Nasopharyngeal carcinoma (NPC) is a common malignant tumor in the head and neck arising from the nasopharynx epithelium. The incidence of NPC exhibits geographical variation with high rates of up to 15 ~ 20 per 10 million (women) or 20 ~ 30 per 10 million (men) in South and Southeast Asia(1–3). In terms of demographic trends, the incidence is about 1.5-fold higher in men than in women in China(4), and the peak age of onset ranges from 50 to 60 years.
Although NPC and other head and neck tumors have similar cell or tissue lineage, NPC has unique biological characteristics. As the nasopharyngeal cavity is deep and narrow, strong local invasion and higher lymph node metastasis rates occur. Besides, it is adjacent to the brainstem, temporal lobe, optic chiasm, cervical spinal cord, temporomandibular joint, parotid gland and other important organs. These characteristics mean that surgical therapy and radical excision are conducted with considerable difficulty.
Morphologically, an abundance of lymphoid cells is often seen intermixed with transformed epithelial cells, but NPC is widely regarded to be squamous in origin. Depending on the degree of differentiation and on the basis of WHO criteria, NPC is categorized into three pathological subtypes. Type I corresponds to differentiated tumors with surface keratin, whereas types II and III are defined as non-keratinizing differentiated and undifferentiated tumors, respectively(2, 3). The non-keratinizing subtypes constitute most cases that are highly malignant and sensitive to radiation, which provides favorable conditions for radiotherapy. Therefore, radiotherapy is the most basic and main treatment for NPC. The 5-year survival rate of early NPC after radiotherapy can reach 80%(5).
For radiotherapy, the clinical target area of NPC includes the area from the upper part of the base of the skull to the lower edge of the clavicle. Moreover, the shape of the large target area is extremely irregular, and the lymphatic drainage area is extremely complex. Therefore, conventional radiotherapy cannot effectively spare the adjacent important organs and normal tissues, especially the parotid gland, which inevitably receives high-dose radiation, and the other salivary glands, which exhibit tissue fibrosis and even shrinkage after irradiation, leading to irreversible impairment of function. One of the most common and long-term sequelae is xerostomia, which seriously affects the quality of life (QoL) of patients as it impacts speaking, nutrition, taste, sleep and so on. Some patients cannot even complete their radiotherapy plan on time, thus decreasing the local control rate of the tumor. In recent years, more attention has been paid to the QoL of NPC patients, focusing on ways to protect the salivary glands and reduce sequelae after treatment, especially xerostomia(6).
Radiotherapy is the primary and only curative treatment for NPC. In recent years, intensity-modulated radiotherapy (IMRT) has been used widely in the treatment of NPC, as it is a technique that allows minimization of the radiation doses to adjacent critical normal structures, while maintaining or increasing the primary tumor site dose. On the basis of this, IMRT can improve target coverage and spare important tissues when applied to NPC patients. The advantages of IMRT make it the preferred treatment method in NPC, compared to conventional three-dimensional conformal radiotherapy (3D-CRT). This has been confirmed in some previous studies. For example, a study by Kristensen et al. indicated that IMRT improved the target volume coverage and protected critical organs compared with 3D-CRT, which suggested that IMRT is better than 3D-CRT in the treatment of patients with NPC(7). Vergeer et al. reported that IMRT treatment resulted in a significant reduction in the radiation dose of the parotid glands and less xerostomia, as well as other side-effect symptoms, compared to conventional 3D-CRT(8). A study by Pow et al. showed that IMRT was significantly superior to CRT in protecting the salivary glands and improved QoL for NPC patients(9). Many other studies have confirmed these results(10–13).
Combined with the above information and based on a previous study, the present study aimed to investigate the relationships between parotid gland dysfunction and radiation dosimetry, xerostomia and QoL in NPC patients with different units after radiotherapy. This study was undertaken in order to demonstrate the superiority of IMRT compared with 3D-CRT in reducing side effects and protecting critical glands.