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"By analyzing the RNA in these serum exosomes, we could determine the mutational profile in the tumor without doing a biopsy," explained Skog. "So, this is an enabling technology to look at what's going on inside the tumor at a given moment."

Like all cancers, glioblastoma cells often contain dozens of mutations. One of these, called EGFRvIII, is a mutation in the gene for a growth factor receptor called EGFR. Skog first screened 30 glioblastoma biopsies for the presence of EGFRvIII; he found it in 14 samples. When he examined blood samples from the same patients, he was able to detect the mutation in 7 samples, including two corresponding to biopsy samples that had come back negative for EGFRvIII.

That particular observation made quite an impression on Fisher. "They were really good at showing that a lot of the tumors that expressed [EGFRvIII], they were able to find it in these microvesicles in the blood, and you go, 'wow'," he said. "And then, even on top of that, they found a few tumors where they weren't able to find it, but we know that EGFRvIII is very specific to glioblastoma in a patient like this, and they were able to find it in the blood but not in the tumor, and you go, 'Hot darn, that's just incredible'."

It may therefore one day be possible to test blood samples from glioblastoma patients to monitor patient progress, check for recurrence, and make individualized treatment decisions. For instance, said Breakefield, one current chemotherapeutic is a monoclonal antibody that specifically targets the EGFRvIII mutation. "Obviously, those people would respond better to this if they had that mutation," she said.

More remotely, it may one day be possible to run early screens for the occurr
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