[A Case of Breast Cancer Complicated by Fibrous Dysplasia That Was Difficult to Differentiate from Bone Metastasis].

A 58-year-old woman with HER2-negative hormone-sensitive postmenopausal breast cancer underwent preoperative bone scintigraphy and CT to search for distant metastasis. Bone metastasis was suspected in the spinous process of the seventh cervical vertebra. MRI revealed a mass that was hypointense on T1- and T2-weighted images and hyperintense on diffusion- weighted images, with intense contrast enhancement, indicating bone metastasis at cT1N0M1, Stage Ⅳ(M: OSS). The patient underwent partial mastectomy and sentinel lymph node biopsy. The postoperative diagnosis was pT2N0cM1, Stage Ⅳ, with the status of bone metastasis being key to staging. PET-CT showed uptake in the spinous process of the seventh cervical vertebra but no other metastatic findings. However, solitary bone metastasis to the cervical spinous process is atypical. CT-guided needle biopsy confirmed benign fibrous dysplasia, and the final diagnosis was breast cancer at pT2N0M0, Stage ⅡA. Fibrous dysplasia is characterized by impaired osteogenesis leading to fibroplasia and commonly occurs in the skull, jaw bones, ribs, and limbs. Solitary fibrous dysplasia in the cervical spinous process is rare. The lesion was asymptomatic and pathologically benign, requiring no treatment. The patient underwent postoperative radiation therapy for the conserved breast and is followed up with endocrine therapy.