Radiation Effects

RT is one of the primary treatment modalities in HNSCC. RT is believed to place HNSCC patients at increased risk of respiratory infection by two mechanisms: 1. damage to respiratory tract cilia and 2. alteration of the microbial landscape of the upper aerodigestive tract. RT to the ciliated epithelium in the upper respiratory tract results in cellular changes that negatively affect mucociliary clearance. In a histopathological study of irradiated nasal mucosa in an animal model, Ohashi et al. observed a progressive decline in the number of cilia as epithelial metaplasia occurred several weeks after radiation.43 At the end of 8 weeks, there was complete loss of cilia.43 Similar histological findings of epithelial change and ciliary loss have been observed in irradiated larynx and nasopharynx tissue from humans.44,45Functional studies of cilia by measuring nasal secretion transport times have shown significantly slower transport in irradiated adults and children compared to healthy controls.46,47 Many patients who have been treated with radiation go on to develop chronic sinusitis.44,48
RT has also been shown to alter the microbial landscape of the oral cavity and oropharynx. Radiation to the salivary glands causes reduction in salivary flow and subsequent xerostomia.49,50Coinciding with decreased saliva in patients treated with radiation for HNSCC, Brown et al. showed significantly altered oral flora including increases in Streptococcus mutans , Lactobacillus ,Candida , and Staphylococcus .49Additional studies on the immunologic impact of these changes have shown alterations in the amount of immunoglobulin A (IgA), which plays a principal role in the local immune response, against virulent streptococcus species. Specifically, there was an increase in IgA activity against Streptococcus pneumoniae , a common cause of lower and upper respiratory tract infections, suggesting increased infections by this species in this population.50 Thus, it is possible that the impact of RT on oral flora may ultimately lead to downstream effects on the microbiology of respiratory infections.