Hi Paula,
Is the atrophy bilateral? I take it it isn't if the suspicion is a tumor. If it is bilateral, though, our top recommendation would be Qing Ying Tang (Clear the Nutritive Decoction). In the absence of a definitive diagnosis, we've had good general success in animals with this symptom, pulse and tongue appearance. The formula's action is quite complex biomedically, but from a Chinese medicine perspective moves Blood, clears Heat and rehydrates the Yin (including the musculature). If it's a tumor, the 'blood moving' effects (i.e. potential anti-neoplastic effects) will probably come in handy, at least if nothing else in relieving inflammation secondary to the tumor. It's quite an effective anti-inflammatory without being immune suppressing. More of an immune modulator in its action.
For a pointer, I'd look at 1/2 to 1 tsp of herbs BID.
Hope that helps you out!
Steve