Welcome to Justanswer! I am Dr. Altman and happy to assist you both today!
I am so sorry your Rottie mix has this lump and I am definitely concerned when you mention that it is growing over the past week...
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
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 the fact that it is rapidly growing. Other symptoms include whether 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 photographing the lump with a ruler or coin next to the lump to measure the size weekly or in dark skinned dogs where it is not as readily viewable measuring with a ruler on a regular basis
Please let me know if this information makes sense and any additional questions I can assist with today
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