Appendix

Grading appendiceal mucinous neoplasms

Tumor grade	In the appendiceal primary tumor	In the peritoneal metastasis
1	Low grade cytology with a pushing margin (low grade appendiceal mucinous neoplasm)	Hypocellular mucinous deposits Neoplastic epithelial elements have low grade cytology No infiltrative type invasion
2	High grade cytology with a pushing margin (high grade appendiceal mucinous neoplasm) Invasive mucinous adenocarcinoma without a signet ring cell component	Hypercellular mucin deposits as judged at 20x magnification High grade cytological features Infiltrative type invasion characterized by jagged or angulated glands in a desmoplastic stroma or a small mucin pool pattern with numerous mucin pools containing clusters of tumor cells
3	Signet ring cell adenocarcinoma with numerous signet ring cells in mucin pools or infiltrating tissue	Mucinous tumor deposits with signet ring cells

Reference: WHO Classification of Tumours Editorial Board: Digestive System Tumours, 5th Edition, 2019

• Note: several different neoplasms are covered by this table
  • LAMN should be grade 1 (primary)
  • HAMN should be grade 2 (primary)
  • Mucinous adenocarcinoma should be grade 2 (primary)
  • Signet ring cell carcinoma should be grade 3 (primary)
  • Pseudomyxoma from LAMN or HAMN can be any grade (peritoneal)
  • Pseudomyxoma from adenocarcinoma can be any grade (peritoneal) but is almost always grade 2 or 3

Which of the following statements is true about mucinous proliferations of the appendix and peritoneum?

1. Low grade appendiceal mucinous neoplasm (LAMN) has a variable grade depending on the morphologic features present
2. Mucinous adenocarcinoma of the appendix is typically grade 1
3. Pseudomyxoma peritonei from appendiceal adenocarcinoma is typically grade 2 or 3
4. Pseudomyxoma peritonei is typically nonneoplastic

C. Pseudomyxoma peritonei from appendiceal adenocarcinoma is typically grade 2 or 3. Pseudomyxoma peritonei can be any WHO grade (using peritoneal criteria) but is almost always grade 2 or 3 when arising from appendiceal adenocarcinoma. LAMN is always grade 1, so answer A is incorrect. Mucinous adenocarcinoma of the appendix is typically grade 2 unless signet ring cells are present, so answer B is incorrect. Pseudomyxoma peritonei is a neoplastic process, so answer D is incorrect.

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Reference: Mucinous neoplasms-general