Imagine you are the parent of a severely ill teenage girl. Her airway is so compromised that she has needed dozens of operations throughout her short life to allow her to breathe. Now the options have run out. Conventional surgery can offer nothing more. She has, doctors tell you, a year to live - at most.
Yet there's an alternative, in the form of pioneering surgery. It's only been performed a handful of times, and only once before on a child.
In that case it has worked - so far - but the procedure is so new that it's impossible to guarantee the outcome. The decision is yours.
We all fervently hope we will never have to weigh such difficult odds, but for doctors and the parents of the sick children at London's Great Ormond Street Hospital, soul-searching dilemmas like this take place every day. Latterly, television viewers have been invited to share in them via an intimate BBC2 documentary series that has taken cameras into the hospital's wards and operating theatres.
In the final episode, the focus shifts to the ethically complex area of experimental surgery, illuminating an ever-present difficulty: for medicine to advance, doctors must experiment, yet they must also constantly question how far they should push the boundaries with each human life. Among them is Professor Martin Elliott who heads the hospital's cardio-thoracic team, and who faces these formidable life and death issues every working day. And it doesn't get any easier.
"When it comes to innovative procedures, every patient who survives is standing on the shoulders of those who didn't and every operation we undertake learns from the experience of the previous one," says Elliott. "But what that means for the parent is that you are signing a contract with uncertainty."
That is the contract faced by the mother and grandmother of the 15-year-old girl whose story is followed in this episode. Born with only one lung, a heart defect and a slowly closing windpipe that had narrowed to only 2.5mm at its narrowest point, the teenager's condition had been deteriorating despite numerous operations. "She'd suffered immensely from having multiple interventions throughout her life to try and keep the airway open, and eventually she just wasn't making any progress," says Elliott. "We didn't feel we could do any more."
It is against this backdrop that Elliott and his team decided she would be right for pioneering surgery in the form of a tracheal transplant, comprising a donor trachea modified with her own stem cells. It's a procedure that has been performed only eight times previously in the world, with mixed results. And so, after extensive deliberation with his team, and with her family, Elliott took the decision that it was worth trying.
"The first question you always as is 'Have we gone as far as we can?' In a situation like this you want to be confident there's nothing else you can do. Then it's a case of calculating risk, and you triangulate a lot of opinions to do that. In this case the risk was about equal - we knew that without treatment she would die within a year, but with treatment she might survive. But it veered towards intervention in the sense that, if she did survive, her quality of life could be outstanding compared to what it was before."
It is, he acknowledges, a difficult enough judgement for a doctor, and extraordinarily more so for a parent. "What they have to get to grips with is that they're part of an experiment, which means we can't give the same kind of reassurance in the way we can when we've performed similar operations - the more you do, the more percentage accuracy you get. But that takes time, and before you reach that point you can't give guarantees, you can only give an understanding of uncertainty, and that's enormously difficult."
As he does in all experimental surgery cases, Elliott first had to face the hospital's Clinical Ethics Committee. Comprised of people from multiple disciplines, it is an arena in which every aspect of the surgical team's thinking is scrutinised. "It's not about permission, it's about debate, although actually it feels like you're getting permission. But the fact is there is never a decision in ethics, there's no right or wrong. It's best judgement."
In this case the Ethics Committee surgery gave the go-ahead and viewers get to share in an operation that was, Professor Elliott recalls, one of the most difficult he has yet performed.
"It was a huge challenge, but once her new trachea had been inserted she was breathing perfectly. Within two weeks she walked out of hospital." Her prognosis looked good, but tragedy followed: two weeks later, Elliott learned that the girl had suffered another cardiac arrest while undergoing a medical investigation at her home hospital. She died three days later. "It was - is - a tragedy," he says quietly. "I feel so sorry for her family, and for her."
And it is a tragedy that, inevitably, raises uncomfortable questions. "You ask yourself all the time when things go wrong: was there anything that we missed? Did we make the wrong decision for good reasons? In this case, we're not sure exactly what happened, which makes it difficult to know what to improve next time. But we know she'd reached the end of conventional therapy, so in that sense I do not regret trying. If you don't try for that individual patient then you can make no progress."
The uncertain nature of these decisions is revealed elsewhere in the same episode: two desperately ill babies undergo the same radical procedure. One survives and thrives, one dies.
Such dilemmas are, Elliott acknowledges, ever-evolving. "Regenerative medicine of the kind we performed with the trachea transplant is a big, emerging discipline - all the big universities around the world are building dedicated departments because they realise this can offer an alternative to traditional transplantation or to organ repair. Use of stem cells should help reduce the number of donors needed and circumvent the need for drugs that often cause enormous problems post transplant. There's been success building the oesophagus, diaphragm, bladder - they've been done and are very close to human implantation."
There's no doubt that this new technology has extraordinary implications: ultimately, Elliott believes, we will be able to bypass donor organs altogether, using artificial "plastic", mixed with the recipients' stem cells. "You are then looking at the plastic dissolving away leaving just the patient's own DNA," he says. "I think when it comes to simple organs this will be in place in ten years, but for more complex organs it will be longer. And there will be problems along the way, of course. But it is a very exciting time for medicine."
For all that, he never loses sight of the fact that behind each surgery is a human life, and a family. "With each case I always think, 'If this was my child, what would do?' Others believe you should never think like that, you should always be detached. But that's the only way I can do my job. To me that's the point."
Series 2 of Great Ormond Street concludes tonight on BBC2 at 9:00pm