Understanding PET

ED: PET imaging is being revolutionised and technological advances are at the forefront of what is happening.

Positron emission tomography (PET) was commercialised in the 1980s with the acquisition of CTI Molecular Imaging by Siemens, and the activities of Philips, General Electric and suchlike. PET is now a standard imaging test in mainly oncological conditions for which fluorodeoxyglucose (FDG) is the principal tracer.

 

FDG tracer

It works on the principle of increased use of glucose by many tumours and FDG is a short-lived tracer with a 2 hour half-life that requires a cyclotron to produce the Fluorine-18 component of this radiopharmaceutical.

FDG PET is now ‘standard’ in preoperative staging for lung and head and neck cancers and is used extensively in staging lymphoma and melanoma as well as in a number of other conditions (Table 1). There has been a worldwide explosion in this technology.

In 2002, a group of dedicated Perth physicians and physicists established the first PET scanner in WA Health at Sir Charles Gairdner Hospital. In 2010, then followed the first private PET CT camera at Hollywood Private Hospital. In 2018, there were PET-CT scanners in central Perth (3 public, 3 private), 2 in outer metropolitan Perth (Joondalup and Rockingham) and 1 in regional WA (Bunbury) – part of the 80+ network throughout Australia.

There is a growing evidence that FDG PET can

be used in many more indications than what is currently covered by Medicare. In many developed countries it is reimbursed for such conditions as staging and restaging of breast cancer and pancreatic cancer, as well as non-oncological indications such as investigation of PUO, vasculitis assessment and dementia assessment.

Major recent advances in PET imaging

These relate to two technological advances:

First, is faster and more accurate PET systems. Large gantry total body PET scanners (not in Australia yet) and fast digital PET-CT systems can acquire whole body images in under 5 minutes with much less radiation dose. As well, PET-MRI systems combine the metabolic information of PET with anatomic and metabolic information of functional MRI and MR-spectroscopy (Figure 1 and 2).

Digital PET CT systems are being installed over east with 3 PET-MRI systems operational there. All these systems show improved sensitivity down to 2-3 mm and the use of artificial intelligence (AI) algorithms are improving speed, reconstruction and image quality as well as improving reporting speed and minimising errors.

Second, is new PET imaging tracers. Gallium-68 agents, produced by a generator (Figure 3. akin to technetium generators found in many traditional nuclear medicine departments) and not by a cyclotron, are now manufactured and used in many public and private PET imaging facilities in the form of Gallium-68 dotatate (targets somatostatin receptors) and Gallium-68 PSMA (targets prostate specific membrane antigen).

Over 40 sites around Australia now offer Ga-68 PSMA for imaging of prostate cancer patients – non-reimbursed imaging fast becoming the gold standard in re-staging prostate cancer patients or the initial staging of high-risk prostate cancer patients (Ref 1.; Figure 4). Ga-68 dotatate, as of May 2018, is Medicare reimbursed for staging/re-staging of neuroendocrine tumours, the gold standard imaging for this type of tumour.

There is also increasing availability of other cyclotron-produced fluorinated agents such as F-18 PSMA for prostate cancer, F-18 ethyltyrosine for brain tumours, F-18 DOPA for Parkinson’s disease and F-18 amyloid agents for diagnosing early Alzheimer’s. Other cyclotron-produced Copper-64 imaging agents will soon become available.

Table 1: Medicare-approved indications for FDG-PET*

Oncology Diagnosis Solitary pulmonary nodule that cannot be pathologically characterised or biopsied, and metastatic squamous cell carcinoma in cervical nodes with unknown primary Staging Non-small cell lung cancer, cervical, oesophageal, gastric, head and neck carcinomas and lymphoma Restaging for suspected recurrence Epithelial ovarian carcinoma, lymphoma and head and neck carcinoma Biopsy guidance Primary brain tumours and bone/soft tissue sarcomas Evaluation of residual structural lesions Primary brain tumours, colorectal carcinoma, sarcoma and lymphoma Assessment before definitive oncology surgery Apparently isolated liver or lung metastasis in colorectal carcinoma, apparently limited metastatic disease in melanoma Other conditions Epilepsy Evaluation of refractory epilepsy being evaluated for surgery where location of epileptogenic focus is not clear Myocardial viability Prior to revascularisation in the presence of impaired left ventricular function when standard viability testing is negative or inconclusive

*only some facilities are eligible under Medicare

References available on request.

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