Profiling heterogenous sizes of circulating tumor microemboli to track therapeutic resistance and prognosis in advanced gastric cancer

Circulating tumor microemboli (CTM) aggregated by ≥ 2 circulating tumor cells (CTCs) are more migratory than single CTCs. Aside from the plasticity in their molecular characteristics, which have been considered tumor migration, CTM also possesses high size heterogeneity. This study, therefore, systematically investigated the heterogeneous sizes of CTM and their involvement in therapeutic resistance in 114 patients with advanced gastric cancer (GC) using a pre-established surface molecule-independent subtraction enrichment (SE)-iFISH strategy. CTM, which was pre-therapeutically detected in 33.3% of GC patients, can further form in another 34.78% of patients following chemo-/targeted therapies. The presence of CTM is relevant to liver metastasis as well as higher CTC levels (≥ 5/6 mL). Further size-based profiling of GC-CTM revealed that CTM with 2 CTCs (CTM(2)) was the dominant subtype, accounting for 50.0% of all detected GC-CTMs. However, CTM with 3–4 CTCs (CTM(3–4)) specifically associates with chemo-/targeted therapeutic resistance and inferior prognosis. Patients with ≥ 1 CTM(3–4)/6 mL have shorter median progression-free survival and median overall survival. Unlike CTM(2) and CTM(3–4), which are detectable in pre-therapy and post-therapy, larger aggregated CTM(≥5) (CTM with ≥ 5 CTCs) was only intra-therapeutically detected in four HER2(+) GC patients, of which three experienced liver metastases. Obtained results suggested that the cluster size of GC-CTM should be dynamically profiled beyond pre-therapeutic whole CTM enumeration in terms of chemo-/targeted resistance or metastasis monitoring. GC-CTM(3–4) could be a potential indicator of therapeutic resistance, while the dynamic presence of GC-CTM(≥5) implies liver metastasis in HER2(+) GC patients.