The exact anatomical position of the functional abnormality detected on PET imaging may be challenging, particularly with small lesions. The superimposition of CT with PET images provides the exact anatomical position of the abnormality. This integrated combination of functional and anatomical PET CT imaging has made a significant contribution to cancer diagnosis, treatment decisions and response to therapy.
Clinical FDG uptake
Experience has shown that not all cancers show significant FDG uptake, and non malignant causes of increased uptake include inflammatory disorders, non-cancerous bone lesions, normal thyroid and reproductive organs, and following radiation treatment. Normal tissues such as fat, bowel and muscles also show FDG uptake under certain circumstances.
Current Indications for PET CT Cancer Examinations
- Lung Cancer
Lung cancer is divided into 2 main groups
- non small cell lung cancer (NSCLC)
Non small cell lung cancer of various types is the commonest variety of lung cancer (86%), and varies in size from a small single nodule less than 3cm in size, to much larger tumour masses, which extend to the chest wall or the central part of the chest, and may cause part of the lung to collapse. These anatomic features of the cancer can be assessed on the CT examination.
However, these cancers may spread to small lymph nodes in the centre of the chest, or beyond the chest to the abdomen or bones, and these growths are often small and not detectable on the CT.
PET CT is able to identify the presence of these lesions, is therefore indicated in this group of cancers. Large or small cancers with no spread seen on the PET CT are amenable to surgery with the possibility of cure, while other tumours with distant spread are treated by other methods such as radiation or chemotherapy.
- small cell lung cancer (SCLS)
Small cell lung cancer occurs in 14% of lung tumours, grows rapidly and spreads quickly to other locations in the body, so this type of cancer is not suitable for surgery. PET CT is therefore not indicated, and treatment is chemotherapy and/or radiotherapy.
- Solitary pulmonary nodules are also investigated with PET CT of the chest.
PET CT is also invaluable in assessing treatment response to determine if there has been any recurrence of the cancer.
Solitary pulmonary nodules are small nodules 3cm or less in size, often incidental findings on the chest X-ray, which may be non-malignant nodules, residual scars from previous infection, primary lung cancers, or small seedlings from cancers elsewhere in the body, such as the bowel. PET CT is performed to detect which of these nodules are cancers, so that the appropriate treatment can be provided.
- Lymphoma
PET is positive in the 3 major histological classes of lymphoma - diffuse large B cell lymphoma, follicular lymphoma, and Hodgkins lymphoma, with less sensitivity in less common subtypes. PET CT is performed for initial staging in Hodgkins disease, especially in stage 1 and 2 disease, and for aggressive non-Hodgkins lymphoma.
PET CT is indicated for assessment of therapy response, both to demonstrate sites of persistent activity requiring further therapy, and to demonstrate possible recurrence of disease after successful treatment.
- Colo-rectal cancer
PET CT is performed for detection of recurrent disease, and to evaluate lesions in the liver and the lungs which may be tumour seedlings and amenable to surgery. If the CEA level in the blood is elevated, then PET CT is indicated to locate the site of possible recurrence. Initial staging with PET CT is also indicated before surgery in some circumstances.
- Head and Neck cancer
These cancers are usually assessed initially by CT and by MRI (magnetic resonance imaging) both of which provide exquisite anatomical detail, and by direct visual examination through a scope. PET CT does have an important role in evaluating neck swelling due to lymph nodes, giving information about the presence or absence of cancer involvement, and also indicating where the actual cancer is if this cannot be visualized through the scope, a not infrequent occurrence. PET CT is also invaluable for detecting possible local recurrence of the cancer following treatment.
- Oesophageal Cancer
PET CT is performed for cancer in the mid and lower thoracic oesophagus, including cancer at the junction of the oesophagus with the stomach, following the diagnosis which is made by direct vision and biopsy through the endoscope. The cancer may be confined to the oesophageal wall, but may spread to small foci in the abdomen, including the liver, or the lungs or bones. PET CT is invaluable in detecting the presence or absence of these deposits which influences treatment, since surgery can be performed if no spread has occurred, and chemotherapy and/or radiation is available if surgery is contra-indicated due to the presence of these distal lesions.
- Melanoma
Melanoma is an aggressive of skin cancer, and can occur anywhere on the body. PET CT is not indicated when there is only a superficial skin lesion, but a deep lesion may have spread to the local lymph nodes in the groin or armpit, or elsewhere in the body. PET CT is therefore indicated in the more advanced stages of this cancer, since distal spread does not follow any specific pattern in melanoma, and PET CT is a very sensitive method for imaging the entire body to detection of these lesions.
- Other malignancies
PET CT is also useful in evaluating a wide range of other less common cancers, such as some kinds of thyroid or bone cancers, or cancers of the liver and reproductive organs, but PET CT is not routinely required. Specifically, cancer of the prostate and many stomach cancers do not take up FDG, so PET CT has no role in their evaluation. Ultrasound and biopsy are often used in breast cancer but FDG used with PET CT does have a role in some cases to evaluate the distal spread of disease in nodes, particularly when the cancer has recurred.
- Inflammation
Body tissues which are involved in inflammation also take up excessive amounts of glucose, so that they also can be seen on the PET CT images. This is not a usual method of investigation in these diseases, but occasionally the exact site of an infection is not found by other tests, and PET CT may then be performed.
- Other diseases
Cardiac and Brain Diseases
The application of PET CT in these diseases is currently being assessed, and the specific instances when this test will contribute to diagnosis and treatment management are awaited.
With our new 64 slice scanner in summer 2007 we have more potential to evaluate cardiac studies in particular.
