Positron emission tomography (PET) utilizing the glucose analogue 2-18fluoro-2-deoxy D-glucose (18FDG) yields physiological information with the majority of malignant tumours having an increased glycolytic rate compared with normal tissues. 18FDG PET has been used successfully in a large number of tumours to diagnose malignancy in regions difficult to biopsy, to stage tumours before therapy and to follow up the effects of treatment.
Breast cancer can be identified but detection depends on the method of interpretation, the size and the histology of the tumour. Avril et al.[7] studied 144 patients with palpable masses and used a conventional and sensitive image reading. The overall sensitivity was 64% and 80%, with 35% and 19% false-negative rates, respectively. The specificity dropped from 94% to 75% using the sensitivity reading. The limitations were: poor detection rate for small (<1 cm) tumours; inability to detect non-invasive tumours (sensitivity 25–41%) and lobular carcinomas (65% false-negatives). FDG-PET also detected only 50% ofmulticentric tumours. Indeterminate lesions will still require biopsy but FDG-PET is not affected by breast density, previous surgery or radiotherapy and benign breast disease, which accounts for false-positive results on MRI, will be negative on FDG-PET.
In patients with known breast cancer, FDG-PET may be helpful in identifying involved axillary nodes with reported sensitivity of 94%[8] although other authors report sensitivity as low as 50%[9], in assessing brachial plexus pathology and detecting distant metastases. Primary chemotherapy is now used in patients with locally advanced disease and FDG-PET is useful for predicting the response based on glucose metabolism. Schelling et al.[10] studied a small group of patients with locally advanced disease and, based on the final histology, divided them into those with minimal residual disease, who have a significantly improved survival, and those with gross residual disease. These authors found a reduction in the standardized uptake value (SUV) of FDG-PET of 55% of baseline predicted which patients would respond. After the first course the sensitivity for assessment was 100%, with specificity of 85%, and accuracy 88% and after the second course the sensitivity was 83%, specificity 94% and an overall accuracy of 91%. In this small series the only patient with minimal residual disease that did not show a significant reduction in SUV had a large in situ component, which is known to be chemoresistant.
Similarly, Tc sestamibi may be of value in assessing response with a decrease in uptake of more than 40% identifying macroscopic complete response (sensitivity 100%, specificity 89%)[11] and using contrast-enhanced MRI, changes in the uptake curves can predict response with a flattening of the uptake curve in responders[12].