Lung Cancer And Pot

Yes. 
 
It's not ideal, but yes.
 
First of all, a link between head, neck, lung cancer and cannabis smoking has never been found. I'll link you to a study in a bit when i'm less busy.
 
edit: http://www.washingtonpost.com/wp-dyn/content/article/2006/05/25/AR2006052501729.html
 
Second, smoking is one of the easiest and most rapid routes of cannabis administration available. In some cases, like crippling nausea, it's the best way to get medicated quickly and effeciently.
 
All that said, smoking will worsen things like emphysema and COPD, and if lung function is already impaired due to a tumor it might exacerbate some of these.
 
The happy medium is a vaporizer- all the advantages of inhaled cannabis with no downsides, other than in some cases difficulty of use (loading, unloading and cleaning the device).
 
The important point to be made is that smoking cannabis won't make the tumor any worse, but it might make a cough or lesions worse.

 

Any smoke is bad for your lungs... cannabis included...
 
Like stated above they can smoke but it's not ideal. Edibles or other tinctures might be a better route.

 

 
The science behind the effects on the lungs from smoking cannabis is not entirely clear, but the studies discussed in this Time article, Study: Smoking Marijuana Not Linked with Lung Damage, indicate that moderate cannabis smoking is slightly beneficial to lung cancer and functioning.

 

 
Oh cool. Donald Tashkin was the researcher for the study I quoted, and yours is done by different people but found pretty much exactly the same thing. Corroborated! 
 
They even quoted Tashkin:
 

 

I'd like to see a study that compares vaping and smoking, with regard to all of these things.  My hunch would be that vaping would be better, and probably have all the same good effects as smoking, but without the aggravation of lesions and coughing.

 

smoking converts the plant matter over to aromatic hydro carbons from the combustion. In the air path/smoke stream you get all the active compounds from the cannabis when you smoke along with thousands of newly created hydro carbons. The anti oxidation etc.. protective properties in the molecules of the cannabis help to protect the airways from the hydro carbons while you smoke.
 
vaping releases none of these, if you vape over 400 degrees it can produce benzene and one other known carcinogen.
 
pros of smoking- smoking destroys some agricultural ferts and chemicals from the heat so they do not make it into your body but you will still absorb the bunt version of the chemical most likely as a more mutated form lessened by heat.
non pros of smoking- the hydro carbons and the heat from smoking will destroy sensitive cannabis compounds like terpenes, they do not survive the heat.
pros of vaporizing. no hydro carbons and you get/absorb almost the full cannabinoid molecular profile. there are still active molecular compounds in the plant matter after vaping because there is not enough heat involved to pull them out of the fibers of the plant. use the left over for cooking with to get all those compounds out of there.
non pros of vaporizing. it is not hot enough to destroy nasty ferts/chemicals in the herb. you will pull the chemicals into the air stream along with the vapor while you inhale. not sure if this is really bad or not but we eat raw foods and absorb ferts and such everyday. this problem is easily fixed by vaping organic herbs.

 

 
Oh, I've read all of this, I just want to see controlled, peer-reviewed actual scientific studies.  It's the thing that bothers me most about the legal status of cannabis.

 

 
This isn't exactly what you're looking for, but in the study, Vaporization as a Smokeless Cannabis Delivery System: A Pilot Study, the Secondary Outcome Measures found that there are significantly increased levels of CO exhalation after smoking cannabis and very little such increases after vaping cannabis.
 
For anyone who missed the UCSF study which found that "[o]ccasional and low cumulative marijuana use was not associated with adverse effects on pulmonary function," here it is: Association Between Marijuana Exposure and Pulmonary Function Over 20 Years. Note that the study looked only at lung function, not lung cancer, chronic bronchitis symptoms, behavioral or other effects, etc.