Lead works very well, but is rarely used anymore. There are just easier, cheaper ways to add mass to an assembly. And no one wants lead in their house.
Unless your dentist clinic is more than 30 or 40 years old, it will not have lead in the walls. X-ray exposure is currently a tiny fraction of what it was back in the day. The last time we did a CT scanner room about 15 years ago, it had lead in the walls though (much more intense radiation in there)