HER2 scoring in gastric cancer and other solid tumors

Sherrie Rice

June 2026—How and why gastric cancer scoring for HER2 positivity differs from breast cancer scoring was the topic of a CAP TODAY webinar early this year presented by Josef Rüschoff, MD, of Kassel, Germany.

He reported also on a study published last year revealing how the gastric cancer interpretive criteria performed in pan-tumor testing for HER2 immunohistochemistry in more than 65,000 solid tumors.

In Fig. 1 is a comparison of the prevalence of HER2 alterations across solid tumors, with each set of overlapping circles representing a range of values found in the literature. (Darker circles in each pair represent the lower end of the range, the lighter outer circles the higher end.) HER2 overexpression, defined by IHC 3+ (shown by turquoise circles), can occur in the absence of its HER2 gene amplification or mutation in many tumor types (shown by pink and purple circles), noted Dr. Rüschoff, who is former chief medical officer and cofounder of Targos Molecular Pathology and senior consultant, Discovery Life Sciences.

“Therefore, in some tumor types,” he said, “HER2 IHC testing may detect cases with HER2 alterations that would not be identified by using other testing methodologies such as next-generation sequencing.”

Both methods used to detect HER2 alterations in breast and gastric cancers—IHC and in situ hybridization—are applied on tissue sections in situ, so the tumor area where HER2 is overexpressed and/or amplified can be identified easily. Overexpression of the protein or amplification of the gene, or both, occurs in about 15 to 25 percent of breast cancers, and in 17 to 20 percent of gastric cancers. “For colorectal cancer,” Dr. Rüschoff said, “the prevalence of HER2 overexpression is much lower—in the range of two to three percent.”

Unlike in breast cancer, where IHC 3+ cases usually show homogeneous strong staining of tumor cells, marked heterogeneity is common in gastric cancers, with only a few glands clearly HER2-positive next to HER2-negative glands.

For HER2 testing in gastric cancer, per the CAP/ASCP/ASCO guideline (Bartley AN, et al. J Clin Oncol. 2017;35[4]:446–464), the primary differences compared with breast cancer are in the staining patterns and cutoff definitions, Dr. Rüschoff said (Fig. 2). “In gastric cancer, a tumor can be scored HER2 IHC 3+ not only when strong, complete [circumferential]membrane staining is present but also when incomplete staining patterns are observed,” he said, such as basolateral or lateral membrane staining.

The cutoff definitions differ among specimen types. In gastric surgical specimens, more than 10 percent of tumor cells must show strong membrane staining to qualify as IHC 3+. For biopsy specimens, a cluster of at least five strongly stained tumor cells is sufficient. This approach, known as the Rüschoff-Hofmann method, is recommended in the gastric cancer guideline.

The key IHC 3+ differences, then, are in membrane staining intensity and pattern, threshold for positive cells, and interpretation. In breast cancer, HER2 positivity requires complete (circumferential) membrane staining; in gastric cancer, incomplete staining patterns are acceptable. In breast cancer, strong membrane staining in 10 percent or more of tumor cells is required irrespective of tumor type; in gastric cancer, the threshold is more than 10 percent of tumor cells in resections or a minimum of five clustered HER2+ tumor cells in biopsy specimens. Whereas a 3+ score in breast cancer is defined by intense circumferential membrane staining, in gastric cancer intense basolateral or lateral membrane staining is also accepted.

“Based on these guidelines,” Dr. Rüschoff said, “the overall incidence of HER2 positivity is comparable between gastric and breast—around 15 to 20 percent. However, a marked difference exists with respect to intratumoral heterogeneity.” About one-third of HER2+ gastric carcinoma patients show HER2 IHC 3+ staining in less than or equal to 30 percent of tumor cells; this pattern is seen in only one to two percent of HER2+ breast carcinomas.

“To avoid missing HER2-positive patients with heterogeneous HER2 expression,” he said, “the 10 percent cutoff is waived for gastric biopsies.”

ISH positivity in gastric cancer is defined by a HER2/CEP17 ratio of two or higher and a gene copy number of six or greater. “In contrast to breast cancer, no additional ISH subgroupings are applied,” he said.

The foundation of the gastric cancer guideline is the two morphological classifications of gastric cancer: Lauren and World Health Organization. The Lauren classification comprises three types: intestinal (54 percent of tumors), diffuse (32 percent), and mixed (15 percent). The WHO classification applies such criteria as tubular, mucinous, and papillary adenocarcinoma and signet ring cell carcinoma.

The Cancer Genome Atlas molecular classification came later and comprises four types: chromosomal instability (50 percent and often HER2+), microsatellite instability (22 percent), Epstein-Barr virus positive (nine percent), and genomically stable (20 percent).

Tubular and papillary adenocarcinomas of the intestinal type are typically HER2 overexpressing or HER2-amplified gastric carcinomas. Diffuse type carcinomas (signet ring cell and mucinous), particularly pure signet ring cell carcinomas, are usually HER2 negative. Study data support this observation, Dr. Rüschoff said, noting the prevalence of positive HER2 expression or amplification: intestinal: 16 to 32 percent; diffuse: six to seven percent; and mixed: 14 to 20 percent.

Dr. Rüschoff and his colleagues studied the HER2 scoring system for gastric cancer, and the results of their laboratory’s validation study were published in 2008 (Hofmann M, et al. Histopathology. 2008;52[7]:797–805). In the study, 168 gastric cancer samples from different sites in China, Mexico, and Germany were evaluated for HER2 expression. The findings led to the recommendation that when staining is moderate to strong, tumors should be scored as IHC 2+ or IHC 3+, “even in instances when staining may be incomplete or basolateral or only lateral, because it turned out that these areas were usually amplified by FISH,” Dr. Rüschoff explained. Under the ASCO/CAP breast cancer guideline at that time, these cases would have been scored IHC negative, zero, or 1+. “So these findings support the rationale for using this modification to assess membrane staining patterns,” he added.

The five-cell cluster recommendation used for scoring in gastric cancer biopsies did not come from the aforementioned study, Dr. Rüschoff said. “In fact, that study recommended that any cohesive IHC 3+ clone be considered positive irrespective of its size.”

The five-cell cluster recommendation, he said, originates from a round-robin study he and colleagues conducted to evaluate the reproducibility of the modified HER2 IHC testing method in gastric cancer (Rüschoff J, et al. Virchows Arch. 2010;457[3]:299–307). “The findings indicated the minimum number of cells that could reliably be assessed between 10 pathologists from three countries at six sites was five. So it’s more or less a statistically defined lower number,” he explained. This led to the recommendation that in gastric biopsies, a focus, usually defined as a clone, that is allowed to be scored positive should have at least five strongly stained evaluable cells.

How does the gastric guideline translate to other solid tumors?

The FDA in April 2024 granted tumor-agnostic approval to trastuzumab deruxtecan-nxki. Published in 2025 was a report, from Caris Life Sciences, of pan-tumor HER2 IHC 3+ prevalence using standardized thresholds and interpretation criteria (Bryant D, et al. JAMA Oncol. 2025;11[8]:919–921).

Caris began performing HER2 IHC testing using the Ventana Pathway anti-HER2/neu (4B5) assay in May 2024 on solid tumors submitted for MI Profile comprehensive testing. Data were pulled from the laboratory information system from June through December 2024, including tumor type, HER2 IHC score, and specimen type (biopsy, resection). For non-breast cases, HER2 IHC 3+ status was determined according to gastric biopsy or resection interpretation criteria. For breast cases, HER2 IHC 3+ was scored according to the ASCO/CAP breast guidelines. HER2 IHC 2+/ISH-positive cases were not included.

“The pan-tumor rate for HER2 positivity was 3.1 percent overall,” Dr. Rüschoff said. In non-breast tumors with an at least 0.4 percent positivity rate or higher using either gastric biopsy or resection interpretation criteria, the HER2 IHC 3+ rate was largely similar across tumor types.

The authors write, “Study data show that positivity rates per tumor type for biopsy vs resection are similar, supporting the validity of these criteria outside gastroesophageal adenocarcinoma.”

And, as Dr. Rüschoff said, “This study demonstrates the utility of standardized thresholds in gastric biopsy or resection interpretation criteria.”

Sherrie Rice is CAP TODAY editor. The webinar was sponsored by Daiichi-Sankyo and AstraZeneca.