Acknowledging the Limitations—as Well as the Strengths–of Imaging is Critical to Patient Care

When it comes to radiation therapy, medical imaging is a very powerful tool, says Lawrence B. Marks, M.D. "But sometimes we get into trouble if we trust the images too much."

By Mike Bassett

Lawrence B. Marks, M.D.

Imaging has profoundly improved radiation therapy over the years, particularly considering how 3D data from CT, MRI and PET scans has allowed radiologists to better target gross lesions and increase their understanding of how they are positioned relative to surrounding tissues, said Dr. Marks, who presented the Wednesday Annual Oration in Radiation Oncology, "Error Bars in Medical Imaging: Stealth and Treacherous."

The value of medical images is inherently limited, and if that point is not acknowledged, it can lead to serious consequences that can undermine patient care, said Dr. Marks, the Sidney K. Simon Distinguished Professor of Oncology Research in the Department of Radiation Oncology at the University of North Carolina at Chapel Hill School of Medicine.

Cancers often extend beyond radiographically-defined lesions, presenting clinicians with targets that typically can't be identified by imaging. "For example, with breast cancer if you excise the radiographically-identified lesion to a negative margin and do nothing else, then the local failure rate in the breast is now 40 percent, so that's an illustration of the shortcoming of imaging," Dr. Marks said. "And we recognize this and have a routine practice to do a lumpectomy to remove the tumor and then add radiation. But, it's worth recognizing that there are multiple diseases—breast cancer in this example—where irradiating cancer that we can't see improves the patient's outcome."

A real problem occurs, Dr. Marks said, when radiation oncologists design radiation beams and make them too tight when relying on medical imaging.

"When I was a young resident in 1986," Dr. Marks recalled, "I remember very vividly saying to my attending, 'I can't see the tumor so well on this scan, the tumor moves, the patient is fidgety and is breathing.' And the answer was always the same—'Add margin, add margin, add margin.'"

Now, Dr. Marks said, new technologies like CT and PET allow radiologists to see a tumor better, while other technologies offer ways to monitor a patient's breathing during treatment and enable the user to control for internal motion.

"And these things make us all very happy, and we think we are doing a better job with these advanced medical imaging technologies available to us," he said. "But these technologies have essentially given us permission to reduce the margin."

Understanding Non-visible Targets

There have been several reports of cancer control rates getting worse when the radiation fields are too tight, Dr. Marks said. "The images will only show the gross disease, they don't show the microscopic extensions. So, if you make the radiation fields too tight, you increase the risk of local failure."

And this is where the term "stealth" comes into play, said Dr. Marks.

"You don't see what you don't see," he pointed out. "I make the analogy to termites in your house—you see the holes in the wood, but you don't see the termites. That's stealth." While a radiation oncologist routinely treats what he can see as portrayed by the images, he is also likely to have to direct a therapeutic dose at a non-visible target.

"I also treat areas that I know from experience, and what I know from anatomy, tumors like to spread," he said. "Surgeons and radiation oncologists designing radiation fields need to understand where cancers like to spread because that influences what we should operate on and where we should aim our radiation beam."

Dr. Marks also pointed out that physiological states will also affect images and their usefulness in radiation planning. For example, it may be necessary to have a patient hold his or her breath in order to maximize image quality, yet it's unlikely that radiation therapy will be delivered under those conditions.

So the "treacherous" part of the title of his oration, Dr. Marks said, comes into play when clinicians fail to acknowledge these limitations in medical images.

Dr. Marks also addressed the issue of communication. "The radiologist's job is not just to read the images but to communicate the findings—and communication is one of the major sources of errors in medicine and we radiologists need to do a better job of communicating."

Improving communication could involve something as simple as enhancing report comprehension by paying more attention to things like fonts, color, punctuation, and the formatting of reports. For example, Dr. Marks pointed out that researchers—particularly those in the advertising industry—have studied the impact of font type on reading comprehension and found that anything other than black type on white print degrades comprehension. The same is true, he said, of type that is completely capitalized. "Yet, radiology reports often end up in capital letters," he said. "That degrades comprehension of the report."

And it's increasingly important for radiologists to communicate with patients as well as fellow physicians. "Patients are going to be reading radiology reports," he said. "And we are going to have to do everything we can to make sure those reports are legible, digestible, and understandable to most people."

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