The golden “Good Luck Bell” hangs on a wall in the lobby of the Francis H. Burr Proton Therapy Center at Massachusetts General Hospital, a braided rope dangling from the clapper. An inscription next to it reads: “When my treatments are done . . . this bell must be rung!!” In a place where good news can be hard to come by, the sound of a cancer patient ringing the bell to mark the end of treatments is cause for joy. Most everyone within earshot — fellow patients, families, and hospital staff — celebrates with loud applause. One recent morning, I met 9-year-old Isabelle Harrison of Arizona and 20-year-old Claire Shaw of Virginia, both of whom finished radiation that day. Each beamed as she yanked the rope back and forth, producing the telltale clang. “I’ve been looking at that bell every day since I walked in here,” said Isabelle, who came to Boston in September and wants to start a foundation back home to help kids like her. Claire, who hopes to return to campus soon at Sweet Briar College, walked out of her final session and announced, “I graduated radiation oncology before I graduated college!” Godspeed, graduates.