Background
Administration of 1-methyl-1-nitrosourea (MNU) is considered a simple and rapid method for inducing breast tumors in rats. While most studies focus on the time frame of tumor development, there are little data on the development of breast tumor in relation to tumor size. Thus the current study was carried out to analyze the phenotype of MNU-induced tumors in relation to tumor size.
Material and Method
Twenty-one 21-day-old female Sprague Dawley rats were administered intraperitoneally with MNU at a dose of 70 mg/kg body weight. The entire palpable tumor was excised when the tumor size reached the diameters of 4.0+/-0.5 mm, 8.0+/-0.5 mm, 12.0+/-0.5 mm, and 16.0+/-0.5 mm and then subjected to histopathological assessment.
Results
Epithelial inclusion cysts and mammary adenomas made up most of the benign tumors, with four cases occurring together with malignant lesions. Ductal carcinoma in situ was seen in tumor sizes of 4.0+/-0.5 mm or less. Among the malignant tumors, the cribriform type was seen predominantly at tumor sizes of less than 12.0+/-0.5 mm, while those with sizes of 12.0+/-0.5 mm or greater were of papillary type. Infiltrating ductal carcinoma-no special type (IDC-NST) commonly seen in humans was also observed at tumor sizes greater than 12.0+/-0.5 mm.
Conclusions
The tumors were found to be mainly of malignant type and the histological features of the induced tumors underwent changes as the tumor grew in size.

Keywords: Phenotype, Methylnitrosourea - toxicity, Mammary Neoplasms, Experimental - pathology, Carcinogens - toxicity