Promises in the Dark (1979)

Numbed by career demands and a recent divorce, Dr. Alexandra Kendall (Marsha Mason) hides behind a hard shell of professional detachment, until she treats Buffy Koenig (Kathleen Beller), a dying 17-year-old cancer patient who reawakens Kendall to life's possibilities. Soon the courageous girl's condition takes a downturn, and the physician must carefully, even agonizingly, weigh the consequences of keeping a promise she made to Buffy.