Basal cell adenoma of the parotid gland; MR features and differentiation from pleomorphic adenoma.

Objectives: Among the benign tumours of the parotid gland, basal cell adenoma (BCA) is far less common than pleomorphic adenoma (PA). MR features of BCA, including diffusion-weighted imaging and dynamic contrast-enhanced study, have not been previously described. Assessment of the crucial MR features of BCA appears to offer beneficial clues for distinguishing BCA from PA.

Methods: We retrospectively reviewed 14 BCAs and 179 PAs in the parotid gland, collected between March 2000 and May 2012, from the MRI database.

Results: Nearly half of the BCAs had cystic components. The average ratio of the maximum diameter of the cysts to the BCAs (cystic ratio) was 0.80 ± 0.11 [standard deviation (SD)]. The BCA cystic ratio was significantly higher (p = 0.00232) than that of PAs. The cystic ratio threshold was 0.65 between cystic BCA and cystic PA. Sensitivity and specificity were 76.5% and 100%, respectively. The average of the apparent diffusion coefficient (ADC) values of the 12 BCAs [1.24 ± 0.18 (SD) ×10(-3) mm(2) s(-1)] was significantly lower than that of the 151 PAs [1.86 ± 0.40 (SD) ×10(-3) mm(2) s(-1)] (p < 0.001) and also lower than that of the cystic PAs [1.83 ± 0.57 (SD) ×10(-3) mm(2) s(-1)] (p < 0.00495). The ADC threshold was 1.31 × 10(-3) mm(2) s(-1) between BCA and cystic PA, with sensitivity and specificity of 81.2% and 91.7%, respectively, and also the same threshold 1.31 × 10(-3) mm(2) s(-1) between BCA and PA, with sensitivity and specificity of 92.7% and 91.7%, respectively. Time-signal intensity curves (TICs) showed various patterns.

Conclusions: A small cystic mass with well-circumscribed borders and slightly lower ADC value may suggest BCA, although TICs showed various patterns.