Abstract P2-23-11: Quantitative gene expression by RT-PCR in histologic subtypes of invasive breast carcinoma: an update in nearly one million cases
Authors: Can Nhu Thuy , Flannery Cynthia A. , Hoag Jess , Akkunuri Alekhya , Bailey Helen , Baehner Frederick 
Published: March 2023

Abstract
Background: Invasive ductal carcinoma (IDC) accounts for ~80% of all invasive breast carcinomas (IBC) with several histologic subtypes comprising the remainder of cases. Some histologic subtypes of IDC (such as tubular carcinoma) have been associated with better prognosis, while other subtypes (such as metaplastic and micropapillary) have been associated with poorer prognosis. Prior studies have shown patients with early stage, hormone-receptor positive IBC that have low Recurrence Score® (RS), as measured by the 21-gene Oncotype DX® (ODX) assay, have little benefit from chemotherapy. Here, we report our experience with the histologic subtypes of IDC and associated patterns of observed gene expression using ODX.
Methods: All US samples submitted for IBC ODX between 2005 to 2021 were reviewed. Ductal carcinoma NOS (DC), tubular carcinoma (TC), cribriform carcinoma (CC), mucinous carcinoma (MUC), lobular carcinoma classic, solid or alveolar type (ILC), pleomorphic lobular carcinoma (PL), medullary/medullary-like carcinoma (MED), metaplastic carcinoma (MET), micropapillary carcinoma (MP), papillary carcinoma (PC) and solid papillary carcinoma (SPC) were included. Quantitative expression of 16 cancer-related genes was measured on a scale from 2 to 15 (relative to reference genes) where 1 unit increment is associated with ~2-fold change in expression. RS was calculated as published. Descriptive statistics for the RS, individual genes (ER, PR, HER2), and gene groups [invasion gene group (IGG) and proliferation gene group (PGG)] were obtained.
Results: A total of 957,624 samples were included in this analysis with 85.4% DC, 9.7% ILC, 2.8% MUC, 0.5% TC, 0.4% PL, 0.4% PC, 0.3% MP, 0.2% CC, 0.2% MED, 0.1% SPC, and 0.02% MET. For all types, a wide continuous range of RS was noted. MET had the highest median RS, followed in decreasing order by MED, PL, DC & ILC, TC, MUC, MP, CC, SPC, and PC. ER and PR were highest among PC and SPC. ER and PR were lowest among MED and MET. HER2 was highest among PC and TC and lowest among MED and MET. IGG was highest in MET and lowest in SPC. PGG was lowest for TC and highest for MED and MET. ER+/PR-/HER2- phenotype occurred more often in MED and PL. ER-/PR+/HER2- phenotype rarely occurred, but was most frequent in MED and MET. ER+/PR+/HER2+ accounted for 0.4% of our total sample.
Conclusions: Here, we demonstrate histologic subtypes of IDC have a wide continuous range of RS. ODX assay may be used to further stratify patients with IDC and its histologic subtypes; however, further studies are needed to better understand the predictive capability of ODX in the histologic subtypes.
Table 1. Quantitative gene expression by RT-PCR in histologic subtypes of invasive breast carcinoma
Table 2. Biomarker profile in histologic subtypes of invasive breast carcinoma
Citation Format: Nhu Thuy Can, Cynthia A. Flannery, Jess Hoag, Alekhya Akkunuri, Helen Bailey, Frederick Baehner. Quantitative gene expression by RT-PCR in histologic subtypes of invasive breast carcinoma: an update in nearly one million cases [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-23-11.