Welcome to Justanswer! I am Dr. Altman and happy to assist you both today!
I am a bit concerned about this lump (thank you for the photos) with its size that even though Wrigley is not bothering it now I am concerned as it grows and the weight of this lump he will start chewing on it soon.
First and foremost, in order to determine if they are benign (noncancerous) lumps versus malignant (cancerous) lumps is to have your veterinarian evaluate and aspirate (poke with a needle) the lumps 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 and with this photo although it is a great one I cannot differentiate between the two.
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 the lump to measure the size weekly
But I would much rather address this mass now while it is this size as it is very challenging to remove in this area of the tail (no skin to close) before he is bothered by it and it potentially abscesses or ulcerates versus later.
Please let me know if this information makes sense and any additional questions I can assist with today
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