no, not all cancer starts in the stem cells, most cancers start in old regular cells.
Take lukemia for instance, it starts in the cytoblasts (which are imature blood cells) not in stem cells
These cancers on the other hand start specifically IN stem cells which WOULD turn into bone marrow cells or breast cells or whatever. The difference is these float through the whole body so they show up in the lymph nodes NOT in the bone marrow (or the breast)
in other words the primary cancer is 1 or 2 cells spread across the whole body. This means the secondries spread MUCH more rapidly AND there are no symptoms which would make early detection possable. Hence the very high death rate from these cancers, though this depends what the cells were ment to be. If they are slow growing cancers like bowl for instance the death rate is still lower than a rapid one like breast cancer even though there is no primary in the breast or bowl