Expert Siemens panel tries to shed new light on CT radiation risk

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The increasing backlash over CT radiation risk ignores several critical factors, said an expert panel convened by Siemens Healthcare on July 1. The current hyper-vigilant debate exaggerates radiation risks, disregards the benefits of appropriate imaging and overlooks ongoing programs to minimize radiation dose, said panelists.

CT imaging has been subjected to increasing scrutiny in recent weeks with policy makers, the medical community and patients all voicing concerns about radiation risk. Siemens sponsored the briefing to address issues and re-frame the risk discussion to incorporate a broader perspective.

A flawed risk formula
The linear no-threshold model (LNT) used to calculate radiation risk is based on a flawed formula that over-estimates cancer risk, said Cynthia H. McCullough, PhD, professor of radiological physics at the Mayo Clinic College of Medicine in Rochester, Minn. The actual cancer risk from low dose radiation, defined as less than 100 millisieverts (mSv), is close to zero, said McCullough.

Scientists use the LNT model that extrapolates data from higher exposures to calculate risk and assumes that risks fall linearly as dose decreases to calculate dose risk. However, the method conflicts with basic principles of radiation biology. For starters, cancer risk from radiation does not fit to a straight line for all types of cancers. In addition, “there are cancers where there is a clear threshold below which no risk is observed,” stated McCullough.

Even using the "flawed" LNT model, a simple cardiac CT study raises lifetime risk of dying from cancer from 22 percent to 22.05 percent, panelists noted.

Non-dose related risks
Currently, more patients are refusing medically needed exams and may be jeopardizing their health, said the panelists.

However, the unasked, key question, asserted McCullough, is: if symptoms are significant enough to warrant a cardiac CT what is the risk of not undergoing an appropriate imaging study. What’s more, the documented rather than theoretical risk associated with alternative studies need to be considered. The risk of mortality during an invasive cardiac catheterization is five times higher than estimated risk of a fatal cancer later in life.

Risk benefit assessment is a critical factor in the image ordering process, said John Lesser, MD, director of cardiovascular CT and MRI at Minneapolis Heart Institute and Abbott Northwestern Hospital in Minneapolis. As Lesser and his colleagues order diagnostic studies they weigh both risks and benefits. Potential benefits of cardiac CT vary and include:

  • Discovery of findings associated with premature death and morbidity;
  • Avoidance of  inaccurate results that lead to other kinds of tests and risks; and
  • Definite exclusion of the heart as the cause of symptoms.

Tech advances and training
The capabilities of CT systems have improved tremendously in the last decade, pointed out Elliot Fishman, MD, director of diagnostic imaging and director of the division of abdominal imaging and CT, at the Johns Hopkins Hospital in Baltimore. Scan times have plummeted to less than one-half a second and the ability to visualize anatomical details like coronary arteries has improved. And the newest scanners make it possible to cut radiation dose from 15 mSv to one to five mSv.

Each advance helps physicians better meet their end goal—improved patient care. Another critical piece of the dose reduction puzzle is training.

Fishman recommended ongoing training of both radiologists and technologists to ensure that scans deliver the lowest possible dose. At Johns Hopkins, the radiology department holds monthly meetings to ensure that techs remains focused on dose reduction. It’s important that protocols are re-designed year to year and scanner to scanner, said Fishman. Advances such as iterative reconstruction and dual-energy can be used to decrease the dose without compromising from image quality, he noted.

The role of the imaging community
The imaging community is taking steps to avoid unneeded exposure and make imaging as safe as possible, said McCullough.

These are:

  • Availability and promulgation of professional Appropriateness Criteria;
  • Increasing emphasis on using decision support to help physicians order the right test for the right patient;
  • Technical innovations to decrease dose and improve image quality; and
  • Heightened emphasis on proper education and training of all personnel involved in operating CT systems.