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Simple Summary  During radiotherapy of head and neck cancer patients, the radiation dose for the major salivary glands (parotids and submandibular glands) should be kept as low as possible to reduce toxicity risks. Nevertheless, volume changes and positional shifts of salivary glands during treatment may result in an increased dose, so adapting the treatment plan is recommended. While previous studies primarily used one or two CTs or CBCTs for imaging during treatment, frequent MRI allows for improved soft tissue contrast and more exact contour adaptation without an additional imaging dose. In this study, twelve patients were treated with MRI-guided radiotherapy with weekly offline adaptations. Significant parotid- and submandibular gland shrinkage, as well as a medial shift of the parotids, were observed. These results support the rationale of MR-guided radiotherapy for head and neck cancer, emphasizing the relevance of future clinical evaluation of toxicity to optimize the benefits of MRI-guided radiotherapy.    Abstract  The aim of this study was to quantify anatomical changes of parotids and submandibular glands and evaluate potential dosimetric advantages during weekly adaptive MR-guided radiotherapy (MRgRT) for the definitive treatment of head and neck cancer (HNC). The data and plans of 12 patients treated with bilateral intensity-modulated radiotherapy for HNC using MR-linac, with weekly offline adaptations, were prospectively evaluated. The positional and volumetric changes of the salivary glands were analyzed by manual segmentation in weekly MRI images and the dosimetric impact of these anatomical changes on the adapted treatment plans was assessed. The mean volume change in parotid and submandibular gland volume was −31.9% ( p  < 0.0001) and −29.7% ( p  < 0.0001) after five weeks, respectively. The volume change was significantly correlated with the cumulative dose for the respective gland at the time of volume measurement. Inter-parotid distance changed by −5.4% (6.5 mm) on average after five weeks ( p  = 0.0005). The distance became significantly smaller only in the left-right direction. The inter-submandibular gland distance changed by 0.7 mm ( p  = 0.38). This study demonstrated significant changes in salivary gland volumes and position following daily MR guidance and weekly plan adaptation. Ongoing clinical trials will provide data on the clinical impact of these changes and novel MR-based adaptation strategies.

MR-Guided Adaptive Radiotherapy for Head and Neck Cancer: Prospective Evaluation of Migration and Anatomical Changes of the Major Salivary Glands