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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n
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.<\/p>\n