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

Abstract

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.

Figure 1

Basal cell adenoma in the left parotid gland of a 56-year-old female. (a) Short-inversion-time inversion recovery image [3000/40 (repetition time/echo time), axial plane] shows a superficial lobe tumour with moderately high intensity in the peripheral region and markedly high intensity in the central area (*). The markedly high-intensity area is a cystic lesion. (b) T₁ weighted image (560/8, axial plane) shows a hypointensity tumour (arrows). (c) T₂ weighted image (4000/104, coronal plane) shows a low-intensity capsule around the tumour (arrows). The ratio of the maximum diameter of the cyst to this tumour (cystic ratio) is 0.68. (d) On axial dynamic enhanced study, the time–signal intensity curve of the solid component of this tumour (solid line) shows Type B early enhancement with poor washout ratio (4%). The cystic region (dashed line) shows no enhancement. The apparent diffusion coefficient value of the solid component is 1.28 × 10⁻³ mm² s⁻¹. (e) Loupe image on the axial section of the specimen with haematoxylin–eosin (HE) staining shows solid and large cystic components (**). The large cyst had lost its contents. (f) Solid component has anastomosing cords of basaloid cells and a small amount of intervening stroma (×100 HE staining).

Figure 2

Basal cell adenoma in the right parotid gland of a 49-year-old female. (a) On short-inversion-time inversion recovery image [3000/40 (repetition time/echo time), axial plane], the deep lobe tumour shows heterogeneous high intensities (arrows). (b) On T₁ weighted image (560/8, axial plane), the tumour shows isointensity for muscle (arrows). The apparent diffusion coefficient value of the tumour component is 1.31 × 10⁻³ mm² s⁻¹. (c and d) Fat-suppression contrast-enhanced T₁ weighted image (540/8.2, axial plane) and time–intensity curve (TIC) on enhanced dynamic study, with the tumour showing heterogeneous enhancement (arrows) with TIC peaks later than 120 s (Type C). (e and f) Loupe and light microscopy (original magnification, ×100) images show the solid component with anastomosing cords of basaloid cells and rich intervening stroma (***).

Figure 3

Cystic pleomorphic adenoma in the right parotid gland of a 37-year-old male. (a) On short-inversion-time inversion recovery image [3000/40 (repetition time/echo time), axial plane], the deep lobe tumour shows heterogeneous moderately high intensities with cystic component in the peripheral region (arrows). (b) T₂ weighted image (4000/104, coronal plane) shows a low-intensity capsule (arrowhead). The cystic component is located in the peripheral region (arrow). The cystic ratio is 0.22. (c d) Fat-suppression contrast-enhanced T₁ weighted image (540/8.2, axial plane) and time–intensity curve (TIC) on enhanced dynamic study, with the tumour showing heterogeneous enhancement and Type C TIC. The cystic component has no enhancement (arrows). The apparent diffusion coefficient value of the solid component is 1.52 × 10⁻³ mm² s⁻¹.