In the book by Jack Herer he talks about it some, chapter on busting
http://www.jackherer.com/chapter15.html">drug war myths, but the doctor who did this study was also one of the main voices behind the cancer scare. He just debunked his own theory and he's been the nations leading lung researcher in the subject for decades. The problem stemmed from more than one small study.
The problem was more that pot and tobacco have certain constituents in common which are thought to be related to cancer so the obvious conclusion drawn was that pot must cause it, or at least there seemed to be a causal link. Over the years he never wavered in that belief but he remained enough of a scientist to wonder why it never seemed to be born out in the results. So he checked, did the largest study yet and got the results mentioned in the report above.
One of the big differences between tobacco and pot is radioactivity, it's the single constituent of tobacco I know of which has been shown to cause cancer on its own. Pot has none. Probably related at least some to how it's grown and fertilized from what I understand, US tobacco is worse than many. Another difference seems to be the contraceptive effect of the thc, Tashkin didn't claim it to be proved in his more recent study but he did grant that the slight negative correlation between cancer and pot only smokers might indicate something more solid. Other studies with cancer tissue tested in lab conditions seem to show similar results.
Even if not contraceptive the positive link to cancer seems dead and I think there's a decent case for research on prevention or treatment as well, at least in lab conditions. If high concentrations or direct to the area applications are needed that might be hard to do with casual use especially with some conditions.