Medical Anomoly, Metatastic B.T. w/no cancer elsewhere

My husband was diagnosed with a small blue cell tumor in the brain in early March. He had a biopsy and the neurosurgeon was able to remove almost all of the tumor. It had started to snake back into the white matter and he wasn't able to safely remove all the cells.
It was diagnosed, as I said, a small blue cell malignant tumor. The tumor itself was extremely necrotic. It was basically mush. The tissue is in such bad shape that the Mayo Clinic has said that it was lucky they could do as many stains on it as they did. There cannot be any more done on what is left because the tissue cannot handle it.
He had had a contrast CT scan done prior to the surgery which did not reveal anything abnormal, except for a tiny spot on his lower left lung. The report calls it a "tiny nonspecific-appearing nodular density".
Since the surgery, he has had a PET Scan, an Octreoscan, and another contrast CT. The PET Scan and Octreoscan did not show any cancer anywhere else in his body, and even the tiny spot on his lung did not show up.
My husband has been in good health before this. The only was we knew something was wrong was that he lost part of his field of vision - when he looked at the "4" on a clock, he could not see "8" thru "1". There was some mild headaches, but he'd been in a very stressful situation (living in another state for a temporary job) and wasn't anything to be suspicious about. He has never smoked, doesn't drink, works with computers as a programmer, never worked with any chemicals or processing plants of any kind. Was in Army Intelligence working in an office environment. No physical symptoms, unusual pain, etc.
What his oncologists have said is that they want to treat it with whole brain radiation with a boost after to the area that the tumor was in.
The anomoly part is that it is acting as a primary tumor, but pathologically, it is metastatic. It looks like it came from somewhere else, but they've run every test they can and all the scan come out clean. The blood work-ups are all normal. He has no pain anywhere else.
Does anyone have any ideas on this? Has anyone ever heard of having a metastatic brain tumor with no sign of cancer anywhere else?
I will add that his case is being submitted to the Tumor Board at Washington University / Barnes-Jewish Hospital in St. Louis tomorrow night.
Any help would be appreciated. We're so unsure and unsettled.