Hi Mike,
I look at the typical location over the Governing Vessel, particularly GV 14, as indicating a severe Heat state. Often times, the lesions are pururlent looking reinforcing this concept of severe toxic Heat. So Hoxsey is something I've always advocated for them. But with cancer, I'm a big believer in using multiple synergistic formulas. Hoxsey is moving and cooling, as is Xian Fang Huo Ming Yin, which also has a reputation for purulent lesions, so the two would go well together.
I think that's what I'd do - use both formulas simultaneously if the cat will allow it. Several anti-neoplastic compounds and mechanisms will be in play when you use both together.
Have fun and good luck!
Steve