A method for determining a subject's metastatic potential, or determining a tumor's or cancer's metastatic stage, or determining a subject's need for metastatic disease therapy, or determining treatment for a subject with a tumor or cancer, or determining a treatment's efficacy for treating a metastatic disease, or detecting metastatic disease or a predisposition thereto, in a subject in need thereof, the method comprising: measuring levels of expression of a panel of genes, RNAs, or proteins in a test sample that has been obtained from a synthetically-engineered pre-metastatic niche (pMN) implanted in the subject, wherein the test sample comprises immune cells, stromal cells, or a combination thereof; wherein the measured expression levels of the panel of genes, RNAs, or proteins in the test sample is compared to control levels or to the level of expression of the panel of genes, RNAs, or proteins in an earlier sample that has been obtained from the synthetically-engineered pMN at an earlier time point, and wherein the panel of genes comprises one or more of S100 Calcium Binding Protein A8 (S100a8), S100 Calcium Binding Protein A9 (S100a9), Peptidoglycan Recognition Protein 1 (Pglyrpl), Lactotransferrin (Ltf), Cathelicidin Antimicrobial Peptide (Camp), Elastase 2, Chitinase (Chi3l3), Bone Morphogenetic Protein 15, C-C Motif Chemokine Ligand 22, and C-C Motif Chemokine Receptor 7, or the panel of RNAs comprises one or more RNAs encoded by S100 Calcium Binding Protein A8 (S100a8), S100 Calcium Binding Protein A9 (S100a9), Peptidoglycan Recognition Protein 1 (Pglyrpl), Lactotransferrin (Ltf), Cathelicidin Antimicrobial Peptide (Camp), Elastase 2, Chitinase (Chi3l3), Bone Morphogenetic Protein 15, C-C Motif Chemokine Ligand 22, and C-C Motif Chemokine Receptor 7, or the panel of proteins comprises one or more cytokines and/or one or more chemokines. measuring levels of expression of a panel of genes, RNAs, or proteins in a test sample that has been obtained from a synthetically-engineered pre-metastatic niche (pMN) implanted in the subject, wherein the test sample comprises immune cells, stromal cells, or a combination thereof; wherein the measured expression levels of the panel of genes, RNAs, or proteins in the test sample is compared to control levels or to the level of expression of the panel of genes, RNAs, or proteins in an earlier sample that has been obtained from the synthetically-engineered pMN at an earlier time point, and wherein the panel of genes comprises one or more of S100 Calcium Binding Protein A8 (S100a8), S100 Calcium Binding Protein A9 (S100a9), Peptidoglycan Recognition Protein 1 (Pglyrpl), Lactotransferrin (Ltf), Cathelicidin Antimicrobial Peptide (Camp), Elastase 2, Chitinase (Chi3l3), Bone Morphogenetic Protein 15, C-C Motif Chemokine Ligand 22, and C-C Motif Chemokine Receptor 7, or the panel of RNAs comprises one or more RNAs encoded by S100 Calcium Binding Protein A8 (S100a8), S100 Calcium Binding Protein A9 (S100a9), Peptidoglycan Recognition Protein 1 (Pglyrpl), Lactotransferrin (Ltf), Cathelicidin Antimicrobial Peptide (Camp), Elastase 2, Chitinase (Chi3l3), Bone Morphogenetic Protein 15, C-C Motif Chemokine Ligand 22, and C-C Motif Chemokine Receptor 7, or the panel of proteins comprises one or more cytokines and/or one or more chemokines.