Many dogs develop illnesses and symptoms that can appear to be symptoms of cancer, but just because cancer dog symptoms are present doesn't mean that they have the deadly disease.
Lack of Appetite
A mast cell tumor will often cause a dog to lose his appetite, however, a dog may lose his appetite due to a cold or indigestion, or it could be as simple as him not being hungry. There are more serious reasons a dog may not eat, which include an obstruction in his digestive system, but that too can be easily treated with surgery.
Lethargy Requires Attention
Canine lymphoma treatment can cause lethargy, as can the cancer itself, but sometimes dogs are lethargic because of the heat or something less serious like a bladder infection or a parasite infection.
Frequent Vomiting
Vomiting in dogs is very common, particularly in dogs that eat grass on a regular basis. A dog that seems to be vomiting more than usual could have a blockage in their digestive track or could have simply eaten something that didn't agree with them.
Most symptoms that dogs display can have multiple diagnoses. Cancer is only one of the possible reasons dogs may appear sick. If your dog is displaying any behavior that is symptomatic of illness it is best that you take your dog to the vet for a check up.