Table 2 The 2016 PSOGI, 2017 AJCC, and 2019 WHO pathological classification and terminology of the PMP

AM:

(1) Mucin without neoplastic epithelium;

(2) Confined to or distant from organ surface

M1a

pM1a

LMCP:

(1) Low-grade cytology;

(2) Rare mitosis;

(3) Few tumoral mucinous epithelium (< 20% of tumor volume)

M1b. G1, well-differentiated

pM1b, Grade 1:

(1) Hypocellular mucinous deposits;

(2) Neoplastic epithelial elements have low-grade cytology;

(3) No infiltrative-type invasion

HMCP:

Features of one or more of the following (At least focally):

(1) high-grade cytology;

(2) Infiltration of adjacent tissues;

(3) Invasion of vascular lymphatic vessels or surrounding nerves;

(4) Cribriform growth;

(5) Neoplastic mucinous epithelium (> 20% of tumor volume);

Sub-classification based on differentiation

(1) well-differentiated:

 Mainly composed of single- tubular glands;

 Tumor cell polarity exists;

 Obvious tumor cell atypia;

 Infiltrative components;

(2) Moderately-differentiated:

 Solid sheet tumor cells mixed with adenoid structures;

 Minimal or no polarity;

(3) Poorly-differentiated:

 Highly irregular to no adenoid differentiation

 Cell polarity disappears

M1b. G2 or G3, moderately- or poorly-differentiated

pM1b, Grade 2:

(1) Hypercellular mucinous deposits as judged at 20 × magnification;

(2) High-grade cytological features;

(3) 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

HMCP-S:

Tumor with signet ring cell component (signet ring cells ≥ 10%)

M1b. G3, poorly- differentiated;

PMCA-S

pM1b, Mucinous tumor deposits with signet-ring cells