Although MR spectroscopy is used to distinguish recurrent tumor and radiation injury in patients previously treated with radiation for brain neoplasm, A review in the September issue of the American Journal of Neuroradiology suggests that further studies need to be done to determine whether that technique alone—or combined with radiologic methods—can obtain the highest sensitivity, specificity and accuracy.
Differentiating between a recurrent tumor or a progressive tumor from radiation injury after radiation therapy is a radiologic dilemma, regardless of the imaging technique used, wrote Pia Maly Sundgren, MD, PhD, of the division of neuroradiology, department of radiology at University of Michigan Health Systems in Ann Arbor, Mich., in her review article.
According to Sundgren, most of the brain neoplasms have been subject to radiation or chemotherapy and many of the tumors do not have specific imaging characteristics that enable neuroradiologists to distinguish between tumor recurrence and injury from radiation treatments or chemotherapy.
The clinical course most often taken, wrote Sundgren, is a brain biopsy or imaging over a lengthy follow-up interval--in order to distinguish between the tumor recurrence and radiation injury—and not the initial imaging itself. And considering the risks involved with radiation therapy—Sundgren pointed to data that suggests that progressive dementia occurs in approximately 20–50 percent of patients with brain tumor who are long-term survivors after treatment with large-field partial- or whole-brain irradiation—a "noninvasive tool that could differentiate these entities when a new enhancing lesion is first identified would be invaluable.”
In her article, Sundgren reviewed the utility of MR spectroscopy in distinguishing between recurrent tumor and radiation necrosis or injury. According to Sundgren, a major problem when reviewing the literature and comparing spectroscopy data are the various ways of calculating metabolite ratios. Different methods have been used, she wrote, but there is no consensus on which type of differently calculated ratios are best able to differentiate between recurrent tumor and radiation injury.
However these ratios are calculated, a drawback in most of the studies in this area, said Sundgren, is a limited correlation with histopathology. One study that she reviewed with correlating histopathology showed that MR spectroscopy is a “helpful” tool in differentiating pure necrosis from normal tumor and from pure tumor, but that it has difficulty differentiating when a specimen is a mixture of recurrent tumor and radiation necrosis.
Although ratio cutoffs could help define the capability of MR spectroscopy to detect statistically significant differences between patients with recurrent tumor and those with radiation injury, Sundgren wrote that “a more clinically applicable measure of the utility of a test, in this case MR spectroscopy, is needed.”
Sophisticated prediction models and larger multicenter studies validating these models, as well as larger prospective studies using methods such as 3D MR spectroscopy are needed to “reach a consensus about the true value of MR spectroscopy as a method of differentiating recurrent tumor from radiation injury,” Sundgren wrote. “Future investigations should be focused on trying to determine if MR spectroscopy alone can be the tool that is needed or if the technique should be used only in combination with other radiologic methods.”