Dr. Atul Gawande just wanted to give a patient some hope. But he ended up saying something he would regret.

In FRONTLINE’s new film Being Mortal, Gawande remembers treating Sara Monopoli, a woman who was diagnosed with Stage IV lung cancer at 34, in the ninth month of her pregnancy. After giving birth to a healthy baby girl, Monopoli was diagnosed with a second disease: thyroid cancer.

In the film, Gawande tells Sara’s husband, Rich, that he knew she would almost certainly die of lung cancer, but he still gave the family hope that an experimental treatment might help treat both cancers. Rich surmises his family’s hope must have been infectious.

“You had joined us,” he tells Gawande. “We had our sunny disposition, hoping for the best.”

“The reason I regret it is because I knew it was a complete lie,” Gawande replies. “I just was wanting something positive to say.”

The conversation captures the dilemma suffered by doctors, families and patients with a terminal illness. The patient faces a painful decision: Whether to keep fighting a disease through every last treatment, trying to live as long as possible, no matter how painfully, or to live out the final days as well as possible.

But his experience with the Monopolis caused Gawande, who also writes for The New Yorker, to reflect: Why is it so hard for doctors to talk about dying with their patients? And how can the medical profession better help people navigate the final chapters of their lives with confidence, direction and purpose?

Being Mortal explores these questions, and the relationships between doctors and patients at the end of life, by following Gawande and his colleagues at Boston’s Brigham and Women’s Hospital and Dana-Farber Cancer Institute.

Rich Monopoli tells Gawande in the film that he wonders about the value of the treatments his wife endured near the end of her life — treatments that made her “sicker and sicker and sicker.” One night, gasping for air, Sara said she couldn’t bear it any longer.

But by then, Rich said, the family had lost time they should have spent focused on each other.

“We should have started earlier with the effort to have quality time together,” he said.

Watch more of the Monopolis’ story in the excerpt below. Being Mortal premieres tonight on PBS (check local listings) and online, starting at 10 pm ET/9 C.