Would you treat with radiotherapy someone with a pacemaker?

For what? Shouldnt be a problem but sometimes precautions are necessary. Talk to your cardiologist about specifics.
Older pacers. And debibrillators had trouble with /f waves produced in linacs, but newer ones do not. The pacer needs to be out of the direct beam, the cardiologist needs to check, make, model, serial number and determine frequency to monitor function. It should not be a problem to treat.
RT and pacemaker. Every pacemaker has a limit in how much radiation it can take.The dosimeter it's will need your pacemaker's numbers and maker and than he/she will call the company.Once your team knows the limits of your pacemaker then the planning can commence. There shouldn't be any real problems or limitations. As you can see the planning has gotten very complex ;).