Human epidermal growth factor receptor 2 expression loss after neoadjuvant chemotherapy in localized gastroesophageal adenocarcinoma: a retrospective study

Introduction

Human epidermal growth factor receptor 2 (HER2) protein overexpression or ERBB2 gene amplification occurs in approximately 10–20% of gastric and gastroesophageal junction (GEJ) adenocarcinomas and represents an established therapeutic target in advanced/metastatic disease (1). While HER2-targeted approaches have transformed management in metastatic settings, their role in localized disease remains under investigation (2,3). Several perioperative studies combining HER2-targeted therapies with chemotherapy and/or immune checkpoint inhibitors have reported encouraging pathological responses, although no definitive survival benefit or standard perioperative strategy has yet been demonstrated (4-10). Accurate HER2 assessment is nevertheless challenged by the marked spatial heterogeneity of gastroesophageal adenocarcinoma. HER2 expression may vary both within and between tumor regions, potentially compromising biomarker reliability (11). Paired-sample analyses have reported relevant discordance rates between diagnostic biopsies and matched surgical specimens (SS), with reported rates ranging from approximately 5% to 20% across retrospective series (12-15). These discrepancies are generally attributed to intratumoral heterogeneity, sampling variability, and methodological differences in HER2 interpretation (16-18). Although current guidelines recommend obtaining multiple tumor-containing biopsies (5–8 biopsies) (Table S1) to improve representativeness, the relationship between sampling adequacy and HER2 discordance remains incompletely characterized. Beyond baseline heterogeneity, HER2 expression may also evolve through time. Dynamic HER2 loss has been described following trastuzumab exposure in advanced disease, supporting the concept of HER2 plasticity and clonal evolution under therapeutic pressure (19,20). However, these observations derive almost exclusively from metastatic settings following HER2-targeted exposure. Whether HER2 status may change under neoadjuvant chemotherapy in localized disease—and whether such changes could have clinical implications—remains insufficiently explored. The potential modulation of HER2 expression through clonal selection in operable gastric cancer therefore represents an important unresolved question. These considerations also raise the need for improved sampling strategies, systematic reassessment of HER2 status, and potentially the integration of circulating tumor DNA (ctDNA) approaches to monitor ERBB2 amplification during disease evolution.

In this single-center study, we aimed to: (I) assess the concordance of HER2 status between endoscopic biopsies and SS in localized gastric/GEJ adenocarcinoma; (II) explore whether biopsy fragment number is associated with HER2 discordance; and (III) describe the dynamics of HER2 expression under neoadjuvant therapy. We present this article in accordance with the STROBE reporting checklist (available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0393/rc).

Methods

Study design and patient selection

We conducted a retrospective observational single-center cohort study including all consecutive patients with localized gastric or GEJ adenocarcinoma who underwent curative-intent resection at a single tertiary cancer center between 2017 and 2021. The study was approved by the Institutional Review Board of Paoli-Calmettes Institute (IRB SHERPAGOL-IPC 2025-026). The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Due to the retrospective nature of the study, individual informed consent was waived.

Inclusion criteria were clinically and histologically confirmed localized gastric or GEJ adenocarcinoma, and availability of HER2 status assessment in both pre-treatment biopsies (PTB) and SS. Patients with metastatic disease at diagnosis or with incomplete medical records (defined as absence of HER2 testing on diagnostic biopsies) were excluded. Clinical, pathological, and treatment data were extracted from electronic medical records.

Treatment characteristics

Neoadjuvant chemotherapy regimens were recorded as categorical variables: FLOT (5-fluorouracil, leucovorin, oxaliplatin, docetaxel), FOLFOX (5-fluorouracil, leucovorin, oxaliplatin), cisplatin/5-fluorouracil ± pembrolizumab, CROSS regimen (chemoradiotherapy: 40.2 Gy plus weekly carboplatin-paclitaxel), or no neoadjuvant therapy (upfront surgery).

Pathological assessment and HER2 testing

Pathological data were extracted from PTB and SS pathology reports. HER2 status was primarily extracted from routine pathology reports. No systematic central pathology review was performed. Cases with missing HER2 status in the pathology report underwent expert pathological reassessment. Collected variables included histological subtype [according to the 5th World Health Organization (WHO) edition classification] (21), tumor differentiation grade, and pathological staging parameters including lymphovascular and perineural invasion. For each case, the number of biopsy fragments used for pathological assessment and HER2 testing was recorded as a surrogate for biopsy sampling adequacy. HER2 was assessed on formalin-fixed paraffin-embedded (FFPE) tissue using standardized immunohistochemistry (IHC) and scored as 0, 1+, 2+, or 3+ according to gastric-specific criteria, on both biopsy and SS (16). IHC analysis was performed using the Ventana anti-HER2/neu (4B5) antibody, and dual-color dual-hapten in situ hybridization (DDISH) was performed using the Ventana HER2 Dual ISH DNA Probe cocktail. All assays were performed on 3 µm tissue sections from FFPE blocks. In SS, a score of 0 was assigned if there was no reactivity or membranous reactivity in <10% of tumor cells. A score of 1+ was assigned if faint or barely perceptible membranous reactivity was seen in ≥10% of tumor cells, with cells reactive in part of their membrane only. A score of 2+ was assigned if weak to moderate complete, basolateral, or lateral membranous reactivity was seen in ≥10% of tumor cells. A score of 3+ was assigned when strong complete, basolateral, or lateral membranous reactivity was seen in ≥10% of tumor cells. Scores of 0 and 1+ were regarded as HER2-negative, score 2+ as equivocal, and score 3+ as HER2-positive. In biopsy specimens, the 10% threshold for tumor cell staining is not applied; HER2 positivity is defined by the presence of membranous reactivity in a cluster of at least five cohesive tumor cells, irrespective of the proportion of stained cells. In situ hybridization (ISH) was used in HER2-IHC-2+ cases to determine ERBB2 gene amplification status. HER2 status was ultimately classified as positive (IHC-3+ or IHC-2+ with amplification), negative, or low according to combined IHC/ISH interpretation.

Concordance analysis

Concordance between biopsy and surgical specimen was evaluated in binary form and by IHC score. For binary concordance analyses, HER2-IHC-2+/ISH-negative tumors (HER2-low/equivocal non-amplified) were classified as HER2-negative. Four concordance patterns were defined: (I) positive/positive (PTB+/SS+): HER2-positive both on biopsy and surgical specimen; (II) positive/negative, or HER2 loss (PTB+/SS−): HER2-positive biopsy, HER2-negative surgical specimen; (III) negative/positive, or HER2 gain (PTB−/SS+): HER2-negative biopsy, HER2-positive surgical specimen; and (IV) negative/negative (PTB−/SS−): HER2-negative both on biopsy and surgical specimen. HER2 loss and HER2 gain were grouped as discordances. For dynamic analyses, we primarily focused on patients with HER2-positive biopsies to assess treatment-associated changes, particularly HER2 loss.

Statistical analysis

Statistical analyses were primarily descriptive. Patient, tumor, and treatment characteristics were summarized using standard descriptive statistics. HER2 concordance between PTB and SS was evaluated using contingency tables, with concordance and discordance rates reported as percentages. The impact of biopsy sampling adequacy was explored by comparing HER2 concordance and discordance rates according to the number of biopsy fragments. Changes in HER2 status under neoadjuvant treatment were described by reporting the proportion of patients with HER2 status stability or variation between PTB and SS. Given the exploratory nature of the study and the limited sample size, all analyses were descriptive and hypothesis-generating, and no formal inferential testing was performed. The sample size was determined by the number of eligible patients during the study period; no formal sample size calculation was performed due to the exploratory nature of the study. No imputation for missing data was performed; analyses were conducted on available data.

Results

Patient, tumor, and treatment characteristics (Table 1)

A total of 57 consecutive patients with localized gastric or GEJ adenocarcinoma and complete HER2 assessment on both biopsy and surgical specimen were included in the analysis. Median age was 67 years [interquartile range (IQR), 55–72 years], and 77% of patients were male. Tumors were predominantly located at the GEJ (cardia; 40%). According to the WHO classification, most tumors were tubular adenocarcinomas (47.4%), followed by mixed adenocarcinomas (24.6%), poorly cohesive carcinomas (21.1%), and undifferentiated carcinomas (3.5%). Six patients (10.5%) underwent upfront surgery. Neoadjuvant chemotherapy was administered to 51/57 patients (89%), with FLOT being the most frequently used regimen (n=30, 53%), followed by FOLFOX (n=12, 21%), cisplatin/5-fluorouracil ± pembrolizumab [n=6, 11%; Keynote-585 trial (22)], and CROSS chemoradiotherapy (n=3, 5%). Among FLOT-treated patients, 24/30 (80%) completed the standard four preoperative cycles. Adjuvant chemotherapy was administered to 42/57 patients (74%). A fully completed perioperative chemotherapy course was reported in 24/51 patients (47%). Pathological staging showed ypT3 stage in 61% and ypN1 in 33% of surgical samples.

HER2 status and biopsy-surgical specimen concordance (Table 2, Figures 1,2)

Figure 1 Distribution of global HER2 transition patterns between biopsy and surgical specimen (stable negative, stable positive, HER2 loss, HER2 gain). HER2, human epidermal growth factor receptor 2; Neg, negative; Pos, positive.

Figure 2 Alluvial diagram showing transitions between HER2 IHC categories on biopsy (HER2 0+/1+/2+/3+) and surgical specimen. Band thickness represents the proportion of patients undergoing each transition. HER2, human epidermal growth factor receptor 2; IHC, immunohistochemistry.

HER2 positivity was observed in 12/57 biopsies (21.1%) and 9/57 SS (15.8%). Overall, 50/57 pairs (87.7%; 95% CI: 76.3–94.9%) were concordant: PTB−/SS− in 43 patients and PTB+/SS+ in 7 patients. Discordant cases included PTB−/SS+ (HER2 gain) in 2 patients (3.5% of the entire cohort; 22.2% of HER2-positive specimens), and PTB+/SS− (HER2 loss) in 5 patients (8.8% of the cohort), yielding an overall discordance rate of 12.3%.

When considering IHC score level, substantial within-category discordance was observed between PTB and SS (Table 2). HER2-IHC-3+ tumors accounted for 8.8% (5/57) of PTB and 10.5% (6/57) of SS. Among the five biopsy HER2-IHC-3+ cases, three remained 3+ on the corresponding surgical specimen. Regarding IHC-2+ tumors, 15 cases were classified as IHC-2+ on PTB, of which 46.7% (7/15) were ISH-positive. In contrast, 14 tumors were scored as HER2-IHC-2+ on SS, with only 21.4% (3/14) demonstrating ERBB2 amplification (Table S2). Lower IHC scores (IHC-1+) also showed reclassification between PTB and SS, highlighting variability at the individual score level beyond binary HER2 status.

Impact of biopsy sampling adequacy on HER2 discordance (Table 3)

Fragment counts were available for 52 of 57 patients (91%), with missing data in 5 cases. The median number of biopsy fragments was 5 per patient (IQR, 4–6), and no cases presented fewer than 3 fragments. There was no clear relationship between fragment number and HER2 concordance between PTB and SS. Discordance rates were 0/7 (0%) for 3 fragments, 2/16 (12.5%) for 4 fragments, 2/9 (22.2%) for 5 fragments, and 1/8 (12.5%) for 6 fragments. Discordant cases were also reported in higher fragment count (≥7) biopsies (Table 3).

Dynamic changes in HER2 status with neoadjuvant therapy (Table 4, Figure 2; Tables S3,S4, Figure S1)

We focused on the 12 patients with HER2-positive PTB to assess changes in HER2 status following neoadjuvant therapy (Figure S1). Overall, 5/12 patients (41.7%; 95% CI: 15.2–72.3%) exhibited HER2 loss on the surgical specimen, while HER2 positivity was maintained in 7/12 cases. When restricted to patients who received neoadjuvant treatment (n=10), HER2 loss occurred in 5/10 cases (50.0%; 95% CI: 18.7–81.3%). Descriptive regimen-specific distributions are reported in Table 4. Patients who underwent upfront surgery without neoadjuvant therapy (n=2) showed no change in HER2-positive status and were not included in this treatment-related analysis. In all HER2-loss cases (n=5), HER2 expression decreased to IHC-0 or IHC-1+ without amplification in most cases, while one tumor shifted from IHC-2+ amplified to IHC-2+ non-amplified, consistent with true loss of ERBB2 amplification rather than minor IHC variability. Two patients demonstrated HER2 gain after neoadjuvant treatment, converting from HER2-negative biopsies to HER2-positive SS. In both cases, HER2 status shifted from biopsy IHC-1+/HER2-negative to surgical specimen IHC-3+/HER2-positive. Both patients had received neoadjuvant radiotherapy combined with systemic treatment, including FOLFOX-based chemoradiotherapy in one case and FLOT plus radiotherapy in the other. Among IHC-2+ tumors on baseline biopsy (n=15), ISH identified ERBB2 amplification in 7 cases (46.7%). In SS, ERBB2 amplification was detected in 3 of 14 HER2-IHC-2+ tumors (21.4%), illustrating variability in ERBB2 amplification status among HER2-IHC-2+ paired samples.

Discussion

In this single-center series of 57 localized gastric and GEJ adenocarcinomas with systematic HER2 assessment on both PTB and SS, we report several key findings. First, we confirmed 87.7% (95% CI: 76.3–94.9%) concordance of HER2 status between PTB and SS, consistent with prior reports describing discordance rates ranging from 5% to 20% and highlighting a small but existing risk of HER2 misclassification in gastric and GEJ adenocarcinomas (13). We also report a discordance in HER2 expression despite adequate biopsy sampling. In our cohort, the median number of biopsy fragments was five per case, consistent with European Society for Medical Oncology (ESMO) recommendations for HER2 assessment (Table S1). However, biopsy fragment number alone represents only an imperfect surrogate of sampling quality and does not necessarily ensure biological representativeness. Variations in fragment size, tumor cellularity, and sampling of non-representative areas such as normal mucosa, fibrosis, or necrosis may contribute to discordant HER2 results. Accordingly, given the exploratory nature of this analysis and the limited number of discordant cases, no firm conclusions can be drawn regarding the relationship between biopsy fragment number and HER2 discordance. Our study confirms the unstable nature of low and intermediate HER2 categories (IHC-1+ and 2+), which displayed extensive discordance between PTB and SS. This finding is consistent with prior reports showing that HER2-IHC-1+ and 2+ scores are the most heterogeneous and least reproducible categories in gastric cancer (11). Conversely, IHC-3+ status remained relatively stable between PTB and SS, reinforcing its strong positive predictive value. Beyond technical considerations, intrinsic tumor heterogeneity and treatment-related effects are also likely to play a major role in HER2 discordance. We observed apparent HER2 loss in 5 of 10 evaluable HER2-positive patients following neoadjuvant therapy. Given the limited number of HER2-positive cases within each treatment subgroup, these observations should be interpreted cautiously and regarded as descriptive only. Whether this apparent HER2 loss reflects true biological evolution or primarily baseline heterogeneity remains uncertain. One possible explanation is that HER2-positive subclones may be more sensitive to systemic treatment, leading to relative depletion of HER2-amplified populations in residual disease. Alternatively, discordance may simply reflect spatial heterogeneity and sampling variability rather than true biological evolution related to treatment. Similar HER2 dynamics have previously been described following trastuzumab exposure in advanced disease, supporting the broader concept of HER2 plasticity under therapeutic pressure (20). These observations may have implications for perioperative HER2-targeted strategies. If HER2 status changes during treatment, baseline assessment alone may not fully capture the biology of residual disease. Conversely, if discordance mainly reflects heterogeneity, excluding patients from postoperative HER2-targeted approaches solely on the basis of a HER2-negative surgical specimen could be problematic. Further studies integrating longitudinal tissue and circulating biomarkers are needed to better characterize HER2 evolution during perioperative treatment and to clarify the potential roles of baseline HER2 assessment, surgical specimen reassessment, and dynamic biomarker monitoring in future perioperative HER2-targeted strategies.

Current evidence indicates that ERBB2 amplification can be detected in ctDNA in gastric cancer and may complement tissue-based assessment (23). However, most data derive from metastatic disease, while evidence in the perioperative or localized setting remains limited (24). Initial pilot studies showed that ERBB2 amplification can be identified in plasma using polymerase chain reaction (PCR)-based approaches, with variable concordance compared with tumor tissue and higher sensitivity in patients with greater tumor burden (25). Subsequent studies using digital PCR and next-generation sequencing confirmed that ERBB2 copy number alterations in ctDNA may correlate with response to anti-HER2 therapies and dynamically reflect treatment-associated clonal shifts, including loss of amplification under systemic therapy (26-28). These findings support the concept that plasma-based HER2 monitoring may capture temporal heterogeneity not fully reflected by single-site tissue sampling (23,29). More recent reports suggest that ctDNA analysis may occasionally detect ERBB2 amplification not identified in archival tissue, possibly reflecting spatial heterogeneity or clonal evolution (26,30). Longitudinal monitoring has also been proposed to identify emerging resistance or reappearance of ERBB2-amplified clones (28,31,32). Nevertheless, these observations predominantly concern advanced-stage patients treated with HER2-targeted therapies (27). In localized gastric cancer, available data remain exploratory, and the clinical role of ctDNA-based ERBB2 assessment in the perioperative setting warrants prospective evaluation.

Strengths of this study include a well-characterized monocentric cohort with single-center pathology assessment, availability of HER2 status on both biopsies and SS, systematic recording of biopsy fragment number, and detailed documentation of neoadjuvant regimens enabling dynamic analyses of HER2 loss. Limitations include the retrospective design, which may introduce selection bias, and the single-center setting, which may limit generalizability. In addition, potential sampling bias related to intratumoral heterogeneity and variability in biopsy representativeness may have contributed to HER2 discordance. The limited sample size, particularly in the HER2-positive subgroup, precludes multivariable analyses and precise estimation of effect sizes. The observed pattern of treatment-associated HER2 loss, together with the limited impact of biopsy fragment number on discordance, highlights that HER2 discordance is likely multifactorial. Future studies in larger, multicenter cohorts are warranted to further explore HER2 dynamics in the perioperative setting. Integrating multi-site sampling, molecular profiling, and longitudinal ctDNA assessment may help better characterize HER2 heterogeneity and plasticity in operable gastroesophageal cancer. These findings should be interpreted in the context of a monocentric European cohort and may not be directly generalizable to other populations or healthcare settings.

Conclusions

In this single-center cohort of localized gastric and GEJ adenocarcinomas, HER2 status showed overall good concordance between PTB and SS, yet clinically meaningful discordances persisted despite biopsy sampling meeting current guideline-based expectations. We observed several cases of HER2 loss among evaluable HER2-positive cases after neoadjuvant therapy, supporting the concept that HER2 expression and ERBB2 amplification may also be dynamic under non-HER2 targeted treatment pressure. These findings underscore the limitations of relying solely on a single pre-treatment biopsy assessment to capture HER2 heterogeneity and suggest that reassessment on SS or on ctDNA may provide additional clinically relevant information in selected patients. While the clinical implications of perioperative HER2 dynamics remain to be defined, our results generate hypotheses that warrant prospective validation in larger multicenter cohorts, ideally integrating ctDNA and complementary molecular approaches to better characterize HER2 evolution under treatment.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0393/rc

Data Sharing Statement: Available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0393/dss

Peer Review File: Available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0393/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jgo.amegroups.com/article/view/10.21037/jgo-2026-0393/coif). C.d.l.F. reports consulting or advisory role with Bristol Myers Squibb, Amgen, Servier, Pierre Fabre, MSD Oncology, Roche/Genentech, Daichi-Sankyo, Astellas Pharma, Gilead Sciences, BeiGene, Jazz Pharmaceuticals, Takeda, AbbVie, AstraZeneca. C.d.l.F. received research funding from Pierre Fabre (Inst), Servier (Inst), MSD (Inst), Agenus (Inst) and support for travel, accommodations, expenses from Pierre Fabre, Servier, MSD Oncology, Amgen, AstraZeneca. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was approved by the Institutional Review Board of Paoli-Calmettes Institute (IRB SHERPAGOL-IPC 2025-026). The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Due to the retrospective nature of the study, individual informed consent was waived.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. Gravalos C, Jimeno A. HER2 in gastric cancer: a new prognostic factor and a novel therapeutic target. Ann Oncol 2008;19:1523-9. [Crossref] [PubMed]
2. Janjigian YY, Kawazoe A, Yañez P, et al. The KEYNOTE-811 trial of dual PD-1 and HER2 blockade in HER2-positive gastric cancer. Nature 2021;600:727-30. [Crossref] [PubMed]
3. Shitara K, Van Cutsem E, Gümüş M, et al. Trastuzumab Deruxtecan or Ramucirumab plus Paclitaxel in Gastric Cancer. N Engl J Med 2025;393:336-48. [Crossref] [PubMed]
4. Hofheinz RD, Merx K, Haag GM, et al. FLOT Versus FLOT/Trastuzumab/Pertuzumab Perioperative Therapy of Human Epidermal Growth Factor Receptor 2-Positive Resectable Esophagogastric Adenocarcinoma: A Randomized Phase II Trial of the AIO EGA Study Group. J Clin Oncol 2022;40:3750-61. [Crossref] [PubMed]
5. Rivera F, Izquierdo-Manuel M, García-Alfonso P, et al. Perioperative trastuzumab, capecitabine and oxaliplatin in patients with HER2-positive resectable gastric or gastro-oesophageal junction adenocarcinoma: NEOHX phase II trial. Eur J Cancer 2021;145:158-67. [Crossref] [PubMed]
6. Wagner AD, Grabsch HI, Mauer M, et al. EORTC-1203-GITCG - the "INNOVATION"-trial: Effect of chemotherapy alone versus chemotherapy plus trastuzumab, versus chemotherapy plus trastuzumab plus pertuzumab, in the perioperative treatment of HER2 positive, gastric and gastroesophageal junction adenocarcinoma on pathologic response rate: a randomized phase II-intergroup trial of the EORTC-Gastrointestinal Tract Cancer Group, Korean Cancer Study Group and Dutch Upper GI-Cancer group. BMC Cancer 2019;19:494. [Crossref] [PubMed]
7. Stein A, Goekkurt E, Al-Batran SE, et al. Perioperative pembrolizumab, trastuzumab and FLOT in HER2-positive localized esophagogastric adenocarcinoma: a phase 2 trial. Nat Med 2025;31:4197-204. [Crossref] [PubMed]
8. Nie RC, Chen XJ, Liang CC, et al. Safety and efficacy of perioperative dual PD-1 and HER2 blockade in HER2-positive gastric cancer. Cell Rep Med 2025;6:102190. [Crossref] [PubMed]
9. Ma Y, Li Z, Wei C, et al. Neoadjuvant camrelizumab plus trastuzumab and chemotherapy for HER2-positive gastric or gastroesophageal junction adenocarcinoma: a single-arm, phase 2 trial. Gastric Cancer 2025;28:652-61. [Crossref] [PubMed]
10. Peng Z, Zhang X, Liang H, et al. Atezolizumab and Trastuzumab Plus Chemotherapy for ERBB2-Positive Locally Advanced Resectable Gastric Cancer: A Randomized Clinical Trial. JAMA Oncol 2025;11:619-24. [Crossref] [PubMed]
11. Zhang H, Wang Y, Wang Y, et al. Intratumoral and intertumoral heterogeneity of HER2 immunohistochemical expression in gastric cancer. Pathol Res Pract 2020;216:153229. [Crossref] [PubMed]
12. Yang J, Luo H, Li Y, et al. Intratumoral heterogeneity determines discordant results of diagnostic tests for human epidermal growth factor receptor (HER) 2 in gastric cancer specimens. Cell Biochem Biophys 2012;62:221-8. [Crossref] [PubMed]
13. Yoshida H, Yamamoto N, Taniguchi H, et al. Comparison of HER2 status between surgically resected specimens and matched biopsy specimens of gastric intestinal-type adenocarcinoma. Virchows Arch 2014;465:145-54. [Crossref] [PubMed]
14. Wang T, Hsieh ET, Henry P, et al. Matched biopsy and resection specimens of gastric and gastroesophageal adenocarcinoma show high concordance in HER2 status. Hum Pathol 2014;45:970-5. [Crossref] [PubMed]
15. Grillo F, Fassan M, Sarocchi F, et al. HER2 heterogeneity in gastric/gastroesophageal cancers: From benchside to practice. World J Gastroenterol 2016;22:5879-87. [Crossref] [PubMed]
16. Hofmann M, Stoss O, Shi D, et al. Assessment of a HER2 scoring system for gastric cancer: results from a validation study. Histopathology 2008;52:797-805. [Crossref] [PubMed]
17. Bartley AN, Washington MK, Colasacco C, et al. HER2 Testing and Clinical Decision Making in Gastroesophageal Adenocarcinoma: Guideline From the College of American Pathologists, American Society for Clinical Pathology, and the American Society of Clinical Oncology. J Clin Oncol 2017;35:446-64. [Crossref] [PubMed]
18. Rüschoff J, Hanna W, Bilous M, et al. HER2 testing in gastric cancer: a practical approach. Mod Pathol 2012;25:637-50. [Crossref] [PubMed]
19. Ishimine Y, Goto A, Watanabe Y, et al. Loss of HER2 Positivity after Trastuzumab in HER2-Positive Gastric Cancer: Is Change in HER2 Status Significantly Frequent? Case Rep Gastrointest Med 2015;2015:132030. [Crossref] [PubMed]
20. Pietrantonio F, Caporale M, Morano F, et al. HER2 loss in HER2-positive gastric or gastroesophageal cancer after trastuzumab therapy: Implication for further clinical research. Int J Cancer 2016;139:2859-64. [Crossref] [PubMed]
21. Nagtegaal ID, Odze RD, Klimstra D, et al. The 2019 WHO classification of tumours of the digestive system. Histopathology 2020;76:182-8. [Crossref] [PubMed]
22. Bang YJ, Van Cutsem E, Fuchs CS, et al. KEYNOTE-585: Phase III study of perioperative chemotherapy with or without pembrolizumab for gastric cancer. Future Oncol 2019;15:943-52. [Crossref] [PubMed]
23. Lengyel CG, Hussain S, Trapani D, et al. The Emerging Role of Liquid Biopsy in Gastric Cancer. J Clin Med 2021;10:2108. [Crossref] [PubMed]
24. Kimura Y, Suzuki K, Tamaki S, et al. Real-time assessment of circulating tumor cells refines the indication for HER2-targeted therapy in metastatic gastric cancer. Sci Rep 2025;15:23906. [Crossref] [PubMed]
25. Shoda K, Masuda K, Ichikawa D, et al. HER2 amplification detected in the circulating DNA of patients with gastric cancer: a retrospective pilot study. Gastric Cancer 2015;18:698-710. [Crossref] [PubMed]
26. Lee J, Franovic A, Shiotsu Y, et al. Detection of ERBB2 (HER2) Gene Amplification Events in Cell-Free DNA and Response to Anti-HER2 Agents in a Large Asian Cancer Patient Cohort. Front Oncol 2019;9:212. [Crossref] [PubMed]
27. Wang H, Li B, Liu Z, et al. HER2 copy number of circulating tumour DNA functions as a biomarker to predict and monitor trastuzumab efficacy in advanced gastric cancer. Eur J Cancer 2018;88:92-100. [Crossref] [PubMed]
28. Zhang C, Chen Z, Chong X, et al. Clinical implications of plasma ctDNA features and dynamics in gastric cancer treated with HER2-targeted therapies. Clin Transl Med 2020;10:e254. [Crossref] [PubMed]
29. Catenacci DV, Kang YK, Uronis HE, et al. Circulating Tumor DNA as a Predictive Biomarker for Clinical Outcomes With Margetuximab and Pembrolizumab in Pretreated HER2-Positive Gastric/ Gastroesophageal Adenocarcinoma. Oncology (Williston Park) 2023;37:176-83. [Crossref] [PubMed]
30. Boldrin E, Mazza M, Piano MA, et al. Putative Clinical Potential of ERBB2 Amplification Assessment by ddPCR in FFPE-DNA and cfDNA of Gastroesophageal Adenocarcinoma Patients. Cancers (Basel) 2022;14:2180. [Crossref] [PubMed]
31. Liu Y, Yang M, Jiang T, et al. Quantitative Analysis of HER2 Amplification by Droplet Digital PCR in the Follow-Up of Gastric Cancer Patients Being Treated with Trastuzumab after Surgery. Gastroenterol Res Pract 2019;2019:1750329. [Crossref] [PubMed]
32. Klein-Scory S, Ladigan-Badura S, Mika T, et al. Liquid biopsy based HER2 amplification status in gastric cancer patients indicates clinical response. Heliyon 2023;9:e21339. [Crossref] [PubMed]