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.