Gravity-based microfiltration reveals unexpected prevalence of circulating tumor cell clusters in ovarian cancer

Circulating tumor cells (CTCs) are rare (few cells per milliliter of blood) and mostly isolated as single cell CTCs (scCTCs). CTC clusters (cCTCs), even rarer, are of growing interest, notably because of their higher metastatic potential, but very difficult to isolate. Here, we introduce gravity-based microfiltration (GμF) for facile isolation of cCTCs. We identify cluster break-up as a confounding cause, and achieve ~85% capture efficiency. GμF from orthotopic ovarian cancer mouse models and from 10 epithelial ovarian cancer (EOC) patients uncovered cCTCs in every case, with between 2-100+ cells. cCTCs represented between 5-30% of all CTC events, and 10-80% of captured CTCs were clustered; remarkably, in two patients, more CTCs were circulating as cCTCs than scCTCs. GμF uncovered the unexpected prevalence, frequency and sometimes large size of cCTCs in EOC patients with either metastatic and localized disease, and motivates additional studies to uncover their properties and role in disease progression.