Unfortunately, cancers are alive, and they have their own unique DNA that is derived from, but not identical to, the DNA of the host. A cancer, however, is not and never will be a person.
I prefer to stick to brain development as a criterion for determining humanity. Cells behave according to the cell type that they are as soon as they differentiate into that cell type. That is, muscle cells contract, heart cells beat, etc. That means that brain cells process information and send signals to the rest of the body. The brain forms between 2 and 5 weeks after conception. That means that between 2 and 5 weeks, an embryo begins to process information about its environment. The older the embryo becomes, the more sophisticated that processing gets. The brain continues to grow in complexity in form and function until about age 25, and its awareness likewise (i.e. information processing) has continuously existed and grown more complex since it formed.
Thanks for the info. Is there a "simple", i.e., common and accepted test for brain cells as there is for DNA? Admissible in court, etc.?