In a long-awaited verdict, Judge Pavel Smirnov decreed Samsonova must spend the rest of her life in a top security psychiatric institution after a diagnosis of 'paranoid schizophrenia'.
However she is suspected of killing up to a dozen people in St Petersburg but police admit the true toll is unlikely ever to be known.
'We may never know the extent of this granny's killings,' said one source.
The institution - known for its severe regime - was used by Josef Stalin's secret police chief Lavrentiy Beria to shut away political prisoners during the Soviet era.
It is now called Kazan Psychiatric Hospital of Special Purpose and has intensive guarding.
She was seen on CCTV carrying out a pan containing the head.
The pensioner was arrested after stray dogs sniffed human remains close to the block of flats where she killed and dismembered the woman for whom she acted as carer.