Treatment of Post-radiation Xerostomia With Allogeneic Mesenchymal Stromal Stem Cells

The goal of this clinical trial is to learn about toxicity of allogeneic mesenchymal stromal stem cells (MSC, derived from umbilical cord tissue) treatment in head and neck cancer patients with for post-radiation xerostomia. The main question it aims to answer is the toxicity of intervention and the secondary objective is to assess preliminary efficacy. Participants will receive MSC in both parotid and submandibular glands: the toxicity of the intervention will be assessed using CTCAE v5.0 and its effect will be evaluated through measuring salivary flow and composition, radiologically (ultrasonography, magnetic resonance imaging), with scintigraphy and questionnaires. In the control group, salivary gland function (salivary flow and saliva composition) will be assessed for comparison with the intervention group.