PET-CT is a hybrid imaging method that has, in recent years, increasingly been brought into consideration by clinicians in cancer diagnosis and treatment monitoring. Since it can evaluate not only the anatomical structure but also the metabolic activity of cells, it differs functionally from conventional CT or MR. This article addresses how PET-CT works, in which clinical scenarios it is taken into assessment and how the results should be interpreted.
What Is PET-CT, How Does It Work?
PET-CT is a hybrid imaging method consisting of the bringing together of Positron Emission Tomography (PET) and Computed Tomography (CT) imaging in a single device. It combines two different techniques simultaneously: while the PET component measures the metabolic activity of cells, the CT component provides detailed information about the anatomical structure.
The substance most frequently used in oncological PET-CT applications is FDG (2-fluoro-2-deoxy-D-glucose). FDG is a radioactively labelled sugar-based molecule. Cells that divide rapidly and consume high energy can take this molecule into their structure more intensively. Since tumour cells also mostly show high metabolic activity, a concentration of signal can occur in the PET image.
This signal is overlaid with the anatomical image of the CT, and answers to both the “where” and “how active” questions are sought simultaneously. This feature is the basic point that functionally distinguishes PET-CT from the conventional imaging methods that show the anatomical structure.
What Distinguishes PET-CT From MR and Conventional Tomography?
Conventional CT (computed tomography) and MR (magnetic resonance imaging) are powerful methods in showing structural changes. They provide valuable information in revealing whether a mass exists, its size and its relationship with neighbouring tissues. However, these methods do not directly show whether the imaged tissue is metabolically active or inactive.
The difference of PET-CT arises exactly here: in addition to the anatomical structure, the level of cellular activity can also be evaluated. This feature can offer a different layer of information particularly in distinguishing whether post-treatment residual tissue is live tumour or fibrosis, or in investigating small metastatic foci.
However, each method also has its own limitations. PET-CT may not give the expected signal in some tumours that do not show FDG uptake or that have low metabolic activity. Which imaging method will give a more suitable answer to which clinical question is evaluated by the oncology team according to the clinical situation.
In Which Situations Can It Come Into Consideration in Cancer Monitoring?
PET-CT is an imaging method that can be used in the oncological process to seek answers to clinical questions different from one another. It is not applied routinely in every patient and every process; at which stage it is evaluated is determined in line with clinical requirements.
Staging: How Far Has the Disease Spread?
After a cancer diagnosis is made, determining how far the disease has spread in the body is one of the basic steps of treatment planning. In some cancer types, in the investigation of lymph node involvement or distant organ metastases, PET-CT can be among the imaging options that can contribute to the clinical assessment process.
Monitoring the Response to Treatment: Is the Drug or Radiotherapy Working?
Evaluating the effectiveness of treatments such as chemotherapy, immunotherapy or radiotherapy is a critical part of oncological monitoring. In this context, PET-CT can be evaluated as a helpful tool in comparing the metabolic activity levels before and after treatment. This comparison can contribute to decisions regarding the continuation or change of the treatment.
Recurrence Investigation: Has the Disease Returned?
In patients who have gone into remission after treatment, some findings during the monitoring period may point to active disease again. Accompanied by clinical symptoms, a rise in tumour markers or other imaging findings, PET-CT can come into consideration in the investigation of possible recurrence foci. Particularly in situations where anatomical imaging cannot give a result of sufficient clarity, the addition of metabolic information can support the clinical assessment.
Investigation of an Unknown Primary Tumour
From time to time, clinicians may, despite detecting a metastatic focus, be unable to determine the primary tumour source with other methods. In such scenarios, PET-CT can be among the methods that can contribute to the investigation of the possible primary source. However, in this context too, the results must be interpreted together with other clinical data.
Is It Used in Every Cancer Type?
Although PET-CT is an imaging method taken into assessment in many cancer types in oncology practice, it does not offer the same level of clinical information in every tumour type. In types such as lymphoma, lung cancer, oesophageal cancer, melanoma and colorectal cancer, FDG-based PET-CT is among the methods frequently evaluated in clinical practice.
On the other hand, some cancer types may give a more limited signal in terms of FDG uptake. In these situations, alternative imaging approaches or PET imaging carried out with different radiopharmaceuticals can come into consideration. For example, in situations where the limitations of FDG-based PET-CT in prostate cancer are noticed, PSMA-targeted approaches are evaluated.
Which imaging method is suitable in which cancer type and at which stage is determined in line with international guidelines and clinical assessment. A single method is not expected to answer all scenarios.
How Does the Application Process Work, Is Preparation Required?
The PET-CT examination is an application carried out by nuclear medicine units. Before the examination, FDG is given to the patient intravenously. A certain waiting period is required for the molecule to be able to distribute to the tissues at a sufficient level; this period is generally around 45 to 60 minutes. The imaging stage then begins and the whole-body scan is completed.
Some preparations may be required before the examination. Foremost among these is a fast of a few hours; this preparation is clinically important because the blood sugar level can affect FDG distribution. Additional assessment may be required in diabetic patients or in situations where blood sugar management creates difficulty. Since the preparation process can differ according to the centre where it is applied and the patient’s clinical condition, the details are shared by the relevant team.
The radiation exposure associated with the use of a radioactive substance is evaluated together with its contribution to the clinical decision-making process. The suitability of the application is addressed separately for each patient.
What Does Seeing “Uptake” or “Signal” in the Results Mean?
One of the expressions that frequently appears in PET-CT reports comes up as “FDG uptake” or “increased metabolic activity”. These expressions indicate that the cells in that region take up FDG more intensively compared to the surrounding tissue. However, a frequently made error needs to be corrected here: increased FDG uptake is not a finding specific to cancer alone.
Infection foci, inflammatory conditions, tissues in the recovery process and some physiological activities can also create a signal in the PET image. For this reason, a PET-CT result is not interpreted on its own; it is evaluated as a whole together with the clinical picture, other imaging, biopsy findings and tumour markers. When there are questions about the findings in the report, consulting the oncology team is the most correct approach.
What Are the Limitations of PET-CT?
Like every imaging method, PET-CT also has certain limitations. Knowing these limitations helps in evaluating the results with a more realistic perspective.
Tumours that do not show FDG uptake may not give a sufficient signal with this method. Some slow-growing cancer types or lesions with low metabolic activity may be missed in imaging. In addition, since structures such as the brain, urinary system and liver can physiologically show high FDG activity, the evaluation of lesions in these regions may require a more careful interpretation.
Millimetric-sized lesions may not be imaged due to the resolution limits of PET. This means that the full extent of the disease cannot be determined with PET-CT under all conditions. In clinical assessment, additional imaging or different methods can come into consideration where necessary, by taking these limitations into account.
Frequently Asked Questions
Is it necessary to perform a PET-CT in every cancer diagnosis?
No, it is not compulsory in every case. Which imaging method is used is determined in line with the cancer type, stage, treatment planning and the clinical question. In some cancer types and some clinical scenarios, MR, CT or different nuclear medicine methods may be more suitable instead of PET-CT.
Is a PET-CT scan harmful?
In PET-CT, there is radiation exposure arising from the radioactive substance and the X-rays associated with the CT component. This exposure needs to be evaluated together with its contribution to clinical decision-making. The suitability of the application is addressed individually for each patient; in special situations such as pregnancy and breastfeeding, additional assessment is carried out.
Does not seeing a signal mean there is no cancer?
Not always. Some tumours that do not show FDG uptake or millimetric-sized lesions may not give a signal in PET-CT. A “negative result” should be interpreted together with the rest of the clinical picture; not making a definitive inference without the assessment of the oncology team is recommended.
Why is fasting necessary?
FDG is a sugar-based molecule. When the blood sugar is high, the uptake of FDG by tumour cells may decrease and the image quality may be adversely affected. For this reason, fasting before the examination is important in terms of preserving the clinical meaningfulness of the imaging.
Are PET-CT and Ga-68 PSMA the same thing?
No. PET-CT is an imaging platform; it can answer different clinical questions according to the radiopharmaceutical used. While standard FDG-based PET-CT evaluates general metabolic activity, Ga-68 PSMA PET-CT uses a targeting mechanism specific to the PSMA protein in prostate cancer. The two methods are taken into assessment in different clinical scenarios.
In what time are the results ready?
After the imaging is completed, the reporting time can vary according to the centre and the scope of the examination. When the report is prepared, evaluating the results together with the oncology team is important in terms of the correct interpretation of the findings in the clinical context.
This text is for general information purposes and does not replace a medical assessment. When and how PET-CT imaging is used in your situation is planned by the physician team together with the cancer type, stage and clinical findings. For your questions regarding when and how PET-CT imaging is evaluated in your clinical picture, you can get information through the contact channels of drhasanmorcali.com.
