Pheochromocytoma may occur as a single tumor or as multiple growths. It usually develops in the medulla (center or core) of one or both adrenal glands. Sometimes this kind of tumor occurs outside the adrenal gland, usually somewhere else in the abdomen. Less than 10% of pheochromocytomas are malignant (cancerous), with the potential to spread to other parts of the body. The tumors may occur at any age, but they are most common from early to mid-adulthood. A common clinical feature is a paroxysm (an attack of symptoms listed below) that may be frequent but sporadic (occurring at unpredictable intervals). The paroxysms may increase in frequency, duration and severity as the tumor grows.
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