There's no easy answer when a dying patient asks their doctor: "How long have I got?"
The response to the toughest of questions is even more difficult when the patient needs it to be precise in order to control their own destiny.
Two months ago NSW became the last Australian state to legalise voluntary assisted dying (VAD), meaning the laws will come into effect universally over the next 18 months.
Determining a person's suitability for VAD puts doctors front and centre of the legal hoops that need to be jumped through in deciding who is eligible to take matters into their own hands.
University of Sydney experts say the calculation is an 'inherently uncertain and imprecise' task made more difficult by inconsistent wording across state legislation.
In an article published in the Medical Journal of Australia, authors from the university's NHMRC Clinical Trials Centre discuss the challenges in determining criteria based on life expectancy requirements.
"Prognosis is difficult and by its very nature uncertain," lead author and oncologist Sharon Nahm said.
"However, differences across states in the wording of current legislation could make determining eligibility even more difficult for doctors."
Among the author's concerns are terms like 'on the balance of probabilities,' a legal concept applied to the burden of proof in civil claims and taken to mean 'more probable than not'.
They compare this to 'expected to die within six months' which appears to imply a higher degree of belief.
Dr Nahm says VAD legislation asks doctors to predict an unspecified probability of a patient dying within a particular period.
The team previously conducted research on the accuracy of oncologists' estimates of life expectancy for patients with advanced cancer participating in clinical trials.
They showed most with an expected survival time of less than six months died within that period.
However, that may not apply to people with other terminal illnesses.
"Doctors are not trained to formulate estimates of expected survival time or to explain them to patients," the team wrote.
"We predict many will find it difficult to answer whether they expect individual patients to die within six months."
The researchers suggest legislation could be improved by having clearer definitions and phrases that better correspond with how prognoses are formulated.
They developed a tool to assist oncologists with communicating them to patients with advanced cancer which incorporates best-case, typical and worst-case scenarios.
VAD state-based life expectancy eligibility requirements
* TASMANIA - eligible if patients have a condition 'expected to cause death within six months'
* VICTORIA, SA - eligible if it is 'expected to cause death within weeks or months, not exceeding six months'
* WA, NSW - eligible if it 'will, on the balance of probabilities, cause death within a period of six months'. A year is allowed for people with a neurodegenerative disease.
* QLD - if the condition is 'expected to cause death within 12 months'.