Radiotherapy (RT) is a mainstay in the treatment of head and neck squamous cell carcinoma (HNSCC). For locally advanced HCSCC, concurrent chemoradiotherapy (CCRT) benefits HCSCC patients in terms of better survival and loco-regional control. In this study, we evaluated changes in oral microbiota in patients, who received CCRT for head and neck cancer. Oral rinsed samples were weekly collected before and during CCRT and at 4 weeks following treatment from HNSCC patients, who had received 70 Gy of radiation delivered to the primary sites for over 7 weeks and concurrent chemotherapy. Oral microbiota changes in three patients were analyzed by next-generation sequencing using 16S rRNA 454 pyrosequencing. On an average, 15,000 partial 16S rRNA gene sequences were obtained from each sample. All sequences fell into 11 different bacterial phyla. During early CCRT, the microbial diversity gradually decreased. In a patient, who did not receive any antibiotics during the CCRT, Firmicutes and Proteobacteria were the most abundant phylum. During the early CCRT, proteobacteria gradually decreased while Firmicutes increased. During the late CCRT, firmicutes gradually decreased while Bacteroides and Fusobacteria increased. In all the patients, yellow complex showed a gradual decrease, while orange and red complex showed a gradual increase during the CCRT. At 4 weeks after CCRT, the recovery of oral microbiota diversity was limited. During CCRT, there was a gradual increase in major periodontopathogens in association with the deterioration of the oral hygiene. Henceforth, it is proposed that understanding oral microbiota shift should provide better information for the development of effective oral care programs for patients receiving CCRT for HNSCC.

Introduction
 Materials and Methods
  Patients
  Concurrent chemoradiotherapy
  Microbial sampling
  DNA extraction
  Pyrosequencing
 Results
  Patient selection for pyrosequencing analysis
  Overall sequence data
  Composition of the bacterial community
 Discussion
 Acknowledgements
 References

• Jin Ho Kim(Department of Radiation Oncology, Seoul National University Hospital)
• Yoon Hee Choi(Department of Hematology-Oncology, Dongnam Institute of Radiological and Medical Sciences)
• Soo-Youn An(Department of Otolaryngology-Head and Neck Surgery, Dongnam Institute of Radiological and Medical Sciences)
• Hee Young Son(Department of Otolaryngology-Head and Neck Surgery, Dongnam Institute of Radiological and Medical Sciences)
• Chulwon Choi(Department of Radiation Oncology, Dongnam Institute of Radiological and Medical Sciences)
• Seyeon Kim(Department of Oral Microbiology, School of Dentistry, Pusan National University)
• Jin Chung(Department of Oral Microbiology, School of Dentistry, Pusan National University)
• Hee Sam Na(Department of Oral Microbiology, School of Dentistry, Pusan National University) Correspondence to