Oral mucositis (OM) is a frequent, debilitating complication in pediatric oncology that impairs quality of life, nutrition, hydration, and treatment adherence. This randomized, prospective, single blind trial in Mexico will evaluate photobiomodulation (PBM) versus a conventional bioadhesive gel for prevention and treatment of antineoplastic therapy-induced OM in children aged 4-17 with leukemia, lymphoma, or head and neck tumors. A total of 49 participants will be enrolled. The study has two components: (1) Treatment - parallel comparison of PBM versus bioadhesive gel for established OM; (2) Prevention - crossover design in which patients receive both interventions across successive chemotherapy cycles. PBM will be delivered with a 660 nm device, 40 mW, 10 J/cm². The primary outcome is OM grade by the WHO scale assessed on days 7, 11, 14, and 21. Expected results include reduced OM incidence, severity, duration, and pain with favorable safety and tolerability, supporting standardized PBM protocols in pediatric oncology in Mexico.

Oral mucositis is a common and debilitating adverse effect in pediatric patients undergoing cancer treatment. It significantly affects quality of life, nutrition, hydration, and treatment adherence. Photobiomodulation is a promising non-invasive therapy, but there is limited evidence in the pediatric population, particularly in Mexico. Objective: To evaluate the clinical utility of FBM in the prevention and treatment of OM induced by antineoplastic therapy in pediatric cancer patients, compared to conventional treatment with bioadhesive gel. This is a randomized, single-blind clinical trial in patients aged 4 to 17 years. The design includes two arms: 1) Treatment (parallel design: FBM vs. gel for established OM) and 2) Prevention (crossover design: patients receive both interventions in successive cycles of chemotherapy). The primary outcome will be the degree of OM according to the WHO scale. FBM is expected to significantly reduce the incidence, severity, and duration of OM, as well as the intensity of pain. In addition, scientific evidence will be generated on the implementation of FBM in pediatric oncology in Mexico, with the potential to standardize protocols that improve patients' quality of life and optimize the management of oral complications of antineoplastic treatment.

Inclusion Criteria:

Prevention Arm - Inclusion Criteria

• Age 4 to 17 years
• Histopathologic diagnosis of malignant neoplasm: leukemia, lymphoma, or solid tumor of the head and neck.
• Within days 1-3 of the start of any chemotherapy cycle; for leukemia patients, in consolidation phase as defined by protocol.
• No signs of oral mucositis at enrollment.
• Availability and willingness to attend scheduled photobiomodulation (PBM) application sessions.
• Parent/legal guardian signed informed consent and child assent when applicable (≥ 8 years).
• No documented primary immunodeficiency.
• No severe concomitant systemic infection or medical condition that, in the investigator's judgment, contraindicates participation.
• No PBM treatment within 14 days prior to enrollment.
• No history of adverse reactions to light therapies or known photosensitivity.
• No history of seizure disorder or diagnosis of epilepsy.

Treatment Arm - Inclusion Criteria

• Age 4 to 17 years
• Histopathologic diagnosis of malignant neoplasm: leukemia, lymphoma, or solid tumor of the head and neck.
• Currently receiving chemotherapy (any cycle of the regimen).
• Presence of oral mucositis of any grade during chemotherapy, identified within the first 3 days from onset of the mucositis episode.
• Parent/legal guardian signed informed consent and child assent when applicable (≥ 8 years).
• No documented primary immunodeficiency.
• No severe concomitant systemic infection or medical condition that, in the investigator's judgment, contraindicates participation.
• No photobiomodulation (PBM) treatment within 14 days prior to enrollment.
• No history of adverse reactions to light therapies or known photosensitivity.
• No history of seizure disorder or diagnosis of epilepsy.

Exclusion Criteria:

• Documented primary immunodeficiency.
• Severe concomitant systemic infection or unstable medical condition.
• PBM treatment within 14 days prior to enrollment.
• Known photosensitivity or prior adverse reaction to light therapy.
• History of seizures or epilepsy.
• Any condition that, in the investigator's opinion, would interfere with study participation or safety.

Participant Withdrawal Criteria

• Attendance to fewer than 80% of scheduled treatment sessions.
• Any adverse event or persistent discomfort attributed to PBM that leads the participant or guardian to decline further intervention.
• Withdrawal of consent by parent/guardian or assent withdrawal by participant when applicable.
• Development of a new medical condition that, per investigator judgment, contraindicates continuation.

Blinding: open for the principal investigator, who will be aware of the intervention, and blinded to the evaluator.