Phospho-histone-H3 immunostaining for pulmonary carcinoids: impact on clinical appraisal, interobserver correlation, and diagnostic processing efficiency

Lung carcinoid tumors are classified as either typical or atypical based on the presence of necrosis and the maximum mitotic count per 2 mm² area. Determining the mitotic count, which is manually conducted on slides stained with hematoxylin and eosin (HE), is time-consuming and subject to high interobserver variability. The objective of this study was to test the sensitivity and specificity of a surrogate mitosis marker, phospho-histone-H3 (PHH3) immunostaining, in the processing of pulmonary carcinoids as compared with the standard HE evaluation. Carcinoid tissue blocks that were available from lung resection specimens were analyzed using HE and PHH3 stains. Two thoracic pathologists and two residents determined the mitotic count on HE and PHH3 stains in accordance with the 2015 WHO guidelines and recorded the time required to complete this task. For both methods, the interobserver agreement among raters for the mitotic count/2 mm² was assessed by conducting intraclass correlation analyses. We found that for both pathologists and residents, the time required to determine the mitotic count using the PHH3 method was reduced compared with the traditional HE method. Furthermore, residents detected more mitoses/2 mm² using the PHH3 stain compared with the HE method. More importantly, the PHH3 method yielded better interobserver agreement than the HE method in terms of mitoses/mm² detection. Overall, our data confirmed that histologic assessments of carcinoid tumors using PHH3 staining provides practical benefits in terms of scoring times, mitosis detection, and reproducibility of mitotic counts. In addition, we found that the benefit was even greater for less experienced pathologists.

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

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.