Metabolomic Imaging for Human Prostate Cancer Detection

Imaging Prostate Cancer: Now You See It

Getting regular checkups for prostate cancer can give a sense of security to men, but the tests used for this screening are not infallible. High concentrations of PSA (prostate-specific antigen) in the blood are a red flag but can be caused by several benign conditions. Even the gold standard, a prostate biopsy, only reveals whether cancer is present in the 8 to 16 small regions of the gland into which the tiny biopsy needle was inserted; tumors between the biopsy sites are often missed. As reported here by Wu et al., sophisticated imaging methods may soon help to improve this diagnostic dilemma by creating three-dimensional metabolic maps of the prostate that pinpoint cancerous regions.

Magnetic resonance spectroscopy can examine an array of compounds within living cells by pulsing the intact tissue with radio-frequency pulses in a magnetic field and measuring the collective reactions of the resident metabolites. A normal prostate will yield a characteristic complex spectrum of metabolites, whereas cancerous tissue will yield an equally complex but altered spectrum. The authors used principal components analysis and discriminant analysis—statistical methods used to extract information from complex data— to identify portions of the signal that could best distinguish normal tissue from tumor tissue and, from this analysis, they calculated a malignancy index from the spectra.

Five surgically removed human prostates, each with confirmed prostate cancer, were imaged in this way with a 7-tesla MRI scanner designed for use on patients. Maps of the malignancy index across the width of the five organs showed a total of 13 hot spots, areas with a high chance of being cancerous. The authors then overlaid a second set of prostate maps identifying tumors in the old-fashioned histological way, by careful staining and study by a pathologist. These histological maps revealed five tumors within the area of the metabolic index maps, each of which corresponded to a region with a high malignancy index by magnetic resonance spectroscopy. The remaining regions with high malignancy indices corresponded to areas that appeared histologically benign. This is an accuracy of 93% when determined on a region-by-region basis.

Before clinical radiologists can apply such a technique to human patients, researchers must solve technical challenges associated with imaging the prostate in living patients in high-field 7-tesla magnets. But eventually, the application of metabolite-based imaging could help with detection and accurate biopsy of prostate cancer, and may ultimately provide specific metabolic information to noninvasively inform diagnosis of cancer subtypes and prognosis of the course of the disease.