Also remember that under  new NIH rules, in the USA the default is using both male and female animals in biomedical research, and strong justification is needed to exclude one of the sexes.

The answer is yes. However, sex-dependent effects are likely to contribute to the development and progression of tumors. Therefore, inclusion of male rats in the same environment is important to identify potential signaling pathways.

Also remember that under  new NIH rules, in the USA the default is using both male and female animals in biomedical research, and strong justification is needed to exclude one of the sexes.

There are so many differences that researchers are starting to discover about gender differences that excluding either sex would probably present a big confound. As already mentioned by the above replies, it is best to include both sexes in doing experiments. This will ensure that the effects observed are true for both sexes (especially for clinically transferable research such as cancer) and if a pattern of difference between the sexes emerge, you might discover something important and ground-breaking. Imagine if a protocol that induces lung cancer in males doesn't do so in females, that would be a very interesting thing to study in itself!