br Discussion br Several kinds of bite

4. Discussion
Several kinds of bite blocks and similar devices have been reported in radiotherapy for head and neck cancer and facial moulage technique is commonly used to fabricate these bite blocks. In the study of Santvoort et al., a vacuum KIN-59 piece resulted in even better patient fixation and smaller rotations, compared with the standard available upper jaw support in stereotactic radiothe-rapy [9]. Several studies have confirmed that fixing and locating patients according to the fine impression of the teeth and tongue could limit position errors to less than 2 mm, allow patients to remain in the same position, and improve the precision of the radiotherapy [24–26]. These results suggest that custom-made bite blocks using silicone-based intraoral impressions fit more accurately and improve comfort, which allows more precise preirr-adiation planning. Based on the reliable and acceptable setup and positioning accuracy, we constructed a new customized bite block through a simple fabrication process using widely used materials in dental clinical practice.
The new custom-made bite block has been used in our hospital for the past four years. The main advantage of this device is that it’s easy-made, easy to use and comfortable for the patients dur-ing treatment. The key point of this method is using the oral stent and silicon rubber through a procedure of making fine intraoral impressions.
As the different-sized oral stents can be prepared beforehand, the total time to make a customized bite block is approximately 20 minutes; it saves time and makes things much easier when occasional need arises to adjust or modify the device during radiation therapy. By compraison, an entire process of making
Fig. 5. A new customized bite block used in radiotherapy of head and neck cancer: CT scans of a patient showing dose distribution maps for intensity-modulated radiotherapy
planning: a: with the customized bite block; b: without the customized bite block.
customized tongue-displacing stent requires more time (between 2 and 3 hours) [11].
The customized bite block must be made before CT simulat-ing positioning. Its cost-effectiveness and simplicity of realization allow it to be made in the mould room by the radiation oncolo-gist instead of a maxillofacial surgeon or orthodontic practitioner, which is an advantage over the other kinds of bite blocks mentioned above. The fine silicone-based impressions of the intraoral tissues also made it possible to individualize the bite block, while being comfortable for the patient and simple in use. After each treatment, the customized bite block is to be rinsed several times in clear water and stored into a dedicated box. Several fixed holes were designed to fit the silicon rubber firmly on the oral stent. Thanks to these advantages of polyester film and silicon rubber-like stable physi-cal and chemical properties, the new custom-made bite block can be kept for many years. In addition, the silicone rubber can easily be differentiated from surrounding tissues under CT scan, and does not affect the outline of the target radiation areas.
For patients receiving radiotherapy targeting the nasal cavity, paranasal sinuses and oromaxillofacial area, type I customized bite block is applicable to cancer of sinuses and nasal cavity, while type II customized bite block is applicable to cancer of the oral cavity, salivary glands, especially tongue cancer to avoid air gap which may lead to underdosage of the surface of the tumour. Type
II customized bite block is also suitable for edentulous patients and patients with maxillofacial malformation, as the inclusion of impressions of the tongue and other intra-oral tissues ensure ade-quate tissue separation.
Furthermore, we evaluated the effect of the customized bite block on the dosimetric factors of patients with head and neck cancer who underwent intensity-modulated radiotherapy, com-paring CT preirradiation plans with and without the use of the customized bite block. The customized bite block had no signifi-cant effect on the conformity and heterogeneity indices, suggesting that it did not change the dose distribution characteristic of the planned target area. By physically separating of the jaws and the depressing of the tongue away from the planning target area, the customized bite block decreased the dose to organs at risk dur-ing intensity-modulated radiotherapy, which may minimize the potential radiation-induced adverse effects. These results have been partly verified in previous studies. Johnson et al. reported the fabrication of customized tongue-displacing stents to achieve a more predictable and consistent radiation dosimetry planning, while sparing greater volumes of healthy tissue from damage in patients receiving head and neck radiotherapy [11].