Parotid Glands

The largest salivary glands, one in front of each ear, wrapped around the ramus of the mandible and extending behind it toward the mastoid.

Each parotid makes saliva and delivers it into the mouth through a duct that opens inside the cheek. Parotid saliva is thin and watery, and it supplies much of the flow while you eat. Saliva moistens food for chewing and swallowing, starts the digestion of starch, helps with taste and speech, and protects the teeth and lining of the mouth.

The parotids produce most of the saliva when eating (stimulated flow). Too much dose causes xerostomia (dry mouth), which affects eating, speech, dental health and quality of life for years. They behave as parallel organs, so limits are written as a mean dose, usually aiming to spare at least one gland well while keeping both as low as possible. Sparing the parotids was one of the main reasons IMRT became standard in head and neck cancer. In whole-brain treatment the lower edge of the lateral fields can pass through the upper parotids, so some protocols ask to keep their dose as low as reasonably achievable.

The Brouwer 2015 consensus defines the parotid on CT by the structures that surround it. In its boundary table:

• Top: the external auditory canal and the mastoid process.
• Bottom: the back part of the submandibular space.
• Front: the masseter muscle, the back edge of the mandible, and the medial and lateral pterygoid muscles.
• Back: the front belly of the sternocleidomastoid muscle, and the outer side of the back belly of the digastric muscle.
• Outside: subcutaneous fat and the platysma.
• Inside: the back belly of the digastric muscle, the styloid process and the parapharyngeal space.

The guideline includes the retromandibular vein in the contour, because it is hard to tell apart from gland tissue on a non-contrast CT, and its remarks list the carotid artery and the extracranial facial nerve as included too. It strongly recommends a planning CT with IV contrast to see where the gland ends. Protocols differ on whether the parotid excludes the part overlapping the target — check your protocol.