1
RARE VARIANTS OF INVASIVE
BREAST CARCINOMA OF NO
SPECIAL TYPE (IBC-NST)
The prior WHO (4
th
ed. 2012) classied several
tumors as separate entities (listed below). Breast
tumors with these “special morphological
patterns” now fall under the umbrella category
of IBC-NST and are no longer considered to be
the following clinically distinct subtypes:
oncocytic, lipid-rich, glycogen-rich, clear cell,
sebaceous, carcinomas with choriocarcinoma-
tous and pleomorphic patterns, melanocytic,
and carcinomas with osteoclast-like stromal
giant cells.
INVASIVE BREAST CARCINOMA
WITH MEDULLARY PATTERN
Medullary carcinoma, atypical medullary
carcinoma, and carcinoma with medullary
features were listed as special subtypes of breast
carcinoma in the prior WHO. This diagnostic
category has poor interobserver reproducibility;
these tumors also show overlapping histologic
features with carcinomas that have basal-like
molecular proles and carcinomas with BRCA1
Issue 18 || May 2022
WHAT’S NEW IN BREAST
PATHOLOGY 2022:
WHO 5
th
EDITION AND
BIOMARKER UPDATES
Kristen Muller,
1
Julie M. Jorns,
2
and
Gary Tozbikian
3
1
Department of Pathology and Laboratory Medicine,
Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA
2
Department of Pathology, Medical College of Wisconsin,
Milwaukee, WI, USA
3
Department of Pathology, The Ohio State University
Wexner Medical Center, Columbus, OH, USA
Corresponding Author: Kristen Muller, DO
Department of Pathology and Laboratory Medicine,
Dartmouth-Hitchcock Medical Center, Lebanon,
NH, USA
E-mail: kristen.e.muller@hitchcock.org
ORCID
Kristen Muller
https://orcid.org/0000-0003-3166-4523
Julie M. Jorns
https://orcid.org/0000-0002-7777-6670
Gary Tozbikian
https://orcid.org/0000-0002-5941-5652
Abstract
The 5th edition WHO Classication of Breast
Tumours (2019) has introduced changes to our
practices. Highlights are presented below, with a
focus on modications to morphological subtype
categorization. In addition, we summarize impor-
tant updates to ER and PR testing made in the
2020 ASCO/CAP guidelines, and briey discuss
PD-L1 and Ki-67 testing in breast cancer.
mutations.
Tumor inltrating lymphocytes (TILs) may
explain the good prognosis of these cancers.
Carcinomas with a basal-like or medullary
pattern (i.e., well-circumscribed, high-grade,
syncytial architecture, necrosis, prominent
TILs, Fig. 1) now represent one end of the
spectrum of TIL-rich IBC-NST; “IBC-NST
with medullary pattern” has been proposed to
replace “medullary carcinoma.”
NEUROENDOCRINE TUMORS
True primary neuroendocrine (NE) neoplasms
of the breast are rare. They are classied as
well-differentiated NE tumors (carcinoid-like
and atypical carcinoid-like) and poorly
differentiated NE carcinomas (small cell
neuroendocrine carcinoma and large cell
neuroendocrine carcinoma).
Distinct NE features and expression of NE
markers by IHC are needed for diagnosis, since
varying degrees of NE differentiation may be
seen in IBC-NST, mucinous carcinomas, solid
papillary carcinomas, and others.
Metastasis must be ruled out before considering
a primary breast NE tumor.
Routine staining on IBC-NST that lacks
characteristic NE morphological features is not
recommended, due to the lack of clinical
relevance.
WELL-DIFFERENTIATED
LIPOSARCOMA IN PHYLLODES
TUMOR
Malignant heterologous elements are among
the diagnostic criteria for malignant phyllodes
tumor.
Adipocytic differentiation in the stromal
component of phyllodes tumor, that is morpho-
logically indistinguishable from well-differenti-
PathologyOutlines.com
WHAT’S NEW
IN PATHOLOGY?
Sponsored by an unrestricted grant from Roche
Fig. 1. IBC-NST with medullary pattern. High-grade
tumor with a syncytial growth pattern and prominent
TILs.