Childhood cancers represent less than 1% of the malignant diseases diagnosed worldwide. Treatment modalities for childhood malignancies will differ depending on the diagnosis. The main treatments may include chemotherapy (QT), radiation therapy (RT), and surgery. The toxicity caused by QT and RT generates adverse effects in the mouth, among them oral mucositis (MO). MO is damage that occurs to the oral mucosa and these lesions are extremely painful, uncomfortable, and cause great morbidity for patients. The main treatment of MO is intraoral photobiomodulation therapy (TBMI), this therapy has been widely used, reducing the severity and pain of MO in patients. The use of extraoral photobiomodulation therapy (TFBME) has been employed in some recent studies and has shown efficacy for reducing and improving the severity of OM. The present study aims to compare the effect of two protocols of photobiomodulation therapy (TFBM) using intraoral and extraoral low power laser in the prevention and treatment of OM in pediatric oncology patients receiving treatment with high dose metrotexate (MTX-HD) and its chemotherapy combinations. Initially, 34 pediatric and adolescent oncology patients receiving intravenous (IV) QT from risk protocols will be selected. Participants will be allocated into 2 groups: Group I: TFBMI (n=17) and Group II: TFBME (n=17). The study will be conducted by a dental surgeon who will perform the initial assessment and reassessments (blinded to the groups) and another who will apply the TFBM. Patients will be evaluated daily from day 1 (D1 - beginning of the chemotherapy cycle) until D10 or until healing of the lesions. The evaluation will consist of oral mucosa examination for grading of OM, pain assessment, quality of life, oral health, depressionand saliva evaluation of patients to assess. The analyses will be performed in the PASW 18.0 program, initially the evaluation of the data distribution will be performed by applying the Shapiro-Wilk and Kolmogorov-Smirnov tests. If, after applying these tests, the data show a normal distribution (p\>0.05), the t-test will be used. If the distribution is non-normal after the application of the tests (p\<0.05), the Wilcoxon test will be used. The p value will be set at 5%. Logistic regression will be used in adjusted models to estimate the probability of occurrence of OM.