Talk to Lung Surgeon on Lungs Cancer

Asslam O Alaikum! My mother is recently diagnosed with lung cancer. The doctor has advised to have a PET scan. The treatment will be started after the scan but till the scan, the patient wants to get relaxed from the stiffness that she feels in her chest along with swelling. Remember please, this is not pain, this is something like stiffness due to a lump. So, she cannot tilt her head or change sides while sleeping. She is also having difficulty moving her arms. Please suggest some injection to temporarily relieve this stiffness and that injection does not have any impact on the PET Scan results. Thank You