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Metas­ta­sis, or metasta­t­ic dis­ease, is the spread of a can­cer or dis­ease from one organ or part to anoth­er not direct­ly con­nect­ed with it. The new occur­rences of dis­ease thus gen­er­at­ed are referred to as metas­tases (some­times abbre­vi­at­ed “mets”). It was pre­vi­ous­ly thought that only malig­nant tumor cells and infec­tions have the capac­i­ty to metas­ta­size (also spelled metas­ta­sise); how­ev­er, this is being recon­sid­ered due to new research. Metas­ta­sis is a Greek word mean­ing “dis­place­ment”, from μετά, meta, “next”, and στάσις, sta­sis, “place­ment”. Can­cer occurs after a sin­gle cell in a tis­sue is pro­gres­sive­ly genet­i­cal­ly dam­aged to pro­duce cells with uncon­trolled pro­lif­er­a­tion. This uncon­trolled pro­lif­er­a­tion, mito­sis, pro­duces a pri­ma­ry het­ero­gene­ic tumour. The cells which con­sti­tute the tumor even­tu­al­ly under­go meta­pla­sia, fol­lowed by dys­pla­sia then anapla­sia, result­ing in a malig­nant phe­no­type. This malig­nan­cy allows for inva­sion into the cir­cu­la­tion, fol­lowed by inva­sion to a sec­ond site for tumori­ge­n­e­sis. Some can­cer cells acquire the abil­i­ty to pen­e­trate the walls of lym­phat­ic and/or blood ves­sels, after which they are able to cir­cu­late through the blood­stream (cir­cu­lat­ing tumor cells) to oth­er sites and tis­sues in the body. This process is known (respec­tive­ly) as lym­phat­ic or hematoge­nous spread. After the tumor cells come to rest at anoth­er site, they re-pen­e­trate the ves­sel or walls and con­tin­ue to mul­ti­ply, even­tu­al­ly form­ing anoth­er clin­i­cal­ly detectable tumor. This new tumor is known as a metasta­t­ic (or sec­ondary) tumor. Metas­ta­sis is one of three hall­marks of malig­nan­cy (con­trast benign tumors). Most neo­plasms can metas­ta­size, although in vary­ing degrees (e.g., basal cell car­ci­no­ma rarely metas­ta­size). When tumor cells metas­ta­size, the new tumor is called a sec­ondary or metasta­t­ic tumor, and its cells are sim­i­lar to those in the orig­i­nal tumor. This means, for exam­ple, that, if breast can­cer metas­ta­sizes to the lungs, the sec­ondary tumor is made up of abnor­mal breast cells, not of abnor­mal lung cells. The tumor in the lung is then called metasta­t­ic breast can­cer, not lung cancer.