Differential induction of squamous cell carcinomas, adenomas, and adenocarcinomas was observed in the lungs of male C57BL/6 and C3H/He mice after repeated intratracheal instillation of benzo(a)pyrene (BP) and charcoal powder suspended in 0.9% NaCI solution. When a high dose of BP (1.0 mg BP and 0.5 mg charcoal powder) was instilled intratracheally once a week for 8 weeks or when a low dose of BP (0.5 mg BP and 0.5 mg charcoal powder) was instilled once a week for 16 weeks, squamous cell carcinomas were induced in high incidence (77 to 87%) in the early period of observation, whereas pulmonary adenomas and adenocarcinomas were induced in low incidence (0 to 48%) in the late period of observation in both strains of mice. On the other hand, when a low dose of BP was instilled intratracheally once a week for 8 weeks, pulmonary adenomas and adenocarcinomas were induced in high incidence (76 to 91%), but squamous cell carcinomas were induced in low incidence (9 to 26%). These results show that a larger quantity of BP instilled intratracheally was needed for induction of squamous cell carcinomas than for induction of adenomas and adenocarcinomas in the lung of mice. Thus, when the carcinogen is administered to a single organ of a single mouse strain by the same route, different amounts of carcinogen have different effects on the incidences of various histological types of tumors.
Supported in part by a Grant-in-Aid for Cancer Research from the Ministry of Education, Science, and Culture of Japan.