Why Do Doctors Rely on CT Over PET for Cancer Diagnosis?

The choice between PET (Positron Emission Tomography) and CT (Computed Tomography) scans in cancer diagnosis is often based on the specific needs of the patient and the application. While both technologies have their unique strengths, PET scans are not always the preferred choice compared to CT scans for diagnosing cancer. Here, we explore multiple reasons behind this decision, highlighting the limitations and advantages of each method.

Multifaceted Reasons for Preference of CT Over PET

Multiple reasons underpin why doctors prefer CT scans over PET scans when it comes to cancer diagnosis:

PET Scans Are Not Suitable for All Applications

1- PET Scans' Limitations: It’s important to note that while PET scans excel in certain applications such as cancer staging, they are not universally applicable. For instance, if you are trying to get a 3D image of a fracture for orthopaedic surgical planning or evaluating a patient with neurological signs for a potential ischaemic stroke, a PET scan would not provide the necessary information. Its primary advantage lies in localizing metabolic activity, making it highly effective for specific applications but not a one-size-fits-all solution.

Accessibility and Cost Considerations

2- Cost and Accessibility: PET scans are significantly more expensive and difficult to perform than CT scans. They require specialist resources and sophisticated technology. The 18-FDG tracer used for PET has a half-life of only 18 minutes, meaning it is not a readily available technology that can be used on demand. These scans are tailored to specific clinical indications and are not meant to replace or supplant the ubiquitous use of CT scans.

Resolution and Interpretation

3- Resolution and Metabolic Activity: While PET scans excel in localizing metabolic activity, their resolution is actually less than that of CT scans. The primary advantage of PET scans comes from their ability to detect metabolic activity, but this is not a universally applicable feature. CT scans, with their superior imaging resolution, provide more detailed anatomical information, making them more versatile in diagnosing a wide range of conditions.

Cancer screening, the primary goal of diagnostic imaging, presents unique challenges. To be effective, cancer screening requires robust evidence that the test can detect cancer at a stage where it can make a practical and meaningful intervention in high-risk populations. Neither PET scans nor CT scans alone meet this criterion across most types of cancer. PET scans are typically used for staging cancer rather than screening, indicating that while they have a specific role, they are not the primary method for early cancer detection.

Regarding the specific scenario mentioned, where a stage 4 colon cancer was completely clear on CT but positive on PET, this is somewhat unusual. Stage 4 colon cancer is typically characterized by widespread metastasis, which would usually be visible on CT scans. This scenario might suggest a different underlying condition or an anomaly in the diagnostic process.

Decisions Based on Clinical Data

When making decisions about which type of scan to use, doctors do not rely on anecdotal evidence. Instead, they base their decisions on clinical data and the specific needs of the patient. For some cases, a PET scan is indeed most appropriate, as it provides important metabolic data that can guide treatment. For other scenarios, such as evaluating anatomical structures or detecting early signs of cancer, CT scans are more suitable.

Doctors are highly trained to choose the most appropriate diagnostic tool for each patient and condition. While advancements in technology continue to improve, the decision to use CT or PET scans remains driven by the clinical context and the unique requirements of each patient's health condition.