Receptors

"Dogs have got this fantastic sense of smell; three-hundred million sensory receptors, us humans have five million. So they're very, very good at finding minute odors. What we now know is that cancer cells that are dividing differently have different volatile organic compounds -- smelly compounds -- that are associated with the cells. And dogs with their incredible sense of small can find these in things like breath and urine," said Dr. Claire Guest who co-founded charity Medical Detection Dogs in 2008 to train specialist dogs to detect human diseases.
foxnews.com

I get it that dogs can be trained for sniffing purposes since they have incredible sense of smell. The cons I believe is that they can also be easily distracted depending on the mood of the dog. If the dog is hungry, his attention is on the food and not on the work tasked upon the dog.

Compared to a mosquito which has few smell receptors.

Researchers discovered a receptor that mosquitoes use to detect both carbon dioxide and skin odor, and they identified compounds that interact with the receptor. The findings may help guide strategies to control mosquitoes and the diseases they transmit.
nih.gov

Mosquitoes can detect carbon dioxide and skin odor from humans and animals and this is their weapon to hunt for food. Life is much simpler for the mosquito.

So what does this teaches us?

The mosquito has a one-track mind. To search for food. So I think we should trust the doctors and modern cancer management researchers in finding the cancer cells because this is their life.

What I am saying is it is okay to have bomb sniffing dogs cause a bomb is not a food. However if when dogs try to sniff cancer cells in humans or flesh, we will have far from conclusive results because meat is also dog food.