Abstract
BACKGROUND: The occurrence of multiple primary cancers is increasing, largely due to better diagnostic tools and longer patient survival. However, managing these cases can be complex, especially when treatments such as chemotherapy lead to complications such as hepatitis B virus (HBV) reactivation, which carries a significant risk of severe illness and death. CASE PRESENTATION: We present the case of a 55-year-old Arab woman diagnosed with a rapidly growing triple-negative breast cancer, found to have a BRCA1/BRCA2 mutation. She began neoadjuvant chemotherapy, but the treatment was stopped early due to severe liver inflammation (transaminitis) and a sharp increase in hepatitis B virus levels, confirming hepatitis B virus reactivation. Antiviral therapy with entecavir was promptly started. Later, the patient developed rectal bleeding, leading to the diagnosis of a mid-rectal invasive adenocarcinoma. She underwent successful surgeries for both cancers: a right modified radical mastectomy and a rectal resection with colostomy. Pathology confirmed two distinct primary tumors. Despite initial concerns due to a positron emission tomography scan, subsequent biopsies showed no recurrence. The patient completed adjuvant radiation and, at a 3-year follow-up, remains healthy and disease-free. CONCLUSION: This case highlights the intricate challenges of managing synchronous primary cancers and the life-threatening risk of hepatitis B virus reactivation during chemotherapy. It strongly emphasizes the need for routine hepatitis B virus screening before chemotherapy, careful monitoring of liver function and viral loads, and the immediate availability of antiviral treatment. A collaborative, multidisciplinary approach is essential for prioritizing treatments effectively and achieving good outcomes in patients with such complex cancer presentations.