Salivary Gland Volume Changes and Dry Mouth Symptom Following Definitive Radiation Therapy in Oropharyngeal Cancer Patients—A Comparison of Two Different Approaches: Intensity-Modulated Radiation Therapy Versus Intensity-Modulated Radiation Therapy/Intensity-Modulated Proton Therapy Combination

Background/Objectives: We aimed to compare the salivary gland volume changes following intensity-modulated radiation therapy (IMRT) alone versus IMRT/intensity-modulated proton therapy (IMPT) combination in oropharyngeal cancer (OPC). Methods: We retrospectively reviewed 78 OPC patients who underwent definitive RT with ipsilateral neck irradiation. RT techniques were either IMRT alone or IMRT/IMPT combination. Salivary gland volumes over time in relation to dry mouth symptom were evaluated. Results: Patients’ characteristics were well balanced between groups. The mean dose to the ipsilateral parotid gland (PG) was significantly lower in IMRT alone than in IMRT/IMPT combination, while those to the contralateral PG and submandibular glands (SMGs) were significantly higher in IMRT alone. The volume ratio of ipsilateral PG showed an initial sharp decline, reaching 0.74, and stabilized thereafter. The ipsilateral SMG showed a continuous decline until 24 months and reached approximately 0.47 by 48 months. The contralateral PG/SMG showed initial decline and subsequent recovery to the initial volume by 48 months. There were no significant differences in salivary gland volume changes between groups. Within 6 months, 60.3% of patients experienced dry mouth symptom, and the dry mouth incidence decreased to 41.0% in 12 months and remained stable thereafter. There were no significant differences in dry mouth symptom between groups. The volume reduction in the ipsilateral salivary glands was greater in patients with dry mouth symptom. Conclusions: No significant differences in salivary gland volume changes and dry mouth symptom were apparent between groups. The critical factor in salivary gland volume change was the delivered dose to the salivary glands, not the RT techniques.