When evaluating this lump, I would advise a veterinary exam and aspiration. This is not a pimple but a mass. In order to determine if it is a benign (noncancerous) versus malignant (cancerous) lump your veterinarian will need to evaluate and aspirate (poke with a needle) to evaluate the material under the microscope
This is really the only way to differentiate the concerning from the non concerning lumps definitively because a cancerous versus a noncancerous lump can appear identical on presentation but I do not like the way this lump looks and would want to rule out a mast cell tumor which is cancerous but with surgical removal is curative.
Evaluation under the microscope will be looking at the lump at a cellular level. Do the cells look normal, is it sebaceous or oil material that is benign or even fatty material indicating a lipoma or are the cells abnormal in appearance? This information is critical in order to diagnose the lump origin and whether it should be removed sooner versus later
Some tell tale signs that a lump might be more of a concern include whether it is rapidly growing, it is bothersome (scratching, licking), it is not easily picked up from the underlying tissue indicating it is attached to the muscle layer. This can be helpful but not 100 % definitive as a fine needle aspiration would be
I always advise continue photographing the lump with a ruler or coin next to it to measure the size weekly but based on the photo you provided this is not a pimple but a mass that should be evaluated under the microscope
Please let me know if this information makes sense and any additional questions I can assist with today. If your dog has Boxer, Pitbull, or Boston Terrier in him then a mast cell tumor is higher probability but other breeds do get these as well.
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