Environment and Breast Cancer: Science Review
Cancer studies: Experimental details
Burek JD, Nitschke KD, Bell TJ, Wackerle DL, Childs RC, Beyer JE, et al. Methylene chloride: a two-year inhalation toxicity and oncogenicity study in rats and hamsters. Fundam Appl Toxicol 1984;4(1):30-47.
Notes
Number of mammary tumor bearing rats did not have an increasing trend, but the total number of mammary tumors per rat did increase in a dose dependant manner (though not evaluated for statistical significance.)
Number of mammary tumor bearing rats did not have an increasing trend, but the total number of mammary tumors per rat did increase in a dose dependant manner (though not evaluated for statistical significance.)
Route
Route of chemical administration: dermal, inhalation, gavage (delivery directly into the
stomach), in feed, subcutaneous injection (under the skin), or intraperitoneal injection (into the
cavity that contains the abdominal organs).
inhalation
Strain
Strain information for the animal species used in the study.
Sprague-Dawley Spartan, Golden Syrian
Doses
Dosage, frequency, and duration of treatment; the sizes of the groups of animals
involved and what age the animals were at the beginning of the study.
0, 500, 1500, 3500ppm, 6 hr/day, 5 days/wk for 2 years. Approx 95 in each group. (More for interim kills). Dosing began at 8 wks old.
Time after cessation of dosing
How long the animals were observed after the chemical was no
longer being administered and before death of the animals.
none
Mammary tumors, benign
Development of benign mammary tumors, reported as a series of
fractions. The numerators represent the number of animals that developed benign mammary
tumors and the denominators represent the total number of animals receiving the particular
dose of chemical. Where available, the denominator will reflect the number of animals alive
when the first tumor developed. Otherwise, it will reflect the number of animals examined. The
order of the fractions reflects the level of chemical treatment, from no dose (controls) on the left
to the highest dose on the right. Where available, the histological type of the tumors will be
indicated, i.e. adenoma or fibroadenoma.
Additional information, in development, includes statistical significance and trend information.
We plan to indicate whether a particular treatment group’s ratio of mammary tumors related to
the control is statistically significantly elevated, as determined by the author. This is indicated
with an asterisk (*). We will also include information indicating whether there was an
increasing, statistically significant dose response trend reported by CPDB. Results that are
statistically significant at p < 0.05 are labeled "T+" and statistical significance between 0.05 and
1.0 is labeled "T~". Where there is no dose-related effect or the trend is identified as decreasing
with dose, results are labeled here as "Tna."
female rats: 79/96, 81/95, 80/96, 83/97; male rats 7/92, 3/95, 7/95, 14/97
total number of rats with tumors not increased, but total numbers of tumors did in male and female rats: males: 8/92, 6/95, 11/95, 17/95; females 165/96, 218/95, 245/96, 287/97
Hamsters- no effect.
Comments
This field contains information on the survival rates of the animals and the body
weight trends in order to evaluate whether these factors were likely to have affected the
generation of mammary gland tumors. Mammary gland tumors tend to develop later in an
animal’s life, so studies with lowered survival could mean that animals died before mammary
gland tumors could develop. Decreased weight (perhaps due to toxicity of the chemical) can
decrease the development of tumors. This field may also contain other comments about the
design or outcome of the study.
Female rats at highest dose had high mortality rate. Hamsters at 1500 or 3500 had decreased mortality rates. Hamsters "lacked evidence of definite target organ toxicity." Animals were palpated monthly starting at the third month. Body weights were consistent across dosed and controls.
"Despite the increased numbers of benign mammary tumors, there was no indication of an increased number or incidence of malignant mammary tumors in either male or female rats exposed to methylene chloride, and thus no indication for progression of benign to malignant mammary tumors." In discussion, notes that SD female rats in their studies generally have 80% mammary tumors.
Other tumors
A list of other tumors that developed in the study that were treatment related.
salivary gland
CPDB TD50 (mg/kg-d)
Data excerpted from the CPDB database that is defined as the "dose-rate in
mg/kg body wt/day which, if administered chronically for the standard lifespan of the species,
will halve the probability of remaining tumorless throughout that period". The CPDB
calculated values for all tumor endpoints listed as well as for total tumors. The range of
mammary gland tumors TD50s is provided, as well as an overall range.
mammary: 4430 (the only TD50 provided)