History: Pt is a 31 y/o female with known MEN2A and hx of surgically cured MTC and surgical hypoparathyroidism being treated with Rocaltrol and Oscal. Pt had a recent occurrence of hypercalcemia, but now has asymptomatic elevation of plasma and urine metanephrines being evaluated with a thin cut dedicated unenhanced adrenal CT to r/o pheochromocytoma. Selected images of the kidneys are submitted as the teaching file.

Findings: Both kidneys demonstrate high attenuation areas outlining the renal medullary pyramids. A single 2 mm focus of greater attenuation is seen within a medullary pyramid of the right kidney (posterolaterally at the interpolar region) that could represent a focal calculus.

Impression: Medullary nephrocalcinosis describes calcific deposits within the medullary pyramids, and is more common than cortical nephrocalcinosis. It commonly results from hypercalcemic states such as hyperparathyroidism, Type 1 or distal renal tubular acidosis, and medullary sponge kidney.
   Renal Tubular acidosis may be primary from an inherited enzymatic defect, or secondary, such as from Fanconi syndrome, Wilson disease, or drug toxicity. In both cases it results from an inability of the distal tubules to secrete hydrogen ions, and therefore to acidify urine. This results in decreased solubility of calcium within the urine, as calcium ions are soluble in acidic urine, but much less so in alkaline urine. This decreased solubility leads to the precipitation of crystalline calcium salts.
   Medullary sponge kidney, also known as renal tubular ectasia, reflects dilatation of the collecting ducts. Approximately 15% of these patients develop calcifications within the dilated tubules. Patients are usually asymptomatic, although they may experience hematuria and flank pain if formed calculi are passed in the renal collecting system.
   Hypercalcemia is most often due to hyperparathyroidism. Primary HPT results from a parathyroid adenoma; secondary HPT most commonly results from renal disease; and tertiary HPT results from ectopic production of parathormone.
Excessive calcium ingestion and absorption from milk-alkali syndrome, sarcoidosis, and vitamin D intoxication are other causes of medullary nephrocalcinosis. In this patient’s case, a leading suspect has to be the Rocaltrol, a synthetic Vitamin D3, in combination with Oscal, a calcium supplement. Calcitriol, the generic name for Rocaltrol, is the most active metabolite of vitamin D3, and has potent effects on intestinal calcium absorption and bone mobilization. One drawback to calcitriol treatment is that it can induce hypercalcemia. This, in turn can lead to hypercalciuria, precipitation of calcium salts and medullary nephrocalcinosis.

References:

Textbook of Diagnostic Imaging, 2nd edition, Putnam and Ravin, W. B. Saunders, 1994
Body CT, A Practical Approach, Slone et al, McGraw Hill, 2000
Slatopolsky, E; Dusso, A; Brown, A. New analogs of vitamin D3. Kidney Int, 56 Suppl 73:S46-51 1999 Dec