The goal of this clinical trial is to test the efficacy of laser photobiomodulation in adult hematologic cancer patients undergoing hematopoietic stem cell transplantation (HSCT). The main questions it aims to answer are: • Is photobiomodulation with laser in the oral cavity, compared to standard care, effective in preventing oral mucositis and functional impairments in adult patients receiving HSCT? • What is the level of patient´s acceptability of photobiomodulation with laser in the oral cavity during HSCT? Participants once a day will receive photobiomodulation (diode laser device) in their oral cavity from the first day of transplantation conditioning until third day post-transplant. Researchers will compare with usual care to see if photobiomodulation helps preventing oral mucositis and functional impairment.

Background: Oral mucositis is a highly prevalent condition in individuals treated for hematologic neoplasms, primarily during hematopoietic stem cell transplantation (HSCT). This condition delays the recovery process, increasing infections, interventions, and hospital stays. To date, there are few experimental trials evaluating the use of photobiomodulation with laser for the management of oral mucositis and reporting its effect on functional outcomes in patients with hematologic cancer undergoing HSCT.

Aim: To evaluate the effectiveness and social acceptability of using photobiomodulation in the oral cavity to prevent mucositis and functional impairments in adult patients undergoing HSCT.

Methods: Randomized controlled clinical trial with parallel groups (intervention group with photobiomodulation versus a control group), assessor and statistician blinded. Participants and setting: 30 patients with hematologic neoplasms aged 18 to 65 undergoing HSCT at the Oncology and Bone Marrow Transplant Unit of Clínica Dávila. Primary outcome measures: Oral mucositis will be assessed on a daily basis with the World Health Organization Scale. Functional capacity will be evaluated with a 2-minute static walking test; handgrip strength using digital Jamar® dynamometry; lower limb strength with 30s Sits-and-Stand test; and quality of life using the Functional Assessment of Cancer Therapy Bone Marrow Transplant questionnaire. Acceptability will be assessed by recording treatment adherence and a Visual Analog Scale. Evaluations will be performed at two time points: on admission to the transplant unit before the start of the conditioning regimen and on the day of hospital discharge. Intervention: Photobiomodulation will be delivered from the first day of conditioning until day 3 post-transplant. A diode laser device with a wavelength of 660 nm, an output power of 40 mW, and an energy density of 4 J/cm2 measured at the end of the optical fiber with 0.04 cm2 of sectional area will be used. The investigators expect that photobiomodulation prevents oral mucositis and enhance functional capacity, muscle strength, nutritional status, and quality of life in patients with hematologic cancer undergoing bone marrow transplantation.

Inclusion Criteria:

• Adults over 18 years of age with hematological neoplasms who require medical indication of an HSCT according to the Transplant Committee at Clinica Dávila
• With sufficient understanding of Spanish.

Exclusion Criteria:

• Observed cognitive deficit
• Patients who do not meet the criteria of clinical stability, progression of the disease, and that do not fulfill requirement of the National Hematopoietic Stem Cell Transplantation Program.
• Participants with an oral infection from any type of Candida prior to HSCT.

Individual participant data may be requested to the researchers by e-mail, in case of relevant justification. All data shared with other researchers will be anonymous.

Data of study protocol will be available from the completion of the study up to 10 years after.

Regulatory institutions such as Ethical Committee Board and Funding Audits will have full access to IPD if they require so.

Assessor will be blinded only for the secondary outcomes. The person responsible for statistical analyses will also be blinded.