CT Dose for Pulmonary Nodule Detection: How Low Can It Go?

When it comes to detecting lung nodules, radiology has long wrestled with the question of how much radiation dose is actually necessary. For part-solid nodules measuring at least 5mm, new research now provides an estimate – 0.9 mGy or 10 quality reference mAs (QRM).

In a study published Sept. 29 in Radiology, researchers from the Mayo Clinic investigated just how low they could push the radiation dose with chest CT while still maintaining the ability to depict pulmonary nodules at a diagnostic-quality level. Based on their findings, the industry has now identified tipping-point after which scans lose their sensitivity.

“Our study showed that sub-solid nodules can be detected without a decrement in performance at the National Lung Screening Trial dose level,” said the team led by radiologist Joel G. Fletcher, M.D. “However, very-low-dose levels resulted in loss of detection of some part-solid nodules.”

Ultimately, the team determined that they could reduce radiation dose by one-seventh – a drop that equates to approximately one-third of that in the National Lung Cancer Screening Trial – before images become inferior to those captured with a routine radiation dose. But, they also discovered the sensitivity for detecting part-solid pulmonary nodules at lower radiation dose levels fell by more than 25 percent, and iterative reconstruction (IR) was not able to improve performance at very-low-dose levels.

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To see how low they could push radiation dose, Fletcher’s team conducted a retrospective study with CT scans from 83 patients. From that group, they identified 66 indeterminate pulmonary nodules of at least 5mm in size in 42 individuals. The average nodule size was 8 mm. They also included a control group of 41 patients who did not have indeterminate pulmonary nodules of at least 5 mm in size. The mean volume CT dose index and size-specific dose estimate at routine dose were 5.3 mGy +/- 2.0 and 6.5 mGy +/- 1.8, respectively.

Axial lung CT images in a 44-year-old man with a 14.8-mm part-solid nodule (arrows) in the left lower lobe. These thoracic radiologist readers detected the nodule on images corresponding to, A, an automatic exposure control setting of 70 quality reference mAs (QRM) reconstructed with filtered back projection, but only one reader detected the nodule at, B, 10 QRM with iterative reconstruction, and no readers detected the nodule with, C, 5 QRM and, D, 2.5 QRM with iterative reconstruction. Note how it is difficult too perceive the ground-glass portions of the nodule at the lower-dose levels shown in C and D. Courtesy: Radiology

Their analysis showed that per-patient and per-nodule, sensitivity dropped slowly as the radiation dose declined, but only 2.5 QRM with IR failed to achieve non-inferiority. And, despite a 25-fold dose difference between 2.5 QRM and 70 QRM, specificity stayed largely unchanged. It is important to note, though, they said, that sensitivity for sub-solid nodules significantly decreased for any dose configuration under 10 QRM.

At very-low-dose levels, they said, IR was no help with part-solid nodules.

“Very-low-dose levels resulted in loss of detection of some part-solid nodules, as demonstrated previously,” the team said. “Radiologists should be aware that even though image quality appears to be improved with IR at these very-low-dose levels, part-solid nodules may be under-detected.”

Knowing the point at which part-solid lung nodules can no longer be effectively imaged is critical, however, because existing research shows these nodules detected at lung screening are more likely to be malignant that solid nodules.

Overall, Charles S. White, M.D., professor of radiology at internal medicine at the University of Maryland School of Medicine and Ella A. Kazerooni, M.D., professor of radiology and internal medicine at the University of Michigan, said, in an accompanying editorial, these results clear up some questions about low-dose CT and lung nodules, and it opens the door for many clinical benefits.

“The most important contribution of this study is that it establishes a lower boundary for radiation dose that can be confidently used to diagnose a pulmonary nodule at chest,” they said.

In particular, they said, as the number of chest CTs increases both in lung cancer screening and general patient care, being able to set the radiation dose at the lowest possible threshold will significantly reduce exposures. For smokers who already have a 15-percent lifetime cancer risk, that can be largely beneficial. But, the impact is more likely to be felt in the general clinic setting, they said, where dose is frequently higher – up to 8 mSv per chest CT.

“On the basis of the findings of this study,” they said, “the potential for dose reduction in some clinical setting is greater than 10-fold.”

Still, White and Kazerooni said, Fletcher’s team left two issues unaddressed – they did not account for body mass index and the large dose needs for larger patients, and they did not consider sex differences. The team acknowledged this shortcoming, adding that they already have plans to conduct larger studies with more patients and readers.

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