SAN DIEGO—Treatment of thyroid cancer with radioactive iodine-131 causes a decrease of key blood cells, necessitating holding off on re-treatment with the isotope for at least a year, researchers reported here at ENDO 2015, the 97th Annual Meeting of the Endocrine Society.
I-131 treatment resulted in a mean reduction of approximately 30 percent of white blood cells from baseline and a 25 percent in blood platelets from baseline within one month of initial therapy, said Athanasios Bikas, MD, a research fellow in endocrinology at MedStar Health Research Institute in Washington, DC.
After the blood count nadir was reached at one month, blood cell recovery begins. Three months after treatment was initiated, there was a decline in both white blood cells and platelets from baseline of approximately 17 percent each, also statistically significant, he reported.
At one year, the white blood cells were still significantly reduced—15.6 percent from baseline—and so were the platelet levels—reduced 13 percent from baseline. Even two to three years later, there was still a significant difference, with white blood cells reduced 5.6 percent and platelets reduced 9.5 percent.
At five years, the white blood cell count was normalized, with a minimal reduction of 0.3 percent, but there was still a significant decline in platelets even out to five years—a 9.3 percent reduction from baseline—although Bikas noted that reductions of less than 10 percent, while statistically significant, are probably not clinically meaningful.
“We did determine that the damage caused by the treatment follows a specific pattern,” he explained in an interview at his poster study. “All parameters decreased to a nadir at one month and then gradually returned to baseline figures. The damage to bone marrow comes up close to baseline after one year.”
He said that treatment with I-131 also causes decreases in other blood parameters. “We also found statistically significant differences with decreases in red blood cell counts, in hematocrit and in hemoglobin, but because those decreases were less than 10 percent we did not consider them clinically meaningful.”
All the decreases reached statistical significance at all time points up to 24 to 36 months, the researchers found.
“Despite finding that damage had been done to the white blood cells and platelets, we did not require treating patients with blood growth factors,” Bikas said. “We did not notice any adverse events that required blood products to restore them to good health.”
The study does raise alarms, however, for the patients with aggressive forms of thyroid cancer that may require re-treatment with the radioactive agent: “What our study shows is that we cannot re-treat them with I-131 for at least a year,” he said.
Asked for his perspective, Nikolaos Stathatos, MD, an instructor in medicine at Harvard Medical School and an endocrinologist at Massachusetts General Hospital, said he has observed similar results in his patients. “You have to prioritize in treating patients with thyroid cancer, and we use I-131 to get rid of the cancer first, and then we watch for any problem due to the effect on the blood cells.
“I would also agree that we need to wait at least a year for the white blood cells to rebound before we could treat these patients again with I-131. These findings are quite important because we are going to be using this therapy more and more as there is a continuing increase in the diagnosis of thyroid cancer.”
For the study, Bikas and his colleagues performed a retrospective analysis of the outcomes of all patients who had received at least one I-131 treatment for differentiated thyroid cancer at MedStar Washington Hospital Center with a prescribed activity of I-131 guided by dosimetry. He said the hope was to find that the guided treatment reduced the impact of I-131 on the blood, but the team was unable to observe that; and in fact found that the treatment did cause an important impact on the blood.
A total of 152 patients were identified who were treated with I-131, including 22 patients who required more than one treatment. That gave the researchers a total of 165 dosimetric treatments to examine in their study.
Complete blood counts values were documented at baseline and at one month and at three, 12, 24 to 36, and 48 to 60 months post-treatment. The researchers also looked at absolute lymphocyte counts and absolute neutrophil counts, and determined that among these white blood cells, the lymphocytes were more affected by I-131 administration. At one-month, the neutrophil count was down 23.6 percent from baseline, but the lymphocyte count was markedly lower, with a reduction of 46.6 percent from baseline.
Both levels remained statistically reduced at one year, with neutrophil counts reduced 14.4 percent and lymphocytes reduced 19.7 percent. After two to three years, the lymphocytes were still 10.4 percent lower than baseline, whereas neutrophils were reduced 3.6 percent, a non-significant reduction from baseline. By the checkup at 48 to 60 months, both lymphocytes and neutrophils were insignificantly reduced.
“Our analysis demonstrated that lymphocytes are more severely affected from I-131 treatment when compared with neutrophils,” Bikas said.
In analyzing the impact of I-131 treatment on blood cells among the few patients who required more than one course of treatment with I-131, the researchers found significant declines in white and red blood cells, platelets, and in the absolute neutrophil count after each consecutive treatment.
“On the contrary, our data demonstrated no significant difference after each treatment for hematocrit, hemoglobin levels, or in absolute lymphocyte counts,” Bikas said, “indicating the absence of an additive effect after each treatment for these parameters.”