Evaluation of normal FDG uptake in palatine tonsil and its potential value for detecting occult head and neck cancers: A PET CT study

Objective The aims of the study were to (1) evaluate the range of physiological FDG uptake in normal pharyngeal palatine tonsil, and (2) investigate the possibility of establishing a cut-off threshold to distinguish between normal pharyngeal palatine tonsil FDG uptake from occult pharyngeal palatine tonsil primary cancer. Methods FDG PET CT of 43 consecutive patients with a low risk of head and neck cancer were reviewed by two observers. Axial PET CT was used to identify foci of FDG uptake related to the pharyngeal palatine tonsil. The highest standardized uptake value, SUVmax of the left and right pharyngeal palatine tonsil was calculated. Similar analysis was performed on 10 consecutive patents with histologically proven occult pharyngeal palatine tonsil primary cancer. Results The mean SUVmax of the 43 right pharyngeal palatine tonsils was 4.82 (range, 1.16-12.74) and 4.68 (range, 0.88-13.65) for the 43 left pharyngeal palatine tonsils with no statistical difference observed (P=0.4). Normal pharyngeal palatine tonsil uptake was generally symmetrical and there was a positive correlation between SUVmax from the left and right sides which was statistically significant (r=0.9, P<0.0001). In the same patient the difference in SUVmax between left and right pharyngeal palatine tonsil ranged from 0.01 to 2.66 and patients with occult pharyngeal palatine tonsil primary cancer it ranged from 0.85 to 11.08. ROC analysis showed that an 'SUVmax difference' cut-off of 0.83 would achieve a sensitivity of 100% and specificity of 81% for detecting occult pharyngeal palatine tonsil primary cancers. Conclusions There is considerable variation of pharyngeal palatine tonsil FDG uptake in patients with no pharyngeal palatine tonsil primary cancer. However, in the same patient there is generally only a small difference in uptake between left and right sides. The absolute difference in SUVmax between left and right pharyngeal palatine tonsil is a potentially useful parameter for distinguishing between normal FDG uptake in pharyngeal palatine tonsil from occult pharyngeal palatine tonsil primary cancer.