When conventional external irradiation is used to treat nasopharyngeal carcinoma, the function of parotid gland is severely damaged due to high dose irradiation, and salivary secretion decreases, leading to common complications such as dry mouth, which seriously affects patients’ quality of life.
The degree of parotid function damage is positively correlated with the irradiation dose received, and the dosimetric advantages of 3D conformal and intensity-modulated radiation therapy make it possible to protect the parotid gland. There is a large body of literature examining parotid dose, effect relationships, and results related to the protection of parotid function with intensity-modulated radiation therapy. Clinically, a 50% decrease in parotid secretion can be observed in the first week of conventional radiation therapy. When the whole parotid gland is irradiated at doses between 25Gy and 30Gy, parotid secretion decreases significantly, and above 40Gy, parotid secretion is very limited.
Nishioka et al. reported non-randomized controlled results of the effect of 3D conformal and conventional radiation therapy on parotid function. 3D conformal radiation therapy was able to reduce the exposure by 1/3 compared to conventional radiation therapy, and 40% of patients had no or only mild dry mouth, while all patients in the conventional group had moderate to severe dry mouth. The mean parotid dose was less than or equal to 24Gy (non-stimulated condition) or less than 26Gy (stimulated condition), and most of the parotid function was protected and continued to recover over time after radiation therapy, with parotid secretion returning to an average of 76% of the pre-irradiation level in the non-stimulated condition and 76% in the stimulated condition. The average recovery of parotid secretion under non-stimulated conditions was 76% and 114% under stimulated conditions. Conversely, above this threshold dose, recovery of parotid secretion is difficult. Below the threshold dose level, salivary secretion does not decrease due to higher average doses. The threshold dose for parotid gland function preservation when some parotid glands are irradiated varies from 15 Gy, 30 Gy, and 45 Gy for 67%, 45%, and 24% of the irradiated parotid volume, respectively. The TD50/5 derived from the normal tissue complication probability model is 28.4 Gy. Munter et al. quantified the relationship between parotid gland function preservation and dose after intensity-modulated radiation therapy for head and neck tumors. The authors concluded that when the mean parotid dose was higher than 26Gy or 30Gy, the relative parotid secretion varied greatly before and after irradiation, and when a 50% and 75% decrease in parotid secretion was observed after irradiation, the dose-effect curves showed that the parotid doses for the probability of a 50% decrease in parotid secretion were 34.8±3.6Gy and Chao et al. analyzed the relationship between dose and function of irradiated parotid glands in 41 patients with head and neck tumors, and the doses to parotid glands ranged from 2 Gy to 71 Gy. Based on different mathematical models, the authors concluded that parotid secretion decreased exponentially after stimulation, and the rate of decrease was 4% of the average dose to parotid glands at 1 Gy. Therefore, if the average bilateral parotid dose is less than 16 Gy, at least 50% of the pre-treatment parotid secretion will be preserved, and if the average dose is 32 Gy, only 25% of the pre-treatment parotid secretion will be preserved. The authors found that even if the parotid gland was irradiated with only 10Gy to 15Gy, parotid secretion could be decreased by about 50%, and the dose that decreased parotid secretion by 50% 7 months after irradiation was 22.5Gy. The authors concluded that in clinical practice, the dose to the parotid gland should be kept below the threshold value of 22.5Gy as much as possible. From this, most authors concluded that to protect parotid function , the average parotid dose needs to be controlled below 16Gy to 26Gy. Some authors have also reported that higher doses can also protect parotid function. Kwong et al. reported the protection of parotid function in 30 cases of early-stage nasopharyngeal carcinoma (T1N0~1M0) with intensity-modulated radiation therapy. 68Gy~70Gy/34F was prescribed for GTV and 64Gy~68Gy for PTV, and the mean parotid dose was 38.8Gy. 19 patients received doses before, 2 months, 6 months, 12 months, and 12 months after radiotherapy. The total stimulated salivary secretion (SWS) and stimulated parotid secretion (SPS) were measured at 6, 12, 18, and 24 months, respectively. At 1 year after intensity-modulated radiation therapy, the SPS of 47.1% of patients and the SWS of 60% of patients returned to at least 1/4 or more of the pre-radiotherapy level, and the percentage increased to 71.4% and 85.7% at 2 years after radiation therapy, and the salivary gland pH and buffering capacity also recovered with time. Wu et al. reported the results of 38 cases of intensity-modulated radiation therapy for nasopharyngeal carcinoma, which showed that intensity-modulated radiation therapy could result in an average dose of 23 Gy to the healthy parotid gland, and the actual parotid function measurement showed no significant change in the healthy parotid function before and after treatment.
These studies provide some guidance for the current prescription dose to the parotid gland for intensity-modulated radiation therapy, which is less than 26 Gy on average (at least in one parotid gland), or less than 20 Gy in at least 20 ml of the total volume of both parotid glands, or less than 30 Gy in at least 50% of the parotid gland (at least in one parotid gland). (at least in one parotid gland). The requirement for the parotid gland in the Cancer Hospital of Chinese Academy of Medical Sciences is 50% of the parotid volume dose less than 35Gy.