This week's case is a biphasic CT exam in a 54 year old man to evaluate for metastatic disease 1 year after left nephrectomy for renal cell carcinoma. He does not have a history of metastases.
The first image shows the scout topogram on the left and top slice of the unenhanced phase of the CT abdomen and pelvis exam on the right.
The second image shows the scout topogram on the left and top slice of the 2 minute delayed phase of the CT abdomen and pelvis exam on the right.
Questions
Both appear to start at the same place. However, there is a difference between the two scans. Can you make the finding?
What is the name for this type of perceptual error?
If you look at the left lower lobe in the top image (unenhanced), there is a new pulmonary nodule that is not included on the field of view on the lower image (2 minute delayed phase). This is despite the fact that based on the scout topogram, both phases start at the same place. But if you look at the heart, it appears slightly different between the two phases, which is the tip off that these two phases actually start in two different places! In this case, a few millimeters makes a significant difference.
In this case, the pulmonary nodule turned out to be a new pulmonary metastasis. The attending reading the case missed it because he looked at the lung bases on the 2 min delayed phase, which doesn't show the nodule. The metastasis was picked up on a later follow-up study and the error was identified in retrospect.
This type of perceptual error is very common, the Edge of the Film error.
The mitigate against this type of error, always pay attention to the edges of the film regardless of whether the study is an x-ray, ultrasound, CT, or MRI. In this case, using the scout topograms would not have prevented the error. The best approach is to eyeball the heart on each phase and see if it looks different. If the heart looks different, you might need to scroll through the different phases to see which one starts the highest.
Don't forget that the most common allegation in malpractice claims against radiologists is missed cancer. A missed metastasis is an important finding that may be amenable to resection prior to the development of diffuse metastatic disease.
