Evaluation of methods to classify ipsilateral breast tumour recurrences as local recurrence or new primary tumour.

Ipsilateral breast tumour recurrence (IBTR) may represent a true local recurrence (LR) from residual malignancy or a new primary (NP) tumour, with important implications for prognosis and treatment. However, no classification system exists to distinguish between these entities. This systematic review of studies evaluating classification methods for IBTR as LR or NP, identified 19 studies reporting 25 systems. Most were clinicopathological (21/25) and four were genomic. Tumour location (72%) and histological subtype (68%) were the most frequently applied criteria. IBTR rates ranged from 2 to 12%, with NP proportions between 13-82% and LR between 18-87%. Time to recurrence was shorter for LR than NP. Across studies, NP was consistently associated with superior survival outcomes. The methodological quality of included studies constrains the certainty of findings. Validation of clinicopathological and genomic criteria is needed before a classification system can be recommended, but pragmatic clinicopathological decisions remain essential in the interim.