Publications relating parotid dose–volume characteristics to
radiotherapy-induced salivary toxicity were reviewed. Late
salivary dysfunction has been correlated to the mean parotid
gland dose, with recovery occurring with time.
Severe xerostomia
(defined as long-term salivary function of <25% of baseline)
is usually avoided if at least one parotid gland is spared
to a mean dose of less than ≈20 Gy or if both glands are
spared to less than ≈25 Gy (mean dose). For complex,
partial-volume RT patterns (e.g., intensity-modulated
radiotherapy), each parotid mean dose should be kept as low
as possible, consistent with the desired clinical target
volume coverage. A lower parotid mean dose usually results
in better function. Submandibular gland sparing also
significantly decreases the risk of xerostomia.
For
complex partial volume RT patterns (e.g.,
intensity-modulated RT), the mean dose to each parotid gland
should be kept as low as possible, consistent with the
desired clinical target volume coverage.
A lower mean dose to
the parotid gland usually results in better function, even
for relatively low mean doses (<10 Gy). Similarly,
the mean dose to the parotid gland should still be
minimized, consistent with adequate target coverage, even if
one or both cannot be kept to a threshold of <20 or <25 Gy.
Published variations in response among different patient
cohorts were probably related to the lack of an accurate
model that correctly includes the effects of multiple
salivary glands and intragland sensitivity variations. When
it can be deemed oncologically safe,
submandibular gland
sparing to modest mean doses (<35 Gy to see any effect)
might reduce xerostomia symptoms.