Challenges of treating incidental synchronous bilateral breast cancer with differing tumour biology

A 59-year-old woman with right breast mass was diagnosed with invasive ductal carcinoma (IDC). Workup consisted of bilateral diagnostic mammogram and ultrasound (US); both showed a right breast mass with normal left breast. Core biopsy showed IDC with estrogen receptor negative (ER-)/progesterone receptor negative (PR-) and HER2/neu positive receptor status. The patient underwent carboplatin-based chemotherapy with Herceptin. The mass completely resolved. The patient desired to proceed with bilateral total mastectomy with right sentinel lymph node biopsy (SLNB). Pathology showed complete resolution of the right-sided breast mass without malignancy in right SLN. Incidentally, IDC was found in the left breast specimen, which was ER+/PR+ and HER 2/neu negative. Tumour board consensus was to obtain a left axilla US with MRI in 6 months if the US was unremarkable. Biologically different synchronous bilateral breast cancer poses a difficult clinical challenge for management due to differing responses to treatment. Use of MRI may be a diagnostic option in women who choose contralateral prophylactic mastectomy.