Researchers explored the struggle to balance current benefits with future risks of medical imaging in a study which estimated that more than 40 percent of pediatric oncology patients exceeded the cumulative radiation dose threshold for risk of significant harm. Published in the October issue of Pediatrics, the study calculated cumulative effective radiation doses in a pediatric oncologic cohort and found considerable variability.

Despite heightened awareness of the risks of ionizing radiation, published data on cumulative effective dose (CED) for pediatric patients are lacking. As survival rates for childhood cancers continue to improve it becomes increasingly important to understand the radiation burden in this population.

Researchers from the department of medical imaging at University of Toronto and Hospital for Sick Children in Toronto, sought to estimate CEDs for a cohort of pediatric oncology patients and designed a five-year retrospective study of 150 pediatric oncology patients who presented with new malignancies in 2001, according to lead author Bilal A. Ahmed, MD, of the University of Toronto.

Thirty patients in five subgroups—leukemia, lymphomas, brain tumors, neuroblastomas and assorted solid tumors—comprised the study population. The median age was 7.6 years (range, 0.1-17.3 years).

In the five-year time period, the cohort underwent a total of 4,338 imaging procedures involving radiation with a median of 19.5 procedures per patient. Individual CEDs ranged 0.0015 to 642 mSv (median, 61 mSv; mean, 113 mSv), reported Ahmed and colleagues.

The researchers found considerable variability among the cohort; 1.3 percent received CEDs more than 500 mSv, 22 percent received CEDs more than 200 mSv and 41 percent received CEDs more than 100 mSv. According to the Biological Effects of Ionizing Radiation (BEIR) VII report, there is good evidence for risk of significant harm at the 100 mSv mark, noted Ahmed and colleagues.

The neuroblastoma subgroup received the highest median CED at 213 mSv, followed by lymphomas at a median 191 mSv. In contrast, median CED for the leukemia subgroup was 5 mSv.

Nuclear medicine and CT accounted for the largest contributions to CED, continued the researchers. “CT constituted 30 percent of the procedures but 52 percent of the cohort CED, whereas nuclear medicine constituted 20 percent of procedures and 46 percent of CED,” wrote Ahmed. Together the two modalities accounted for more than 95 percent of the total cohort CED.

The authors identified two primary limitations to the study. They may have underestimated CEDs if patients underwent imaging at other sites. In addition, dose estimates use five-year age categories, which might result in overestimation and underestimation for patients at either end of the age bracket.

The researchers used the BEIR VII estimates to calculate increased cancer risk among pediatric oncology patients. They estimated that exposure to the median CED of 61 mSv at 10 years of age might result in an excess lifetime cancer risk of 1.2 percent. A CED of more than 100 mSv correlates with an excess lifetime risk of more than 2 percent, and children receiving more than 200 mSv might have an excess risk of 4 percent.

Ahmed and colleagues offered several strategies to minimize radiation dose. These include:

• An assessment of the risk/benefit balance;
• Consideration of nonionizing modalities—ultrasound and MR--as appropriate and available;
• Use of age-related dose reduction protocols;
• Development of ultra low-dose protocols; and
• Appropriate follow-up intervals for high-dose procedures.

The study outlined a critical conundrum in pediatric oncology. “The benefits of accurate, timely imaging are immense and must not be underestimated,” offered the researchers. However, with five-year survival rates for pediatric cancers exceeding 80 percent for some malignancies, physicians need to “consider all possible sources of potentially harmful, long-term effects.”