I'm in the US and have accompanied a friend to her chemo and radiation appointments.
There was no bell to ring at the chemo location, but there was a big noticeboard where survivors could leave a card to encourage others, thank the nurses, oncologists, front desk staff, God, etc. My friend left a card when her treatment was over, though she still goes for regular checkups, blood tests, and mammograms twice a year.
There was a bell at the radiation location. My friend rang it, and the people in the waiting room cheered and wished her well, as did the desk staff. She got a laminated certificate marking the last day of radiation, signed by the desk staff and the radiology techs and doctor, which she was happy to accept.
The bell, cert, and noticeboard are not supposed to be a celebration of "winning" or "kicking cancer in the ass". They do offer the hope of ending treatment, which is often enough for people to look forward to, and encourage others to hope for the light at the end of the treatment tunnel too.
Of course everyone knows that not all the people in the waiting room will get to write a little card and stick it onto the board, or ring the bell with hope for full remission, but equally, it's accepted that people can celebrate their own personal progress through treatment. Nobody going through cancer treatment believes they are out of the woods as soon as their treatment ends. It's OK to publicly mark the milestones along the way all the same, and I don't think allowing a little symbol of hope into a place where people can feel very low is a bad thing.
Some of the noticeboard cards were written by family members of patients, perhaps because their loved ones had died. They were cards expressing gratitude to the staff and encouraging others to have courage, faith, etc.