She wouldn't have been as the symptoms of a brain tumor large enough for the sufferer to notice that they have one would not have escaped a medical checkup required for a mission. The "I have a brain tumor' narrative was added to give the story greater dramatic effect and not because it would fit logically into the overall narrative of the program.
A brain tumor of any size would require immediate treatment to prevent its spread. The surgery, radiation treatments or chemotherapy necessary to do this would all be easily detectable by any trained physician and they would be impossible to conceal. If the tumor could not be removed, then the accompanying medical effects (headaches,seizures,etc) would be difficult to conceal and would indicated that an CAT scan or MRI of the patient's brain was necessary.
Finally, to be "medically cleared" for an assignment the presumption could be made that the person involved would have to provide recent medical records. Unless those records were falsified in some manner or somehow not review prior to any medical clearance screening, it is doubtful that a brain tumor sufferer could be cleared for an assignment.