The purpose of this study is to find out whether photobiomodulation/PBM therapy using the Thor LX2.3 therapy system is a safe and effective treatment for oral Graft-Versus-Host Disease/GVHD.

Inclusion Criteria:

• Allo-HCT recipients
• Age ≥ 4 years-old
• Oral cGvHD with NIH-modified OMRS score ≥ 3 who have failed ≥ 1 line of therapy, including topical corticosteroids..
• No change in systemic immunosuppressive therapy (type or intensity level) within 2 weeks prior to enrollment.
• If a patient is currently using another oral topical treatment for mouth lesions/symptoms, patient must be stable on this treatment for ≥2 weeks prior to study enrollment. Patient may continue same topical dose/frequency during the study period.

Exclusion Criteria:

• Presence of an active uncontrolled infection. Subjects with a controlled infection receiving definitive therapy for 48 hours prior to enrollment are eligible.
• Personal history of mucosal head and neck cancer in the past 5 years.
• Pregnant or breastfeeding.
• The subject or guardian is unable to give informed consent or unable to comply with the treatment protocol including appropriate supportive care, follow-up, and research tests.
• Any serious medical or psychiatric illness that could, in the Investigator's opinion, potentially interfere with the completion of treatment according to this protocol.

• Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.

Double-blind