Background: "Hot skull", or diffuse increased activity of bone seeking radiotracers, is frequently seen in the bone scans of some patients, especially elderly women. This finding has been attributed to enhanced bone metabolism in old age.
Material and methods: We semi-quantitatively studied 342 normal bone scans (161 male and 181 female within the age range 12 to 82 years). We divided the patients into 7 age groups: 10-19, 20-29, 30-39, 40-49, 50-59, 60-69, and 70 and above. The geometric means of the anterior and posterior background corrected skull and mid-femoral ROI values were used for calculation of the skull to femoral ratio (SFR).
Results: The skull to femoral ratio was significantly higher in female patients in the age groups 30-39 and above. In females, the five upper age groups (30-39, 40-49, 50-59, 60-69, and 70 and above) had significantly higher SFR than the lower age groups. In males, the two upper age groups (60-69, 70 and above) had significantly higher SFR than the lower age groups. The findings in males were not concordant with the previous studies addressing this issue, which could be explained by different bone mineral density in the Iranian population.
Conclusion: Our data showed that "hot skull" is not necessarily an abnormal finding, especially in elderly women. We suggest that every nuclear medicine department uses its own normal values and reference samples for quantitative evaluation, due to ethnic or socio-economical variations.