Imaging cancer with PET-CT

It is a valuable tool but should not be used for screening purposes and is not infallible

Glucose analogue: PET-CT for imaging cancer uses a positron emitting radionuclide called Fluorine-18 (Fl-18) . Come May, Chennai will have a PET-CT (Positron emission tomography-computed tomography) tool combined with a cyclotron facility for molecular imaging. To begin with, the centre will concentrate on imaging and diagnosing cancer.

PET scans “detect the metabolic signal of actively growing cancer cells in the body while CT scans provide a detailed picture of the internal anatomy that reveals the location, size and shape of abnormal cancerous growths.” Though PET and CT have their advantages, they offer more information on cancer location and metabolism when the two are ‘fused’ together.

The PET-CT for imaging cancer uses Fluoro-deoxyglucose (FDG) containing a positron emitting radionuclide called Fluorine-18 (Fl-18). The cyclotron facility will produce Fl-18.

Glucose analogue

Fl-18, which is a glucose analogue, enables visualisation of increased glucose uptake by the cancer cells. Since cancer cells grow uncontrollably, they consume more glucose and hence Fl-18 gets accumulated in such cancer cells.

The inflamed areas, much like the cancer cells, consume more glucose and hence take up more of Fl-18.

“But the malignant cells [unlike the inflamed areas] show up Fl-18 even after two hours,” said Dr. B.S. Ajai Kumar while explaining how the two cells behave after accumulating Fl-18. Dr. Kumar is Chairman of the Bangalore based HealthCare Global Enterprises Ltd (HCG) and HCG along with the TVH Health Care, Chennai, is setting up the facility in Chennai.

False positives

“But sometimes PET-CT can give false positive results even in the case of inflammation,” said Dr. Sankar Srinivasan, Consultant Medical Oncologist at the Chennai based Apollo Speciality Hospital.

“Also in the case of active TB where the bacteria are actively dividing, PET-CT can sometimes give false positives. And PET-CT should be done only after 8 weeks after radiation, as the inflammation caused by radiation can give false positive results.”

Dr. Vineet Gupta concurs with Dr. Srinivasan. “The false positives in the case of chest are almost 50 per cent,” said Dr. Gupta, Medical Oncologist and Haematologist at the Apollo Hospitals, Hyderabad.

“At present Fl-18 goes both to the tumour and infection. Yes, it can create problems in interpretation. But we are going to introduce many new molecules which will go only to the tumour,” noted Dr. Kumar G. Kallur, Director of the Department of Molecular Imaging, HCG, Bangalore.

When used in conjunction with other conventional tools such as biopsy and pathological reports and CT, PET-CT helps in understanding whether the disease is at an initial or advanced stage, and the extent to which the disease has spread. This, in medical parlance, is known as staging.

Staging

Understanding the staging at diagnosis is important because treatment is based on this information.

“In specific cases, PET-CT is useful in knowing if the tumour is confined or has spread to other organs,” said Dr. Srinivasan. “Particularly in the case of lung cancer, the CT may show multiple lesions but PET can indicate if they are confined to the lungs or have spread.”

Studying response

Another application is in evaluating the patient’s response to chemotherapy. Since not all patients respond the same way to chemotherapy, how do we know if it is really working?

According to Dr. Kumar, normal CT scans do not show subtle changes in the tumour after a few chemotherapy sittings, so doctors prefer to study how patients respond to chemotherapy only after completing the entire course.

“But molecular imaging shows even subtle changes. So can do it after three sittings. It helps to know if doctors are on the right track. It will tell if the active disease is responding to chemotherapy,” said Dr. Kumar.

Exception lymphoma

“It is not the case in all forms of cancer,” Dr. Srinivasan contested, “but is helpful in the case of lymphoma.”

“Doing a PET-CT is not useful in making a clinical decision. We can know if there is reduction from CT results itself,” Dr. Srinivasan noted. “But it is really useful in the case of lymphoma. Even after chemotherapy [of a lymphoma] we may still see residual mass. Before PET became available, there was no way of knowing if the mass was a tumour."

According to Dr. Gupta, it is very good to use it after a few chemotherapy sittings in the case of lymphoma.

“There is increasing evidence with breast cancer, but not so much evidence in the case of head & neck, lung and prostate cancers,” Dr. Gupta said.

But its usefulness is limited in the case of certain types of cancers found in kidney, pancreas, uterus, and to some extent in the brain. Since the brain cells actively divide, it is not possible to get any useful information.

“But after radiation we can use it for looking at recurrence in the same area,” said Dr. Srinivasan. “And finally there are some caveats. PET-CT should not be used as a screening tool. And it is not infallible,” stressed Dr. Gupta.