Transcranial Photobiomodulation (tPBM) for Somatic Symptoms in Treatment-Resistant Depression

Inclusion Criteria: Must have a current diagnosis of Major Depressive Disorder (MDD). The clinical severity must be evaluated by a psychiatrist as moderate or above, defined as a Clinical Global Impression-Severity (CGI-S) score \> 4 and a 17-item Hamilton Depression Rating Scale (HDRS-17) total score ≥ 18. Must be currently receiving stable antidepressant treatment for at least four weeks but showing inadequate response (treatment-resistant). This study will be conducted as an add-on therapy. Must have full behavioral capacity, normal intellectual functioning, and the ability to comprehend and sign the informed consent form. Exclusion Criteria: Individuals diagnosed with Bipolar Disorder or Schizophrenia. Individuals with current or recent Substance Use Disorder. History of organic brain lesions (e.g., neurodegenerative diseases, epilepsy, stroke) or any medical conditions affecting central nervous system function. Individuals with abnormal intellectual functioning based on clinical judgment (e.g., suspected intellectual disability, severe learning difficulties). Individuals who are currently pregnant (due to limited evidence regarding the safety of tPBM during pregnancy). Any other condition that, in the investigator's judgment, would render the participant unable to cooperate, unsuitable for the study, or unwilling to sign the informed consent.