A 45-year-old, no smoker Caucasian woman was diagnosed with metastatic breast cancer in September 2021. In August 2021, she had undergone a right breast core biopsy, and histological examination diagnosed invasive ductal breast cancer: hormonal receptor status (ER and PgR) was positive, HER2 was not overexpressed, Ki-67 was 60%. In September 2021, staging with 18FDG PET/CT detected breast disease, axillary and mediastinal lymph node metastases, humerus, iliac and ischium bone metastases; contrast-enhanced breast MRI and bone scintigraphy both confirmed metastatic disease. Combination therapy with Ribociclib 600mg/die for 21 days with 28-days cycle plus Letrozole 2.5mg/day plus Triptorelin 3.75mg every four weeks was adopted as first-line treatment for this pre-menopausal, hormone receptor-positive and HER2-negative MBC. Patient had no other comorbidities and did not assume drugs before starting therapy. Before proceeding with treatment, we evaluated infectious markers, and found hepatitis B serology positive for infection as reported below: T0 –September 2021. - HBsAg positive - HBV DNA 4383 IU/mL - HBsAb Negative - HBcAb IgG Positive - HBcAb IgM Negative - HBeAg Negative - Normal transaminases and liver function tests; no HDV coinfection. The assessment of hepatic fibrosis by a transient elastography (fibroscan), reported a value of hepatic stiffness of 3.3 kPa and of CAP (Controlled Attenuation Parameter) of 199dB/m, indicative of absence of fibrosis and steatosis. Following the hepatologist’s recommendations, the patient started treatment with Tenofovir disoproxil fumarate 245 mg/day for her diagnosis of hepatitis B HBeAb positive with the recommendation to check hepatitis B status (quantitative HBV-DNA) and liver function weekly, especially during the first month of treatment with Ribociclib. As such, during the first cycle of treatment with Ribociclib in September 2021, we carried out weekly evaluations of HBV DNA levels, which significantly decreased (28 UI/ml) and subsequently negativized (<10 UI/ml) (). After three cycles of treatment with Ribociclib, in January 2022, 18FDG PET plus contrast-enhanced CT and breast MRI were repeated (). The patient achieved a complete metabolic response and a partial response of disease (PR), according to Response Evaluation Criteria for Solid Tumors [RECIST1.1 ()]. Compared to September 2021, there were no areas of uptake at the 18FDG PET and there was a significant reduction of the breast site, lymph node and bone metastases in the contrast-enhanced CT and breast MRI; revaluations were performed in May 2022, when 18FDG PET, contrast-enhanced CT and breast MRI confirmed disease stability (), and in September 2022 (stable disease). Both times, HBV DNA levels continued to be undetectable. Most importantly, treatment was well tolerated - with hematological toxicity not more than grade 2 according to CTCAE criteria and no need for dose reduction. No febrile neutropenia or QTc prolongation were reported; no liver toxicity emerged and the patient did not experience episodes of fatigue. Moreover, despite undergoing such an intense treatment, patient’s mood was constantly good; she did not ask or manifest need of psychological support and, to the date, patient shows a positive thinking and feelings of gratitude ().