Diffusion-Weighted Imaging Does Not Distinguish Benign versus Malignant Parotid Lesions

Background
Many recent studies have investigated the use of DWI and apparent diffusion coefficient (ADC) maps in distinguishing various disease states in the head and neck.

The objective of a recent study was to determine the ability of echo-planar DWI in differentiating types of primary parotid gland tumors.

The study has shown that diffusion-weighted imaging (DWI) with apparent diffusion coefficient maps could distinguish pleomorphic adenomas from most other primary neoplasms, but could not distinguish Warthin tumors from malignant lesions.

Design
Prospective study.

Participants
136 consecutive patients with suspected primary parotid neoplasm who underwent surgery or biopsy during a five-year study interval.

Methodology
MRI was performed before surgery or biopsy. MR 1.5-T imaging was performed with axial T1, T2-weighted 3D fast imaging with steady state precession (FISP), and an echo planar fat-suppressed DWI with b factors of 0 s/mm2, 500 s/mm2, and 1000 s/mm2. ADC maps were generated, and an irregular region of interest (ROI) was placed on these maps over the entire tumor. A mean of each ROI was determined, and a mean of all the ROIs in a patient was computed.

Results
There were 43 pleomorphic adenomas with a mean ADC value (in 10-3 mm2/s) of 2.09 ± SD of 0.16. This was statistically different from all other types except for myoepithelial adenoma, with a mean ADC of 1.86 ± 0.18 (P =0.054). Mean values for Warthin tumors (n=32) were 0.89 ± 0.16. Warthin tumors were not significantly different from mucoepidermoid carcinomas (n=16), acinic cell carcinomas (n=10), or basal cell adenocarcinomas (n=9) but were different from myoepithelial adenomas (n=6), lipomas (n=3), and salivary duct carcinomas (n=11).

Conclusions
Echo-planar DWI can differentiate pleomorphic adenoma and myoepithelial adenomas from all other primary parotid neoplasms, but significant overlap between other benign and malignant lesions remains. Further studies combining DWI, morphologic criteria, and other MRI techniques are warranted.

Reviewer’s Comments
This study included a relatively large number of patients. As the authors point out, it is most important to distinguish Warthin tumors, which are benign and have a low rate of recurrence, from malignant tumors and pleomorphic adenomas, which have a high rate of recurrence and significant potential for malignant transformation. As such, the inability to distinguish Warthin tumors from malignant tumors is unfortunate.

However, combining morphologic features with ADC maps may prove valuable. I would have preferred the authors to have included 13 patients with lymphoma and inflammatory pseudolesions, since one does not know in advance if one is dealing with a primary parotid neoplasm.

Author: Yaron Lebovitz, MD

Reference:
Habermann CR, Arndt C, et al. Diffusion-Weighted Echo-Planar MR Imaging of Primary Parotid Gland Tumors: Is a Prediction of Different Histologic Subtypes Possible? AJNR Am J Neuroradiol; 2009;30 (March): 591-596.