As a long time breeder, former vet's assistant & animal control field officer I can tell you that you should have the sport looked at by your vet right away, although it sounds more like your dog may have an abscess rather than a tumor. The patch of missing fur might be leading to an eruption which would be painful and messy.
As you probably know, if it is cancer, early detection and treatment is better than waiting. If it turns out to be an abscess, your vet can manage it much more safely and clear it up faster than letting it mature on its own.
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