DecisionDx®-Melanoma’s i31-SLNB: Report from the Largest Prospective Multicenter Study to Date Confirms 2.6% Nodal Positivity in Patients Predicted to Have Less Than 5% Risk

Castle Biosciences (Nasdaq: CSTL) reported prospective multicenter results showing DecisionDx-Melanoma’s i31-SLNB accurately predicts sentinel lymph node (SLN) positivity and identifies low-risk patients.

Rhea-AI Summary

Castle Biosciences (Nasdaq: CSTL) reported prospective multicenter results showing DecisionDx-Melanoma’s i31-SLNB accurately predicts sentinel lymph node (SLN) positivity and identifies low-risk patients. In 912 patients across 30 U.S. centers, those with 5% predicted risk had a 2.6% actual SLN positivity rate and 97.8% three-year recurrence-free survival.

Positive

• Study enrolled 912 patients across 30 U.S. centers
• Low-risk group (5% predicted) had SLN positivity
• T1b–T2a low-risk subgroup had only 1.4% nodal positivity
• Low-risk patients showed 97.8% three-year recurrence-free survival

Negative

• Only 430 of 912 patients underwent SLNB, limiting direct nodal verification for all
• High-risk (>10%) group had notable nodal positivity of 21.4%, implying substantial residual risk

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Key Figures

Patients in study: 912 patients Participating centers: 30 U.S. centers SLNB performed: 430 patients +5 more

Patients in study

912 patients

Prospective multicenter i31-SLNB melanoma study

Participating centers

30 U.S. centers

Prospective multicenter i31-SLNB melanoma study

SLNB performed

430 patients

Patients who underwent sentinel lymph node biopsy

SLNB not performed

482 patients

Patients who did not undergo sentinel lymph node biopsy

Low-risk SLN positivity

2.6%

Actual SLN positivity in patients with <5% predicted risk by i31-SLNB

T1b–T2a low-risk positivity

1.4% (1/74)

SLN positivity in T1b–T2a patients with <5% predicted risk

High-risk SLN positivity

21.4%

SLN positivity in patients with >10% predicted risk undergoing SLNB

Three-year RFS (low-risk)

97.8%

Three-year recurrence-free survival for low-risk i31-SLNB patients

Historical Context

5 past events · Latest: Mar 04

5 events

1. Mar 04

   Clinical data preview

   24h Move

   -0.9%

   Preview of SSO 2026 data on i31-SLNB identifying T1b–T2a patients avoiding SLNB.

2. Mar 03

   Operations / facilities

   24h Move

   +2.2%

   Announcement of new Friendswood headquarters supporting growth and clinical testing operations.

3. Feb 26

   Earnings results

   24h Move

   -7.3%

   2025 revenue of $344.2M exceeding guidance and strong core test volume growth.

4. Feb 19

   Clinical validation

   24h Move

   -6.8%

   Prospective JAAD study validating AdvanceAD-Tx for predicting JAK inhibitor response.

5. Feb 05

   Earnings date notice

   24h Move

   -7.3%

   Scheduling of Q4 and full-year 2025 earnings release and conference call.

24h Move is the share-price change in the day after each event; other market factors may also have contributed.

Key Terms

sentinel lymph node biopsy, SLNB, NCCN, Breslow thickness, +4 more

8 terms

sentinel lymph node biopsy medical

"NCCN Cutaneous Melanoma Guidelines recommend avoiding a sentinel lymph node (SLN) biopsy"

A sentinel lymph node biopsy is a surgical procedure that removes and tests the first lymph node(s) that drain fluid from a tumor to see if cancer has spread. Think of it as checking the first security checkpoint after a breach; a negative result often means less extensive surgery and lower treatment costs, while a positive result can change therapy, prognosis, regulatory decisions and market demand for related diagnostics and treatments, making it important to investors.

SLNB medical

"recommend avoiding a sentinel lymph node (SLN) biopsy (SLNB) if the predicted SLN positivity"

A sentinel lymph node biopsy is a surgical test that removes and examines the first lymph node(s) likely to receive cancer cells from a tumor, like checking the first bucket under a leaking pipe to see whether the problem has spread. Investors care because SLNB results shape treatment decisions, clinical trial endpoints and regulatory outcomes for cancer drugs and devices, so those findings can materially affect a company’s clinical prospects and market value.

NCCN medical

"NCCN Cutaneous Melanoma Guidelines recommend avoiding a sentinel lymph node (SLN) biopsy"

The NCCN is a U.S.-based coalition of leading cancer centers that produces widely used clinical practice guidelines for cancer diagnosis and treatment. Its recommendations act like a trusted rulebook for doctors and insurers, shaping which therapies are prescribed and reimbursed; inclusion or favorable guidance can therefore boost a drug or device’s market uptake and revenue prospects, while exclusion can limit access and dampen sales.

Breslow thickness medical

"factors, including Breslow thickness, ulceration, mitotic rate, and age."

Breslow thickness is a pathologist’s measurement of how deep a melanoma tumor has grown into the skin, reported in millimeters. It matters to investors because deeper tumors predict worse outcomes, influence treatment choices, determine clinical trial eligibility and regulatory decisions, and therefore can affect demand for therapies, diagnostic tools and reimbursement — like measuring the depth of a crack to judge how costly a repair will be.

mitotic rate medical

"factors, including Breslow thickness, ulceration, mitotic rate, and age."

Mitotic rate is a measure of how quickly cells in a tissue sample are dividing, reported by a pathologist after counting dividing cells under a microscope. For investors, it signals how aggressive a tumor or disease may be—like measuring how fast weeds are spreading in a garden—and can influence prognosis, treatment choices, clinical trial outcomes and the commercial value of therapies or diagnostic tests.

gene expression profile medical

"integrates the independently predictive 31-gene expression profile (GEP) score with established"

A gene expression profile is a snapshot of which genes in a cell or tissue are switched on and how strongly they are producing their products, like a theater marquee showing which plays are running and how popular each is. For investors, these profiles matter because they can indicate whether a drug or diagnostic is likely to work, reveal which patient groups will benefit, and reduce development time and risk—factors that influence a biotech company’s value and commercial prospects.

recurrence-free survival medical

"low-risk i31-SLNB result (less than 5% risk) demonstrated a 97.8% three-year recurrence-free survival"

Recurrence-free survival is the length of time after a treatment during which a patient shows no return of disease. It’s used in clinical trials to measure how well a therapy prevents the illness from coming back — think of it like the time a repaired roof stays leak-free. For investors, longer recurrence-free survival suggests a treatment is more effective, which can improve chances of regulatory approval, commercial uptake, and long-term revenue.

American Joint Committee on Cancer medical

"identify patients at low risk of SLN positivity compared with American Joint Committee on Cancer"

A cancer staging organization that creates the standard system doctors use to describe how advanced a cancer is; think of it as a common ruler for measuring tumor size, spread, and severity. Its staging rules shape treatment decisions, clinical trial eligibility, and how outcomes are reported, so changes can influence demand for therapies, trial design, regulatory decisions and revenue forecasts for companies developing cancer diagnostics and drugs.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Study results were recently shared at the Society of Surgical Oncology (SSO) 2026 Annual Meeting in Phoenix.

NCCN Cutaneous Melanoma Guidelines recommend avoiding a sentinel lymph node (SLN) biopsy (SLNB) if the predicted SLN positivity rate is less than 5%. This report shows that patients with a less than 5% predicted risk of SLN positivity by the DecisionDx-Melanoma test had an actual SLN positivity rate of just 2.6%, and only 1.4% in patients with T1b–T2a tumors specifically, again confirming the clinically actionable performance of DecisionDx-Melanoma.

FRIENDSWOOD, Texas, March 09, 2026 (GLOBE NEWSWIRE) -- Castle Biosciences, Inc. (Nasdaq: CSTL), a company improving health through innovative tests that guide patient care, today announced new data from the largest prospective multicenter study to date evaluating DecisionDx-Melanoma’s integrated sentinel lymph node biopsy (i31-SLNB) test result. The study data, recently shared at the SSO 2026 Annual Meeting, confirm that the i31-SLNB accurately predicts SLN positivity and identifies low-risk patients who may safely consider forgoing SLNB while maintaining favorable long-term outcomes. These results expand upon earlier publications from the same prospective multicenter clinical study and further strengthen the growing body of evidence supporting the role of DecisionDx-Melanoma in guiding SLNB decision-making.^1-3

“This study of DecisionDx-Melanoma’s i31-SLNB result confirms that the test delivers clear and clinically meaningful separation between low- and high-risk patients in real-world practice,” said Timothy Beard, M.D., FACS, lead author and surgeon at Summit Medical Group in Bend, Oregon. “Importantly, patients identified as low risk not only had very low rates of nodal positivity but also demonstrated high recurrence-free survival over three years. That level of prospective validation can give clinicians and patients greater confidence when deciding whether to proceed with or forgo sentinel lymph node biopsy.”

Current National Comprehensive Cancer Network^® (NCCN) Cutaneous Melanoma Guidelines recommend forgoing SLNB when the likelihood of finding a positive SLN is less than 5%, considering SLNB when the risk is between 5-10% and offering the surgery when the likelihood of positivity is above 10%. DecisionDx-Melanoma’s i31-SLNB algorithm integrates the independently predictive 31-gene expression profile (GEP) score with established clinicopathologic factors, including Breslow thickness, ulceration, mitotic rate, and age. For any GEP test to be clinically meaningful, the molecular score should demonstrate independent predictive value beyond traditional staging, ensuring it contributes distinct biological insight rather than duplicating clinicopathologic criteria. By combining validated tumor biology with select clinicopathologic features, the i31-SLNB algorithm generates a personalized likelihood of SLN positivity to support risk-aligned shared decision-making consistent with NCCN Guideline thresholds.

In this prospective, multicenter study of 912 patients with T1–T4 cutaneous melanoma enrolled across 30 U.S. centers, 430 patients underwent SLNB and 482 did not, allowing for evaluation of both nodal positivity and recurrence outcomes. Among those who underwent SLNB, patients with a less than 5% predicted risk of SLN positivity by the i31-SLNB had an actual SLN positivity rate of 2.6% compared to 21.4% in those with greater than 10% predicted risk.

In the T1b–T2a subgroup, where SLNB decisions often have the greatest clinical uncertainty, risk stratification by the i31-SLNB was particularly pronounced. Only 1.4% (1/74) of patients with less than 5% predicted risk had a positive node. Patients with a high-risk i31-SLNB result (greater than 10% risk) had an 18.5% SLN positivity rate. These patients were 13.2 times more likely to be SLN-positive.

Patients with a low-risk i31-SLNB result (less than 5% risk) demonstrated a 97.8% three-year recurrence-free survival (RFS) rate, indicating a very low risk of recurrence.

Collectively, these results support the ability of the integrated i31-SLNB to more precisely identify patients at low risk of SLN positivity compared with American Joint Committee on Cancer (AJCC) staging criteria and other predictive GEP tests.

Together, the study findings underscore two key conclusions from the prospective study:

1)	DecisionDx-Melanoma identifies low-risk patients who have favorable outcomes, with very low nodal positivity rates and high three-year RFS.
2)	DecisionDx-Melanoma also identifies patients at high risk of nodal positivity, providing clear and clinically meaningful separation between low- and high-risk groups.

“The strength of these prospective data clearly differentiates the risk prediction provided by DecisionDx-Melanoma from staging criteria and other predictive GEP tests and reinforces our leadership in precision risk assessment for patients with melanoma,” said Derek Maetzold, president and chief executive officer of Castle Biosciences. “By integrating DecisionDx-Melanoma’s independently validated 31-GEP score with key clinicopathologic factors, the i31-SLNB result provides clinicians with a more comprehensive, accurate and clinically grounded foundation to guide more confident SLNB decision-making.”

This study has been accepted for publication at Future Oncology and will be available online soon.

About DecisionDx-Melanoma
DecisionDx-Melanoma is a gene expression profile (GEP) test designed to analyze tumor biology to deliver a personalized risk assessment for patients with stage I–III cutaneous melanoma, enhancing risk stratification beyond American Joint Committee on Cancer (AJCC) staging alone. By combining molecular insights with select clinicopathologic features, the test provides two distinct outputs: a personalized risk of sentinel lymph node (SLN) positivity and a personalized risk of recurrence and/or metastasis. This clinically actionable information is designed to help guide risk-aligned patient management decisions, including SLN biopsy consideration, follow-up intensity, imaging and referrals.

DecisionDx-Melanoma is supported by more than 50 peer-reviewed publications, including prospective studies and meta-analyses, and was developed in collaboration with more than 100 leading U.S. institutions. The test has been clinically validated in more than 10,000 patient samples, ordered more than 220,000 times since launch, and has been shown to be associated with improved patient survival. Learn more at www.CastleBiosciences.com.

About Castle Biosciences
Castle Biosciences (Nasdaq: CSTL) is a leading diagnostics company improving health through innovative tests that guide patient care. With a primary focus in dermatologic and gastroenterological disease, we develop personalized, clinically actionable solutions that help improve disease management and patient outcomes.

We put people first—empowering patients and clinicians and informing care decisions through rigorous science and advanced molecular tests that support more confident treatment planning. To learn more, visit www.CastleBiosciences.com and connect with us on LinkedIn, Instagram, Facebook and X.

DecisionDx-Melanoma, DecisionDx-CMSeq, i31-SLNB, i31-ROR, DecisionDx-SCC, MyPath Melanoma, AdvanceAD-Tx, TissueCypher, DecisionDx-UM, DecisionDx-PRAME and DecisionDx-UMSeq are trademarks of Castle Biosciences, Inc.

Forward-Looking Statements
This press release contains forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended, which are subject to the “safe harbor” created by those sections. These forward-looking statements include, but are not limited to, statements concerning: the ability of DecisionDx-Melanoma’s i31-SLNB test to (i) generate a personalized likelihood of SLN positivity to support risk-aligned shared decision-making consistent with NCCN guideline thresholds; and (ii) identify patients at both low and high risk of nodal metastasis. The words “designed,” “may”, “can”, and similar expressions are intended to identify forward intentions or expectations disclosed in our forward-looking statements, and you should not place undue reliance on our forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations disclosed in the forward-looking statements that we make. These forward looking statements involve risks and uncertainties that could cause actual results to differ materially from those in the forward-looking statements, including, without limitation: subsequent study or trial results and findings may contradict earlier study or trial results and findings or may not support the results obtained in these studies, including with respect to the discussion of our tests in this press release; actual application of our tests may not provide the aforementioned benefits to certain patients; and the risks set forth under the heading “Risk Factors” in our Annual Report on Form 10-K for the year ended December 31, 2025, and our subsequent Quarterly Reports on Form 10-Q, each as filed or to be filed with the SEC, and in our other filings with the SEC. The forward-looking statements are applicable only as of the date on which they are made, and we do not assume any obligation to update any forward-looking statements, except as may be required by law.

1) Yamamoto M, Sickle-Santanello B, Beard T, et al. The 31-gene expression profile test informs sentinel lymph node biopsy decisions in patients with cutaneous melanoma: results of a prospective, multicenter study. Curr Med Res Opin. 2023;39(3):417-423. doi:10.1080/03007995.2023.2165813
2) Guenther JM, Ward A, Martin BJ, et al. A Prospective, Multicenter Analysis of Recurrence-Free Survival After Sentinel Lymph Node Biopsy Decisions Influenced by the 31-GEP. Cancer Med. 2025;14(7):e70839. doi:10.1002/cam4.70839
3) Guenther JM, Ward A, Martin B, et al. A prospective, multicenter analysis of the integrated 31-gene expression profile test for sentinel lymph node biopsy (i31-GEP for SLNB) test demonstrates reduced number of unnecessary SLNBs in patients with cutaneous melanoma. World J Surg Oncol. 2025 Jan 3;23(1):5. doi: 10.1186/s12957-024-03640-x

Investor Contact:
Camilla Zuckero
czuckero@castlebiosciences.com

Media Contact:
Allison Marshall
amarshall@castlebiosciences.com

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What did Castle Biosciences (CSTL) report about i31-SLNB results on March 9, 2026?

The company reported that i31-SLNB predicted low-risk patients with a 2.6% actual SLN positivity rate. According to Castle Biosciences, the prospective study of 912 patients across 30 centers showed strong risk separation and favorable three-year RFS for low-risk patients.

How does DecisionDx-Melanoma’s i31-SLNB affect SLNB decisions for patients with predicted <5% risk (CSTL)?

The test identifies patients with very low actual SLN positivity who may consider forgoing SLNB. According to Castle Biosciences, patients with <5% predicted risk had 2.6% nodal positivity and high three-year recurrence-free survival, aligning with NCCN thresholds.

What were the key patient numbers and study scale for the i31-SLNB prospective study (CSTL)?

The prospective multicenter study enrolled 912 patients at 30 U.S. centers with 430 undergoing SLNB. According to Castle Biosciences, this scale enabled evaluation of nodal positivity and three-year recurrence outcomes across clinical subgroups.

What were i31-SLNB results specifically for T1b–T2a melanoma patients in the CSTL study?

In the T1b–T2a subgroup, low-risk patients had only 1.4% nodal positivity (1/74). According to Castle Biosciences, this pronounced risk separation addresses a common area of clinical uncertainty for SLNB decisions.

How did high-risk i31-SLNB results correlate with sentinel node positivity in the CSTL data?

Patients with >10% predicted risk had substantially higher SLN positivity, about 21.4% overall. According to Castle Biosciences, high-risk patients were approximately 13.2 times more likely to be SLN-positive than low-risk patients, indicating clear risk stratification.