Introduction/Background Evaluating hormonal receptor and the human epidermal growth factor receptor 2(HER2) status in recurrent breast cancer(RBC) have proven to have significant clinical impact.

In some cases, a conversion of receptor status is reported.We aim to determine the conversion rate of HER2, estrogen receptor(ER) and progesterone receptor(PR) between primary tumors and RBC and define clinical significance.

Methodology This study included 45 patients diagnosed with RBC in Salah Azaiez Institut between 2010 and 2021.

Results Fourty-five patients were included in our study, of whom 30 and 15 had loco-regional and distant recurrence, respectively, after a median follow-up of 34 months.

Twelve patients(26.7%) had a change in tumor phenotype during recurrence: a switch to a triple negative status was observed in 11 cases, whereas 4 discordant patients showed either HR or HER2 positivity at recurrence. Only two patients, who changed from HR negative to HR positive, were subsequently treated with hormonotherapy and the three patients who converted to HER2 positivity subsequently received trastuzumab.

Discordance in HR status was observed in 16 cases(35.6%). The highest rate of discordance was observed for PR(42.2%), with PR loss as the main change(40%). ER and HER2 discordant rates were 33.3% and 15.6%, respectively.

For patients with HER2 discrepancy, more patients were ‘HER2 loss’ rather than ‘HER2-gain’.

Discordance rates between primary and metastatic and between primary and locally recurrent lesions, respectively, were 26.7% (4/15) and 36.7%(11/30) for ER, 46.7%(7/15) and 40%(12/30) for

PR, and 20%(3/15) and 13.3%(4/30) for Her2neu.

With regards to adjuvant therapy application, univariate analysis found that adjuvant hormonotherapy(P=0.033) was associated with ER conversion between primary and recurrent/metastatic lesions;

Radiotherapy was associated with PR conversion(P=0.033) and no clinico-pathological factor was related with HER2 conversion(P>0.05) except the use of trastuzumab therapy for primary tumor(P=0.012).

Conclusion These results confirm the importance of re-evaluating ER, PR and HER2 expression between primary and RBC to help guide treatment.

Disclosures he authors have nothing to disslose