PET-CT (Positron Emission Tomography): The Uses and Examples in Cancer and Other Diseases

What is PET-CT?

PET-CT is an advanced oncological imaging method, which is frequently used to identify lesions suspected of cancer, to detect the spread after cancer diagnosis, and to determine the response to treatment. In fact , the radiological method that best shows the response to cancer treatment is PET-CT.

The most important difference of cancer cells from normal cells is their rapid and uncontrolled proliferation. The energy needs of highly active cells are also different from normal. If we can view the substances (sugar, protein and some special building blocks) that cells use in this process, we can find cancerous tissues in living organisms. The PET-CT scanning device works on this logic.

History of PET-CT

The success of the method increased significantly when the device used as PET (Positron Emission Tomography) in the 1990s was combined with CT (computerized tomography) in 2001. One of the first uses of PET-CT was to detect suspicious lesions with cancer. The first official use of the device was in 1998 in the USA to evaluate the suspicious findings for lung cancer.

PET-CT Usage Areas and Examples

1. Biopsy planning for the tissue with cancer suspicion 

Cancerous tissue may cause damage to its own structure due to excessive and uncontrolled proliferation (this is called necrosis). Samples taken from the necrotic area will not yield results; PET-CT shows areas where high activity cells are concentrated and guides the biopsy.

2 Evaluation of cancer spread / stage 

PET-CT technology is successfully used in many types of cancer to evaluate the spread of cancer (in medical term, staging) for patients diagnosed with cancer.

Determination of the treatment approach: In one out of three patients on average the treatment approach is changed after PET scan. Sometimes, instead of a heavy surgical application, medical treatment can be performed and unnecessary surgery can be prevented. In some cases, PET-CT may show that there is no spread in a patient who is thought to be metastatic, and may lead to the decision of surgery instead of medical treatment. For example, PET-CT successfully detects regional and distant metastases in breast cancer. On the left, the patient had a breast tumor and multiple axillary lymph nodes (green arrows).

3. Determination of cancer treatment response 

After cancer treatments such as chemotherapy, radiotherapy and immunotherapy, PET-CT is performed to evaluate the treatment response. PET-CT evaluates treatment response by showing both cancerous cell activity and the changes in size. It provides guiding information about both the size change of a tumour and whether there is a decrease in its activity.

4. Planning for  radiotherapy

With PET-CT mapping of active tumor cell areas to be treated with radiation, a more accurate and effective treatment is given, and it prevents unnecessary radiation to healthy tissues and reduces side effects.

5. Early detection of cancer recurrence

After treatment is completed, patients are followed up with tumor biomarkers, ultrasound and tomography, and blood tests. If a suspicious situation occurs in these tests, cancer recurrence and prevalence are successfully detected by PET-CT method.

In this patient, tumor in the left breast and spread of lymph nodes in the left axilla and inside the rib cage were detected, and it was evaluated as stage 4. While there was a very good response to chemotherapy, extensive new active lymph nodes were detected in the follow-up. However, PET-CT detected that this new finding was not a cancer recurrence but a "sarcoid reaction", a response of the immune system, and this was proven by taking a biopsy.

6. Diagnosis and characterization

It is used to display and define lesions that are detected in any part of the body with methods such as ultrasound, tomography or MRI (magnetic resonance imaging) and are suspicious for cancer according to international standards.

7. Prevention of unnecessary surgeries and planning of correct surgery

PET-CT can prevent unnecessary surgeries by detecting the spread (stage) of the tumor more accurately. Sometimes, it allows us to determine the correct stage of the disease and apply the necessary surgical method to the patient by correcting the images that cause us to evaluate it as a misleading widespread disease with traditional methods. On the left, there is a malignant melanoma (skin cancer) tumor in the soft tissue in the 3D PET-CT images that guide the surgical intervention.

8.  Differentiation between malignant and benign tumors

While PET-CT is performed, it gives the chance to measure the amount of uptake of the substance given to the body in the lesion and to evaluate the appearance characteristics of the lesion. It is the imaging method that can provide the most comprehensive information for a suspicious lesion (especially in lung, pancreas, soft tissue areas) before biopsy.

9. PET-CT imaging in prostate cancer: PSMA-PET

Standart PET-CT success is relatively low in cancers with low proliferation potential (low grade) such as prostate cancer, and in some specific cancer cell types (mucinous adenocarcinoma) and tumors below 1 cm. However, PSMA PET-CT, which is a different PET scanning method, is the most successful method in detecting prostate cancer metastases and evaluating the response to treatment. The green arrows in the image indicate prostate tumors that have metastasized to the bone.

10.  Guide for treatment plan

It guides radiotherapy applications by showing the spread of cancerous tissue more accurately for treatment methods that may damage the tissues adjacent to the tumor, such as radiotherapy. PET-CT is one of the application methods that has been increasingly used by experienced centers in recent years.

11.  Early recognition of treatment side effects

An important issue in oncological imaging is to recognize the toxicity (harmful effects) of chemotherapy and immunotherapy early. In the side image, the patient has bilateral diffuse infiltrations due to lung toxicity after chemotherapy for breast cancer, and in PET-CT, it is observed quite active in parallel with the severity of toxicity (bright areas).

12.  Diagnosis and treatment plannging for neuroendocrine cancers

Gallium PET-CT (DOTATATE) is the best method to evaluate pancreatic neuroendocrine cancer (well-differentiated type). Gallium PET showed that in this patient, the tumor had spread to the surrounding tissue and was inoperable, and there was no distant spread. Three courses of intra-arterial (intravenous) lutetium treatment gave a very good response.

13. PET-CT in non-cancer diseases (infections)

When PET-CT technology is used effectively, it is beneficial not only in cancer but also in microbial inflammatory diseases. It can be used both in determining the foci and diagnosing directly in some microbial diseases, and in distinguishing these diseases from cancer, by providing metabolic and radiological examination with a comprehensive evaluation. On the left, a microbial disease in the lumbar bones called vertebral osteomyelitis was detected in this patient who was being treated for esophageal cancer.

Some Notable Examples of PET-CT

Above is the case of lung adenoid cystic carcinoma, one of the rare cancers. PET-CT clearly shows that the tumor is attached to the right main bronchus and carina (red arrow on the left). The original PET-CT image on the right, the image obtained image after processing on the left.

Above, a case of recurrent ovarian cancer. Green arrows show the intra-abdominal spread of cancer. PET-CT offers very valuable information in the detection of recurrences.

Above, three cancers detected after treatments in the same patient: PET-CT scan was performed for the purpose of staging first with the diagnosis of non-Hodgkin lymphoma (NHL), and then after evaluating the response to chemotherapy treatment, mesenteric gastrointestinal stromal tumor (GIST), one of the rare tumors was detected. And later, metastases of thyroid cancer were detected in the lung.

In the patient diagnosed with lung cancer, second primary colon cancer was detected in PET-CT.