Benign spindle cell lesions of the breast: a diagnostic approach to solitary fibrous tumour, nodular pseudoangiomatous stromal hyperplasia and nodular fasciitis

Diagnostics and Treatment of Extrameningeal Solitary Fibrous Tumors

Solitary fibrous tumors (SFT) are rare mesenchymal neoplasms that account for less than 2% of all soft tissue masses. In the latest WHO 2020 Classification of Soft Tissue Tumors, extrameningeal SFT was listed as intermediate (rarely metastasizing) or malignant neoplasms. Due to the lack of characteristic clinical features, their diagnosis and treatment remain challenging. The pathogenesis of SFT is often associated with the presence of fusions of the NAB2-STAT6 gene on the 12q13 chromosome. Cytoplasmic CD34 positive staining is considerably characteristic for most SFTs; less frequently, factor XII, vimentin, bcl-2, and CD99 are present. A key factor in the diagnosis is the prevalent nuclear location of STAT6 expression. Radical resection is the mainstay of localized SFTs. In the case of unresectable disease, only radiotherapy or radio-chemotherapy may significantly ensure long-term local control of primary and metastatic lesions. To date, no practical guidelines have been published for the treatment of advanced or metastatic disease. Classical anthracycline-based chemotherapy is applicable. The latest studies suggest that antiangiogenic therapies should be considered after first-line treatment. Other drugs, such as imatinib, figitumumab, axitinib, and eribulin, are also being tested. Definitive radiotherapy appears to be a promising therapeutic modality. Since standards for the treatment of advanced and metastatic diseases are not available, further investigation of novel agents is necessary.

Solitary fibrous tumor of the breast: A case report

INTRODUCTION

Solitary fibrous tumors (SFTs) are uncommon mesenchymal neoplasms that comprise <2 % of all soft tissue tumors. They are a diagnostically challenging group of neoplasms that can occur essentially anywhere. Molecular or genetic testing of soft tissue tumors will increasingly add to the foundation of distinctive histologic features, as accurate diagnosis is critical for appropriate treatment.

CASE PRESENTATION

A 28-year-old woman was referred to our hospital for a left breast mass. Ultrasonography showed an oval hypoechoic mass with partially obscured boundaries. Surgical specimens revealed spindle tumor cells surrounding the mammary ducts and were immunoreactive for both CD34 and STAT6, suggesting SFTs. However, the infiltration of spindle tumor cells into the surrounding fat, and the storiform-like pattern made us consider dermatofibrosarcoma protuberans (DFSP) as a differential diagnosis. Lack of amplification of the COL1A1-PDGFB fusion gene, a characteristic feature of DFSP, led to our definitive diagnosis of breast SFT.

DISCUSSION

The presence of STAT6 in tumor cell nuclei is a highly sensitive immunohistochemical marker for SFT. In our case, morphological features evoked the differential diagnosis of DFSP and we investigated the COL1A1-PDGFB fusion gene. The diagnostic process of reliably performing careful morphological examination and immunohistochemical marker test, and then obtain conviction by molecular cytogenetic technique is more and more important for soft tissue tumors.

CONCLUSIONS

We report a quite uncommon case of breast SFT and excluded DFSP as a differential diagnosis. If it is difficult to distinguish between these diseases, molecular cytogenetic analysis would be required for accurate diagnosis.

Bilateral multicenter pseudohemangiomatous interstitial hyperplasia of the breast: a case report

Bilateral multicenter breast pseudohemangiomatous stromal hyperplasia (PASH) is a rare, benign breast disease. Here, we report on a female patient with bilateral multicenter PASH who underwent a mastectomy and prosthetic reconstruction. The surgery was successful, and no recurrence was observed during the 18 months of follow up.

A Rare Case of Breast fat-Forming Solitary Fibrous Tumor With Molecular Confirmation

Solitary fibrous tumor (SFT) is an uncommon fibroblastic neoplasm which may arise in a wide range of anatomic location and can occur across all ages. Fat-forming SFT is a rare morphological variant of SFT. Primary breast fat-forming SFT is exquisitely rare. Here, we report a case in a 51-year-old Chinese woman with a palpable painless mass in the left breast. A color Doppler ultrasound scan examination demonstrated a 3.4-cm oval, well-circumscribed, hypoechoic solid mass with several peripheral and internal color flow signals. Magnetic resonance imaging (MRI) showed a focal lobulated solid nodular lesion displaying geographical enhancement but no architectural distortion. Subsequently, she underwent a left breast lumpectomy. In histopathologic examination, there was a well-circumscribed, cellular spindle cell tumor consisting of short fascicles of bland, fusiform, ovoid to spindle cells disposed in a patternless architecture around branching vascular spaces within a fibrous stroma with wispy collagen. Cells revealed mild nuclear atypia. Mitotic figures were up to 4/10 high-power fields (HPFs) in the hot spot. Mature adipocytes intermixed with spindle cells were also observed. The tumor cells were diffusely positive for CD34 and STAT6. Some S100-expressing adipocytes co-expressed STAT6. Next-generation sequencing (NGS) revealed the presence of the NAB2exon6::STAT6exon2 fusion. The histological, immunohistochemical (IHC) and molecular examinations confirmed the diagnosis of fat-forming SFT. Post-excision, the patient showed no signs of tumor recurrence or metastasis in a 7-month follow-up. Here, we describe a rare case of a fat-forming SFT involving the breast and highlight the comprehensive pathological evaluation and necessary ancillary testing.

Tiger-Striped PASH: Recognition of a Unique Morphology Allows for a Zippered-Up Diagnosis of Pseudoangiomatous Stromal Hyperplasia of Breast

Pseudoangiomatous stromal hyperplasia (PASH) of the breast is histologically characterized by anastomosing and slit-like spaces invested by collagenous stroma and lined by flattened, spindle cells. These clear spaces that may mimic microscopic vascular channels do not contain red blood cells. Immunohistochemistry (IHC) studies may also help to confirm a diagnosis of PASH, with the spindled cells marking positively with CD34 and PR while demonstrating no reactivity with more specific endothelial antigens such as CD31 and ERG. In the current case, a 39-year-old female was diagnosed with cellular PASH of the right breast with unique histological patterns showing “tiger-striped” and “zippered” histologies. To our knowledge, this is the first report of these unique variant PASH morphologies.

Nodular fasciitis of the breast: clinicopathologic and molecular characterization with identification of novel USP6 fusion partners

Nodular fasciitis is a benign, self-limited, pseudosarcomatous neoplasm that can mimic malignancy due to its rapid growth, cellularity, and mitotic activity. Involvement of the breast is rare and diagnosis on biopsy can be challenging. In this largest series to date, we examined the clinicopathologic and molecular characteristics of 12 cases of nodular fasciitis involving the breast/axilla. All patients were female, with a median age of 32 years (range 15-61). The lesions were 0.4 to 5.8 cm in size (median 0.8). All cases presented as palpable masses, and two patients had overlying skin retraction. Microscopically, lesions were relatively well-circumscribed nodular masses of bland myofibroblastic spindle cells within a variably myxoid stroma. Infiltrative growth into adipose tissue or breast epithelium was frequent. Mitotic figures were present in all cases, ranging from 1 to 12 per 10 high-power fields (median 3). Immunohistochemically, all cases expressed smooth muscle actin and were negative for pan-cytokeratin, p63, desmin, CD34, and nuclear beta-catenin. Targeted RNA sequencing performed on 11 cases identified USP6 gene fusions in eight; one additional case was positive by break-apart fluorescence in situ hybridization. The common MYH9-USP6 rearrangement was detected in four cases; another case had a rare alternative fusion with CTNNB1. Three cases harbored novel USP6 gene fusions involving NACA, SLFN11, or LDHA. All fusions juxtaposed the promoter region of the 5' partner gene with the full-length coding sequence of USP6. Outcome data were available for eight patients; none developed recurrence or metastasis. Five patients elected for observation without immediate excision, and self-resolution of the lesions was reported in three cases. Albeit uncommon, nodular fasciitis should be considered in the differential diagnosis of breast spindle cell lesions. A broad immunohistochemical panel to exclude histologic mimics, including metaplastic carcinoma, is important. Confirmatory detection of USP6 rearrangements can aid in classification, with potential therapeutic implications.

The many faces of solitary fibrous tumor; diversity of histological features, differential diagnosis and role of molecular studies and surrogate markers in avoiding misdiagnosis and predicting the behavior

Background

Solitary Fibrous Tumor (SFT) is a distinct soft tissue neoplasm associated with NAB2-STAT6 gene fusion. It can involve a number of anatomic sites and exhibits a wide spectrum of histological features.

Main body

Apart from diversity in morphological features seen even in conventional SFT, two histologic variants (fat-forming and giant cell-rich) are also recognized. In addition, a malignant form and dedifferentiation are well recognized. Owing to diverse histological features and involvement of diverse anatomic locations, SFT can mimic other soft tissue neoplasms of different lineages including schwannoma, spindle cell lipoma, dermatofibrosarcoma protuberans, liposarcoma, gastrointestinal stromal tumor (GIST), malignant peripheral nerve sheath tumor (MPNST), and synovial sarcoma. SFT is classified as an intermediate (rarely metastasizing) tumor according to World Health Organization Classification of Tumors of Soft tissue and Bone, 5th edition. The management and prognosis of SFT differs from its malignant mimics and correct diagnosis is therefore important. Although SFT expresses a distinct immunohistochemical (IHC) profile, the classic histomorphological and IHC profile is not seen in all cases and diagnosis can be challenging. NAB2-STAT6 gene fusion has recently emerged as a sensitive and specific molecular marker and its IHC surrogate marker signal transducer and activator of transcription 6 (STAT6) has also shown significant sensitivity and specificity. However, few recent studies have reported STAT6 expression in other soft tissue neoplasms.

Conclusion

This review will focus on describing the diversity of histological features of SFT, differential diagnoses and discussing the features helpful in distinguishing SFT from its histological mimics.

Practical approach to diagnosis of bland-looking spindle cell lesions of the breast

The diagnosis of bland-looking spindle cell lesions of the breast is often challenging because there is a close morphological and immunohistochemical overlap among the different entities. The present review will discuss reactive spindle cell nodule/exuberant scar, nodular fasciitis, inflammatory pseudotumor, myofibroblastoma (classic type), lipomatous myofibroblastoma, palisaded myofibroblastoma, benign fibroblastic spindle cell tumor, spindle cell lipoma, fibroma, leiomyoma, solitary fibrous tumor, myxoma, schwannoma/neurofibroma, desmoid-type fibromatosis, dermatofibrosarcoma protuberans, low-grade fibromatosis-like spindle cell carcinoma, inflammatory myofibroblastic tumor and low-grade myofibroblastic sarcoma arising in the breast parenchyma. The pathologist should be aware of each single lesion to achieve a correct diagnosis to ensure patient a correct prognostic information and therapy. Accordingly representative illustrations and morphological/immunohistochemical diagnostic clues will be provided.

Controversial issues in soft tissue solitary fibrous tumors: A pathological and molecular review

The clinical evolution of solitary fibrous tumor (SFT) remains unclear. Although various clinical, morphological and molecular criteria may indicate increased risk of malignancy, some SFT can still progress despite having a clearly benign appearance. Various risk stratification systems have been proposed, but unfortunately they are not sufficient to precisely determine the malignant potential. In this review, we discuss current knowledge on SFT, focusing on the following controversial issues: (i) the diverse morphologic spectrum: 'the great simulator;' (ii) malignant transformation or dedifferentiation; (iii) current risk stratification systems; and (iv) molecular factors associated with clinical evolution. The morphological spectrum of SFT and the list of differential diagnoses continue to expand. Both have increased considerably since the first descriptions of specific molecular alterations. A classification of malignant SFT should not be based on histology alone. The correlation of all pathological and molecular factors is recommended; its inclusion in risk stratification systems may help to improve diagnosis and prognosis.