Vascellari et al., 2004: "Liposarcoma at the site of an implanted microchip in a dog." The Veterinary Journal. 2004;168:188--190.
Summary: An 11-year-old dog developed a cancerous tumor (liposarcoma) around a microchip that had been implanted approximately 19 months earlier. The tumor was removed and the dog recovered.
Excerpts: "The intact microchip was found completely embedded within the mass...[and] a diagnosis of low-grade liposarcoma was made." // "Veterinary surgeons are... encouraged to check the microchips that have been implanted in pets at least annually, such as when they come in for vaccinations, and report any adverse reaction."