“I thought I was completely healthy.”

Ryuichi Sakamoto spoke in a whisper at a bistro steps from his West Village home. Silver hair framed his tortoise shell glasses, and the sleeves of his black shirt were pulled over his hands. Over four decades, this 65-year-old pianist and composer worked to become the most popular Japanese musician in the West. He was a founding member of the synth-pop pioneers Yellow Magic Orchestra; his 1980 single “Riot in Lagos” became a linchpin in the development of early electro and hip-hop; his song “Behind the Mask” was covered by both Michael Jackson and Eric Clapton; he won an Oscar for his score to Bernardo Bertolucci’s 1987 film, “The Last Emperor.”

When he turned 40, he also focused his hard work on his own health, adopting a macrobiotic diet and eating organic food. So when a lump appeared on the left side of his neck in 2014, he thought it was just a sign of aging. It turned out to be oropharyngeal cancer. “My faith in ‘health’ was crushed,” he said, grinding the palms of his hands together with a clap. “I could have lost my voice, so I feel very lucky that I didn’t.”

Radiotherapy followed, and a full year of recovery was planned, which meant scrapping a solo album. During treatment, he was brought to tears by vocalists who had previously never moved him, like the Cuban singer Omara Portuondo. He turned down every work request save one, scoring Alejandro G. Iñárritu’s 2015 film, “The Revenant,” for which he was nominated for a Golden Globe.

“Ryuichi is for me a poet and a painter,” Mr. Iñárritu said. “During the last 30 years, Ryuchi’s music has filled my life, and working with him was a great honor.”