How should a benign adrenal gland cyst be treated ?

It shouldn't: There is no treatment. If it truly is a cyst. If it is an adenoma, then there is still no real treatment, but sometimes the doctor will check hormone levels to make sure that it is not a functioning adenoma.

Hormone check: I would check hormones to make sure it is not producing extra hormones. Otherwise if hounsfield unit indicates a cyst, then no need to do anything else except follow.