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I am very pleased to hear that Toby is doing so well.
While hemangiosarcoma is one of the more common masses that we see on the spleen in dogs it is possible that Toby has a more benign mass such as a hemangioma, or a benign fibrous hyperplastic nodule. Other much less common tumors of the spleen include lymphoma or mast cell tumors. Without a biopsy it is impossible to tell which one of those he has.
Given that he is feeling well 6 months after diagnosis I would think he has one of the more benign conditions rather then hemangiosarcoma as even with aggressive treatment odds are that he would have passed away by now from that very aggressive cancer. While I have heard some promising things about I'm Yunity ( polysaccharopeptide or PSP) the sample sizes have been very small, as you stated, and I don't think that he would feel this well even with that therapy.
I can understand that it must be scary for you to think about possibly seeing something that you don't want to on an ultrasound. However if you see the nodule is the same size, or even smaller would that make you rest a little easier? I think I would want to know where my dog stood. Because if the mass is still quite small then you can breathe a sigh of relief and if it is large then you know that you don't have much time left with him, even though the supplement has obviously slowed growth of the mass. It all depends upon your perspective and whether you are a person that needs to know and prepare themselves for what may happen.
I don't think either answer is wrong, because I cannot imagine that you would change anything for him now, it is simply your personal preference.
Please let me know if you have any further questions.