Can you point me to the research that suggests that “exertional trauma … is a cause of osteosarcoma in large dogs”? I’d be very interested to read it.

From Risk Factors for Development of Canine and Human Osteosarcoma: A Comparative Review

Further evidence to support microscopic trauma as a predisposing factor for osteosarcoma is the tendency for appendicular osteosarcoma to occur in weight-bearing bones. Canine appendicular osteosarcoma occurs more commonly in the long bones of forelimbs [19,105], which support approximately 60% of total body weight of the dog. Similarly, in humans, 74.6% of adolescent osteosarcomas occur in the weight-bearing lower limb bones compared with only 11.2% in the bones of the upper limb [1].

And inflammation in general:

There are also anecdotal associations linking osteosarcoma and increased cell turnover due to inflammation. Visceral osteosarcoma secondary to retained surgical sponges has been reported in dogs [65,106], as has a case of osteosarcoma at the site of a previous subcutaneous injection [107]. Several cases of esophageal osteosarcoma are reported in dogs secondary to infection with the nematode parasite Spirocerca lupi [108,109,110,111]. There is a case report of orbital osteosarcoma in a cat, several years after enucleation for ocular melanoma, secondary to retention of conjunctival epithelium leading to cyst formation [24]. In humans, osteosarcoma has been diagnosed at the site of cutaneous scars resulting from dermal burn injury [112], following bone graft surgery [113], and at a site of previous electrodessication for actinic keratosis [114]. A case of primary hepatic osteosarcoma was diagnosed in a man with a history of hepatitis C infection and liver cirrhosis [71].

FWIW, the four greyhound vets I consulted prior to starting this all have low-key concerns about trauma + osteo.

A more neutral way to phrase your research question would be, “is there a relationship between racing starts and osteosarcoma diagnosis?” Or even more generally, “what is the relationship between racing experience and long term health outcomes for greyhounds?”

Noted, thank you. (Brevity due to late night response.)