Stage IV lung carcinoids: spectrum and evolution of proliferation rate, focusing on variants with elevated proliferation indices

Abstract

The spectrum and evolution of proliferation rates in stage IV lung carcinoids is poorly defined. In particular, there are limited data on the prevalence and characteristics of tumors exceeding the standard upper proliferative criteria-as defined largely based on early-stage carcinoids-in metastatic setting. Sixty-six patients with stage IV lung carcinoids were identified, and all evaluable samples (n = 132; mean 2 samples per patient) were analyzed for mitotic counts and Ki-67 rate. Clinicopathologic and genomic features associated with elevated proliferation rates (>10 mitoses per 2 mm² and/or >20% hot-spot Ki-67), and evolution of proliferation rates in serial specimens were analyzed. We found that mitoses and/or Ki-67 exceeded the standard criteria in 35 of 132 (27%) samples, primarily (31/35 cases) at metastatic sites. Although neuroendocrine neoplasms with >10 mitoses per 2 mm² are currently regarded as de facto neuroendocrine carcinomas, the notion that these cases are part of the spectrum of carcinoids was supported by (1) well-differentiated morphology, (2) conventional proliferation rates in other samples from same patient, (3) genetic characteristics, including the lack of RB1/TP53 alterations in all tested samples (n = 19), and (4) median overall survival of 2.7 years, compared to <1 year survival of stage IV neuroendocrine carcinomas in the historic cohorts. In patients with matched primary/metastatic specimens (48 pairs), escalation of mitoses or Ki-67 by ≥10 points was observed in 35% of metastatic samples; clonal relationship in one pair with marked proliferative progression was confirmed by next-generation sequencing. Notably, escalation of proliferation rate was documented in a subset of metastases arising from resected typical carcinoids, emphasizing that the diagnosis of typical carcinoid in primary tumor does not assure low proliferation rate at metastatic sites. In conclusion, stage IV lung carcinoids frequently exceed the standard proliferative criteria established for primary tumors, and commonly exhibit proliferative escalation at metastatic sites. Despite the overlap of proliferation rates, these tumors show fundamental morphologic, genomic and clinical differences from neuroendocrine carcinomas, and should be classified separately from those tumors. Awareness of the increased proliferative spectrum in metastatic carcinoids is critical for their accurate diagnosis. Further studies are warranted to explore the impact of proliferation indices on prognosis and therapeutic responses of patients with metastatic carcinoids.

Figure 1.

Distribution of mitotic counts and Ki-67 rates in stage IV carcinoids at primary and metastatic sites: dot-pots (A, C, E; bars show mean and standard deviation) and histograms (B, D, F).

Figure 2.

Well-differentiated morphology characteristic of the majority of carcinoids with elevated proliferation rate. Liver metastasis (A-C) and ovarian metastasis (D-F) demonstrating nested/trabecular architecture and bland, monotonous plasmacytoid cytomorphology with low nuclear/cytoplasmic ratios (A, D) but elevated mitotic counts (arrows) and Ki-67 of >20% in hot-spot areas (B, E) in the background of low-proliferative Ki-67 areas (C, F).

Figure 3.

Examples of histologic disorganization and hypercellularity in a minority of carcinoids with elevated proliferation rates. Panels A-F illustrate a liver metastasectomy specimen with well-demarcated areas of increased proliferation associated with increased cell crowding and higher nuclear/cytoplasmic ratios (D, F) in the background of conventional nested/organoid carcinoid morphology with low mitotic and Ki-67 rates (C, E). This was the only specimen in this series where areas of increased proliferation were sharply demarcated from low-proliferative areas by H&E. G and H illustrate specimens from lung (G) and liver (H) in which focal areas of singe cell/cord-like infiltration were intermixed with areas of conventional carcinoid morphology. Arrows in D highlight mitotic figures. Despite increased cellularity and loss of organoid architecture, none of the cases exhibited geographic/confluent necrosis; only limited comedo-like necrosis was present (H, asterisks).

Figure 4.

Histopathologic features and next-generation sequencing in a case showing marked proliferative progression and increased cell crowding at metastatic site (patient 8 in Figures 6A). H&E section from the endobronchial biopsy (A) shows nested pattern and bland nuclei with no identifiable mitoses and uniformly low Ki-67 index (<5%; panel C). Core biopsy from supraclavicular metastasis (B, D) shows elevated mitotic counts (arrows; 11 per 2 mm²), high hot-spot Ki-67 of 40–60% (seen in the majority of the specimen; D-left) but with focal low-proliferative regions (D-right); H&E section shows increased cell crowing with the loss of organoid morphology (B). By next-generation sequencing, primary and metastatic samples harbored identical somatic KDM5C E646A mutation and CDKN1B deletion (E).

Figure 5.

Prevalence of samples with elevated proliferation rate by metastatic site. *P<0.05; **P<0.005.

Figure 6.

Evolution of proliferation rate in stage IV carcinoid patients with >1 evaluable sample. In a heat-map (A), each row represents a single patient and columns represent the values for primary (P) and metastatic samples (M, # in chronological order). White spaces represent absence of value or specimen. “R” indicates resection specimens. Flow chart (B) summarizes proliferation rate changes in matched metastatic (Met) vs primary samples. Specimen types in each group are summarized in a table below the flow chart. Res = resections/excisions, small = non-resection (biopsy/fine needle aspirate) specimens. (C) Graphic representation of evolution and heterogeneity of proliferation rates in multiple samples. Bx biopsy, met metastasis, Res resection

Figure 7.

Comparison of mitotic counts versus hot-spot Ki-67 index in the same sample (n=93) as a function of specimen type. A. Scatter dot plots. Black dots represent resection specimens, gray – biopsy specimens. Bars indicate a mean. B. Summary for the distribution of mitotic counts vs Ki-67 index by specimen type highlighting over-representation of low mitotic counts (<2 per 2 mm²) in biopsies compared to resections (60% vs 22%, P=0.0003).