Your doctors may deem a biopsy to be too risky. It could also be that there would be too much risk when weighed against the likelihood of making a diagnosis. You should request that a
thoracicEchocardiogram
Lung needle biopsy
Thoracic aortic aneurysm
Thoracic ct
Thoracic organs
Thoracic outlet anatomy
Thoracic outlet syndrome
Thoracic spine x-ray
Vertebra, thoracic (mid back) or pulmonary radiologist review your CT scan. This is a radiologist who specializes in the interpretation of disease within the chest. Ask them to be sure that this is not a hernia.
If it is not possible to tell with reasonable certainty, that this mass is not cancer, even with an MRI, exploratory surgery should be considered. Maybe your doctors are already reasonably certain that this is not a serious problem. However a mass this size may be a problem, even if it isn’t cancer.
A cancerous mass of the diaphragm is rare, but spread of cancer to the diaphragm from another body site, is not rare. Your age and apparent good health makes these diagnoses unlikely.
Repeating studies of a mass this size every 3 to 6 months is not your best option. Ask your doctors what needs to be done to establish a diagnosis. Also ask them to discuss the risks of waiting and repeating CT or MRI.