Radiation-Induced Oral Mucositis

Radiation-induced oral mucositis (RIOM) is a major dose-limiting toxicity in head and neck cancer patients. It is a normal tissue injury caused by radiation/radiotherapy (RT), which has marked adverse effects on patient quality of life and cancer therapy continuity. It is a challenge for radiation oncologists since it leads to cancer therapy interruption, poor local tumor control, and changes in dose fractionation. RIOM occurs in 100% of altered fractionation radiotherapy head and neck cancer patients. In the United Sates, its economic cost was estimated to reach 17,000.00 USD per patient with head and neck cancers. This review will discuss RIOM definition, epidemiology, impact and side effects, pathogenesis, scoring scales, diagnosis, differential diagnosis, prevention, and treatment.


DeFiNiTiON
Radiation-induced oral mucositis (RIOM) (Figures 1, 4 and 5C) is one of the major ionizing radiation toxicities and normal tissue injuries that result from radiotherapy (1). RIOM was first termed in 1980 as a side effect of radiotherapy (RT) in cancer patients (2). RIOM is a normal tissue injury lasting between 7 and 98 days, which starts as an acute inflammation of oral mucosa, tongue, and pharynx after RT exposure (1,3). This coincides with recruitment of various inflammatory cells and release of inflammatory cytokines, chemotactic mediators, and growth factors. RIOM can progress to an acute life-threatening stage as a result of severe physical obstruction of food and water intake with subsequent weight loss and septic complication due to lost protective epithelial and basement membrane barriers. This leads to limitations of local tumor control due to cancer treatment interruption and alterations in radiation dose fractionation (4)(5)(6)(7). Studies suggested the stages of progression of RIOM as initial hyperemia and erythema during the preulcer phase, during which there is a release of various pro-inflammatory cytokines from epithelial, vascular, and connective tissue cells at the site of tissue injury. This is followed by the epithelial phase with various degrees of desquamation and basement membrane damage with loss of the protective barrier, which ends with the physical appearance of the ulceration. The postulcerative phase varies depending on the extent of the tissue toxicity. A secondary infection with Gram-negative bacteria or yeast may occur, with microcoagulation of the vasculature that worsens the inflammation by the local ischemia with more necrotic tissue yield. The final stage will be the healing phase and fibrosis (2,8,9).

RiOM ePiDeMiOLOGY (iNCiDeNCe, PReDiCTORS, AND RiSK FACTORS)
Radiation-induced oral mucositis occurs in up to 80% of head and neck cancer irradiated patients and reaches up to 100% in patients with altered fractionation head and neck cancer. RIOM of grade 3 and 4 have been recorded in 56% of head and neck cancer patients treated with radiotherapy (1,12).    (15).

Age
Increased risk in very young age (high cell turnover rate) and old age (slower healing rate) Gender Trends to increased risk in females Oral health and hygiene Maintaining good oral hygiene and oral health lowers radiation-induced oral mucositis (RIOM) risk Salivary secretory function Decreased saliva leads to increased RIOM risk

Genetic factors
Potential for high RIOM risk in certain individuals still to be identified Body mass index Delayed healing and increased breakdown in malnourished individuals Renal function Increased mucotoxicity linked with high serum creatinine level (poor renal function) Smoking Delays the healing Previous cancer treatment History of mucositis due to previous cancer treatment increases the risk Many risk factors have been identified for RIOM. These risk factors include concomitant chemotherapy (CT), bad oral hygiene, below average nutritional stratus, lack of antibiotic use at early stage mucositis, and smoking (13). Table 1 shows the significant predictors for the prevalence of severe RIOM and the symptoms of RIOM in a longitudinal study of patients with oral cavity cancer among head and neck cancer outpatients of a radiation department at a major medical center in Taiwan (14). This study used the Generalized Estimating Equations to analyze the predictive factors of prevalence of severe RIOM and RIOM-related symptoms. They found that the significant predictors for the prevalence of severe RIOM included type of treatment [RT vs. concomitant chemoradiotherapy (CCRT)], cumulative radiation dose, smoking, and body mass index (BMI). Patients who received CCRT (Coef. 0.145, p < 0.05), who have a higher cumulative radiation dose (Coef. 0.000, p < 0.01), who are smokers (Coef. 0.090, p < 0.01), and who have lower BMI (Coef. 0.005, p < 0.05) were at high risk to develop severe RIOM. RIOMrelated symptoms were also predicted by type of treatment (RT vs. CCRT) (Coef. 1.618, p < 0.05), cumulative radiation dose (Coef. 0.003, p < 0.05), and smoking (Coef. 1.759, p < 0.001). These significant predictors are implemented by radiation oncologists to minimize and/or prevent the RIOM. June Eilers and Rita Million have summarized the patient-linked factors leading to increased risk for RIOM ( Table 2) (15). They found that very young age, female gender, poor oral health and hygiene, decreased saliva secretion, low BMI, poor renal function with elevated serum creatinine level, smoking, and history of RIOM are risk factors predicting the development of RIOM in head and neck cancer patients (15).

RiOM iMPACT AND SiDe eFFeCTS
Radiation-induced oral mucositis side effects and sequels include oral pain in 69% of patients, dysphagia in 56% of patients, opioid use in 53% of patients, weight loss of 3-7 kg, feeding tube

PATHOGeNeSiS AND SuGGeSTeD MeCHANiSTiC PATHwAYS
The pathophysiology of RIOM is not fully understood. Recent studies proposed that the pathogenesis of RIOM is composed of four phases: an initial inflammatory/vascular phase, an epithelial phase, a (pseudomembranous) ulcerative/bacteriological phase, and a healing phase (2,5).
At the inflammatory phase, the RT-induced tissue injury results in the release of inflammatory cytokines; e.g., interleukin (IL)-1β, prostaglandins (PGs), and tumor necrosis factor-α (TNF-α) from the resident cells such as epithelial, endovascular, and connective tissue. These mediators might increase the damage by increasing the vascular permeability, leading to more infiltration and recruitment of inflammatory cells. Stem cells travel to the site of the tissue injury with other innate immunity components, e.g., MPO-positive leukocytes, macrophages, and neutrophils (18). On the other hand, there are some anti-inflammatory cytokines, such as IL-10 and IL-11, that work to minimize the injury (18).
The epithelial phase is initiated within a week by the apoptotic and cytotoxic effects of RT on the proliferating basal cells. This is why the recovery period is dependent on the rate of epithelial turnover, which could be enhanced by growth factors like epidermal growth factor and keratinocyte growth factor (KGF) (19).
After a week, the epithelial breakdown ends with the beginning of the ulceration. This occurs, when epithelial loss leads to disrupted basement membrane, formation of ulcer pseudomembrane, and inflammatory exudate. The ulceration stage is very painful, since the protective barrier that covers the nerve endings at the lamina propria is lost (19). The resulting microcoagulation and neutropenic state facilitate the Gram-negative bacteria and yeast colonization with the production of secondary infection. Bacterial exotoxins aggravate the inflammatory reaction by inducing mononuclear burst with the release of more IL-1β, TNF-α, and nitric oxide (8,9,16,20).
In 2009, Redding summarized Sonis' RIOM pathobiology phases (Figure 2). The initiation phase with RT and/or CT injury results in direct and lethal DNA damage with the release of reactive oxygen species (ROS) from epithelial and vascular endothelial cells, fibroblasts, and tissue macrophages. This is followed by amplification of this signal (11). During the primary damage response, the DNA damage and ROS act through three major pathways: (1) fibronectin breakdown, which stimulates the macrophages leading to activation of the matrix metalloproteinases (MMPs), (2) nuclear factor-κB (NF-κB) activation, which stimulates the gene expression and the release of proinflammatory cytokines, e.g., TNF-α, IL-1β, and IL-6, and (3) ceramide pathway through sphingomyelinase and ceramide synthase. The end result will be more tissue injury and stimulated apoptosis (11). During the signal amplification phase, there is restimulation of tissue damage and apoptosis by the major pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6), NF-κBmediated gene expression, and ceramide and caspase pathways. The basement membrane protective barrier is lost during the ulceration phase. This leads to Gram-negative and yeast secondary infection potential, which adds more pro-inflammatory reactions and complicates the already existing inflammation. The healing phase starts by matrix signaling to basal epithelial cells to migrate, proliferate, and differentiate (11). Signal amplification during RIOM or CT-induced OM is a main step in this treatment-induced injury, according to Sonis et al. (17). RT and/or CT activate the transcription factor NF-κB in epithelial, endothelial, and mesenchymal cells and macrophages, resulting in upregulation of genes and production of pro-inflammatory FiGuRe 2 | Redding's summary of RT and/or chemotherapy (CT)-induced oral mucositis pathobiology (11). Redding has summarized the pathobiology phases of radiation-induced oral mucositis induced by RT and/or CT. In brief, initiation phase with RT and/or CT results in direct and lethal DNA damage, which leads to release of reactive oxygen species (ROS) from epithelial, vascular endothelial, fibroblasts, and tissue macrophages with cycles of amplifications. Within such primary damage response, the DNA damage and ROS lead to three major steps: (1) fibronectin breakdown that activates macrophages ending with stimulation of matrix metalloproteinase; (2) nuclear factor-κB (NF-κB) activation that stimulates the gene expression and release of pro-inflammatory cytokines, e.g., TNF-α, interleukin (IL)-1β, and IL-6; and (3) ceramide pathway through sphingomyelinase and ceramide synthase. The result will be more tissue injury and stimulated apoptosis. During the signal amplification phase, there is restimulation of tissue damage and apoptosis by the major pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6), NF-κB-mediated gene expression, and ceramide and caspase pathways. During the ulceration and loss of the protective barrier, secondary infection adds more pro-inflammatory reactions and complicates the already existing inflammation before the healing phase starts by matrix signaling to basal epithelial cells to migrate, proliferate, and differentiate. Republished with the permission of Dr. Redding. (A) Initial phase, (b) primary damage phase, (C) signal amplification phase, and (D) ulcerative phase. cytokines: tumor necrosis factor-α (TNF-α) and IL-1β, which amplify the primary signal and activate NF-κB. This leads to transcription of genes responsible for MAPK, cyclooxegenase-2 (COX-2), and tyrosine kinase signaling molecules. These signaling pathways activate MMPs 1 and 3 in the epithelial and lamina propria cells, which collectively cause tissue injury (10) (Figure 3).

RiOM GRADiNG AND SCORiNG SCALeS
There has been more than one grading scale for RIOM. Table 4 shows the comparison of different RIOM scoring scales (14,(21)(22)(23). World Health Organization Oral Toxicity Scale measures the anatomical, symptomatic, and functional elements of OM (Figure 4). The Radiation Therapy Oncology Group (RTOG) determined the acute radiation morbidity scoring criteria for mucous membranes. Finally, the Western Consortium for Cancer Nursing Research describes only the anatomical changes associated with OM (24). Radiation Therapy Oncology Group developed the Acute Radiation Morbidity Scoring Criteria for the evaluation of RT effects (another criterion was generated for late effects of RT) (25). The National Cancer Institute (NCI) Common Toxicity Criteria (NCI-CTC) scores CT-related side effects. The RTOG was gathered with the NCI-CTC to produce version 2.0, which has been used in all NCI clinical trials since March 1998 (Table 5) (2,25,26).
The OM Index (OMI) scores the severity of OM by the erythema, ulceration, atrophy, and edema (a scale of 0-3 was designated for each element: 0 = none and 3 = severe). The OMI is considered internally consistent with high test-retest and interscorer reliability, and it shows solid validity (27).
The OM Assessment Scale (OMAS) is a highly reproducible scoring scale for RIOM, responsive over time, and accurate in detecting OM-associated elements (25). OMAS records the objective assessment of OM depending on the scoring of the presence and size of the ulceration or pseudomembrane (score 0-3: 0 = no lesion; 1 = lesion of <1 cm 2 ; 2 = lesion of 1-3 cm 2 ; FiGuRe 3 | Signal amplification during OM induced by RT and/or CT (10). Signal amplification during RT-and/or CT-induced OM is mediated by activation of NF-κB that is reactivated by IL-1β. NF-κB induces the expression of genes responsible for the MAPK, COX-2, and tyrosine kinase pathways to finally activate the MMP1 and MMP3 signaling at the injured tissue cells. TNF-α, tumor necrosis factor-α; IL-1β, interleukin-1β; NF-κB, nuclear factor-κB; MAPK, mitogen-activated protein kinase; COX-2, cyclooxegenase-2; MMP1, matrix metalloproteinase 1; MMP3, matrix metalloproteinase 3; OM, oral mucositis; CT, chemotherapy. Republished with the permission of Dr. Sonis. 3 = lesion >3 cm 2 ) and erythema (score 0-2: 0 = none; 1 = not severe; 2 = severe) on the upper and lower lips, right and left cheeks, right and left ventral and lateral tongue, floor of the mouth, soft palate, and hard palate (23,28). All these scoring scales are validated and are required in assessing RIOM and the therapeutic benefits of any new treatment of RIOM.

DiAGNOSiS OF RiOM
Radiation-induced oral mucositis can develop within or after 2 weeks from the beginning of RT. Oral assessment guide could be a useful tool for detection of early OM ( Table 6) (20). Apart from the early clinical signs and symptoms, CBC with differential is considered the baseline to help radiation oncologists to determine the most susceptible time for developing OM or oral infection. Radiation oncologists can start the RT provided that there is no evidence of any periodontal disease. If at any point RIOM develops, oral lesion culture and antimicrobial therapy are recommended as soon as possible (29). Since renal diseases are considered contributing factors for OM (15), chemistry levels should be regularly monitored by the treating physician (29).

DiFFeReNTiAL DiAGNOSiS OF RiOM
Because similar conditions can coexist in immunocompromised patients including cancer patients receiving RT and/or CT, differential diagnosis for RIOM is critical. Table 7 shows possible similar conditions ( Figure 5) (20,31).

PROGNOSiS OF RiOM
The general long-term prognosis is reasonably good since most lesions resolve within 2-4 weeks after stopping the RT or CT. Although RIOM is considered a self-limited injury in some patients, it could be a lethal injury in moderately to severely ill patients, which could lead to ICU admission with obligatory cessation of RT. Patient losses are a common event under these circumstances (32).

PReveNTiON OF RiOM
Maintaining good oral care is the main preventive measure for RIOM to minimize the risk for candidiasis or secondary bacterial infection, especially in hyperfractionated radiotherapy, combined CCRT regimens, or RT combined with a targeted agent due to increased mucositis severity (3). We will summarize the most recent agents and measures to prevent RIOM.

Good oral hygiene
Good oral hygiene has been found to be one of the most effective ways to lower the risk of RIOM and minimize its progression. Pre-existing oral pathology, e.g., dental caries, periodontal lesions, pulpal disease, and oral xerostomia, has been linked with increased bacteria colonization and severe RIOM. It is recommended to do early oral examination before starting any mucosal toxic therapy for cancer patients. To help minimize the oral side effects of antineoplastic therapy, it is recommended to eliminate any oral pathology before the beginning of RT. This may be accomplished by performing early histological, cytological, microbiologic, and serologic examinations (2). The Multinational Association of Supportive Care in Cancer (MASCC) and the International Society of Oral Oncology (ISOO) guidelines  recommend the use of a standardized oral care protocol, e.g. brushing with a soft toothbrush, flossing and the use of non-medicated rinses (saline or sodium bicarbonate rinses) ( Table 9) (33-36). The good oral care can be summarized as follows: -Rinsing with a non-irritating solution, e.g., saline to increase the quality of saliva. -Daily ultrasoft tooth brushing with fluoride toothpaste.
-Scaling and cleaning.
-Very soft diet with low sugar and non-acidic food and drinks ( Table 8). -Flossing is not recommended due to low platelet count.
-No smoking or alcohol.
-Other preventive procedures include minimizing the microbial load (will be discussed more in the treatment section) and educating the patient on good oral hygiene, which is mandatory.
2. Cryotherapy has been recommended for CT-induced OM, but no proven role in RIOM due to insufficient evidence (33). 3. Keratinocyte growth factor is an epithelial mitogen that reduces the levels of ROS by activating nuclear factor (erythroidderived 2)-like 2 and had been used in RIOM with promising results (37-53). It appears to be one of the promising treatment and prevention options for RIOM that has been investigated in clinical trials (39, 43). Palifermin (IV recombinant human  KGF-1) had been approved by the US-FDA for minimizing OM in hematologic malignancies' patients who receive myelotoxic therapies and require hematopoietic cell support after its reliable results in alleviating WHO grade 3 and 4 OM in these patients. Palifermin is delivered IV 3 days before of CT/RT and for 3 days after CT. Palifermin should be avoided on the same day of CT/RT (33). 4. Amifostine is a free-radical scavenger, antioxidant, and cytoprotective agent that was administered subcutaneously (SC) and IV in many clinical trials for RIOM. Amifostine is conventionally given IV before RT or CT. It is approved by the US-FDA to reduce the cumulative renal toxicity associated with repeated administration of cisplatin in patients with advanced ovarian cancer. In addition, it was approved for by the US-FDA to reduce the incidence of moderate to severe xerostomia in patients undergoing postoperative RT for head and neck cancer, where the radiation port includes a substantial portion of the parotid glands (33, 54-62). Although there was a reduction in the pro-inflammatory cytokine production, its side effects, e.g., hypotension and nausea, were recorded, particularly with IV route. Nevertheless, SC injection 60 min before RT in head and neck cancer patients showed marked reduction of these side effects, unfortunately, with reduced efficacy and patient compliance. Only cutaneous toxicity was noted in SC route of amifostine delivery (54, 55). For moderate to severe RT-induced xerostomia in head and neck cancer patients, the recommended dose of amifostine is 200 mg/m 2 once daily over 3 min IV, starting 15-30 min before standard fraction RT (1.8-2.0 Gy). Blood pressure should be monitored before, during, and after the IV infusion. Oral 5-HT3 receptor antagonists with/without other antiemetics are recommended before amifostine therapy (63-65). 5. Radiation shields (intraoral devices), midline mucosa-sparing blocks, 3-D and RT field design, intensity-modulated radiation therapy (IMRT), and removal of separable prosthetics are shown in preclinical studies to reduce the radiation scatter and the RIOM injury (66-69). 6. Low-energy helium-neon laser applied before RT showed significant reduction in the duration and the severity of RIOM in head and neck cancer patients (70). MASCC/ISOO guidelines suggest the use of low-level laser therapy in CT-induced OM at centers that can provide the necessary technology and training (33).

SYMPTOMATiC TReATMeNT OF RiOM
No single agent has been approved by the US-FDA for the treatment of RIOM. Symptoms reduction and complications prevention of RIOM, including nutritional support, pain control, prophylaxis, and/or treatment of secondary infections, are considered the main cornerstone in the management of RIOM (34-36). Agents that were investigated and/or applied in RIOM treatment are discussed in the context of recently updated evidence-based preclinical and clinical studies.
I Locally applied agents 1. Glycyrrhetinic acid/povidone/sodium hyaluronate gel has mechanical action implemented in the relief of pain in RIOM. It adheres to the mucosal surface of the mouth, soothing oral lesions. Nevertheless, the preclinical studies  are controversial, and only one clinical trial on unknown results was conducted to date (71). 2. l-Glutamine is a non-essential amino acid that counteracts RT-induced metabolic deficiencies (72). Locally applied l-glutamine reduced the RIOM in a randomized clinical trial (73). Glutamine powder for oral suspension was approved by the US-FDA for topical application in management of CT-induced OM, mainly IOMyet (74). 3. Manganese superoxide dismutase is a detoxifying agent that removes ROS. It was shown to have radioprotective effects against RT-induced colitis, esophagitis, hepatic cells apoptosis, and intestinal and eye injury . Phase I dose escalation study of GC4419 (manganese-containing macrocyclic ligand complex similar to naturally occurring superoxide dismutase enzymes) in combination with CT/RT for squamous cell cancer of the head and neck has just been completed waiting for results release (NCT01921426). 4. Local anesthetics, e.g., diphenhydramine, viscous xylocaine, lidocaine, and dyclonine hydrochloride, are used for short-term relief of pain associated with RIOM, despite the fact that they can interfere with the taste sensation leading to hypoalimentation (99,100). Swishing and gargling the anesthetic viscous gel containing 2%   lidocaine and holding 5 mL of it in mouth for 1 min then spitting it out before meals have been shown to be helpful for better alimentation (101). One clinical trial showed dyclonine hydrochloride to have a superior effect among all other agents without significant difference recorded (102). The most effective anesthetic agent is still to be determined. Benzocaine gel is another locally applied bioadhesive agent containing benzyl alcohol (10%) and is used to relieve pain and facilitate eating and drinking in mild and moderate RIOM (103). Benzocaine-containing lozenges are diluted to alleviate the pain sensation and mechanical sensitivity in mild to moderate OM (104,105). The "magic mouthwash" (lidocaine, diphenhydramine, magnesium aluminum hydroxide) and morphine mouth washes are preferable and have been reported by patients to be effective in alleviating pain in RIOM (106-108). 5. The application of corticosteroids mouthwashes has shown promising results. The limited availability of a large-scale data is a gap that should be bridged through relevant clinical studies (109). 6. Allopurinol and uridine were shown to be effective in reducing 5-fluorouracil oral toxicity in preclinical studies (110)(111)(112)(113)(114). Despite these results, they were ineffective approaches in randomized clinical trials as a therapy to reduce the treatment-related oral toxicity (115,116). 7. Chlorhexidine is a bisguanidine exhibiting broadspectrum antibacterial and antimycotic activities. The clinical trials done with chlorhexidine concluded that it cannot be recommended for the prophylaxis or the treatment of RIOM (117)(118)(119)(120). Alcohol-containing chlorhexidine mouth rinse should be avoided during clinical oral ulceration. Therefore, the MASCC/ISOO guidelines recommend against the use of chlorhexidine mouth rinse for prevention or treatment OM (33). 8. Artificial saliva spray is an over-the-counter agent frequently used to alleviate mucosal dryness in mild cases of RIOM (121). 9. Chamomile has anti-inflammatory, antipeptic, antispasmodic, and antibacterial effects. It was investigated with encouraging results as an emulsion therapy for CT-induced mucositis (122)(123)(124)(125)(126). Studies are needed for its application in RIOM to determine its efficacy. 10. Honey has been investigated in many preclinical studies due to its mucosal protective effect that was confirmed as a reduction in the incidence and severity of RIOM (127)(128)(129)(130)(131)(132). However, the available clinical trial used only Manuka honey, and it appears to contradict the preclinical studies' results (133). More studies are needed to confirm the therapeutic potential of honey in RIOM. 11. Sucralfate is a basic aluminum salt of sucrose sulfate that was used as mouthwash to reduce the intensity of RIOM and CCRT-induced mucositis as well (39, 100,[134][135][136][137][138][139][140][141][142][143][144][145][146][147][148][149][150][151]. Despite its long application history, it is considered to have little effect in RIOM when compared to oral hygiene and symptomatic mucositis therapy (2). MASCC/ISOO Mucositis Guidelines did not find enough evidence for the beneficial application of sucralfate in OM (34). 12. Vitamin A and its derivatives have anti-inflammatory and epithelial proliferative effect (152). Topical tretinoin has been shown to reduce the oral complications during bone marrow (BM) transplantation (153). 13. Vitamin-E (tocopherol) has been shown to lower the oxidative damage of the oral mucosa and reduce the incidence of symptomatic RIOM in head and neck cancer patients in a randomized double-blind clinical trial (152,154). 14. Sodium alginate was shown to reduce the discomfort and the severity of RIOM in a randomized clinical trial (155). 15. Benzydamine hydrochloride is a non-steroidal antimicrobial, anti-inflammatory, anesthetic, and analgesic agent that reduces pro-inflammatory cytokine production, scavenges the ROS, and acts as membrane stabilization and as an antimicrobial agent (118,156,157). When compared with chlorhexidine, patients with RIOM treated with benzydamine hydrochloride found more discomfort (118). Benzydamine hydrochloride failed to be approved by the US-FDA for OM management.
Because of a negative interim analysis, a recent phase III trial for benzydamine hydrochloride therapy in RIOM was stopped (33). 16. Povidone-iodine is an antiviral, antibacterial, and antifungal agent. Randomized clinical study showed that povidone-iodine reduces the incidence, severity, and duration of CCRT-induced OM, in addition to its advantages of being cheap and easily applied (118,(158)(159)(160). 17. Capsaicin is an inhibitor of neutrophils that reduces the pain sensation. One clinical trial showed that orally applied capsaicin caused temporary relief of pain in mucositis caused by RT and CT (161). However, more studies are needed for optimization of its analgesic effect.
II Systemically applied agents 1. Cyclooxegenase-2 inhibitors that have different mechanisms of action were applied in the management of RIOM. They suppress NF-κB, reduce pro-inflammatorycytokine production, and inhibit angiogenesis (162)(163)(164). A randomized placebo-controlled trial showed that prophylactic systemic administration of indomethacin, a COX-2 inhibitor, significantly lowered the severity and delayed the onset of RIOM (165). In addition, PG E1 and E2 showed improvement in OM induced by RT and CT in few studies; however, their application is still controversial (166-172). 2. N-acetylcysteine is an antioxidant that has been shown to suppress NF-κB activation (173,174). Because of its proven radioprotective role in RT-induced dermatitis, bone injury, liver toxicity, and intestinal injury (173)(174)(175)(176)(177)(178)(179)(180)(181)(182)(183)(184)(185)(186)(187)(188), N-acetylcysteine was recommended as a candidate for a trial in RIOM. In a placebo-controlled phase II trial of patients with head and neck cancer, N-acetylcysteine significantly reduced the severity of RIOM (33). 3. Colony-stimulating factor and granulocyte-macrophage colony-stimulating factor (GM-CSF) systemic therapy recruit neutrophils to the tissue injury site (189). Local application of GM-CSF mouthwash was shown marked alleviation of RIOM in several studies (2). Nevertheless, in clinical trials, its systemic application therapeutic value appears controversial (190,191). SC GM-CSF reduced the severity of OM in patients treated with accelerated RT (192). In another randomized clinical study, systemic GM-CSF reduced the incidence of RIOM; however,   another study did not show the same result (140,193). Systemic GM-CSF therapeutic potential is still controversial and requires further investigation. 4. Transforming growth factor-β3 inhibits the oral basal cell proliferation. It was shown to reduce the incidence of CT-induced mucositis (194). However, a reliable clinical trial is needed to assess its therapeutic potential with RT. 5. Beta-carotene's antioxidative effect (195,196) was implemented in a randomized clinical trial where there was a significant reduction in the incidence of severe OM in CCRT (197). 6. Analgesics are strong candidates for alleviating the pain related to RIOM. A retrospective study showed that opioid therapy remains a corner stone for OM pain management in CCRT, as suggested by the MASCC/ ISOO guidelines (33, 198). 7. Azelastine is a potent second-generation selective histamine antagonist that is used as an anti-inflammatory and antioxidant agent. One clinical trial showed significant reduction in the incidence and the severity of OM with CCRT (199). 8. Propantheline is an anticholinergic agent that reduces the salivary flow. One clinical trial showed that propantheline and oral cryotherapy may be feasible and effective in reducing mucosal toxicity in cancer patients receiving high-dose CT (200). However, studies are needed for RIOM. 9. Immunoglobulins have lower salivary and systemic levels in patient receiving antineoplastic therapy. They have immune-modulating and anti-inflammatory properties. Intravenous or intramuscular immunoglobulins are frequently applied as prophylactic and therapeutic options for RIOM (158,201). 10. Systemic corticosteroids were used in RIOM management. A double-blind placebo-controlled randomized trial has shown a tendency toward reduced RT interruption in prednisone-treated relative to placebo-treated patient groups without evidence of reduced RIOM incidence or severity (202). 11. Pentoxifylline regulates endotoxin-induced production of TNF-α. Although the preclinical studies showed significant reduction in the severity of RIOM with pentoxifylline (203), the clinical trials show that it is not effective in reducing the antineoplastic oral toxicity (204)(205)(206)(207)(208). 12. Salicylic acid derivatives should be avoided due to the increased risk for bleeding (34-36). 13. Sphingomyelinase and ceramide synthase inhibitors can be a potential candidate for RIOM. They inhibit the ceramide pathway-mediated RT-induced apoptosis (209)(210)(211)(212)(213)(214)(215)(216). No current clinical trials have been started for them yet.
III Oral microbial load reduction agents 1. Antimicrobial agents showed beneficial effect in prophylaxis and reduction of the severity of RIOM. RT injury leads to a change in the mucosal membrane barrier, salivary flow, and composition which favor the growth and colonization of different bacterial species, mainly Gram-negative bacteria. Many preclinical studies have investigated the therapeutic effect of different antimicrobial agents in RIOM (217)(218)(219)(220). The FDA has granted fast track designation for brilacidin-OM, an oral rinse formulation of defensin-mimetic brilacidin (221)(222)(223), for the prevention of OM. There is a current phase II clinical trial to evaluate the safety and efficacy of brilacidin oral rinse in patients with head and neck cancer (NCT02324335). 2. Fungal infections are not involved directly in the development in RIOM, rather they can complicate the situation, especially in immunocompromised patients, and that is why the use of antifungal agents have been applied in RIOM treatment. A clinical study has shown that systemic fluconazole prophylaxis caused a significant beneficial effect on the severity of OM and on radiotherapy interruptions (224). The same effect was noted in randomized clinical trials investigating clotrimazole (2). Some oral mouthwashes containing amphotericin B have shown similar effects; however, due to carrier allergy, there might be a limitation in its application (225).

Antibacterial agents have been investigated in mucositis
depending on a hypothesis stating that aerobic species (e.g., Pseudomonas spp. and Staphylococcus epidermidis), anaerobic bacteria (e.g., Bacteroides spp., and Veillonella spp.), and endotoxin of aerobic Gram-negative bacilli are considered a main contributor in the development of the secondary infection phase in RIOM (2,226). Antibiotic lozenges with polymyxin-E and tobramycin have protected against severe mucositis when compared to placebo or chlorhexidine (227). In addition, ciprofloxacin-and ampicillin-containing mouthwashes showed similar effect (228,229). 4. Antiviral agents against herpes simplex virus (HSV) type I and varicella zoster virus (VZV) were applied topically and systematically. HSV and VZV are the most common viral infections that aggravate RIOM in seropositive and myelo-suppressed patients (230)(231)(232). Systemic and topical acyclovir was investigated and applied in RIOM management and caused a reduction in the oral herpetic infections without an evident prophylactic role against OM itself (233)(234)(235)(236)(237)(238).

CeLLuLAR THeRAPieS FOR RiOM
Bone marrow-derived mesenchymal stromal cells (bmMSCs) therapy have been applied in fractionated radiation-induced OM where the administration of a systemic single dose of six million MSCs resulted in a significant decrease in ED50 (the RT dose that produces ulcer in 50% of irradiated mice) (239). The first MSCs therapy for RIOM was done in 2014 by Schmidt et al. (239). They concluded that transplantation of BM or bmMSCs could modulate RIOM in fractionated RT, depending on the time of transplantation (239). Nevertheless, in another study, the authors concluded that bmMSCs transplantation had no therapeutic benefits on RIOM in single-dose RT when compared to the therapeutic effect of mobilization of endogenous BM stem cells (240). More studies are needed in this field building on the initial studies, which showed significant and clinically relevant therapeutic gain of MSCs therapy for RIOM (Table 10).