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Hi. Welcome to Just Answer. I'm sorry to hear about your loss. I did some looking into your question. I researched on a veterinary specific website to see if there were any articles or experiences by other vets with that product and an increased level of cancer. I didn't find any specific papers that listed a correlation between the two. There aren't any specific studies that have shown an increase in TCC's in dogs with exposure to the chemical that is in the zodiac tic and flea collars. Most likely the development of the TCC in her was totally unrelated to that products use. I did come across this from a material safety data sheet on the chemical.
"Propoxur was investigated for carcinogenic effects in a 2-year feeding study in mice. Dietary concentrations of 500, 2000 or 8000 ppm were employed in the study. An increased incidence of benign liver adenomas occurred in male mice at 2000 ppm or greater. When rats were fed for 2 years in a single type of diet, urinary bladder neoplasias were observed at concentrations of 1000 ppm and above. Propoxur was not carcinogenic in other types of diets administered to rats at high doses up to and including the maximum tested concentration of 8000 ppm."
Here is the link to it.
It is a different product, but it has the proposer in it so it is relevant.
Here is the actual material and safety data sheet on it.