HER2 Endometrial Cancer: Diagnosis & Targeted Therapies

HER2-positive endometrial cancer is a biologically aggressive subtype in which tumor cells carry extra copies of the HER2 gene or overexpress the HER2 protein on their surface, making them candidates for targeted drugs originally developed for breast cancer. HER2 amplification is most common in serous and other high-grade endometrial cancers, where it shows up in roughly 10 to 13 percent of cases, though it is rare in the more typical low-grade endometrioid type. The diagnosis involves a specific staining and scoring process, and the targeted treatment landscape has expanded from a single antibody added to chemotherapy to a growing menu of antibody-drug conjugates that may work even in tumors with lower HER2 levels.

Which Endometrial Cancers Are HER2-Positive

Endometrial cancer is not one disease. The most common subtype, endometrioid carcinoma, makes up the majority of cases and tends to be lower grade. Serous carcinoma, clear cell carcinoma, and carcinosarcoma are less common but behave more aggressively. HER2 amplification clusters heavily in these aggressive subtypes. A large study found HER2 amplification in about 13 percent of serous carcinomas, roughly 11 percent of carcinosarcomas and clear cell carcinomas, but under 1 percent of endometrioid carcinomas overall.¹ Among the small number of endometrioid cancers that were HER2-amplified, most were high grade, where the rate reached about 4.5 percent.

This distribution matters practically. If you have a low-grade endometrioid cancer, HER2 testing is unlikely to change your treatment. But if your pathology report shows serous histology, carcinosarcoma, clear cell, or a high-grade endometrioid tumor, HER2 status becomes a meaningful question that can open the door to additional therapies.

How HER2 Fits Into Molecular Classification

Modern endometrial cancer classification goes beyond what a tumor looks like under the microscope. Research from The Cancer Genome Atlas identified four molecular subgroups based on the tumor’s genetic profile: POLE-ultramutated (which tends to have an excellent prognosis), microsatellite instability-high, copy number-low, and copy number-high (sometimes called the serous-like group). In clinical practice, pathologists approximate these groups using a combination of protein staining and targeted gene sequencing.

HER2-positive tumors are overwhelmingly concentrated in the copy number-high group, which also tends to have abnormal p53. Genomic sequencing data have shown that HER2 amplification is almost exclusive to tumors carrying TP53 mutations.² This overlap has led to proposals to screen all p53-abnormal endometrial cancers for HER2, since that would efficiently catch the vast majority of amplified cases without needing to test every tumor.

An analysis of high-grade endometrioid cancers specifically found that even within that subtype, the molecular breakdown was varied: about 44 percent were microsatellite unstable, 29 percent were copy number-high, and 18 percent were POLE-mutated.³ The takeaway is that molecular subtype does not perfectly map to what the cancer looks like under the microscope, and testing both HER2 status and molecular class gives the most complete picture for treatment planning.

How HER2 Testing Works in Endometrial Cancer

If you are familiar with HER2 testing in breast cancer, the endometrial version borrows heavily from it but has its own scoring rules. Testing starts with immunohistochemistry (IHC), a staining method that shows how much HER2 protein sits on the surface of tumor cells. Pathologists assign a score of 0, 1+, 2+, or 3+ based on the intensity and pattern of staining. A score of 3+, which requires strong, complete membrane staining in more than 30 percent of tumor cells, counts as HER2-positive. A score of 2+ is equivocal and triggers a follow-up test, usually fluorescence in situ hybridization (FISH), which directly counts the number of HER2 gene copies in the tumor’s DNA. A score of 0 or 1+ is considered negative.⁴

One complication is that the scoring criteria used in endometrial cancer do not perfectly match those used in breast cancer. Two guideline sets have circulated: one based on the 2018 breast cancer HER2 guidelines, and another adapted from the enrollment criteria of the clinical trial that first proved trastuzumab works in endometrial serous carcinoma.⁵ These two systems can classify borderline cases differently, which means your HER2 result could depend partly on which scoring system your pathology lab uses. If your result is equivocal, it is reasonable to ask whether FISH was performed and which scoring criteria were applied.

The NCCN guidelines now recommend HER2 testing for advanced or recurrent serous carcinoma and carcinosarcoma, and testing has become routine in large cancer centers.⁶ Smaller or community hospitals may not yet have established workflows for endometrial-specific HER2 testing, so patients diagnosed outside academic centers should confirm that their tumor was tested with appropriate criteria.

Why Heterogeneity Makes Testing Tricky

Unlike some cancers where a biomarker is either on or off throughout the whole tumor, HER2 in endometrial cancer is often patchy. This phenomenon, called intratumoral heterogeneity, means that one region of the tumor might be strongly HER2-amplified while an adjacent area shows normal HER2 levels. A study using FISH found this kind of heterogeneity in 36 percent of endometrial cancers examined, with HER2 amplification present in only 5 to 50 percent of tumor cells rather than uniformly.⁷ Two patterns emerged: a “cluster” pattern with distinct HER2-positive zones next to HER2-negative zones, and a “mosaic” pattern where cells with different HER2 levels were scattered throughout.

This heterogeneity matters for two practical reasons. First, it can cause test results to disagree with each other. About 69 percent of cases with discrepant IHC, FISH, or next-generation sequencing results turned out to have this kind of patchwork amplification pattern.⁷ Second, the primary tumor and a metastatic site do not always agree on HER2 status. One study found an 87.5 percent concordance rate between primary and metastatic sites, while a broader literature review put the concordance rate at about 68 percent.⁸ Tumors with mosaic or clustered heterogeneity in the primary were more likely to be non-amplified in their corresponding metastases. For this reason, retesting a biopsy from a recurrence or metastatic site can sometimes change the treatment plan.

Trastuzumab Added to Chemotherapy

The landmark trial that established HER2-targeted therapy in endometrial cancer randomized patients with advanced or recurrent HER2-positive uterine serous carcinoma to receive standard carboplatin-paclitaxel chemotherapy either with or without trastuzumab. Updated results showed that adding trastuzumab extended median progression-free survival from 8 months to about 13 months and median overall survival from roughly 24 months to about 30 months.⁹ The benefit was especially strong in patients with stage III or IV disease at initial diagnosis, where the median overall survival on the trastuzumab arm had not yet been reached at the time of analysis.

A systematic review and meta-analysis pooling available data confirmed that adding trastuzumab to carboplatin-paclitaxel significantly prolonged both overall and progression-free survival in HER2-positive uterine serous carcinoma. However, the combination came with more side effects: the rate of both total adverse events and high-grade adverse events was higher in the trastuzumab group.¹⁰ Overall response rate did not differ significantly between the groups, suggesting that trastuzumab’s advantage shows up more in durability of response than in initial tumor shrinkage.

Antibody-Drug Conjugates and the HER2-Low Frontier

Antibody-drug conjugates (ADCs) represent a newer class of HER2-targeted therapy. These drugs link a HER2-targeting antibody to a potent chemotherapy payload. The antibody steers the drug to HER2-expressing cells, where the payload is released to kill them. One advantage of ADCs is the “bystander effect”: when the payload is released inside a HER2-positive cell, it can leak into neighboring tumor cells that have little or no HER2, potentially killing them too. This is particularly relevant in endometrial cancer given the heterogeneity discussed above.

Trastuzumab deruxtecan (T-DXd) and disitamab vedotin (RC48) are two ADCs generating clinical interest. A case report documented a complete response to disitamab vedotin in a patient with HER2-low metastatic endometrial carcinoma, meaning her tumor was scored as IHC 2+ without gene amplification. After ten cycles of the drug, all tumor lesions had disappeared, and the patient remained in remission at the time of publication.¹¹ While a single case report is far from proof of broad efficacy, it illustrates an important shift: ADCs with bystander activity may benefit patients whose tumors express some HER2 but not enough to qualify as HER2-positive by traditional criteria.

This concept of “HER2-low” has already reshaped treatment in breast cancer, and it is gaining traction in endometrial cancer too. The large prevalence study cited earlier found that HER2-low expression (IHC 2+ without amplification) was common in non-endometrioid histologies, appearing in about 10.5 percent of serous, clear cell, and carcinosarcoma cases.¹ If ADCs prove effective in HER2-low endometrial tumors in larger trials, the pool of patients eligible for HER2-targeted treatment could expand considerably.

Why HER2-Targeted Drugs Sometimes Stop Working

Not every HER2-positive endometrial cancer responds to targeted therapy, and some that respond initially develop resistance over time. Researchers have identified several mechanisms driving this resistance:

Outgrowth of HER2-negative cells: In tumors with mixed HER2 expression, trastuzumab kills the HER2-positive cells but leaves HER2-negative cells untouched. Over time, those negative cells take over, and the tumor essentially becomes invisible to the drug.
Changes to the HER2 protein itself: Part of the HER2 protein can be shed from the cell surface, or mutations can alter the antibody’s binding site, preventing trastuzumab from latching on.
Activation of bypass pathways: Tumor cells can switch on alternative growth signals that keep them dividing even when HER2 is blocked.
Overexpression of other HER-family receptors: Increased activity of related receptors like HER3 or EGFR can compensate for HER2 blockade.

These mechanisms have been specifically identified in uterine serous carcinoma.¹² The first mechanism, outgrowth of HER2-negative cells, ties directly back to intratumoral heterogeneity: tumors that were patchy from the start are more likely to develop resistance this way. ADCs with bystander activity may partially address this problem, and pan-HER inhibitors like neratinib, a small-molecule drug that blocks multiple members of the HER family, have shown preclinical efficacy in HER2-amplified endometrial tumors.¹³ Whether these strategies will translate into durable clinical benefit remains under investigation.

Managing Side Effects of HER2-Targeted ADCs

Traditional trastuzumab is generally well tolerated, with its main concern being heart function. ADCs carry a different safety profile because of their chemotherapy payload. Trastuzumab deruxtecan, in particular, has been associated with interstitial lung disease (ILD), a type of lung inflammation that can range from mild to life-threatening. A meta-analysis across breast cancer patients receiving the drug found an overall ILD rate of about 12 percent. Most cases were mild, but around 13 percent of ILD cases were severe (grade 3 or 4), and a small number were fatal.¹⁴ Higher doses were linked to substantially higher ILD rates. The incidence of heart-function decline was low, under 2 percent, and most of those cases had no symptoms.

Clinical guidelines for managing T-DXd toxicity call for regular CT scans to catch ILD early. For mild, asymptomatic ILD, the drug is paused until the inflammation resolves. If it clears within four weeks, treatment can resume at the same dose; if it takes longer, the dose is reduced. For symptomatic ILD of grade 2 or higher, T-DXd is permanently discontinued and steroids are started promptly.¹⁵ Heart function is also monitored: specific ejection fraction thresholds determine whether to continue, pause, or stop therapy.¹⁶ These protocols were developed primarily in breast cancer but are being applied to endometrial cancer patients receiving the same agents, since the toxicity profiles are expected to be similar.

For patients, the practical message is that any new cough, shortness of breath, or chest discomfort while on an ADC should be reported immediately. Early detection of ILD makes the difference between a temporary treatment interruption and a serious complication.

Monitoring Recurrence With Blood Tests

Standard follow-up for endometrial cancer relies on imaging and clinical exams, which can miss recurrences until the tumor grows large enough to see on a scan. Circulating tumor DNA (ctDNA), fragments of tumor DNA shed into the bloodstream, offers a potentially earlier signal. A pilot study in endometrial cancer patients found that ctDNA analysis detected recurrence and progression earlier than conventional methods, and that ctDNA levels tracked dynamically with treatment response, rising when the cancer grew and falling when treatment worked.¹⁷

For HER2-positive endometrial cancer specifically, ctDNA could serve a dual purpose: detecting recurrence and tracking whether the tumor’s HER2 status has changed. Given the heterogeneity discussed earlier and the fact that recurrent tumors can lose or gain HER2 amplification, a blood-based test that captures the evolving molecular profile could help clinicians decide whether to continue, switch, or add HER2-targeted therapy at the time of relapse, without always needing a new tissue biopsy. This technology is still in its early stages for endometrial cancer, but it is an area of active research that may become part of routine monitoring within the next several years.

Cost-Effectiveness and Who Benefits Most

Adding targeted therapies to treatment inevitably raises costs. A cost-effectiveness analysis compared several strategies for newly diagnosed stage III or IV endometrial cancer: chemotherapy alone, universal immunotherapy for all patients (with or without trastuzumab added for HER2-positive serous or carcinosarcoma cases), and a targeted approach where immunotherapy and trastuzumab were reserved for patients whose tumors tested positive for the relevant biomarkers. The targeted strategies, guided by both HER2 and mismatch repair testing, were cost-effective compared to chemotherapy alone. Universal immunotherapy for all patients, regardless of biomarker status, was never cost-effective.¹⁸

The analysis also found that the biomarker-targeted approach was more cost-effective in non-Hispanic Black women than in non-Hispanic White women, largely because the subtypes that benefit from these targeted therapies, serous carcinoma and carcinosarcoma, are disproportionately more common in Black women. The most cost-effective single strategy was targeted immunotherapy for mismatch-repair-deficient tumors combined with trastuzumab for HER2-positive serous and carcinosarcoma cases.¹⁸ These findings reinforce the case for systematic biomarker testing at diagnosis: testing costs are modest compared to the drugs themselves, and testing ensures that the patients most likely to benefit actually receive the right treatment, while patients who would not benefit are spared unnecessary toxicity and expense.

Unanswered Questions and Ongoing Trials

The landscape is moving fast, but several important questions remain unresolved. Trastuzumab’s survival benefit was established in a phase II trial of moderate size, not a definitive phase III. That trial enrolled patients with uterine serous carcinoma only, so the evidence base for using trastuzumab in HER2-positive carcinosarcoma or clear cell carcinoma is extrapolated rather than directly proven. Whether the same survival advantage holds in those histologies is still being studied.

The most consequential open question is where ADCs will land in the treatment sequence. Should they replace trastuzumab as first-line therapy, or are they better reserved for patients whose cancer progresses after initial treatment? Trials are ongoing, and the HER2-low category adds a further layer of complexity. If ADCs prove effective in HER2-low endometrial tumors in controlled trials, the definition of “HER2-targetable” will widen, and an IHC score of 1+ or 2+ without amplification, currently classified as negative, could become a therapeutic green light. Separate trials are exploring combinations of HER2-targeted agents with immunotherapy drugs like checkpoint inhibitors, a pairing that has shown synergy in other cancer types and could address the immune evasion that some endometrial tumors rely on for survival.