DOS 513 - Week 3 Discussion
Initial Post: Contouring for a Parotid Case
The parotid glands are a pair of large salivary gland located superficial to and partly behind the ramus of the mandible covering the masseter muscle, extending posteriorly to the sternocleidomastoid muscle.1 Superiorly, the parotids can extend to the zygomatic arch, inferiorly, to the angle of the mandible, and medially, approximately to the styloid process.
The parotids play an important role in saliva production. Saliva coats and lubricates the mouth, providing tooth protection, infection protection, and playing an early role in digestion. When the parotids are compromised by surgery, radiation, or disease, xerostomia (mouth dryness) can occur, resulting in an increased incidence of dental cavities, oral infections, and difficulty in eating, potentially resulting in nutritional deficits.
When planning a parotid patient, it is important to know the extent of the disease, since this will guide the range of beam coverage and the extent of structures that need to be contoured. According to Quang Dang, CMD (December 3, 2014), a typical photon plan would include, at a minimum:
- Right Parotid
- Left Parotid
- Brain Stem
- Right Cochlea
- Left Cochlea
- Spinal Cord
Quang went on to explain that if non-coplanar beams or other complex arrangements are used, it would be worthwhile to also include:
- Right Eye
- Left Eye
- Right Lens
- Left Lens
At the physicians discretion, it may be useful to contour the oral cavity or pharynx. Josh Howard, CMD and Upendra Parvathaneni, MD make a case that if both parotid glands are potentially compromised, it can be important to attempt to spare at least one of the submandibular glands, which also produce saliva, and in particular, a type of saliva that is rich in mucin, which has lubrication and infection control properties.2
Once normal anatomy is contoured, a physician will provide target volumes such as GTV, CTV, and PTV, which may extend into the neck if lymphatic spread is suspected. Quang suggested (December 3, 2014) that if IMRT or VMAT is used, it is important to create a plan optimization volume that is pulled 3 mm back from the skin surface to allow the beam optimizer to not have to fight to get dose to the surface of the skin. Bolus could also be used if surface dose is desired.
If there are significant dental artifacts on the planning CT, it can be important to contour out the streaks and override their density so that the beam optimizer does not think there is air or high density material in the tissue, which will both affect the calculation of the beam distribution.
If the treatment fields extends into the neck, the physician may determine that it is appropriate to contour the larynx, thyroid, and trachea. If there is extra time available, contouring the carotid artery on the affected side(s) would be very helpful for future planners if the disease ever recurs, since the carotid will become an important dose limiting structure (Kent McCune, oral communication, December 3, 2014).
If I were contouring this for a proton plan, I would also include the right and left retinas, right and left optic nerves, optic chiasm, pituitary depending on patient age, and right and left temporal lobes of the brain. Although these structures would almost certainly receive zero dose with a proton plan with the possible exception of the ipsilateral temporal lobe, it may be important to be able to show documentation of zero dose, which could be a strong argument for proton treatment if an insurance company is wavering one way or the other on reimbursement. In fact, most of the normal structures listed above would receive zero dose with a proton plan, because a superficial structure like the parotid is an excellent example of a case where the Bragg peak can be used to its best effect.
- Lozano RG. Head and neck cancers. In: Principle and Practice of Radiation Therapy. 3rd ed. St. Louis, MO: Mosby; 2010.
- Howard J, Parvethaneni U. Talk presented at: AAMD Annual Meeting; June 1, 2014; Seattle, WA. http://www.medicaldosimetry.org/pub/ad9e1b67-e381-76a7-2948-0b7408a80f24. Accessed December 3, 2014.