Because vs. In that

You use in that to introduce an explanation of a statement you have just made. 

e.g. I'm lucky in that I've got four sisters

When we use because, we are focusing on the reason. Because is used to express a simple cause effect relationship 

e.g. The boy was absent because he was ill.

Teratomas are unusual forms of cancer in that they are composed of tissues such as tooth and bone, tissues not normally found in the organ in which the tumor appears. - correct

Teratomas are unusual forms of cancer because they are composed of tissues such as tooth and bone, tissues not normally found in the organ in which the tumor appears. - incorrect