Combustion of many things, tobacco, wood, marijuana, coal etc. will produce polycyclic aromatic hydrocarbons which are believed to contribute to the development of some cancers. Though marijuana smoke does contain PAHs as well as other chemicals found in tobacco smoke, there has yet to be a reported case of lung cancer attributed solely to smoking marijuana. A significant difference between marijauna and tobacco smoke is that marijuana smoke contains no radioactive agents. Tobacco smoke does, a result of phosphate fertilizers. One such chemical found in tobacco smoke is Polonium 210, the only component in the smoke shown to cause cancer alone in lab tests. Many people fail to point this out when they are comparing smoking tobacco to marijuana.
Consider all the people smoking light/low tar cigarettes, they are at the same risk of developing lung cancer because the filters don't stop you from inhaling polonium 210.