Variation in parotid gland size, configuration, and anatomic relations.

Purpose: To examine the variation in the anatomy of parotid glands discerned by magnetic resonance imaging. Methods: Head and neck magnetic resonance scans of 16 patients (representing 32 glands) whose studies consisted of 5 mm contiguous sections were selected at random. The T1 weighted scans were thresholded and outlined to only encompass the parotid tissue. A volumetric analysis program (ISG Technologies, Inc.) was used to compute the parotid volume in cubic millimeters. Each of the 32 glands was measured independently by two observers. Results: The difference between observers averaged 4.8%. The median volume was 25 262 mm3, range 9225–54 080 mm3. In four patients there were considerable differences in the volumes of the right and left parotid glands, with variations of 9, 10, 14 and 29%. In nine patients, (18 glands) the depth from the medial edge of the gland to the spinal cord ranged from 19–32 mm. However, the maximum variation between the two sides in a single patient was 4 mm. Observations made include: (1) parotid glands extending anterior to the masseter muscle, or posterior to the posterior margin of this muscle; (2) parotid glandular tissue extending above the zygoma and the external auditory canal; (3) parotid tissue extending posteriorly to overlap the spinal cord; (4) parotid glands extending below or remaining above the angle of the mandible; and (5) wide variation of the transverse dimension of the parotid glands, with one measuring 4.8 cm. Discussion: To ensure that the entire parotid is or is not in a treatment field a computerized tomography or magnetic resonance scan is necessary. If a specific portion of the gland must be in the field a volume histiogram must be available.