A 65-year-old postmenopausal woman; treated case of recurrent right-breast carcinoma was referred for 18F-NaF PET/CT in view of right sterno-clavicular joint swelling to rule out skeletal metastasis. She was on hormonal treatment until 2012. Her sr alkaline phosphatase levels were within normal limits.
18F-NaF PET/CT showed symmetric increased tracer uptake in bilateral frontal region on MIP image (Fig. 1). Other notable findings are increased tracer uptake in right sterno-clavicular joint (where patient was symptomatic); multiple vertebrae, left greater trochanter and bilateral knee joints. These are consistent with degenerative and arthritic changes. Transaxial CT in bone window showed symmetric thickening of frontal bones with clear boundaries (Fig. 2). Fused transaxial image showed symmetric increased tracer uptake in thickened endocranial surface of frontal bones (Fig. 3).
Hyperostosis frontalis interna (HFI) is a benign entity which is characterised by the thickening of inner table of frontal bone; which is often diffuse and symmetric. It is most commonly reported in postmenopausal women as an incidental finding; with reports stating as much as 60-88% of the cases. The aetiopathogenesis is unclear; however, prolonged exposure to oestrogen has been postulated to cause this kind of thickening. Hershkovitz et al hypothesised that the bone deposition starts from the inner table and involves the dura in later stages [1]. There is increase in cancellous bone deposition with organised trabecular pattern [2] on histopathology.
Usually this is an incidental finding while the patient is being worked up for any neurological, endocrinopathy, psychiatric or metastatic evalation and commonly associated with obesity and endocrine dysfunction. However, some patients with this finding do present with headache, seizures or psychiatric symptoms. Increased serum alkaline phosphatase and serum calcium may also be seen in this disorder.
Certain peculiar features of HFI on CT include [3]:
A systematic classification of HFI into 4 categories (A-D) was used based on the various morphologic characteristics [1]. A type E category was added later representing severe form of HFI with soft-tissue expansion [4]. May et al have proposed a practical CT based system for identifying and classifying HFI. Accordingly, an objective scoring system with 3D volume rendered images and the CT characteristics can characterise HFI with 91% positive predictive value.
Scintigraphic studies consistently demonstrate increased tracer uptake in thickened areas of frontal bone. Increased 18F-NaF uptake has been reported without any radiographic abnormality; suggesting an early marker of HFI [5]. Tracer uptake can be diffuse, focal or heterogenous in bifrontal region; symmetric or rarely asymmetric on planar or maximum intensity projection (MIP) images. This can be confounded with metastases [6]. However, hybrid imaging like PET/CT or SPECT/CT resolves the issue by localising and characterising the tracer uptake in areas of fontal bone thickening. Awareness of such pattern can minimise the reporting of false positive metastases as well as avoid unnecessary investigations.
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Hyperostosis frontalis interna
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From the provided PET/CT images and CT cross-sectional scans, there is thickening of the inner table of the bilateral frontal bones with a relatively symmetrical distribution. In the fused images (PET/CT), this region shows significantly increased radiotracer uptake, consistent with bone hyperplasia. No prominent parenchymal lesion is noted in the brain tissue, and there is no obvious bone defect or erosive change.
This patient has a prior history of breast cancer, but the frontal changes appear more indicative of thickening of the inner table rather than focal bone destruction or lesions with malignant morphological features of metastatic disease. The normal serum alkaline phosphatase level further reduces suspicion of widespread bone metastasis or other active bone pathologies.
Considering the patient’s age, prior breast cancer history, normal serum alkaline phosphatase, and imaging findings of bilateral symmetrical thickening of the inner frontal bone with increased tracer uptake, the most likely diagnosis is:
Hyperostosis Frontalis Interna (thickening of the frontal inner table).
No additional biopsy is necessary at present. Further evaluation may be warranted if new clinical symptoms or abnormal laboratory results arise.
For asymptomatic Hyperostosis Frontalis Interna, no specific treatment is typically required, and regular outpatient follow-up is advised. If the patient develops headaches or other neurological or psychological symptoms, other causes should be excluded before symptomatic management. For patients with a history of breast cancer, routine oncological follow-up remains essential.
Regarding rehabilitation and exercise, as the patient does not demonstrate significant structural or functional impairment or fracture risk (frontal bone thickening generally does not affect daily activities), the following suggestions based on the FITT-VP principle (Frequency, Intensity, Time, Type, Progression, Individualization) are recommended:
In cases of degenerative bone changes or reduced cardiopulmonary function, increasing the exercise load should be done with caution and any discomfort during workouts should be monitored closely.
Disclaimer: This report is for reference only and cannot replace in-person consultation or professional medical advice. Patients should seek prompt medical attention for any abnormal changes.
Hyperostosis frontalis interna
