## 4 Discussion  
In this study, we evaluated the usefulness of silicone 3D printed bite blocks as fixation devices for head and neck RT patients. Dosimetry of patients, treated with the silicone 3D printed bite blocks, showed better coverage of the PTVH and surface of the tumour bed, and less scattering of the RT dose to adjacent normal tissues, than conventional universal corks. Good alignment in the occlusal surface was observed by port film imaging verification. Patients felt more comfortable using the silicone 3D printed bite blocks than other rigid materials. Figure 6: Imaging verification by port film images. Orthogonal (**a**) anterior-posterior; and (**b**) lateral images were obtained to verify isocentre placement for patients receiving head and neck RT. The port film images showed good alignment in the occlusal surface without displacement. Oral fixation in the RT process has been an important issue for decades, and various oral devices, such as bite blocks have been widely use in clinical treatment. [PERSON] et al. examined the use of bite blocks to reduce setup error and intrafractional movement in the head and neck RT with a real-time tumour tracking RT system [6]. [PERSON] et al. reported the use of an intraoral stent as a mouth-opening device to reduce the radiation dose to healthy structures, especially bone tissue and salivary glands during RT [16]. For patients with HNC who could not tolerate the intraoral devices, [PERSON] et al. proposed RT methods in which the tongue is stuck out. Decreasing the RT dose was noted, but the reproducibility of such a dose decrease should be a concern [17]. [PERSON] et al. declared decreased radiation-induced mucositis with the use of a customized bite block for carbon-ion RT [18]. [PERSON] et al. stated the effectiveness of water-equivalent bite blocks during external beam RT for oral cancer. In-homogeneity correction was achieved by the use of a water-equivalent material [19]. [PERSON] et al. recommended the use of custom-made bite blocks incorporating tongue depressors or elevators to reduce radiation-induced mucositis during carbon-ion RT. According to different origins of HNC, depressors or elevators were used to reduce incidental exposure of the tongue and healthy structures during RT [20]. These kinds of 3D printed bite blocks were manufactured recently, and their clinical use is still being investigated and evaluated [21; 22; 23]. We have designed silicone 3D printed bite blocks to support opening of the mouth and fixation of the upper and lower dental arches, and tongue in CT simulations and RT. For tongue cancer, the tumour bed or surgical bed is presented in the air cavity. The skin-sparing effect of high energy photon beams may reduce the dose on the surface of the tongue. A higher dose was needed to compensate for the skin-sparing effect with unacceptable non-uniformity of the dose within the PTVH, and caused excessive irradiation to sensitive structures, which resulted in severe toxicities. The silicone 3D printed bite blocks with tongue depressors may work as boluses to overcome the skin-sparing effect so that a uniform dose may be obtained. The bolus effect still exists for patients with HNC other than tongue cancer. A homogeneous dose in the PTVH that presented in the air cavity was also observed in the 3D printed bite blocks, and not containing the conventional oral corks. This is the first publication in which silicone 3D printed bite blocks have been customized in clinical RT treatment. The material and shape of the 3D printed bite blocks can be customized to fit every patient's oral cavity. The dosimetric parameters, CI and HI, were used to evaluate the RT plans. Patients stated that they felt more comfortable when silicone 3D printed bite blocks were used, and we speculated that this meant greater fixation and better compliance in the RT process, which was confirmed by port film images verification. In this investigation, we mainly aimed to establish the technique and processing procedures with optimization of intraoral materials for 3D printing.