Risk for Thyroid Cancer Persists Decades After Childhood Radiation

From Medscape Medical News > Oncology

Fran Lowry

January 3, 2011 — People who received radiation for an enlarged thymus when they were babies have an increased risk of developing thyroid cancer that lasts throughout their lives, according to a new study published in the December issue of Radiation Research.

The study results might provide some insight as to why the rates of thyroid cancer continue to rise, lead author Jacob Adams, MD, from the University of Rochester in New York, told Medscape Medical News. "They also mean that the risk for thyroid cancer is going to be life long. We can't ignore that fact."

The finding comes from an update of a longitudinal study of individuals who were irradiated for thymic enlargement with lower doses of radiation (median, 0.95 Gy; mean, 1.29 Gy) between 1926 and 1957 and who were followed until 1987 for the occurrence of cancer.

The subjects were part of the Hempelmann cohort; Louis Hempelmann, MD, began the study in 1951 at the University of Rochester.

At final follow-up, when the individuals were, on average, 38 years old, those exposed to lower-dose thymic radiation had a risk for thyroid cancer that was roughly 19 times higher than that of their unexposed siblings and 24.3 times higher than that of unrelated subjects of the same age and sex living in New York State.

Irradiating the thymus gland is no longer done, but in the early part of the last century, a misconception about the normal size range of infant thymus glands and the mistaken belief that an enlarged thymus could lead to status lymphaticus and suffocation caused thousands of infants and children to be exposed to thymic radiation.

Dr. Adams and his team sought to determine if the increased cancer risk seen in the Hempelmann cohort persisted. From 2004 to 2008, they resurveyed the surviving members of the cohort, as well as their unexposed siblings. The mean age of the irradiated subjects at the second survey was 58 years.

The researchers found that the risk of developing thyroid cancer was still higher for irradiated subjects than for their siblings. Thyroid cancer occurred in 50 of the 1303 irradiated subjects, who received a mean thyroid dose of 1.29 Gy, and in only 13 of the 1768 nonirradiated siblings.

After adjustment for age, Jewish religion, sex, and history of goiter, the rate ratio for thyroid cancer was 5.6 (95% confidence interval [CI], 3.1 to 10.8).

The risk was also dose dependent. The adjusted excess relative risk per gray was 3.2 (95% CI, 1.5 to 6.6) and the adjusted excess absolute risk per gray was 2.2 cases (95% CI, 1.4 to 3.2) per 10,000 person-years.

"This was the longest longitudinal follow-up of any cohort exposed to chest radiotherapy. The results clearly show that the risk of thyroid cancer remains for a very long time," Dr. Adams said. "They also suggest that young children undergoing multiple [computed tomography] scans that expose the thyroid to radiation are at increased risk for thyroid cancer well into adulthood. Even as we cut radiation doses, we can't assume that we're going to eliminate cancer risk, in this case thyroid cancer risk. We're going to decrease it, but we're probably never going to eliminate it."

Even when you're old, you're not out of the woods.

Commenting on this study for Medscape Medical News, Albert Blumberg, MD, chair of the American College of Radiology Commission on Radiation Oncology, and vice chair of the Department of Radiation Oncology at the Greater Baltimore Medical Center in Maryland, said: "It just reinforces things we've known about for a long time. We really ought to think 2, 3, 4, and 5 times before we irradiate anybody for a benign condition."

Dr. Blumberg said that radiologists are cognizant of the importance of using as little radiation as possible in appropriate settings and that the profession has recently launched initiatives to increase awareness about the issue among its members and the public.

"This study reinforces the importance of the efforts that have been underway — organized by the American College of Radiology, the Society for Pediatric Radiology, and other concerned groups — with the Image Gently campaign for children. This has led to the Image Wisely campaign for adults, which was inaugurated this fall," he said.

"The bottom line is before we order scans willy-nilly on anybody, regardless of their age, we need to think twice. Do we really need the study? How can we do a requested study in the most expeditious fashion with the least radiation dose to be able to interpret the study correctly? Because, as this article says, you're not out of the woods even 58 years after the fact."

Dr. Blumberg added that "no radiation comes without a price. Radiologists need to be participating fully in the Image Gently and Image Wisely campaigns and to be mindful in our own personal practices of what we are doing, because there's no free lunch here. Every time you do a study on somebody, it has an implication."

Dr. Adams and Dr. Blumberg have disclosed no relevant financial relationships.

Radiat Res. 2010:174:753-762. Abstract