Experimental Rhabdomyosarcomas in Rats and Guinea Pigs

The present investigation deals with the morphological study of the subcutaneous sarcomas which developed in rats and guinea-pigs, following a single injection of polyciclic hydrocarbons. Sixty-one rats of both sexes and different ages were injected with 5 mg or 3 mg of 9,10-dimethyl-l,2-benzanthracene (DBMA) (groups I - VI), and 3 groups, each of 8 male rats, 21 weeks old, with 5 mg of 20-methylcholanthrene (MC) (group VII), 5 mg of 3,4-benzopyrene (BP) (group VIII), and 5 mg of 1,2,5,6-dibenzanthracene (DB) (group IX). Four groups of 8-10 guinea-pigs, 26 weeks old, received an injection of 5 mg of DMBA (group X), 5 mg of MC (group XI), 5 mg of BP (group XII), 5 mg of DB (group XIII). Olive oil was used as vehicle. Fourthy-three rats treated with DMBA, 1 rat with BP, and 1 rat with DB, developed subcutaneous tumors. No tumors were obtained in the rats treated with MC. Subcutaneous tumors arose in 6 and 4 guinea-pigs treated with DMBA and with MC respectively, but not in guinea-pigs treated with BP or DB. The reason of the negative results in rats injected with MC and of the low incidence of tumors in rats injected with BP and DB, is to be related to the high early mortality of the animals of these groups, due to the toxicity of the treatment. All the subcutaneous tumors resulted to be rhabdomyosarcomas. The diagnosis was made on the basis of the presence of typical cells, such as round or oval giant cells, strap-shaped and racquet-shaped cells, of the nuclear and cytoplasmic characters, and of the arrangement of the reticular fibers. Cross striations were observed only rarely, but their presence has not been considered necessary for the diagnosis of rhabdomyosarcoma. Development of rhabdomyosarcomas in rats treated with various oncogenic agents has been reported by other authors: it is the first time, however, that these tumors are seen to arise in such a high incidence. As far as we know, these tumors have never been reported to occur in guinea-pigs.