Abstract 6022: Investigating the mechanisms behind salivary gland tissue sparing in response to ultra-high dose-rate FLASH proton radiotherapy
Authors: Morcos George S. , Chowdhury Priyanka , Kim Michele M. , Shoniyozov Khayrullo , Metz James , Dong Lei , Koumenis Constantinos 
Published: March 2024

Abstract
Background: Radiotherapy (RT) is a primary treatment for cancers of the head and neck. However, although highly efficacious in eliminating tumors, incidental damage to underlying tissue remains a major limitation of RT. In the case of head and neck cancer, damage to the otherwise healthy salivary glands is often unavoidable even at low dosages, leaving patients with lifelong xerostomia and other comorbidities. FLASH Proton RT (F-PRT) is a form of ultra-high dose RT and has been reported to spare healthy tissues while retaining its tumor-controlling efficacy. The first system to deliver dosimetrically identical FLASH Proton RT (F-PRT; 60- 110 Gy/sec) or Standard Proton RT (S-PRT; 0.5-1 Gy/sec) using double-scattered protons was designed by our group.
Aim: In this study, we aimed to investigate the effect of F-PRT on radiation-induced salivary gland dysfunction and the potential mechanism of this sparing effect by analyzing the effects of FLASH on two distinct salivary gland cell types: acinar and ductal cells.
Methods: The head and neck area of C57BL/6 mice were irradiated with a single dose of 16 Gy of F-PRT (128Gy/s) or S-PRT (0.95 Gy/s). Radiation-induced xerostomia was studied by measuring the saliva flow rate of mice at 5, 10, 14, and 28 days post-PRT. Expression of AQP5, SOX9, and Keratin- 18, 19 was studied at 2, 5, 10, 14, and 28 days post irradiation by both immunofluorescence and protein immunoblotting.
Results: Following irradiation with a single dose of 16 Gy, saliva flow was reduced by both treatments. However, the F-PRT-treated mice showed a significant improvement in salivary flow at 14 and 28 days post-irradiation, compared to those treated with S-PRT. Expression of AQP5 was significantly downregulated at 2, 5, 10 and 14 days post irradiation with S-PRT while F-PRT-treated mice showed a significant restoration of the AQP5 expression during the same time points. Expression of SOX9 showed no significant differences between S-PRT and F-PRT treated mice at earlier time points (2, 14 days), however, there was a significant difference in expression of SOX9 between the irradiated and unirradiated group. The difference in expression of SOX9 was visible from day 28 post-irradiation, where the F-PRT treated group showed increased expression compared to S-PRT.
Conclusion: Our findings demonstrate that F-PRT reduces radiation-induced hyposalivation and reinstates the expression of AQP5 in the acinar cells of the submandibular gland of mice. Furthermore, it demonstrates that on day 28 post-irradiation, F-PRT-treated mice displayed more SOX9-expressing progenitor cells in the submandibular salivary glands compared to S-PRT.
Acknowledgements: This work was supported by a Sponsored Research Agreement from IBA to Drs. C. Koumenis and L. Dong.
Citation Format: George S. Morcos, Priyanka Chowdhury, Michele M. Kim, Khayrullo Shoniyozov, James Metz, Lei Dong, Constantinos Koumenis. Investigating the mechanisms behind salivary gland tissue sparing in response to ultra-high dose-rate FLASH proton radiotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6022.