Aims The aim of this study was to analyse a series of borderline and malignant phyllodes tumours (PTs) of the breast by whole-genome profiling to identify genomic markers that could help to recognise potentially malignant tumours within borderline tumours.

Methods We evaluated the genetic imbalances of a series of 53 PTs (30 borderline, 23 malignant) using the Human CNV370 BeadChip microarray (Illumina), containing 370 000 SNP markers and correlate this alterations with clinicopathological features.

Results Forty-five PTs (85%) showed chromosome copy number variations (CNVs). Twenty PTs (37%) showed five or more chromosomal imbalances (8/30 borderline (27%) and 12/23 malignant (52%)). The large-scale genetic changes associated with malignant were+7p (9/23), +1q (8/23), −10p (8/23), −13q14 (7/23), +8q (6/23) and +10q (6/23) and borderline were+1q (13/30), −13q14 (9/30), −6q (8/30) and −10p (8/30). Losses in 9p21.3, encompassing CDKN2A/B gene, were present in three tumours (malignant), whereas deletions of 13q, with a minimal region in 13q14.2 encompassing the RB1 gene, were found in 9/30 borderline and 7/28 malignant tumours. High-level amplifications were seen in eight tumours (seven malignant and one borderline): in 7p in three tumours (including EGFR in two), 7q31.2 (including TFEC and MET), 8q24.21 (including MYC) and 8q23.3 (including CSMD3) in one tumour each.

Conclusions Whole-genome profiling by SNP arrays in PTs leads to identify a high number of CNV, gains of 7p and 8q, losses of 13q and 10, losses in 9p21.3 (CDKN2A/B) and the presence of amplifications, especially involving EGFR, as markers of potentially malignant tumours.