In this study, the therapeutic effects of PBM for radiochemotherapy-induced OM and its clinical implications to nutritional status in patients with HNC were evaluated. This study is innovative because it highlights the role of PBM in the improvement of nutritional impacts caused by OM.
Male individuals, with an average of 40 years or more, low education and income levels, with alcohol drinking habits and/or smokers characterize the population of patients diagnosed with HNC [19–23]. Regarding skin color, the findings of this study differ from literature data, which demonstrates prevalence in white individuals [24, 25]. However, according to the Brazilian Institute of Geography and Statistics (IBGE) (2019) , the territorial contingent of the Bahian population is predominantly marked by African descendants, which would justify the large percentage of brown and black participants.
Gonçalves et al. (2018)  demonstrated, similarly to our study, a predominance of tumor location in the oropharynx and stated that tumors in this region are often diagnosed late and in advanced stages, due to difficulties in the examination and scarcity or non-perception of signs and symptoms. Casati et al. (2012)  also stated that the primary locations most commonly diagnosed for HNC are the oral cavity, oropharynx and larynx, as they are structures in direct contact with the risk factors of tobacco and alcohol. The involvement of these anatomical structures culminates in a high functional impairment, thus promoting greater sequelae to the nutritional and health status of this population.
Weight loss is often an unavoidable condition for the cancer population . For patients with HNC, this loss happens very quickly, affecting 80% of diagnosed patients. Invariably, one of the main outcomes of this condition is malnutrition . Advanced stages and combined therapeutic regimens are more likely to have negative nutritional impacts, in particular, the reduction or total incapacity of oral food intake. In addition, these large are tumors that can cause obstructions in the oral feeding route and demand enhanced energy and metabolic supply for its growth . The combined therapy, acting locally and systemically, can induce or exacerbate potentially harmful side effects, such as OM, xerostomia, poor oral cavity condition, anatomical and functional mechanical obstructions, malabsorption, constipation, diarrhea, emesis, nausea, dysgeusia, dysphagia and pain . These conditions are listed as predisposing factors for weight loss and lead to worsening of nutritional status and prognosis, in addition to a decrease in quality of life [32, 33].
For individuals who were affected by OM it was noticed that there was a deficit in body weight and BMI. Using a similar methodology, Goobo et al. (2014)  demonstrated analogous results regarding the use of PBM in the prevention and treatment of OM, and its effects on body weight and BMI of patients with HNC, however, that the type of laser used by the authors has a higher power and a wavelength of 970 nm. According to the authors, the equipment was used in an unfocused manner, with the goal of obtaining biomodulatory effects, however, they did not indicate whether any device was used for this purpose, or what was the distance between the equipment and the oral mucosa. It is also important to emphasize that defocused high-power laser acts by decreasing the tissue vaporization effect, thus resulting in increased incision diameter and, thus, reduction its depth, a primary property of this category of lasers [34, 35]. MASCC/ISOO recommends the use of low-power laser for the prevention and treatment of OM, due to its therapeutic properties of positive biostimulation, anti-inflammatory, analgesia and healing, without causing harm or toxicity reactions in its use . The visible red light from the low-intensity laser is readily absorbed by endogenous chromophores. It acts by triggering biological reactions, which are not thermal nor cytotoxic, through photochemical, photophysical and photobiological events, leading the cell metabolism to beneficial physiological changes. The mechanism of action of PBM is related to its action on the cytochrome c oxidase (CcO) complex, acting in the facilitated transport of electrons in the mitochondrial respiratory chain. This transport provides an increase in the transmembrane proton concentration gradient, which will boost the production of adenosine triphosphate (ATP) whose bioavailability will be increased to promote the functions of cell metabolism . In this study, PBM was not able to prevent OM. There was, however, a reduction in the severity of the condition. The greater the gravity of mucositis, the greater the amplification of its symptoms and the greater the interference with the individual's ability to feed orally and with the nutritional status. According to Sonis (2007) , OM is an inflammatory reaction marked by the participation of cytokines with a pro-inflammatory profile. Thus, the greater the pro-inflammatory gradient concentration, the greater painful symptomatology and, consequently, the non-beneficial impacts on body weight and BMI. It is believed, that PBM reduced the severity of OM by decreasing the production of inflammatory cytokines, preventing the evolution to more advanced stages. Findings of severe SIWL for patients in the SHAM group indicate that they were more vulnerable to the effects of OM than individuals in the PBM group. The assessment of SIWL is an important parameter for understanding weight variations as a function of time, as it is possible to measure the severity of weight loss, identifying the risk and the various stages of malnutrition [8, 16].
The PBM protocol recommended to prevent OM is the use of a low-power laser with a wavelength of 660nm, energy density of 6.2J, in 72 application points, 10 seconds per point, during 03 days a week, alternately . Other studies [38–42] presented different PBM protocols, which ranged in energy density from 1J to 6J, with application points from 09 to 58 points, alternating to continuous and different conductive media. Previous data from our group , whose PBM protocol included low-power AsGaAl laser, 660nm, 86.7 mW, energy density of 2J, applied to 28 points in the oral cavity for 3 seconds at each point, was also unable to prevent the emergence of OM. However, as observed in this study, the severity of OM was reduced. These laser peculiarities make the studies difficult to be compared, not invalidating, however, the relevance of its findings.
The anthropometric measurements, mainly through assertive and quick methods such as measuring body weight, checking the variation of body weight and calculating the BMI, as well as its classification, is extremely important for the early diagnosis of the most frequent nutritional disorders in cancer patients. This quick identification can help in counseling and possible nutritional interventions, preventing nutritional damage, thus providing a better prognosis for these patients [13, 31].
Previous data [32, 33], showed a strong correlation between pre-treatment BMI and weight loss, however, the findings are conflicting as they show divergences in relation to the BMI classification. For the first study, it is stated that individuals classified as overweight, before starting antineoplastic therapy, are more likely to maintain or have less reduction in body weight during treatment, reducing possible interruptions, because their immune function is not compromised, when compared to individuals with low weight and obesity, and, therefore, overweight individuals tend to have better results for quality of life and future nutritional status. On the other hand, for Zhao et al. (2015) , overweight or pre-treatment obesity was a predisposing factor for weight loss, when compared to eutrophic individuals. This condition was justified by the tendency of professionals to prescribe nutritional supplements for low weight and eutrophic patients, aiming at the recovery or maintenance of body weight. This practice, however, is not performed with overweight and obese patients, making their weight loss more evident.
Our study had limitations related to the sample size, that can interfere the absence of statistical significance when evaluating the influence of PBM on the initial and final difference in body weight and BMI, according to the presence or absence of OM, as well as in the classification of BMI. It was observed, however, that the numerical difference in the group of patients with OM who underwent PBM was expressive, both for the difference in weight loss and for the difference in BMI, when compared to individuals in the control group. The treatment of cancer patients has some particularities, such as interruption due to worsening side effects, indisposition and death, which end up substantially reducing the sample size.