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2007 Pètra Braam

Parotid gland sparing radiotherapy

Introduction
Radiotherapy is a common form of treatment for head-and-neck malignancies. One of the most prominent complaints after radiotherapy is a dry mouth, which is caused by irradiation of the salivary glands. The main contributors of saliva during stimulation are the parotid glands, which are positioned near the neck nodes and in most cases in the vicinity of the primary tumor. Consequently irradiation of part of the parotid gland cannot be prevented and radiation-induced hyposalivation will occur. The dose to the parotid gland and therewith the degree of function reduction, depends on the irradiated target volume. Not only the primary tumor, but also the nodal target volume is important, especially the level II lymph nodes. Lowering the border of the nodal target volume consequently results in a reduction of dose to the parotid gland. Another way of reducing the dose to the parotid gland might be irradiation by intensity-modulated radiotherapy (IMRT). This is a new radiation technique that theoretically shows advantages for dose reduction to the parotid gland.

Materials and methods
First the long-term objective parotid gland function and subjective outcome after radiotherapy was investigated. We used stimulated bilateral parotid flow measurements and questionnaires. Thereafter we investigated the distance from the base of the skull to the most cranial metastatic lymph node in patients with oropharyngeal, hypopharyngeal and laryngeal carcinoma, This distance was used to specify the superior extent of the elective nodal target volume to guide the definition of target volumes for head-and-neck irradiation. Third, we compared conventional radiotherapy (CRT) with IMRT for parotid salivary output in patients with oropharyngeal cancer.

Results
The parotid salivary output can still recover years after radiotherapy. The mean stimulated flow rate 5 years after radiotherapy was 0.25 mL/min, which was an increase of 32% compared to 12 months after treatment. But at 5 years follow-up 41% of the patients complained of moderate to severe dry mouth. When investigating the cranial distribution of metastatic lymph nodes in oropharyngeal and hypopharyngeal cancer, different results were found compared with laryngeal cancer, implicating that guidelines should be tailored to specific primary sites. Advices for elective level II target volume delineation were given. Using IMRT, the parotid gland mean dose reduced from 48.1 Gy using CRT to 33.7 Gy, resulting in a significant reduction in flow complications. At 12 months follow-up the number of complications was 43% using IMRT compared with 83% using CRT. A complication was defined as stimulated flow rate <25% of the pre-radiotherapy flow rate. However, there was no significant difference in subjective xerostomia between the treatment modalities. At 12 months 76% of patients treated with IMRT complained of partial or complete persistent dryness, compared with 72% treated with CRT, respectively.

Conclusion
Reducing the dose to the parotid gland is the main concept of parotid gland preserving radiotherapy. Using clinically optimized guidelines for target volume delineation a dose reduction can be achieved. Combined with intensity-modulated radiotherapy instead of conventional techniques, a further parotid sparing treatment can be realized.