Lung cancer from cigarette or cigar smoke is caused by the content of the cigarette itself. Because people rarely smoke pure tobacco, they are susceptible to cancer from the artificial chemicals in the cigarette. People who smoke marijuana are usually smoking it in its pure form, which like pure tobacco causes no harm.
Some claim that marijuana may cause lung cancer, but there is no concrete evidence of such. if it were to cause cancer in anyway, I believe the method of smoking would be what causes it, ie., smoking it with tobacco. Marijuana itself does not cause lung cancer, and in fact, THC has been shown to attack and shrink cancer cells.
Although scientists may suspect that marijuana causes lung cancer, there have been no definitive studies that back up the claims that this is in fact true. There have been few studies addressing this question, thus we simply cannot state with confidence that marijuana causes lung cancer. These circumstances might change in the future, particularly with legalization of marijuana gaining ground in Western countries.
Many studies have been done showing the relationships between smoking cigarettes and lung cancer. Of all of the studies done I have never seen one that says smoking marijuana can give you cancer. Marijuana is a natural herb and it seemingly does not contain the cancer causing components that other cigarettes have.