Purpose: Irradiation of the parotid glands causes
salivary dysfunction, resulting in reduced salivary
flow. Recovery can be seen with time; however, long-term
prospective data are lacking. The objective of this
study was to analyze the long-term parotid gland
function after irradiation for head-and-neck cancer.

A
total of 52 patients with head-and-neck cancer and
treated with radiotherapy (RT) were prospectively
evaluated. Stimulated bilateral parotid salivary flow
rates were measured before RT and 6 weeks, 6 months, 12
months, and at least 3.5 years after RT completion.
A complication
was defined as a stimulated parotid flow rate of <25% of
the pre-RT flow rate. The normal tissue
complication probability model proposed by Lyman was fit
to the data. Multilevel techniques were used to model
the patterns of flow rates with time.

Results: The mean stimulated flow rate of the parotid
glands before RT was 0.31 mL/min (standard deviation
[SD], 0.21). This was reduced to 0.14 mL/min (SD, 0.15)
at 6 weeks after RT and recovered to 0.20 mL/min (SD,
0.22) at 6 months and 0.19 mL/min (SD, 0.21) at 12
months after RT. The mean stimulated flow rate was 0.25
mL/min (SD, 0.28) 5 years after RT.
The mean dose to
the parotid gland resulting in a 50% complication
probability increased from 34 Gy at 6 weeks to 40 Gy at
6 months, 42 Gy at 12 months, and 46 Gy at 5 years
after RT. Multilevel modeling indicated that both dose
and time were significantly associated with the flow
ratio.

Conclusion: Salivary output can still recover many years
after RT. At 5 years after RT, we found an increase in
the salivary flow rate of approximately 32% compared
with at 12 months after RT.