Table 1

Differential diagnosis for diagnostically challenging bland-appearing spindle cell lesions of the breast.

DiagnosisKey histological featuresSMACD34Other IHCAncillary testsBehaviour
Reactive spindle cell noduleHaemosiderin deposition, fat necrosis, foreign body giant cell reaction and inflammatory cell infiltrate+Reactive
Inflammatory pseudotumourInflammatory cell infiltrate+~50% ALK+Rearrangement of 2p23 (ALK) in ALK+ casesReactive
Desmoid-type fibromatosisLong sweeping fascicles of spindle cells with infiltrative growth, open thin-walled vessels+β-catenin+ CTNNB1 mutation analysesLocally aggressive
Low-grade fibromatosis-like spindle cell carcinomaSmall clusters of epithelioid cells among spindle cells with mild atypia and infiltrative growth+pancytokeratin +p63+Malignant
Low-grade myofibroblastic sarcomaMild focal atypia and infiltrative growth+fibronectin+Fibronectin fibrils and fibronexus junctions in ultrastructural studiesMalignant
Nodular fasciitisTissue culture-like appearance and extravasated erythrocytes+Rearrangement of 17p13 (USP6)Reactive
Pseudoangiomatous stromal hyperplasiaPseudovascular spaces+/−+PR+, CD31–Reactive
MyofibroblastomaWell-circumscribed spindle cell lesion with collagen bands and adipose tissue component+/−+PR+, RB1−Deletion of 13q14 (RB1)Benign
FibroadenomaAdmixed benign glandular and stromal elements+/−+Benign
Phyllodes tumourAdmixed glandular and stromal elements with leaf-like fronds, stromal overgrowth, cytologic atypia, increased mitoses and heterologous elements+/−+Benign or malignant
Benign fibroblastic spindle cell tumourHaphazardly arranged fascicles of spindle cells and keloid-like collagen fibres+RB−Deletion of 13q14 (RB1)Benign
Spindle cell lipomaAdmixed spindle cells and mature adipocytes+RB1−Deletion of 13q14 (RB1)Benign
Solitary fibrous tumourBranching ‘staghorn’ vasculature+STAT6+Rearrangement of 12q13 (STAT6)Benign or malignant
Dermatofibrosarcoma protuberansStoriform arrangement of spindle cells with infiltrative growth+t(17;22)(q22;q13) producing COL1A1-PDGFB fusionLocally aggressive, rare metastasis
Low-grade fibromyxoid sarcomaCollagenised hypocellular areas and cellular myxoid nodulesRearrangement of 16p11 (FUS)Malignant
  • .+, positive; –, negative; ALK, ALK receptor tyrosine kinase; COL1A1, collagen type I alpha 1 chain;CTNNB1, catenin beta 1;FUS, FUS RNA binding protein;IHC, immunohistochemistry; PDGFSB, platelet derived growth factor subunit B; PR, progesterone receptor;RB1, RB transcriptional corepressor 1;SMA, smooth muscle actin;STAT6, signal transducer and activator of transcription 6;USP6, ubiquitin-specific peptidase 6.