CASE 16596 Published on 10.02.2020

Hyperostosis frontalis interna (HFI) on 18F-NaF PET/CT


Musculoskeletal system

Case Type

Clinical Cases


Ashwini Kalshetty, Sagar S. R

Affiliation: Radiation Medicine Centre (R.M.C), Bhabha atomic Research Centre (B.A.R.C).
Corresponding author: Ashwini Kalshetty,
Address: Radiation Medicine Centre, C/o Tata Memorial Annexe, JerbaiWadia Road, Parel, Mumbai 400012.
Email I.D:
Tel. No. (+91) (022) 24134960/24146059
Fax: (+91) (022) 24157098


65 years, female

Area of Interest Hybrid Imaging ; Imaging Technique PET-CT
Clinical History

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.

Imaging Findings

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]:

  1. Bifrontal symmetry
  2. Sharp boundaries without crossing the sagittal sinus
  3. Endocranial expansion
  4. Continuity with inner table and diploic space.

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.

Written informed patient consent for publication has been obtained.

Differential Diagnosis List
Hyperostosis frontalis interna
Morgagni-Stewart-Morel syndrome
Paget’s disease
Fibrous dysplasia
Final Diagnosis
Hyperostosis frontalis interna
Case information
ISSN: 1563-4086