 PecFent and Placebo will be delivered by Archimedes Pharma When starting the radiotherapy sessions 2 days after the preliminary scanner session, the medic will collect the patient signed informed consent, will deliver the treatment 1 dose of fentanyl transmucosal Pecfent 100μg in naive patients or 1 dose of Pecfent 200μg in patient with a stable opioid background pain treatment or placebo and will collect pain intensity value with a patient VAS collected before and at the end of the radiotherapy session. Finally patient radiotherapists global satisfaction will be collected An assessment a week the end of the radiotherapy session will be assessed by investigator consisting in a Patient global satisfaction 