Cambodia\'s UN-backed war crimes tribunal on Tuesday ruled against freeing the Khmer Rouge\'s ailing former \"First Lady\" on health grounds, and said she would undergo medical treatment instead. The court\'s highest appeal body overturned a decision last month to unconditionally release Ieng Thirith, 79, after experts said she was unfit for trial because she has dementia and most likely Alzheimer\'s disease. \"The supreme court chamber concluded that the original ground for keeping the accused in provisional detention, namely to ensure her presence during the proceedings, remains valid and relevant,\" judges said in a statement. They added that the former social affairs minister would be transferred to a hospital or similar facility for medical treatment \"which may help improve her mental health to such an extent that she becomes fit to stand trial\". Her case will then be reviewed \"no later than six months\" after the start of her treatment, they said. Court spokesman Lars Olsen said it was unclear how soon Ieng Thirith would be moved from the detention facility where she has been held since 2007 with three other former regime leaders. \"I can\'t say how long it will take but obviously it\'s a pressing matter,\" Olsen told AFP. Judges admitted, however, that the chance of Ieng Thirith recovering enough to answer to charges of war crimes, genocide and crimes against humanity was \"slight\" as medical experts have said her condition was unlikely to improve. Freeing Ieng Thirith -- who was Pol Pot\'s sister-in-law -- would have dismayed many Khmer Rouge survivors still haunted by the horrors of the 1975-1979 regime, blamed for the deaths of up to two million people. Led by \"Brother Number One\" Pol Pot, who died in 1998, the Khmer Rouge wiped out nearly a quarter of the Cambodian population, through starvation, overwork and execution in a bid to create an agrarian utopia. Last month\'s ruling on Ieng Thirith\'s health came just days before the start of her long-awaited trial alongside her husband and former foreign minister Ieng Sary, \"Brother Number Two\" Nuon Chea and ex-head of state Khieu Samphan.