in each situation; 4) the long-term outlook for patients with low-grade tumors and peritoneal spread is guarded with just over half dying of disease after 10 years; 5) appendiceal mucinous tumors with destructive invasion of the appendiceal wall, complex epithelial proliferations, or high-grade nuclear atypia generally pursue an aggressive clinical course and should be classified as mucinous adenocarcinomas; 6) peritoneal tumor can be classified as involvement by LAMN or MACA, and this distinction is of prognostic significance; 7) bulky peritoneal tumor worsens prognosis; and 8) LAMNs associated with high-grade peritoneal tumor behave as adenocarcinoma.