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PET CT Scan (Nuclear Medicine)

Presentation (Technical Aspects 2)

How can the PET technique improve the care of the cancer patient ?

The technique allows to assess the spread (stage) of the disease.
For most of the types of malignant tumours, the spread of the disease is the most important prognosis parameter. Therefore, it is very important to know if there are metastases – regional or distant – in order to choose the most appropriate treatment. Actually, cancerous proliferation determines the indication for a local treatment (resection / radiotherapy) or for a general treatment (systemic chemotherapy). Here, the PET SCAN plays an important role.

 

Various publications pointed out that the PET determines more precisely the spread of the disease than conventional imaging techniques; this is particularly important for patients that have already followed other treatments which could make difficult the analysis of conventional radiology imaging. Indeed, former treatments often alter the anatomy or structure of tissues; an inflammatory reaction or non-specific healing reaction (fibrosis) often appears after surgery or radiotherapy. With conventional examinations, it is often difficult to show a tumour recurrence while the PET SCAN can sometimes do it.

 

Another example of the interest of the PET SCAN is to allow – with only one examination – to check if there is any suspicious lesions in the whole body, especially when the tumour markers circulating in the blood have increased, which can be the sign of a recurrence in a asymptomatic patient. In the past, that case often asked for the clinical practitioner to make another battery of additional examinations in order to try finding the possible recurrence area.

 

The use of the PET for determining and circumscribing the areas to irradiate in case of chemotherapy is particularly innovating. Within the Jules Bordet Institute, a close cooperation between the PET and the Radiotheraphy Departments was made in that field. To that purpose, a set of softwares was recently installed; this will allow to take metabolic information into account in order to determine if some areas have to be irradiated or not. The goal is to only irradiate the active tumour tissue, which will enable us to use stronger x-ray doses – but more targeted – with less harmful effects on healthy surrounding tissues.

 

The PET allows to assess the treatment response

In the near future, this will probably be the most important application of the technique since there is a very important advantage of metabolic imaging in comparison with structural imaging. A clear distinction has to be made between both uses of the PET scan in that approach; the first assesses the final results of the treatment while the second tries to quickly determine – by means of images – the efficiency of chemotherapy soon after its introduction so as to avoid the administration of ineffective and useless treatments. Nevertheless, studies are still necessary before the introduction of this last use in the daily practice (apart from clinical studies).

• Assessment of the presence of residual disease after a defined treatment
  The final prognosis of the disease after an initial treatment (‘front-line treatment’) is mainly determined by the presence or absence of an activity at the end of the treatment. This residual pathological activity either points out that an insufficient dose of an effective treatment is given, or that some parts of the tumour resist the treatment. It is important to quickly identify these pathological remnants since they require additional treatment. For example, the PET can be used after the treatment of a lymphoma. After the initial treatment (e.g. combined chemotherapy), a residual mass often remains – mainly made of scar (granulation?) tissue – ; nevertheless, it can contain some tumour foci that did not receive the treatment. Only the PET technique can show if there is still a residual tumour activity within such a mass. According to many examination centres, recurrence risks are much important when the PET shows residual activity than when it is fully normalized after the treatment. On the basis of the PET results, the haemato-oncologist will thus be able to decide if additional treatment is necessary.
  
  
• Early assessment of the treatment response
  The final goal of an oncology treatment is to eradicate malignant cells (cytotoxic therapy) or to stop their growth (cytostatic therapy). Both types of treatment induce a series of molecular alterations at the level of the malignant cells – that change the tumour growth – which quickly influences the global metabolic system of the tumour. After some weeks (often from 6 to 8 weeks), these metabolic changes lead to measurable structural changes – among others, to the reduction of the tumour mass. In a classical way, the treatment efficiency is evaluated in function of these late structural changes such as measured by ultrasound scan, CT scan or MRI.
  
  

  As the PET imaging is based on metabolic changes, it will quickly show the efficiency of the initiated treatments.
  Why is it so important ? The main goal is to early identify the patients for whom the treatment is ineffective, which would avoid to spare the patient the toxicity of an inefficient treatment and to start quicker with another potentially efficient treatment. This principle has already proved useful in clinical practice in case of stomach cancer; actually, a PET scan done after 2 weeks of chemotherapy permitted to identify the patients that did not respond to the treatment (i.e. non-responding patients). The PET is also interesting for the early imaging (even after 24 hours) of the metabolic response of patients affected by a certain type of sarcoma during a Glivec treatment. This new generation medicine stops tumour growth thanks to a targeted action on a defined molecule – the tyrosine kinase – which can specifically be found at the level of that tumour. The Iressa is another example of these new generation treatments; it is efficient for pulmonary tumours – among others.

What are the future prospects of the PET technique?

It will probably achieve a great development in clinical oncology. The technological evolution will allow to reduce the duration of the examination to 10 minutes (instead of 30 minutes today).

The availability of new radiotracers represents the most important development source for the PET. At present, we have a very powerful marker that synthesizes DNA, i.e. the fluorothymidine marked with fluorine-18 (FLT). Our ever-improving knowledge of the underlying molecular mechanisms of cancer will make a long series of new molecules available – which can be marked with a positron emitter (isotope) –, as for example monoclonal antibody marking, like for the herceptine.

 

Obviously, the possibilities of that technique are enormous. What mainly slows down its development is its cost for our health care system. Already currently, we are seeing – in our country – some reluctance from the financing authorities and the will to control the savage growth of PET centres in Belgium. For the five coming years, the biggest challenge for the current nuclear medicine team will be to succeed in gathering the necessary financial means within that climate or restriction, so that the patients of the Institute and of the Iris network could benefit from these new fascinating developments in the best conditions.

 

- Back to : Technical Aspect : Introduction

- Back to :

• Some technical aspects of the PET : click here
• Why is the 18-fluorodeoxyglucose a unique tracer ? click here

 

Person in charge : Pr Patrick Flamen

Last reading of this page : January 2008