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 . There is increase in cancellous bone deposition with organised trabecular pattern  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 :
A systematic classification of HFI into 4 categories (A-D) was used based on the various morphologic characteristics . A type E category was added later representing severe form of HFI with soft-tissue expansion . 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 . 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 . 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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