Hypercalcemia and osteoclast proliferation in adult T-cell leukemia.

Eighteen autopsy cases of adult T-cell leukemia (ATL) were investigated clinicopathologically. Thirteen of the patients had hypercalcemia during their clinical course. Nine of the thirteen had a high level of serum calcium at the terminal stage, even after extensive chemotherapy. Microscopic examination of the bone revealed proliferation of osteoclasts and bone resorption in eight patients. No osteoclast proliferation or bone resorption was found in the other nine normocalcemic patients. The infiltration of ATL cells was observed in only two patients--one was hypercalcemic and the other, normocalcemic. The factors affecting the serum calcium level were examined in two hypercalcemic patients. Hypercalcemia could not be accounted for by parathyroid hormone or prostaglandins E levels, which were in the normal range, or by 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, which were low. Our findings are consistent with the mechanism proposed by several investigators, that the malignant T-lymphocytes produced an osteoclast-activating-factor-like substance that caused osteoclast proliferation and hypercalcemia.