It's either a human or it's dead
Assuming that the miraculous in fact did not happen and the DNA within this tumor-organism is 100% the cancer host's DNA, then we're faced with one of two explanations of how exactly this 'embryo' has formed.
Possibility 1: Someone the cancer has managed to turn a somatic body cell back into the equivalent of a zygote. Currently, modern science has no idea how to do this, we go about cloning differently, but its theoretically possible to take a cell with all the DNA of its host and grow a perfect clone. And, through some weird process of stimulation, the tumor has done so. This is unlikely.
Possibility 2: Somehow the cancer has managed to evolve through the entirety of embryonic development by accident into a separate unique species. That is, somehow, it's managed to mutate its own sub-species of human such that it develops perfectly, despite the fact that it took evolution a good few billions years to get from the first stages of life to the complexity of humans. Any mistake in the development process will kill the fetus. This isn't merely 'unlikely', this is 'unlikely on a scale which cannot be comprehended and might as well be two to the power of infinity minus one'.
(It should be noted that it both cases we need the tumor to stop being a tumor because it'll kill the developing embryo once the tumor forms. So we're just going to assume that once the tumor forms the embryo, it'll stop being a tumor. Technically possible.)
So, looking at that, the answer is 'Yes, it's possible though unlikely that a tumor could accidentally trigger conditions to create an embryo and somehow in the process also stop being a tumor'. It's possible.