Castle Biosciences Presents New Data at ASTRO 2023 Highlighting Risk-Stratification Performance of DecisionDx®-SCC in Patients with Cutaneous Squamous Cell Carcinoma Eligible for Adjuvant Radiation Therapy

Castle Biosciences, Inc. (Nasdaq: CSTL), a company improving health through innovative tests that guide patient care, will share new data demonstrating the ability of its DecisionDx^®-SCC test to identify node-negative cutaneous squamous cell carcinoma (SCC) patients at a higher risk of metastasis who may benefit from adjuvant radiation therapy (ART). The data will be shared in an oral presentation during the American Society for Radiation Oncology (ASTRO) 2023 Annual Meeting, being held Oct. 1-4 in San Diego.

“Risk stratification for patients with SCC can be challenging due to broad staging guidelines based on a wide range of clinicopathologic risk factors,” said Jason Newman, M.D., study author and director of the Head & Neck Cancer Division at Medical University of South Carolina. “When a patient’s risk of metastasis is unclear, tough decisions as to which treatment approaches to pursue become that much more difficult. DecisionDx-SCC complements traditional risk stratification with independent and objective information on a patient’s biologic risk of metastasis that can be invaluable in guiding risk-aligned treatment decisions, such as consideration of radiation therapy for a patient.”

Details regarding Castle’s presentation are as follows:

Abstract ID 1180: Risk-stratification using the 40-gene expression profile (40-GEP) test identifies patients with node negative cutaneous squamous cell carcinoma (cSCC) at higher risk of metastasis who may benefit from adjuvant radiation therapy (ART)

Session #: QP 16

Session: H&N 3: Innovative Approaches to Individualizing Therapy for Head & Neck and Skin Cancer

Date & Time: Tuesday, Oct. 3 from 5:15-6:15 p.m. Pacific Time

The study aimed to evaluate whether DecisionDx-SCC (40-GEP) test results could refine the ability to select node negative SCC patients at a higher risk of metastasis who are most likely to benefit from ART. An intermediate risk population in which ART is often considered was defined as Brigham and Women’s Hospital (BWH) stage T2a or higher (n=489). DecisionDx-SCC significantly stratified the cohort according to risk of metastasis, with metastasis-free survival (MFS) rates of 92.4%, 76.1% and 59.4% for patients with Class 1 (low biological risk of metastasis), Class 2A (moderate risk) and Class 2B (high risk) test results, respectively (p<0.0001). Cox regression analysis demonstrated a significant difference in Class 2A and 2B MFS compared to Class 1, with a 3.2-fold and 6.4-fold increase in metastasis, respectively (p<0.0001).

Nearly half of the ART-eligible cohort (46%) received a DecisionDx-SCC Class 1 test result and had a less than 10% risk of metastasis, identifying a population whose treatment could potentially be de-intensified. Conversely, patients with low-risk BWH T1 stage, who are traditionally not considered for ART, that received a Class 2A or 2B test result had a greater than 10% risk of metastasis and could be considered for adjuvant therapy.

Abstracts will be published in the International Journal of Radiation Oncology • Biology • Physics (Red Journal).

About DecisionDx®-SCC

DecisionDx-SCC is a 40-gene expression profile test that uses an individual patient’s tumor biology to predict individual risk of cutaneous squamous cell carcinoma metastasis for patients with one or more risk factors. The test result, in which patients are stratified into a Class 1 (low), Class 2A (moderate) or Class 2B (high) risk category, predicts individual metastatic risk to inform risk-appropriate management. Peer-reviewed publications have demonstrated that DecisionDx-SCC is an independent predictor of metastatic risk and that integrating DecisionDx-SCC with current prognostic methods can add positive predictive value to clinician decisions regarding staging and management. More information about the disease and test can be found at www.CastleTestInfo.com.

About Castle Biosciences

Castle Biosciences (Nasdaq: CSTL) is a leading diagnostics company improving health through innovative tests that guide patient care. The Company aims to transform disease management by keeping people first: patients, clinicians, employees and investors.

Castle’s current portfolio consists of tests for skin cancers, uveal melanoma, Barrett’s esophagus and mental health conditions. Additionally, the Company has active research and development programs for tests in other diseases with high clinical need, including its test in development to predict systemic therapy response in patients with moderate-to-severe psoriasis, atopic dermatitis and related conditions. To learn more, please visit www.CastleBiosciences.com and connect with us on LinkedIn, Facebook, X and Instagram.

DecisionDx-Melanoma, DecisionDx-CMSeq, DecisionDx-SCC, MyPath Melanoma, DecisionDx-UM, DecisionDx-PRAME, DecisionDx-UMSeq, TissueCypher and IDgenetix 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 potential of DecisionDx-SCC test results to identify node-negative cutaneous SCC patients at a higher risk of metastasis who may benefit from ART and provide information that can be invaluable in guiding risk-aligned treatment decisions, such as consideration of radiation therapy for a patient. The words “can,” “may” and similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain these identifying words. We may not actually achieve the plans, 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 our 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 shown in this study, including with respect to the discussion of DecisionDx-SCC in this press release; actual application of our tests may not provide the aforementioned benefits to 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, 2022, 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.

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