Cancer treatment outcome is notoriously difficult to predict. The disease spreads rapidly in some and slowly in others. In some cases, the disease is destroyed after treatment, but in others, it returns even stronger and spreads. These factors and others can complicate treatment strategies.A technique that uses MRI on patients about to undergo external-beam radiation therapy for prostate cancer may provide valuable clues as to whether the disease is likely to return and spread. An ill-defined mass (*) of low-signal intensity in the left peripheral zone is associated with extracapsular extension (arrow) that measures 6 mm in radial diameter. This patient developed metastases 41 months after therapy. Reprinted with permission of Radiology.Dr. Fergus V. Coakley and colleagues at the University of California, San Francisco, studied 80 test subjects with prostate cancer, each of whom underwent endorectal MR imaging with a GE Medical Systems unit. Characteristics of their baseline tumors were studied and recorded.Investigators determined that the presence and degree of extracapsular extension (which measures the spread of cancer beyond the membrane surrounding the prostate gland) were important posttreatment indicators of the disease's future course.Patients with an extracapsular extension of >5 mm before treatment were likely to develop future problems. Three of five patients with this level of extension developed metastases 24, 43 and 63 months after therapy. Patients with relatively large extensions may be candidates for more aggressive treatment early on. The investigators said that their results are preliminary and that they want to examine a larger study group. Details of the research were published in the April issue of Radiology.