Risk factors can be classified as patient-related, tumor-related, and treatment-related factors.

Patient-related factors

Sex has been reported to have an influence on mucositis risk because, in most studies, female sex is linked to increased risk.24 However, this may be driven by dosimetric parameters (ie, calculating the risk per kilogram of body weight or per square meters of body surface area).24, 25 An individual's low baseline performance status may be related to increased mucositis risk.25 There is inconsistent evidence regarding the influence of age, smoking, oral hygiene, and body mass index on mucositis risk.24, 26 The risk of mucositis may be influenced by genetic variants in drug-metabolizing pathways, immune signaling, and cell injury-repair mechanisms, although the evidence is limited or conflicting (see Supporting Table 1). Systemic factors, such as comorbidities, medication use, and previous therapy, have been associated with increased risk (see Supporting Table 2). Numerous variables had conflicting evidence, such as neutropenia/leukopenia. Uncontrolled diabetes mellitus was suggested as a potential risk factor of persistent chronic OM related to RT.22

Tumor-related factors

Clinical features of the tumor, such as site and stage, may influence the risk of mucositis and severity.24 In patients with H&N cancer who are receiving RT, these characteristics determine the radiation plan (field and dose) and thus influence the exposure of the mucosal tissues and the development and severity of mucositis.

Treatment-related factors

An increased risk of mucositis has been reported with increasing doses of radiation,27 myeloablative conditioning before HCT,28 and chemotherapy, such as methotrexate and melphalan.25 Altered, fractionated RT (eg, accelerated fractionation of 6 fractions per week or 2 daily fractions of 2 Gy each) may be associated with increases in mucositis frequency and duration.29, 30 Conversely, intensity-modulated radiation therapy, which enables the design of radiation scattering, can allow a reduction in toxicity, for example, by planning a cumulative exposure <30 Gy, when possible.27

Risk factors in pediatric patients

Among children, low body weight, anxiety level, nausea/vomiting, and previous OM are associated with an increased risk of developing OM.31 An association was reported between OM and leukopenia and neutropenia, as well as between OM and lymphopenia in patients with solid tumors.31, 32 There is conflicting evidence regarding the association between platelet level and OM.31, 32 Like in adult patients, genetic variants may determine mucositis risk (see Supporting Table 1).31, 33 Reported microbial risk factors include HSV type 1, oral Candida species, and unspecified bacterial infections.32, 34

Hematologic malignancies like lymphoma and germinal tumors like neuroblastoma, nephroblastoma, and retinoblastoma reportedly increased the risk of mucositis induced by chemotherapy.34, 35 Protocols using high-dose methotrexate, daunorubicin, doxorubicin, vincristine, etoposide, busulfan, and cytarabine were presented as potential risk factors for OM in pediatric cancer patients.13, 31

Pathobiology

Historically, mucositis pathogenesis was described using a simplistic approach, which ascribed normal tissue injury of mucosa as the sole consequence of the indiscriminate effects of radiation or chemotherapy on the rapidly dividing normal cells of replenishing tissues like the gastrointestinal mucosa. This idea was overturned by the results of many studies indicating that direct cell injury (direct DNA double strand breaks [DSBs]) could only account for approximately one-third of the observed injury. Consequently, the complexity of the pathogenesis of chemotherapy-associated or RT-associated normal tissue injury was realized and summarized in a model that broadly compartmentalizes the biological sequence into 5 broad stages. Although such a model is convenient, it minimizes the intricacies of each stage and does not adequately relate the biological or clinical dynamics of mucositis development, especially for cases in which the tissue is repeatedly challenged, as with fractionated radiation. Nonetheless, the model does provide a snapshot summary of the molecular and cellular events and pathways that terminate in mucositis development. The 5 stages are (Fig. 5): initiation, up-regulation and activation (primary damage response), signal amplification, ulceration, and healing.36 This offers a convenient summary of the complex, nonlinear progression of injury as the various stages and associated biological processes interrelate and overlap.

Five Stages Delineate the Biological Sequence of Mucositis Pathogenesis. Although convenient as a tool to describe the key biological elements involved, the process is more complex and interactive than is suggested by a simple set of illustrations. Nonetheless, both radiation-induced and chemotherapy-induced oral mucositis share similar components, including targeting the basal epithelial layer. Once apoptosis or necrosis affects the basal cells, epithelial renewal ceases, and ulceration ensues. (A) In the initiation phase, direct cell injury in the form of radiation-induced or chemotherapy-induced double strand breaks accounts for approximately 30% of basal cell injury. Most injury, however, is initiated by the production of ROS and activation of the innate immune response. These 3 events, which occur simultaneously, are interactive in initiating the biological cascade that results in apoptosis and necrosis of basal stem cells. Cells destroyed by the direct effects of cytotoxic agents produce DAMPs. ROS resulting from the effects of agents on cellular water are highly biologically active. (B) In the up-regulation and activation phase, both ROS and DAMPs activate key transcription factors, such as NF-κB, and DAMPs activate transcription factors by binding to pattern-recognition receptors, such as TLR-4. Consequently, genes are activated and expressed, resulting in the production of proinflammatory cytokines and signaling molecules. At the same time, the ceramide pathway is activated after lipoperoxidation of the cell membrane. Damaging enzymes (matrix metalloproteinases) affect the connective tissue. Naturally occurring defense mechanisms, such as the control of oxidative stress by antioxidant enzymes, are overwhelmed. (C) In the signal-amplification phase, as the process snowballs, the number of signaling molecules (including cytokines) increases, providing positive feedback to amplify the effects. Increased epithelial permeability resulting from tight junction breaks provides a conduit for surface bacteria cell wall products (PAMPs) to fuel the biological fire. (D) In the ulceration phase, with no replenishment, the epithelium becomes atrophic and ultimately ulcerates. Colonizing bacteria continue to spew out PAMPs, damaging enzymes continue to affect the connective tissue, and compositional changes in the cellular infiltrate are noted. (E) In the healing phase, once the cytotoxic challenge is completed (no additional radiation or chemotherapy), spontaneous healing occurs, with messaging from the submucosa stimulating epithelial proliferation and guiding differentiation.

The molecular events that characterize the initiation phase happen almost immediately after patients receive chemotherapy or RT. Although there is biological havoc within the tissue, clinically, the mucosa seems unaffected. This finding speaks to the need to initiate steps to attenuate the risk of mucositis before and at the time of treatment.

The initiation stage occurs throughout the cellular and tissue compartments of the mucosa and submucosa. This stage is characterized by direct damage to DNA, oxidative stress responses mediated by reactive oxygen species (ROS), and activation of the innate immune response:37, 38 Both direct and indirect damage to cellular DNA precipitates DSBs, leading to apoptotic and necrotic tissue changes. These degeneration products (damage-associated molecular pattern molecules [DAMPs]), such as alarmins, bind to pattern-recognition receptors, such as Toll-like receptors (TLRs) (ie, TLR-4), to escalate injury by activating the innate immune response. Simultaneously, water in cells undergoes hydrolysis to generate damaging superoxides and hydrogen peroxide. In the early stages of cellular stress, internal defense mechanisms, such as the deoxidizing enzymes superoxide dismutases, are ramped up. This happens either through activation of transcription factors, such as nuclear factor κ-B (NF-κB) and nuclear factor erythroid 2-related factor 2 (Nrf2), or through intracellular superoxide. However, these are quickly overwhelmed by the aggressiveness of the regimen-related challenge.

During the second stage, ROS and the innate immune response further activate several transcriptional factors, including NF-κB, Wnt, p53, and their associated canonical pathways. The NF-κB pathway is one of the most studied. Once activated, NF-κB–mediated gene expression results in increased production of proinflammatory cytokines (such as tumor necrosis factor α [TNF-α] and interleukin [IL]-1β, IL-4, IL-6, and IL-18) and cytokine modulators, stress responders (such as cyclooxygenase 2 [COX-2]), and cell adhesion molecules, which, in turn, can lead to cell apoptosis. The TLR signaling pathway and various kinase pathways (such as mitogen-activated protein kinase [MAPK]) are playing a role in this process. Cross-talk between all of the active elements is robust.

Other identified pathways that may lead to apoptosis include the ceramide pathway, which affects the cell membrane. DNA DSBs can directly activate ceramide synthase, with the consequent generation of ceramide. In addition, ROS lead to lipid peroxidation, sphingomyelinase activation, and the hydrolysis of membrane sphingomyelin to yield ceramide. Although ceramide is considered a proapoptotic molecule, its accumulation is a signal for increased membrane permeability and ultimately break of the epithelial cells.

During the signal-amplification phase, the molecules induced by this primary response further alter local tissue response by amplifying NF-κB and other pathways through feedback mechanisms. For example, released TNF-α sustains NF-κB activity and, at the same time, initiates the activation of MAPK signaling. The early breakdown of cells spills additional DAMPs into the tissue, which, in turn, increases NF-κB. The DAMPs also drive the innate immune response. Furthermore, while the intercellular bridges break, the epithelial cells start to spread apart, and the bacteria penetrate from the oral cavity into the tissue. The immune system fights these bacteria and, in turn, generates pathogen-associated molecular pattern (PAMPs) molecules, which induce additional NF-κB activity. The end result is a vicious cycle that amplifies the damage. Furthermore, while the RT or chemotherapy continues, the influx of ROS endures and constantly feeds the development of proinflammatory cytokines. Simultaneously, cytotoxic therapy-induced mucosal damage occurs through connective tissue fibrinolysis and the stimulation of tissue-damaging matrix metalloproteinases, which damage the extracellular matrix. On the epithelial surface, the oral flora undergoes a shift toward more pathogenic bacteria.

Cumulatively, these early biological events lead to progressive tissue injury and loss of epithelium continuity, which manifest clinically as ulceration and atrophic changes. The ulcerative stage is the most clinically significant because patients develop symptomatic, deep ulcers that are susceptible to infection. Oral bacteria colonizing the ulcers play a role in extending mucosal damage and increasing its severity by direct stimulation of infiltrating macrophages, neutrophils, and lymphocytes to secrete additional proinflammatory cytokines.39 This harmful process continuously increases, leading to a storm of ROS, PAMPs, DAMPs, and proinflammatory cytokines.

Finally, at the end of the RT or chemotherapy, spontaneous healing of the ulcers occurs. Once the trigger for this process is held, the redox equilibrium starts the shift back, and the process gradually becomes self-contained. Stimulated by signaling molecules from the extracellular matrix, epithelial migration, proliferation, and differentiation occur, and local microbial flora is reestablished. This final stage leads to renewal of the mucosa, restoring its continuity; however, the genetics of the new epithelium differ from the original epithelium. This may result in a lower threshold for subsequent cycles of RT or chemotherapy.