Aims Fibromatosis-like spindle cell carcinomas (FLSCCs) are rare metaplastic breast cancers (MBCs) that are characterised by bland spindle cells in a collagenous stroma. Although some MBCs are highly malignant, FLSCCs have indolent behaviour with low potential for lymph node or distant metastasis. Owing to their rarity, there are limited genomic data on FLSCCs. In this study, we analysed the clinicopathological features and molecular characteristics of four FLSCCs to elucidate the pathogenesis of these rare tumours.

Methods and results Four pure FLSCCs were sequenced by DIAN (Hangzhou Lab) using a 324-gene platform (FoundationOne CDx) with licensed technologies. The results showed that most FLSCCs harboured the pathogenic H1047R mutation in PIK3CA (3/4, 75%) and the −124C>T mutation in the telomerase reverse transcriptase (TERT) promoter (3/4, 75%). No copy number variations were observed in any cases in our study.

Conclusions Our study showed that PIK3CA and TERT promoter mutations were common genetic features of FLSCCs. These findings contribute to our understanding of FLSCCs biology.