- Composed of variable proportions of atypical

spindle cells, adipocytes, univacuolated or

multivacuolated lipoblasts, pleomorphic to

multinucleated cells, and myxoid to collag-

enous stroma.

- Lack of MDM2 or CDK4 amplication.

- Rb expression is generally lost.

- Low rate of local recurrence (10%–15%); no

known risk of dedifferentiation.

• Myxoid pleomorphic liposarcoma

- Occurs predominantly in children and young

adults with a predilection for the mediastinum.

- Admixture of areas resembling myxoid

liposarcoma with more cellular areas contain-

ing overt nuclear pleomorphism, which resem-

bles pleomorphic liposarcoma.

- Lacks recurrent chromosomal changes, namely

MDM2 amplication and DDIT3 gene fusion.

- Clinical behavior akin to pleomorphic liposar-

coma.

Fibroblastic/myoﬁbroblastic tumors

• EWSR1::SMAD4 positive broblastic tumor

- Small dermal and subcutaneous acral nodule,

with indolent biological behavior.

- Histologic zonation with acellular hyalinized

center and peripheral fascicular monomorphic

spindle cell growth.

- Diffuse ERG nuclear expression in the absence

of CD34 and SMA expression.

- EWSR1::SMAD4 fusion.

• Angiobroma of soft tissue

- Benign neoplasm with rare local recurrence.

- Uniformly bland short spindle cells in variably

myxoid to collagenous stroma, with promi-

nent vascular network of small thin-walled

branching blood vessels (Fig. 1).

- NCOA2 gene rearrangements in up to 80%.

• Supercial CD34-positive broblastic tumor

- Rare, slow-growing, indolent neoplasm.

- Supercial location, typically in the lower

extremities.

- Large eosinophilic cells with granular to glassy

cytoplasm; marked pleomorphism with low

mitotic count (Fig. 2).

- CD34 and frequent keratin expression.

Issue 20 || November 2022

WHAT’S NEW IN SOFT

TISSUE AND BONE

PATHOLOGY 2022–

UPDATES FROM THE

WHO CLASSIFICATION

5TH EDITION

Erica Y. Kao

, Jose G. Mantilla

Department of Pathology, Brooke Army Medical

Center, San Antonio, TX, USA

Department of Pathology, University of Washington,

Seattle, WA, USA

Corresponding Author: Erica Y. Kao, MD

Department of Pathology, Brooke Army Medical

Center, San Antonio, Texas, USA

E-mail: erica.kao.mil@health.mil

ORCID

Erica Y. Kao

https://orcid.org/0000-0001-6005-3559

Jose G. Mantilla

https://orcid.org/0000-0003-4752-6459

Abstract

The 2020 release of the WHO Classication of

Soft Tissue and Bone Tumors, 5th edition,

contains several changes driven by new knowl-

edge in the eld. These include reclassication of

some entities, renement of risk classication

systems, and the inclusion of novel disease

processes, many of which are driven by recurrent

gene fusions. The most notable changes are

described here.

SELECT NEW ENTITIES

Lipomatous tumors

• Atypical spindle cell/pleomoprhic lipomatous

tumor

Smooth muscle tumors

• Inammatory leiomyosarcoma

- Rare, and thought to be relatively indolent

compared to conventional leiomyosarcoma.

- Typically arise in the deep extremities.

- Variably atypical eosinophilic spindle cells in

fascicles, with mitotic activity and prominent,

usually diffuse, mixed (predominantly mono-

nuclear) inammatory inltrate.

- Near-haploid karyotype.

• EBV-associated smooth muscle tumor

- Associated with EBV infection, usually in the

setting of immunosuppression.

- Can arise in any anatomic location, most often

in visceral sites and CNS.

- Prognosis depends on the patient’s immune

condition. Most tumors do not metastasize.

- Cytologic atypia is highly variable. In half of

cases, a second population of more primitive

appearing round cells are seen. T-cell inam-

matory inltrates are common.

- Invariably positive expression of EBER.

PathologyOutlines.com

WHAT’S NEW

IN PATHOLOGY?

Sponsored by an unrestricted grant from Roche

Fig. 1. Angioﬁbroma of soft tissue.

Fig. 2. Superﬁcial CD34-positive ﬁbroblastic tumor.