Rationale: In 2020, 115,000 Dutch patients were diagnosed with cancer. Up to 85% of patients treated with radiotherapy involving the head and neck, chemotherapy or stem-celltransplantation (SCT) suffer from taste disorders (dysgeusia). Dysgeusia is one of the most distressing adverse effects of cancer therapy, may be long-lasting and may contribute to malnutrition and decreased QoL. Dysgeusia pathobiology is complex and relates to direct damage to taste buds by anticancer therapies, neuropathy and/or mucosal infection and inflammation. Hyposalivation and concurrent medications may also play a role as well as smoking and poor oral health. Zinc suppletion, clonazepam and delta-9-tetrahydrocannabionol have only limited success. Thus, dysgeusia in cancer patients represents a significant unmet clinical need.

Photobiomodulation therapy (PBMT) using specific wavelengths of red/near infrared light reduces oxidative stress and increases ATP in cells, which improves cell metabolism and reduces inflammation. PBMT is safe and effective for the prevention of oral mucositis and is linked to pain reduction, nerve damage recovery and improved wound healing. There is emerging evidence for PBMT to improve taste, likely based on its regenerative effects on taste buds and nerves involved in taste function. However, there is need for more reliable data on the effect of PBMT on taste. Objective: Assess the efficacy of PBMT to prevent/ameliorate dysgeusia in patients with multiple myeloma treated in Amsterdam UMC with conditioning chemo(radio)therapy followed by autologous stem-cell-transplantation.

Study design: Single centre, prospective, longitudinal, double-blinded, randomized, controlled study. Study population: Recipients of autologous hematopoietic stem cell transplantation (SCT) for the treatment of multiple myeloma in Amsterdam UMC. Intervention: Patients will be blinded to receive either PBMT or sham-PBMT. Main study parameters/endpoints: Objective and subjective taste function and taste associated covariables and their impact on QoL will be assessed.

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The application of PBMT to the (peri)oral region is safe and comes with no relevant side effects. The application of PBMT or sham-PBMT will take about 10-15 minutes per treatment. The measurements at the start of the study and at the visit six weeks after SCT will last about 30 minutes. Scoring the PROMS (2 questionnaires of 1-5 questions) during hospitalization will take about five minutes per day; 2 questionnaires (30+15+ 5 questions), 10 minutes, weekly. Patients do not need to come to the hospital specifically for the study, as they already have an appointment in the hospital.

Inclusion Criteria:

• Patients diagnosed with a haematological malignancy
• > 18 years of age
• Receiving HDM followed by SCT
• Able and willing to give informed consent

Exclusion Criteria:

• Having taste disorders not related to SCT (e.g. COVID-19)
• History of a head and neck tumor treated with surgery and/or (chemo)radiation
• Neurological diseases (e.g. Parkinson's disease)