Role of PET-CT in Managing Cancers

Introduction

Positron Emission Tomography and Computed Tomography (PET-CT) combine two advanced imaging techniques to provide more accurate results with respect to the presence and recurrence of cancer. The CT scan provides detailed images of the tissues and organs, while a PET scan is a molecular imaging technique, which determines abnormal metabolic activity or protein expressions on cancer cells. The fusion of these two modalities provides unique information that is widely used to manage cancers. PET scan typically makes use of radioactive substances attached to various molecules of interest, which upon intravenous injection, selectively target pathologies in the body. 

How has PET-CT Changed Cancer Management?

PET-CT not only is used to diagnose cancer but also provides valuable information regarding the cancer stage, the efficacy of treatment, recurrence of cancer, determining the site of biopsy, radiation treatment planning, the aggressiveness of cancer and much more. This fusion modality provides pivotal information to oncologists throughout the management of cancer patients. With the advent of newer chemotherapeutic drugs, precise radiotherapy technology and advances in surgical techniques, this modality has gained very high importance in guiding treating clinicians.

How Does a PET-CT Scan work?

The basic principle behind PET-CT scan is an increased accumulation of the radiotracer molecules by cancerous cells due to their increased energy requirement or abnormal protein expression. It is a non-invasive imaging technique. These radiotracers are handled by trained nuclear medicine doctors and technologists. After administering intravenous injection of an appropriate radiotracer, a scan is acquired after approximately one hour, during which the radiotracer localises in pathologic cells. The areas with increased radiotracer concentration are visualised as bright spots in the scan, which are then correlated and complemented with CT scan characteristics, ultimately creating a comprehensive report by nuclear medicine specialists.

Know the PET-CT Radiotracers:

• FDG (fluorodeoxyglucose): Worldwide, FDG is the most commonly used PET-CT scan in cancer management of solid tumours. It involves the use of glucose analogue tracer fluorine-18 fluorodeoxyglucose (FDG) to detect widely prevalent cancers, such as colorectal cancer, lung cancer, breast cancer, melanoma, multiple myeloma, lymphoma, and head/neck cancers. The FDG-PET scan is also used to evaluate the response of radiation therapy and chemotherapy. It provides surrogate markers for assessing the aggressiveness of particular cancer types. It also serves as an important surveillance modality for early detection of potentially treatable recurrent cancers, as well as assists in accurate radiotherapy planning. Lastly, FDG PET is also being employed in monitoring response to novel immunotherapy agents. 
• Ga-68 DOTATATE: An analogue of the protein somatostatin, this radiotracer is used for the detection and characterisation of neuroendocrine tumours of the stomach and intestinal lining, lung, liver, pancreas, as well as that of paragangliomas, meningiomas and medullary thyroid cancers. It is extremely crucial to guide the selection of surgery, somatostatin therapy, chemotherapy and liver-directed therapies in these groups of cancer. 
• Prostate-specific membrane antigen (PSMA): Prostate-specific membrane antigen (PSMA) is a glycoprotein, which is overexpressed in prostate cancer cells. PSMA PET scan is an advanced technique that identifies the location of prostate cancer lesions. PSMA PET assists in staging, response evaluation, and recurrence detection of prostate cancer. 
• Ga-68 EXENDIN-4: It is used to detect a potentially dangerous tumour called Insulinoma, which causes a sudden fall in blood glucose levels and leads to associated symptoms, such as tremors, sweating, palpitations and loss of consciousness. It is a very rare tumour that is completely curable with surgical resection. 
• FluoroDOPA: An analogue of dopamine, this tracer is used to diagnose Parkinsons disease and related disorders. It is also used to image medullary thyroid cancers and brain gliomas.

A new array of PET radiotracers, like RGD and FAPI, are in development and under clinical trials. PET-CT as a modality has opened a new horizon in treating cancer patients and providing crucial information to oncologists all over the world. In the era of precision oncology, PET is providing valuable information about cancers at the molecular level, elevating our understanding of the behaviour of this destructive disease. They are also providing newer treating avenues with high-dose radioactive agents in metastatic cancers, with minimal side effects to patients. We hope these advances in PET continue to deliver benefits to cancer patients everywhere. 

This article is contributed Dr. Akshay Bedmutha, Consultant - Nuclear Medicine and PET Imaging, HCG Cancer Centre, Ahmedabad