Liu, Minetta C.

3516 Background: ICIs produce durable benefit in MSI-H CRC, yet optimal treatment duration and monitoring strategies remain unclear. ctDNA enables minimally invasive assessment of molecular response during and after ICI. We evaluated real-world treatment patterns, outcomes, and longitudinal ctDNA dynamics in pts with MSI-H CRC treated with ICI. Methods: Pts with MSI-H mCRC treated with first-line ICI were identified from Natera’s Real-World Database with linked longitudinal ctDNA results (Signatera, Natera, Inc.) and commercial claims data (Forian’s Hybrid data ecosystem, CHRONOS). Overall survival (OS) was estimated using available mortality data (Veritas Data Research, Fact of Death Mortality Data Index). ctDNA status was assessed pre-ICI when available and longitudinally following ICI initiation. Outcomes included OS and time to next trmt (TTNT); ctDNA dynamics following ICI discontinuation were descriptively evaluated. Results: Among 559 pts treated with first line ICI (pembrolizumab [70.1%], ipilimumab+nivolumab [15.7%], and nivolumab [11.8%]), the median duration of ICI was 266 days. Baseline (pre-ICI) ctDNA results were available for 180 pts, including 38% who were treated with chemotherapy, and most pts (71.7%, 129/180) were ctDNA-positive (ctDNA(+)) prior to ICI initiation. Of these, 89.9% (116/129) had ctDNA-timepoints after the start of ICI, and 81.0% (94/116) achieved anytime ctDNA clearance (from ctDNA(+) to negative), while 19.0% (22/116) remained ctDNA(+). ctDNA(+) at anytime after the start of ICI (HR: 8.0, p < 0.0001) or on first result after the start of ICI (HR: 3.4, p < 0.0001) was associated with inferior OS compared ctDNA-negativity (ctDNA(-)) at these time points. Among pts whose first post-ICI test was ctDNA(+), 32% switched treatments, with a median TTNT of 302 days, compared to only 6.6% of pts (median TTNT 683 days) whose first post-ICI test was ctDNA(-). First post-ICI ctDNA status was associated with longer ICI duration (median 192 days for ctDNA(+) vs 303 days for ctDNA(-)), and ctDNA(+) was predictive of inferior OS (HR: 4.9; p < 0.001). ctDNA after the end of ICI was available for 392 patients; it was persistently negative in 77.2% (303/392), persistently positive in 12.8% (50/392), cleared after the end of ICI (positive to persistently negative) in 4.1%, and the remaining 5.9% had variable patterns (e.g. positive to negative to positive). Conclusions: In this large real-world cohort of MSI-H mCRC treated with ICI, longitudinal ctDNA dynamics strongly correlated with treatment durability, time to subsequent therapy, and OS. These findings support further evaluation of ctDNA to inform treatment duration and surveillance strategies in MSI-H mCRC.

11177 Background: Early detection of recurrence after curative-intent treatment is a critical priority, traditional imaging often detects relapse when disease burden is high, potentially limiting treatment efficacy. ctDNA has emerged as an established prognostic biomarker for molecular residual disease (MRD) detection and early relapse. We performed a real-world meta-analysis on published datasets that utilized personalized, tumor-informed ctDNA testing to assess its performance across tumor types and clinical management approaches. Methods: Real-world studies published from December 2022 to December 2025 using the personalized, tumor-informed Signatera ctDNA assay (Natera, Inc.) were included. Eligibility required ctDNA assessment in the MRD (2–16 weeks post-surgery (histology dependent)/pre adjuvant therapy (ACT)) and/or surveillance (post-ACT/end of the MRD window if no ACT was given) settings, with outcomes stratified by ctDNA status and hazard ratios (HR) reported from univariable or time-dependent Cox regression analysis. Exclusions included: clinical trials, biobanks, metastatic disease treated with palliative intent, case reports, reviews, editorials, or commentaries. Clinical outcomes were harmonized into a composite event-free survival (EFS) endpoint (time to recurrence, progression, or death). Pooled HRs and 95% CIs were estimated using random and fixed effects models and heterogeneity between studies was assessed using the I 2 and Cochran’s Q test. Median lead time from ctDNA-positivity to radiographic or clinical recurrence was summarized when available. Results: We identified 18 eligible publications comprising 3,004 unique patients across 15 solid tumor types. During the MRD window, ctDNA positivity was associated with a significantly increased risk of an EFS event compared with ctDNA negativity (pooled HR: 8.15; 95% CI: 6.12–10.85, I 2 = 0.00%, P = 0.844, n = 13). During the surveillance window, inclusive of post-definitive treatment, ctDNA positivity conferred an even greater EFS risk (pooled HR: 18.30; 95% CI: 14.17–23.65, I 2 = 0%, P = 0.911 for heterogeneity, n = 16). Across studies, ctDNA detection during surveillance preceded radiographic or clinical recurrence by a median of 3.20 months (95% CI: 2.50-4.10, P < 0.0001, n = 10), although this varied substantially by study/tumor type (I 2 = 80.9%). Conclusions: In this pan-cancer real-world meta-analysis, ctDNA positivity assessed using a tumor-informed assay was highly prognostic following curative-intent therapy at all timepoints evaluated with low heterogeneity between studies. These findings support the broad clinical utility of tumor-informed ctDNA testing for post-treatment risk stratification and longitudinal disease monitoring across solid tumors.

PURPOSE The feasibility of Oncotype DX Breast Recurrence Score testing on diagnostic core needle biopsy (CNB) samples has been demonstrated through analysis of a large central laboratory experience. However, concordance with paired samples from the same primary breast tumor has not been comprehensively evaluated. The primary objective of this study was to determine the degree of concordance of Recurrence Score (RS) results between paired CNB and surgical excision (SE) specimens from the same untreated primary breast tumor of patients with early-stage, hormone receptor–positive, human epidermal growth factor receptor 2–negative invasive breast cancer. MATERIALS AND METHODS Patients with a clinical RS result on either a CNB or SE specimen were included. Lin concordance correlation coefficient (CCC) with 95% CIs was estimated to assess agreement of continuous RS results between paired samples. Cohen Kappa with 95% CIs was used to evaluate categorical RS result concordance. RESULTS A total of 134 patients were identified with paired CNB and SE specimens. The median age was 62 years (range, 33-99; 23 patients [17%] were age ≤50 years v 111 [83%] age >50 years). The mean RS results were 15.6 (range, 0-56) for CNB and 16.6 (range, 0-59) for SE ( P = .003), with no statistically significant differences in RS categories. Agreement between continuous RS results from paired samples was strong, with a Lin CCC of 0.86 (95% CI, 0.80 to 0.90). The overall percent agreement in RS categories was 91.8% (95% CI, 87.1 to 96.4). Categorical agreement was further supported by a Cohen Kappa statistic of 0.64 (95% CI, 0.44 to 0.83), indicating substantial agreement between CNB and SE specimens. CONCLUSION The results of this study demonstrate high concordance of RS results between paired samples, which supports the use of CNB for Oncotype DX testing.

Abstract Introduction: Among Non-Hodgkin lymphomas (NHL), DLBCL is the most prevalent subtype (~30% of diagnoses). First-line (1L) chemoimmunotherapy cures many patients (pts), but 30-40% will relapse. Prognostic and predictive biomarkers are needed to identify pts refractory to 1L therapy or at high risk of relapse, enabling timely intervention and improved outcomes. This study evaluated tumor-informed WGS-based longitudinal ctDNA monitoring in a real-world DLBCL cohort. Methods: We retrospectively analyzed ctDNA data from 49 pts with DLBCL. Plasma samples were collected before, during, and at the end of 1L therapy (EOT, defined as ± 21 days from the last infusion (preferred when available) or ± 14 days from post-treatment PET imaging) and during surveillance/subsequent treatments. A WGS-based tumor-informed assay (Signatera™ Genome, Natera, Inc.) was used for ctDNA detection and quantification. The association of ctDNA dynamics with event-free survival (EFS) was evaluated using Cox regression analysis, restricted to pts completing at least 5 cycles of 1L therapy. Results: The median age of the cohort was 62 years (range: 56-74), and median follow-up was 22 months (range: 14-30). Pre-treatment ctDNA-positivity rate was 100% (31/31). A total of 19 pts achieved ctDNA clearance during 1L therapy (early clearance within 3 cycles, n=5; late clearance between cycle 4 and EOT, n=14). Pts with no clearance or ctDNA-positivity at EOT (n=7) had significantly inferior EFS compared to those achieving ctDNA clearance (HR 8.04, 95%CI: 1.87-34.62, p=0051). On further stratifying the clearance group and comparing with no clearances, pts with early clearance had superior outcomes (HR 13.22, 95%CI: 1.48-1741.94, p=0.0166), relative to late clearances (HR 5.65, 95%CI 1.46–25.16, p=0.0129). Notably, all 5 pts with early ctDNA clearance remained serially negative and relapse-free. Of the 49 pts, 9 proceeded to chimeric antigen receptor T-cell therapy (CAR-T). Among 6 of these pts with available ctDNA results pre and post therapy, all 3 who stayed ctDNA-positive after CAR-T relapsed (1.3, 2, and 4.5 months post-infusion); 1 pt with pre- and post-CAR-T ctDNA-negativity and 2 pts with ctDNA clearance post-CAR-T remained relapse-free for 9.6, 15.9, and 22.4 months, respectively. Taken together, the PPV and NPV for a single ctDNA time point post CAR-T infusion were 100% for predicting long-term clinical outcomes. Conclusions: Our data support longitudinal ctDNA monitoring as a valuable prognostic and predictive biomarker in DLBCL. ctDNA clearance, particularly early clearance within the first 3 cycles of 1L therapy, is associated with improved pt outcomes. Longitudinal ctDNA monitoring and dynamics can enable identification of high-risk pts who do not clear ctDNA and may benefit from earlier referral for CAR-T. Furthermore, ctDNA monitoring pre- and post-CAR-T infusion is predictive of treatment response and can identify pts at high risk of relapse.

Abstract Introduction: Circulating tumor DNA (ctDNA) has emerged as a promising biomarker for treatment response monitoring, risk stratification, and early recurrence detection for patients with lymphoma. Tumor-informed ctDNA testing for detecting molecular residual disease (MRD) is seeing early adoption in this setting, demonstrating technical feasibility and clinical need for personalized monitoring tools. However, the prognostic utility of ctDNA across indolent and aggressive subtypes of lymphoma remains poorly understood. Here, we present real-world data on ctDNA detection and clearance dynamics in patients with various lymphoma subtypes, including patients who are newly diagnosed and those in the relapsed/refractory setting receiving chimeric antigen receptor (CAR)-T therapy. Methods: We performed a retrospective, real-world analysis of ctDNA testing in 148 patients with either B-cell (n=129) or T-cell lymphoma (n=19). Among the 129 patients with B-cell lymphoma, 113 patients had aggressive subtypes (n=84 with DLBCL; n=29 with other), and 16 had indolent lymphoma subtypes. ctDNA was analyzed using a tumor-informed 16-plex mPCR-NGS assay (SignateraTM, Natera, Inc.) from September 2020 - May 2025. Plasma samples (n=1,122; median: 7 per patient, range: 1-31) were collected at baseline, during therapy, at end-of-treatment (EOT), and during follow-up per physician's discretion. ctDNA status at baseline and EOT, and clearance kinetics during first-line (1L) induction therapy were correlated with clinical outcomes. Results: The median patient age was 61 years (range: 18.3–84), and median follow-up was 18.7 months (range: 0.6–54.4 months). Disease stage at diagnosis was available for 139 patients: stage I (n=15, 11%), stage II (n=31, 22%), stage III (n=16, 12%), and stage IV (n=77, 55%). Stage was unknown in 9 patients. Overall, ctDNA detection at baseline was 95.7% (90/94), including 100% in patients with indolent B-cell (11/11), and T-cell lymphomas (6/6), and 94.8% (73/77) in patients with aggressive B-cell lymphomas. Across all subtypes, EOT ctDNA status was strongly prognostic for event-free survival (EFS). Patients with ctDNA-positivity at EOT had significantly worse EFS (HR: 7.33, 95%CI: 2.8-19.18, p&lt;0.0001). In aggressive lymphomas, particularly in DLBCL, ctDNA positivity at EOT was associated with a markedly worse prognosis (Aggressive: HR: 7.69, 95%CI: 2.62-22.56, p=0.0002; DLBCL only: HR: 5.74, 95%CI: 1.71-19.23, p&lt;0.01). Among patients with evaluable ctDNA time points relative to 1L therapy (N=60), ctDNA clearance at any time point during treatment correlated with improved EFS. (HR: 18.55, 95%CI: 4.82-71.32, p&lt;0.0001). All patients who did not clear ctDNA relapsed, yielding a positive predictive value of 100% (11/11); the negative predictive value for those who did clear was 88% (43/49). The remaining six patients turned positive during surveillance prior to relapse. Of 15 patients treated with CAR-T cell therapy, 11 had pre- and post-infusion timepoints for ctDNA analysis. Of these, 82% (9/11) were ctDNA-positive prior to CAR-T, and 55% (5/9) achieved post-infusion ctDNA clearance and a complete clinical response, though one became ctDNA-positive and relapsed (13.3 months post-CAR-T), and another suffered non-disease mortality (22.3 months post-CAR-T). In contrast, all 4 patients remaining ctDNA-positive post-CAR-T relapsed or died within a median of 3.3 months (range: 1.3–6.4 months). Two patients persistently ctDNA-negative remain disease-free. Taken together, the PPV and NPV for 12-month EFS following CAR-T therapy using a single ctDNA timepoint were both 100%. Conclusions: This is the first report of the largest, real-world study evaluating the utility of ctDNA across a broad spectrum of heterogeneous lymphoma subtypes in patients receiving various standard of care therapies. ctDNA status at EOT and on-treatment clearance were strong predictors of clinical outcomes across multiple histological subtypes of lymphoma treated with standard-of-care agents including CAR-T cell therapy. The robustness of these findings across biologically distinct subgroups highlight the broad applicability of ctDNA testing for risk stratification and recurrence monitoring for both aggressive and indolent disease. Further prospective studies are needed to validate these findings.

Objective: In this study, we evaluate the utility of ctDNA analysis in a large cohort of patients for whom ctDNA testing was ordered commercially with real-world application. Summary Background Data: Circulating tumor DNA (ctDNA) has emerged as a prognostic and predictive biomarker for assessing post-surgical molecular residual disease (MRD) and response to treatment. Methods: A real-world data analysis was performed using commercial ctDNA testing (SignateraTM) from patients (N=795, n=5,971 plasma samples) with stage I-III colon cancer treated at multiple US institutions. The association of ctDNA detection within the MRD window, during surveillance, and the impact of ACT was correlated with patient outcomes. Results: ctDNA-positive patients during the MRD window and surveillance showed significantly shorter DFS compared to ctDNA-negative patients (hazard ratio (HR): 9.85, P<0.0001; HR: 26.91, P<0.0001). Multivariate analysis of the MRD window revealed ctDNA-positivity as the most significant factor associated with inferior DFS (adjHR: 7.7, P<0.001). MRD-positive patients who received ACT showed improved DFS compared to patients observed post-surgery (adjHR: 6.1, FDR adj P=0.0007). No ACT benefit was observed in MRD-negative patients (adj HR: 1.20, FDR adj P=0.768). On evaluating ctDNA dynamics from MRD to surveillance, patients who remained ctDNA-positive or converted from negative to positive showed a significantly inferior DFS (HR: 34.40, P<0.0001; HR: 13.65, P<0.0001) compared to persistently ctDNA-negative patients. Conclusions: Postsurgical ctDNA detection is prognostic of relapse and potentially predictive of ACT benefit in patients with resectable colon cancer, which may enable personalized surveillance, intervention, and/or trial options, ultimately improving patient outcomes.

PURPOSE Neoadjuvant immunotherapy (IO) has become the standard of care for early-stage triple-negative breast cancer (TNBC), but not yet for other subtypes. We previously developed a clinical-grade mRNA-based immune classifier (ImPrint) predicting response to IO that is now being used in I-SPY2.2 as part of the response predictive subtypes. We report the performance of ImPrint in hormone receptor–positive and human epidermal growth factor receptor 2–negative (HR+HER2–) patients from five IO arms. METHODS A total of 204 HR+HER2– (MammaPrint high-risk) patients from five IO arms (anti–PD-1, anti–PD-L1/poly [ADP-ribose] polymerase inhibitor combination, anti–PD-1/toll-like receptor 9 dual-IO combination, and anti–PD-1 ± lymphocyte activation gene 3 dual-IO combination) and 191 patients from the chemotherapy-only control arm were included in this analysis. Patients were classified as ImPrint+ (likely sensitive) versus ImPrint– (likely resistant), using pretreatment mRNA. Performance of ImPrint for predicting pathologic complete response (pCR) to IO-containing arms was characterized and compared with tumor grade (III), MammaPrint (ultra) High2 risk (MP2), and estrogen receptor (ER)-low (ER ≤ 10%). RESULTS Overall, the pCR rate across the five IO arms was 33%. 26% of HR+HER2– patients were ImPrint+, and pCR rates with IO were 75% in ImPrint+ versus 17% in ImPrint–, with the highest pCR rate >90% in a dual-IO arm. In the control arm, pCR rates were 33% in ImPrint+ and 8% in ImPrint–. Tumor grade (III), MP2, and ER-low showed pCR rates in IO of 45%, 56%, and 63%, respectively, with lower pCR odds ratios (OR < 7.5) compared with ImPrint (OR = 14.5). CONCLUSION Using an accurate selection strategy, HR+HER2– patients could achieve pCR rates similar to what is seen with best neoadjuvant therapies in TNBC and HER2+ (ie, pCR rate >65%-70%). ImPrint, an Food and Drug Administration IDE-enabled assay, may represent a way to identify HR+HER2– patients for IO that best balances likely benefit versus risk of serious immune-related adverse events.

e13042 Background: The conventional treatment paradigm for metastatic breast cancer (MBC) is to continue systemic therapy until disease progression. With improved systemic therapy options, some patients (pts) with MBC attain radiographic remission. Tumor-informed circulating tumor (ct)DNA detects molecular disease burden and may guide decision-making. We report on an institutional cohort of pts with MBC in which tumor-informed ctDNA testing was utilized prospectively to guide decisions on systemic therapy or surveillance imaging frequency. Methods: Utilizing an IRB-approved protocol, we retrospectively identified pts monitored with tumor-informed ctDNA using the Signatera™ mPCR-NGS assay (Natera, Inc) as part of routine clinical care in Mayo Clinic, Rochester. We then identified a subset with biopsy-proven MBC who attained radiographic remission of disease following systemic therapy, deemed by the treating medical oncologist as “exceptional responders”. Real-time ctDNA testing was used to inform a decision to either discontinue systemic therapy, treatment de-escalation (remove a component of multi-drug therapy), increase the time interval between restaging scans, or stay off therapy previously stopped due to toxicity . Data on breast cancer characteristics, imaging results, ctDNA test results, and outcomes were collected. Descriptive statistics were used to analyze the data. Results: 10 pts met our study criteria. The median age at MBC diagnosis was 42 years (range 28-61); 5 pts had TNBC, 3 pts had HER2+, and 2 pts with ER+/HER2- MBC. Most pts (60%) were on first or second line therapy (range 1-5). The median duration of the most recent regimen was 27.5 months (range 10-70 months). All pts were without radiographic evidence of active cancer at the time of ctDNA testing with negative results (0 MTM/mL) at the time of clinical decision-making. 4 pts stopped all systemic therapy and proceeded with observation (1 trastuzumab, 1 trastuzumab deruxtecan, 1 pembrolizumab, and 1 talazoparib). 2 pts discontinued a targeted therapy (1 olaparib, 1 abemaciclib) while continuing endocrine therapy. 1 patient stopped targeted therapy (abemaciclib) due to a blood clot, and ctDNA testing guided the decision to not start another CDK4/6 inhibitor while continuing endocrine therapy. In 3 pts the frequency of restaging scans was decreased to either annually or semi-annually. The median follow-up after a ctDNA-informed decision was 11 months (range 1-41 months). At this time, no patient has converted to ctDNA-positivity, experienced resurgence or progression of disease, needed to change systemic therapy or increase imaging frequency. Conclusions: Tumor-informed ctDNA may offer additional confidence to oncologists and pts to deescalate systemic therapy or decrease the interval of imaging frequency in exceptional responders with MBC. Additional data to support this approach is needed.

Abstract Background: Tumor-informed circulating tumor (ct) DNA testing is recognized as a powerful predictor of CRC recurrence. However, the benefit of systemic therapy to prevent or delay clinical recurrence in patients (pts) with molecular recurrence after curative surgery remains uncertain. Methods: Pts with CRC who had undergone curative resection of primary and/or metastatic sites + standard of care (SoC) adjuvant treatment, if applicable, were enrolled in ALTAIR study if they (1) prospectively tested positive for ctDNA using a clinically validated, personalized assay (Signatera™, Natera, Inc.) within 3 months before enrollment and (2) had no recurrence on radiological (CT) imaging. Pts were randomized to receive FTD/TPI or placebo for 6 months. CT scans and ctDNA analyses were conducted every 2 months in the first year, every 3 months in the second year, and every 6 months in the third year. The primary endpoint was disease-free survival (DFS), with secondary endpoints including ctDNA clearance, overall survival (OS), and adverse events. The study assumed a median DFS of 8 months in the placebo group, an HR of 0.667 for the FTD/TPI group, 0.05 significance level, 0.80 power, 2-year enrollment, and 1-year follow-up, requiring 240 pts and 190 DFS events. Baseline ctDNA levels were evaluated by mean tumor molecules (MTM)/ml. Results: Between July 2020 and June 2023, 243 pts were enrolled and randomized to FTD/TPI (n=122) or placebo (n=121). Baseline characteristics were balanced, and 96.3% of pts received SoC treatment postoperatively. FTD/TPI group had a median DFS of 9.30 months vs. 5.55 months in the placebo group, which did not reach statistical significance in the primary population (HR, 0.79; 95% CI, 0.60-1.05; P = 0.107). The benefit was highly pronounced, however, in stage IV pts (HR: 0.53, P = 0.012). In addition, after excluding pts with low ctDNA levels (&lt;0.179 MTM/mL, threshold determined by ROC analysis) from both arms, significant benefit with FTD/TPI was observed in the primary population, resulting in an improved DFS (N=154, HR: 0.61 95% CI: 0.43-0.85; P = 0.003) with a median DFS of 8.12 months for FTD/TPI vs. 3.89 months for placebo; consistent with these findings, a trend toward higher ctDNA clearance rate was observed for the FTD/TPI group (8.33%), compared to placebo (4.05%).OS data remain immature, with 24 events reported across both arms. Grade ≥3 adverse events occurred in 73.0% of the FTD/TPI arm versus 3.3% in the placebo arm, with no new safety signals. Conclusions: Although statistical significance was not reached in the primary population, FTD/TPI showed benefit in pts with high molecular tumor burden and/ or Stage IV disease, resulting in improved DFS. Citation Format: Hideaki Bando, Jun Watanabe, Masahito Kotaka, Nobuhisa Matsuhashi, Eiji Oki, Yoshito Komatsu, Manabu Shiozawa, Keiji Hirata, Yuji Miyamoto, Kun-Huei Yeh, Shruti Sharma, Vasily Aushev, Adham Jurdi, Minetta C. Liu, Alexey Aleshin, Ichiro Takemasa, Daisuke Kotani, Akihito Sato, Toshihiro Misumi, Yoshiaki Nakamura, Qian Shi, Hiroya Taniguchi, Takeshi Kato, Takayuki Yoshino. A randomized, double-blind, phase III study comparing trifluridine/tipiracil (FTD/TPI) versus placebo in patients with molecular residual disease following curative resection of colorectal cancer (CRC): The ALTAIR Study [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr CT131.

Abstract Purpose: The aim of PREDICT was to confirm clinical validity and the potential for clinical utility of serial circulating tumor cell (CTC) enumeration in patients with metastatic breast cancer, focusing on its prognostic value in different breast cancer subtypes and clinical settings. Experimental Design: In total, 4,436 individual patient-level data with CTC results from both baseline and one follow-up (CellSearch; Menarini Silicon Biosystems) were analyzed to evaluate the association between CTC detection and overall survival (OS) in the full patient cohort and separately for tumor and treatment types. Results: Using the cutoff ≥1 CTC for CTC positivity, 913 (20.6%) patients had 0 CTCs at both time points (neg/neg) and 325 (7.3%) and 1,189 (26.8%) patients converted from CTC negative to CTC positive (neg/pos) or vice versa (pos/neg), whereas 2,009 (45.3%) patients had at least one CTC at both time points (pos/pos). The median OS for the neg/neg, neg/pos, pos/neg, and pos/pos group was 45.6, 26.1, 32.3, and 17.3 months, respectively (P &lt; 0.0001, global log-rank test). CTC responders (pos/neg) showed a lower risk of death compared with CTC nonresponders (pos/pos; HR, 0.48; 95% confidence interval, 0.44–0.53). Similar results were obtained in subgroup analyses according to hormone receptor and HER2 subtype, treatment type, and with a ≥5 CTC cutoff for CTC positivity. Conclusions: Follow-up CTC assessments strongly predict OS independently from tumor subtype and treatment. New randomized trials to define the clinical utility of CTC monitoring for risk stratification and as an early response marker in metastatic breast cancer are urgently needed.