An Atypical Presentation of Hyperostosis Frontalis Interna With Cyclical Bone Growth.

Hyperostosis frontalis interna (HFI) is a poorly understood osteogenic malformation of the internal surface of the frontal bone. Hyperostosis frontalis interna is characterized by nodules of hyperplastic bone and thickening of the internal surface of the frontal bone. Abnormal growth typically progresses along the frontal bone and can extend to other components of the cranial vault and, in extreme cases, dura mater. Here we describe the gross morphology and microstructure of a novel presentation of HFI. During routine cranial vault dissection, a 66-year-old female cadaver with history of breast cancer was found to have unusual midline intracalvarial ossifications originating at the internal frontal crest and crista galli. The lesions demonstrated bilateral nodular symmetry with involvement of the anterior falx cerebri. Histologic analysis revealed the nodule grew cyclically with periods of slow apposition interrupted by lines of arrested growth. The gross and histologic features of the lesion align with an eruptive lesion of a HFI type B classification. However, the involvement of midline dura mater is typically associated with far more advanced type D/E patterns, representing an atypical, previously undescribed early-stage pattern of HFI. The cyclical growth of the lesion is likely due to altering estrogen levels in association with breast cancer and potential treatments. This case underscores the need for nuanced descriptions of early lesions and deeper investigation into metabolic and hormonal contributors to disease progression. Broader documentation of atypical or early-stage HFI presentations may ultimately refine existing classification systems and enhance understanding of the pathophysiological mechanisms underlying HFI.