CANCER METASTASIS AND INVASION
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Things seems to become blurry rapidly when you approach this subject because of the intertwined steps involved in these processes. Cancer cells do not spread with intent to kill the host. In solid tumors,  Evidences suggest however that in order to survive its local expansion, it needs to find food, building materials and Oxygen   elsewhere.  Indeed local growth can only support so much, migration of stressed  cancer cells will eventually Occur.
Like Many cellular processes, Metastasis and invasion are molecular based and involves:
1. Genes of metastasis initiatiation
2. Proteins formation to trigger and support the necessary functions needed for metastasis
3. Receptors both cytoplasmic and at the cellular membranes to amplify metastasis and cellular growth, migration and invasion.  also receptors of autocrine function, cell defense, attachment to future location (Preferential Homing and vascular doors to allow extravascular localization)
4.  Through those Receptors, signal transduction pathways are mostly activated but sometimes suppressed and this lead of course to amplifications of certain transcription factors. 
5.protection against immune mediated defense, protection against harmful substrate in the Extracellular matrix, protection against host tissue cellular defense)
6.Migration will not start without Detachment from the local site, protection against Anoikis, generation of adhesion molecule to blood vessel, formation of of enzyme to break the path of migration, toward the new home, and
7. Seeding and ensuring new Growth in the new location.

In hematologic malignancies, unless we speak of lymphoma, the malignant cells own the blood environmemts which include the bone Marrow matrix.  Homing becomes more of an issue. The skin is one of the major target.  Plasmacytoma and Granulocytic Sarcoma have a somewhat wider or different list of locations.