Application of a Spacer Medial to the Left Parotid Gland to Improve Dosimetry for Head and Neck Radiotherapy-A Swine Model

Abstract

Purpose: Radiation therapy is an important treatment method for head and neck cancer. The parotid gland, submandibular gland and minor salivary gland are often irradiated due to being adjacent to the target volumes. Although advancing radiotherapy techniques have reduced the dose of radiation to normal tissues and toxicity, the radiation dose distributed to the parotid gland remains high, causing a considerable amount of xerostomia. The aim of this study is to reduce the dose of radiation to the left parotid gland by inserting a spacer into the medial parotid space and separating it from the target volume. Materials and Methods: A minimally invasive procedure was developed to create a space medial to the left parotid gland in experimental swine, and a spacer was inserted into this space. A computed tomography simulator and radiation therapy planning system was used to analyze the distribution of the radiation dose to the putative gross disease, high risk clinical target volume, and parotid glands. Statistics obtained by using a paired t test were used to compare the dosimetry parameters with or without the insertion of the spacer. Results: The dose of radiation to the parotid glands was found to be significantly reduced by the demand for a similar coverage and conformity of the putative target volume for the gross disease (PTV 70 Gy) and high risk nodal region (PTV 63 Gy) with 2 Gy per fraction. Before the space was created, the left and right parotid glands were estimated to receive a mean dose of 2808 ± 205 cGy and 2901 ± 189 cGy, respectively. After the spacer was inserted into the left medial parotid gland space, the left and right parotid glands were estimated to receive a mean dose of 2027 ± 364 cGy and 2972 ± 114 cGy respectively. Therefore, the spacer had made a significant difference in the dose to the ipsilateral (left) parotid gland (p=0.003) without affecting the contralateral (right) one (p=0.36). Conclusion: An experimental model was developed by using spacers to create a space between the left parotid gland and the target volume, which markedly reduced the dose to the left parotid gland. Both short and long term functional assays for parotid damage in animals is warranted.