Difference between taking tissue of cancer from secondary place or primary place
I would disagree a bit with the previous answer. There are absolutely differences between the metastasis and the primary tumor. One result of this is that people very rarely die of primary tumors, but rather from the metastases of those tumors. The primary tumors will be quite heterogeneous and contain largely cells that have not undergone steps necessary for migration like epithelial to mesenchymal transition. After cells have metastasized they will be separate from the original primary tumor, and contain unique genomes. Because this is the more 'progressed' group of cancer cells, these are often the cells that need to be targeted. So, determination of the proper treatment strategy should be done on the metastasized cells.