Answer:
Aluminum is a common adjuvant for injectable vaccines for animals and humans and is added to the vaccine because it helps stimulate the immune system to react against the vaccine and build a stronger immune response with fewer booster vaccinations. A fibrosarcoma is a malignant, often aggressively invasive tumor that arise from fibrous connective tissue, which is what forms scar tissue. There is a hypothesis that administration of a vaccine with an aluminum adjuvant can cause overstimulation of the fibrous scar tissue response which then results in the development of a fibrosarcoma.
Cats in particular seem to be sensitive to this and about 1 in 10,000 cats will develop a fibrosarcoma in response to a vaccination.