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

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.

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

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.

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