This episode's headline: Diet and Cancer. The guests include Dr. ERNST WYNDER, American Health Foundation; Dr. CLIFFORD GROBSTEIN, National Academy of Sciences; Dr. JOHN HIGGINSON, Cancer Researcher. Byline: In New York: ROBERT MacNEIL, Executive Editor; In Washington: JIM LEHRER, Associate Editor; MONICA HOOSE, Producer; MAURA LERNER, Reporter