医療用放射線によるがんのリスクは過大評価されていた可能性がある

コンピュータ断層撮影による被曝のリスクはこれまで考えられていたよりも低い可能性がある
Radiation risk from computed tomography may be lower than previously thought
コンピュータ断層撮影（CT）による放射線誘発性がんを発症するリスクはこれまで考えられていたよりも低い可能性があるとのスタディ結果が2010年RSNAで発表された。研究者らは米国メディケア請求を用いてレトロスペクティブスタディを行い、CTスキャンの分布を解析し電離放射線被曝量を計測し関連するがんを推定した。データには1998～2001年および2002～2005年の2グループの100万以上の記録が含まれていた。彼らはCTスキャンの数および型を解析し、各々の患者の被曝量を“低”用量（50～100mSv）または“高”用量（100mSv超）に分類した。1998～2001年に42%の患者、2002～2005年には49%の患者がCTスキャンを施行された。放射線を被曝した患者の割合は低および高用量のいずれも1998～2001年の群から2002～2005年の群までに、2倍に増加した。この結果は診断および管理に高速CTの使用が増加したことと一致している。CTの電離放射線による発がんはそれぞれの群で0.02%および0.04%と推定されたのに対し、過去のスタディではそれぞれ1.5～2.0%と推定されていた。
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The risk of developing radiation-induced cancer from computed tomography (CT) may be lower than previously thought, according to a study presented at the 2010 annual meeting of the Radiological Society of North America (RSNA).

"Radiation from medical imaging has gotten a tremendous amount of attention in recent years," said Aabed Meer, an M.D. candidate at Stanford University in Palo Alto, Calif. "This is one of the first studies to track CT utilization in such a large population."

The researchers conducted a retrospective study using US Medicare claims from 1998 through 2005 to analyze the distribution of CT scans, determine the ionizing radiation exposure associated with the exams and estimate the associated cancer risk in a population of older adults.

"The study focused on the elderly Medicare population, which receives the highest amount of per capita radiation," Meer said. "We analyzed more than 10 million records from the Medicare claims database."

The data were studied in two groups, including 5,267,230 records from 1998 through 2001 and 5,555,345 records from 2002 through 2005. For each group, the researchers analyzed the number and types of CT scans that each patient received to determine the percentage of patients exposed to "low" radiation doses of 50 millisieverts (mSv) to 100mSv and "high" radiation doses, in excess of 100mSv. They then used standard cancer risk models to estimate the number of cancers induced.

CT scans of the head were the most common examinations, representing 25 percent of the first group and 30 percent of the second group. However, abdominal CT delivered the greatest proportion of radiation, accounting for approximately 40 percent of the total radiation exposure in each group. Imaging of the pelvis and chest represented the second and third largest sources of radiation.

From 1998 to 2001, 42 percent of patients underwent CT scans. From 2002 to 2005, 49 percent of patients underwent CT scans. The percentage of patients exposed to radiation doses in both the low and high ranges approximately doubled from the first group to the second group. The researchers found this to be consistent with the increasing use of high-speed CT in patient diagnosis and management.

Cancer incidences related to ionizing radiation from CT were estimated to be 0.02 percent and 0.04 percent of the two groups, respectively.

"Our findings indicate a significantly lower risk of developing cancer from CT than previous estimates of 1.5 percent to 2.0 percent of the population," said coauthor Scott Atlas, M.D., chief of neuroradiology at the Stanford University Medical Center. "Regardless, the increasing reliance on CT scans underscores the importance of monitoring CT utilization and its consequences."

Other coauthors are Laurence Baker, Ph.D., and Pat A. Basu, M.D.