PET scanning, in lung cancer and in Ballarat

Professor Alex Pitman from Lake Imaging , at St John of God Hospital in Ballarat, graced us with his presence in Hamilton last night, and provided us with an engaging and informative outline of PET scanning.

PET stands for Positron Emission Tomography. This mode of whole-body scanning is very useful in cancer medicine, having a role in the diagnosis and staging of a wide ranger of tumours. For our purposes, it is an indispensable part of the management of almost all lung cancer.

How does PET scanning work? Now I am no expert in this field. However…. a PET scan is a nuclear-medicine test. It involves the injection of a ‘tracer’ into the patient. In Australia, this tracer is universally F-18 fluoro deoxy glucose, or FDG. Essentially this is F-18 fluorine attached onto a glucose molecule. FDG tracks around the body in the same way as glucose; ie, it is taken up by organs to be metabolized. Tumours take up lots of glucose, but metabolize it pretty poorly – so FDG tracer hangs around in them. The F-18 fluorine bit of FDG is radioactive, and emits positrons, which collide with and annihilate electrons, releasing photons. After the contrast injection, and a bit of a rest, a patient goes throught the PET scanner. The photons are detected by the PET scanner, allowing mapping of the tumour (and the body in general). A PET scan is combined with a CT scan, which allows better visual localization of the tumour within the body.

When do we use it?

1. PET scanning is used in combination with CT scanning and anatomical biopsy for the evaluation of solitary pulmonary nodules. If a nodule is too difficult to biopsy without considerable risk to the patient, a PET scan is a useful way of ‘ruling in’ a malignant diagnosis. In Australia, if a nodule is ‘hot’ on PET scan then there is over 90% likelihood of it being a cancer. (In other countries, fungal infections and TB in particular also often turn up and are hot on PET scan. I have seen a patient with a fungal infection in Australia also undergo surgery because a lesion was ‘hot’ on PET scan, so we need to be a bit careful).

2. PET scanning is also used for staging non-small cell lung cancer prior to surgery. In a patient with lung cancer a PET scan is useful if combined with CT and mediastinal biopsy procedures to evaluate the possibility of malignant involvement of mediastinal lymph nodes prior to surgical treatment. Indeed, a recently published small study has suggested that PET combined with diagnostic CT scan, without mediastinal biopsy sampling, has a very high negative predictive value (NPV) (ie if PET/CT is clearly negative then there is not going to be cancer, NPV in this study 97%) for mediastinal spread, They suggest that PET/CT should be relied on to indicate if a patient with NSCLC is anatomically suitable for surgical resection without the patient needing an invasive mediastinoscopy or endobronchial ultrasound biopsy. However, a study published a couple of weeks earlier suggested that there is considerable risk of false negative and false positive results if PET is relied on alone to evaluate the likelihood of malignant involvement of the mediastinal nodes.

The PET scan operators would very much like us to use PET to re-stage cancer after therapy, and to monitor disease in lung cancer. Medicare will not reimburse PET in those situations (but will for the evaluation of a solitary nodule or for the staging of cancer prior to treatment). The clinical reality is, however, that the benefit of treating recurrent non-small cell lung cancer with radiotherapy and chemotherapy is not sufficient that it can clearly be regarded as a good thing to diagnose recurrent disease early and treat it. That is to say, even if a PET scan picks up recurrent disease, we don’t necessarily know what to do about it. There are some new chemotherapeutic agents that may change the playing field in that regard, but are yet to have their early promise confirmed. If and when they do, the indication for PET scan will become even more broad. For the time being I think that the use of restage lung cancer after treatment routinely would be expensive and indulgent.

Undoubtedly PET scanning has an important, and increasing, role in management of patients with lung cancer. It is terrific for us that there is now a PET scanner in Ballarat. The service they have provided us to date has been excellent. For the benefit of our patients, however, evaluation of the PET/CT in lung cancer still needs to occur in a multidisciplinary environment, with radiologist, nuclear medicine specialist, pathologist, oncologist, thoracic surgeon, respiratory physician, radiation oncologist all present and all able to discuss the patient’s situation. It is difficult to assemble such a team outside of the major metropolitan areas. The team at St Vincent’s Hospital / Peter Macallum in Melbourne have provided a great service for our Victorian patients to date, and continue to do so.

Andrew