Atypical multivacuolated lipoblasts and atypical mitoses are not compatible with the diagnosis of spindle cell/pleomorphic lipoma—reply

Atypical Pleomorphic Lipomatous Tumor: Expanding Our Current Understanding in a Clinicopathologic Analysis of 64 Cases

Atypical pleomorphic lipomatous tumor (APLT) is a recently recognized adipocytic neoplasm that lies on a spectrum with atypical spindle cell lipomatous tumor (ASCLT). APLT/ASCLT are included together in the fifth edition World Health Organization Classification of Soft Tissue and Bone Tumours; however limited data on APLT have emerged since its first description in 2017. This study aims to further define the clinicopathologic features of APLT in a large series of 64 cases. Histologic features and ancillary studies were reviewed, and clinical and follow-up data were obtained from referring institutions. Immunohistochemistry for MDM2, CDK4, CD34, Rb, S100, and desmin was performed in cases with available material. Patients were 24 females and 40 males, and the median age was 61 years (range, 20 to 89 y). Tumors arose in upper limb (33%), lower limb (31%), trunk (23%), head and neck (8%), breast (3%), and inguinal region (2%), with a median size of 5.4 cm (range, 1.5 to 14.5 cm). Tumor depth was mostly subcutaneous or deep/subfascial. Microscopically, APLTs were variably composed of atypical spindle and pleomorphic cells, adipocytes, and lipoblasts, often showing infiltrative growth, myxoid or collagenous stroma, and multinucleate floret cells. Mitoses were infrequent and necrosis was consistently absent. By immunohistochemistry, tumors expressed CD34 (63%), S100 (19%), and desmin (28%). APLT showed frequent loss of Rb (79%; 46/58). Rare cases showed staining for MDM2 (2%) or CDK4 (12%); however, fluorescence in situ hybridization was negative for MDM2 amplification in all cases tested (0/22). Follow-up in 28 patients (median duration: 21 mo) revealed a single patient with local recurrence (4%); no patient developed metastatic disease. Despite its frequently infiltrative growth, nuclear pleomorphism, and hypercellularity which can mimic sarcoma, APLT behaves in an indolent manner and should be distinguished from its more aggressive mimics, including atypical lipomatous tumor/well-differentiated liposarcoma and pleomorphic liposarcoma. Immunohistochemistry for CD34, S100, desmin, and Rb (demonstrating loss), as well as exclusion of MDM2 amplification, can support the diagnosis.

Updates from the 2020 World Health Organization Classification of Soft Tissue and Bone Tumours

The fifth edition of the World Health Organization (WHO) classification of soft tissue and bone tumours was published in May 2020. This 'Blue Book', which is also available digitally for the first time, incorporates an array of new information on these tumours, amassed in the 7 years since the previous edition. Major advances in molecular characterisation have driven further refinements in classification and the development of ancillary diagnostic tests, and have improved our understanding of disease pathogenesis. Several new entities are also included. This review summarises the main changes introduced in the 2020 WHO classification for each subcategory of soft tissue and bone tumours.

A Reappraisal of Superficial Pleomorphic Liposarcoma

OBJECTIVES

Superficial pleomorphic liposarcoma (PL) has a favorable prognosis compared to deeply seated PL. Given developments in the classification of lipomatous neoplasms, we reappraised a series of cases.

METHODS

Retrospective clinicopathologic evaluation and genome-wide single-nucleotide polymorphism (SNP) microarray studies were performed for cases previously designated superficial PL.

RESULTS

Four cases were identified (age, 48-70 years). Two were dermally confined, whereas two were superficial subcutaneous; no recurrences or metastases were reported. Tumors demonstrated pleomorphic spindled morphology with variable cellularity. Multivacuolated atypical lipoblasts were focal in 3 and abundant in 1. Dermal tumors demonstrated atypical cells within sclerotic collagen. Genome-wide SNP microarray studies revealed consistent gains and losses, including losses at the 13q14.2 locus encompassing RB1 and DLEU2 and deletion/disruption of the TP53 locus. Although subcutaneous examples showed genomic changes similar to deep PL, the dermal examples showed fewer genetic alterations, including changes reported in the spectrum of atypical spindle cell/pleomorphic lipomatous tumors (ASPLT). All lacked MDM2 amplification.

CONCLUSIONS

Careful integration of histologic and genetic features may improve classification of lipomatous neoplasms with atypia, allowing reclassification of some superficial PL as ASPLT.

Update on Lipomatous Tumors with Emphasis on Emerging Entities, Unusual Anatomic Sites, and Variant Histologic Patterns

This article reviews the histologic patterns of spindle cell/pleomorphic lipoma, well-differentiated liposarcoma, and dedifferentiated liposarcoma in the context of both usual and atypical anatomic presentation. The utility of molecular and immunohistochemical diagnostic modalities to distinguish these entities is described. In addition, more recently described and controversial entities, including atypical spindle cell lipomatous tumor and anisometric cell lipoma, are discussed.

Well-differentiated liposarcoma and dedifferentiated liposarcoma: An updated review

Well-differentiated liposarcoma (WDL)/atypical lipomatous tumor and dedifferentiated liposarcoma (DDL) together comprise the largest subgroup of liposarcomas, and constitute a histologic and behavioral spectrum of one disease. WDL and DDL typically occur in middle-aged to older adults, particularly within the retroperitoneum or extremities. WDL closely resembles mature adipose tissue, but typically shows fibrous septation with variable nuclear atypia and enlargement. WDL does not metastasize, but can dedifferentiate to DDL, which is associated with more aggressive clinical behavior, with a greater propensity for local recurrence and the capacity for metastasis. Although distant metastasis is rarer in DDL compared with other pleomorphic sarcomas, behavior is related to location, with a significantly worse outcome in retroperitoneal tumors. DDL typically has the appearance of undifferentiated pleomorphic or spindle cell sarcoma, and is usually a non-lipogenic sarcoma that is adjacent to WDL, occurs as a recurrence of WDL or which can arise de novo. WDL and DDL share similar background genetic aberrations; both are associated with high-level amplifications in the chromosomal 12q13-15 region, which includes the CDK4 and MDM2 cell cycle oncogenes. In addition, DDL harbor further genetic changes, particularly 6q23 and 1p32 coamplifications. While surgical excision remains the treatment mainstay with limited medical options for patients with aggressive recurrent disease or metastases, novel targeted therapies towards the gene products of chromosome 12 are being evaluated. This review summarizes the pathology of WDL and DDL, discussing morphology, immunohistochemistry, genetics and the differential diagnosis.