A 30 year old patient presents to a clinician because of intermittent, severe headaches accompanied by perspiration, palpitations, and pallor. Blood pressure on the initial examination was within normal limits, but, when the patient came in later with a headache, it was 180/135 mm Hg. Urinary vanillylmandelic acid (VMA) levels were elevated. Roughly, what percentage of the tumors causing this pattern is malignant?