Possible Ribociclib-Associated Focal Nodular Hyperplasia-Like Lesion Mimicking Liver Metastasis in a Chemotherapy-Naïve Patient: First Case Report.

Hepatic focal nodular hyperplasia (FNH)-like lesions mimicking metastasis have been reported as rare complications of cytotoxic chemotherapy, but they have not previously been described in association with CDK4/6 inhibitors alone. We present the case of a 32-year-old chemotherapy-naïve woman with de novo hormone receptor (HR)-positive/HER2-negative metastatic breast cancer who was treated with first-line ribociclib and letrozole. After four cycles, imaging revealed a complete response of her bone metastases alongside a newly developed 5.5-cm hepatic lesion suspicious for visceral progression. Liver function tests remained normal. Two ultrasound-guided percutaneous core biopsies, performed three months apart and directly targeting the lesion, confirmed a benign FNH-like lesion without evidence of malignancy. Ribociclib was continued without modification; at last follow-up the patient remained in sustained complete oncologic response, with the hepatic lesion stable in size and no biochemical or clinical evidence of hepatic dysfunction. To our knowledge, this appears to be the first reported case of an FNH-like hepatic lesion temporally associated with ribociclib in a patient who had not received prior cytotoxic chemotherapy. The principal clinical message is that newly detected hepatic lesions arising during otherwise effective targeted therapy do not necessarily reflect disease progression, and that, in radiologically discordant cases, histopathologic confirmation may be required to avoid premature discontinuation or switching of effective treatment.