“…Different definitions of CUP have been used by different authors over the years, complicating the identification of a homogeneous entity in literature. BM-CUPs have been variably defined either as BM with no previously known cancer (i.e., BM as the first manifestation of a systemic cancer, with the primary site not always remaining unknown) [ 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ], BM in which the primary site has not been identified within a temporal interval from onset (more often 2–3 months) [ 14 , 15 , 16 , 17 , 18 ], or as BM with no primary tumour identified after standard or extensive work-up [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ]. In some studies, the definition of CUP is not clearly stated [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ].…”