The phrase "almost invariably fatal" is correct and usable in written English.
It can be used in contexts discussing outcomes or consequences that are highly likely to result in death or severe harm.
Example: "In cases of untreated advanced cancer, the prognosis is almost invariably fatal."
Alternatives: "nearly always deadly" or "typically lethal".
Malignant mesothelioma is almost invariably fatal.
Pancreatic ductal adenocarcinoma (PDAC) is a genomically diverse, prevalent, and almost invariably fatal malignancy.
Management should be surgical, since the outcome under conservative management seems almost invariably fatal.
Although chronic granulomatous disease was once almost invariably fatal, recent advances in diagnosis and treatment have improved the prognosis.
It is a cancer that can be treated if action is taken promptly; otherwise it is almost invariably fatal.
When they went for a routine 5-month sonogram, they discovered that their baby was suffering from a minuscule but almost invariably fatal condition; the baby's posterior urethral valve was malfunctioning and his bladder wasn't emptying.
Finally, when Jonathan was a year old, a neurologist discovered that he had Canavan disease, a rare genetic disorder that is invariably fatal and almost exclusively strikes Ashkenazi Jews, those who trace their origins to Eastern and Central Europe.
Fair enough, but should someone with a basal cell carcinoma – the commonest skin cancer, which is never fatal and almost invariably easily treated – avoid using the C-word for fear of offending someone with a melanoma?
Almost 5 million Americans suffer from this slow and invariably fatal deterioration of the heart muscle.
There are increasing reports of mesothelioma, tumors of the lung & stomach casing, which are invariably fatal, take 20-40 years to develop, & almost never occur without some, even slight, exposure to asbestos.
