Benign placental mass with fetal growth retardation in a bull mastiff

A 4-year-old bull mastiff presented due to premature labour. The referring veterinarian elected to perform a caesarian delivery and at the time of surgery a 4×4×2 cm round, smooth, red to tan, lobulated soft mass was identified attached to the allantoic surface of the zonary placenta of one pup. Microscopically, this mass was composed of loosely arranged confluent undulating cords of polygonal to columnar epithelioid cells separated by a fine fibrovascular stroma resembling the placental labyrinth. The labyrinthine structure and epithelioid nature of the cells suggested that the mass was of trophoblastic origin. Due to the non-invasive nature of the mass and relatively low mitotic activity, this proliferative trophoblastic mass was considered to be benign. The absence of morphological features supporting malignant behaviour and the recapitulation of the normal labyrinthine architecture led to the diagnosis of a trophoblastic hamartoma. To the authors' knowledge this is the first report of a placental hamartoma in the dog.