Project Summary Radiation-induced xerostomia (RIX) is the most frequent and permanent late side-effect of RT in head and neck cancer patients that leads to compromised speech, difficulty in eating and swallowing and an overall reduction in quality of life in patients. Sadly, no definitive therapy or effective mitigating strategy is available for routine clinical management of RIX. The development of safe and effective strategies to mitigate RIX has been hindered by limited mechanistic insight and lack of objective methods to characterize the trajectory of normal tissue injury. In this regard, our preclinical studies have revealed that the temporal evolution of radiation- induced vascular injury and DNA damage response is influenced by the host immune status. Our preliminary studies have also revealed that vitamin D (VitD) deficiency exacerbates radiation-induced vascular injury in vivo. Conversely, VitD treatment protects salivary glands from radiation injury in 3D organoids. These observations along with the known anti-inflammatory and antioxidant effects of VitD have led us to hypothesize that correction of VitD deficiency through diet or administration of the active metabolite, calcitriol, can protect salivary glands from radiation damage and alleviate RIX in vivo. To test these hypotheses, we propose to characterize the dynamic changes in vascularity and immune profiles of salivary glands in response to radiotherapy in mice (Aim 1) and evaluate the impact of VitD on preventing/mitigating RIX (Aim 2). A preclinical large animal trial to examine the effects of VitD on response of head and neck tumors and salivary glands to volumetric modulated arc therapy is also proposed (Aim 3). Using novel experimental models and imaging technologies, the application will systematically examine the vascular and immune mechanisms underlying salivary gland radiation injury and define the therapeutic potential of VitD as a radioprotective agent. The proposed studies will enable development of VitD supplementation regimens for prevention of RIX in head and neck cancer patients in the near future.