title: The association between phenotype and size of breast tumors induced by 1-methyl-1-nitrosourea (MNU) injection in rats.
abstract: 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.
  
  
  METHODS
  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.
authors.affiliation: Department of Pathology, School of Medical Sciences, Science University of Malaysia, 16150 Kubang Kerian, Kelantan, Malaysia. hasnan@kb.usm.my
authors.forename: Hasnan
authors.initials: H
authors.lastname: Jaafar
authors.forename: Fauziah
authors.initials: F
authors.lastname: Mohamad Idris
authors.forename: Siti Norasikin
authors.initials: SN
authors.lastname: Mohd Nafi
mesh_terms: D000818:Animals; D002273:Carcinogens / Q000633:toxicity*; D005260:Female; D008325:Mammary Neoplasms, Experimental / Q000139:chemically induced / Q000473:pathology*; D008770:Methylnitrosourea / Q000633:toxicity*; D010641:Phenotype; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D002273:Carcinogens; D008770:Methylnitrosourea