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PURPOSE EA1181 (ClinicalTrials.gov identifier: NCT04266249 ) is a single-arm trial evaluating neoadjuvant taxane, trastuzumab, and pertuzumab (THP) in patients with clinical stage II/IIIa human epidermal growth factor receptor 2 (HER2)–positive breast cancer. This report focuses on the secondary end points of pathologic complete response (pCR) rates and associated factors. The primary end point—3-year recurrence-free survival among patients achieving a pCR (ypT0/Tis, ypN0)—will be reported when data mature. METHODS Patients received four cycles of trastuzumab and pertuzumab (HP) with either once per week paclitaxel (12 weeks) or docetaxel (every 3 weeks for four cycles), followed by surgery. Clinicopathologic characteristics were assessed in all patients. The HER2DX pCR score was determined using diagnostic biopsy samples in a representative subset. Logistic regression models identified factors associated with pCR. RESULTS A total of 2,175 patients were enrolled (781 HER2+/estrogen receptor‑negative [ER–]; 1,394 HER2+/ER+). Of 2,141 patients who initiated THP, the overall pCR rate was 43.8%: 63.7% in HER2+/ER– and 32.4% in HER2+/ER+ tumors. Higher pCR rates were observed in tumors that were ER-negative or low ER expressing, progesterone receptor-negative or low expressing, HER2 immunohistochemistry (IHC) 3+, and in patients treated with once per week paclitaxel. T3 disease was associated with a lower pCR rate in HER2+/ER– tumors; otherwise, tumor (T) and nodal (N) stage did not significantly affect pCR. Among 569 patients assessed for HER2DX pCR scores, a high score was independently associated with higher pCR rates, after adjustment for clinicopathologic variables. CONCLUSION Neoadjuvant THP achieved pCR in nearly two-thirds of patients with HER2+/ER– and one third with HER2+/ER+ breast cancer. Low hormone receptor expression, HER2 IHC 3+ status, once per week paclitaxel use, and a high HER2DX pCR score were independent predictors of pCR. These findings should help optimize risk stratification and treatment personalization for patients with HER2-positive breast cancer.

Importance Limited granular data exist on breast cancer in transgender, nonbinary, and/or gender-diverse (TGD) individuals. Objective To describe patient demographic and tumor clinicopathological variables, clinically relevant risk factors, method of breast cancer detection, treatment characteristics, and locoregional outcomes and to estimate a 5-year breast cancer–specific survival (BCSS). Design, Setting, and Participants This cohort study obtained data on patients treated from 1990 to 2023 from 22 US academic medical centers using institutional databases, electronic medical records, and tumor registries. Included participants were TGD individuals 18 years or older with stage 0 to IV breast cancer. A comparative breast cancer cohort was selected from the 2016 to 2021 Surveillance, Epidemiology, and End Results Program (SEER) breast cancer dataset. Data were analyzed from January to October 2025. Exposure Breast cancer. Main Outcomes and Measures Estimated 5-year BCSS and locoregional recurrence rates. Wilcoxon rank sum and χ 2 tests were used to compare continuous and categorical variables. Kaplan-Meier analysis was used to estimate 5-year BCSS. Results A total of 112 TGD persons (median [IQR] age at diagnosis, 42.5 [36.5-51.0] years; 104 individuals [92.9%] were assigned female at birth) with breast cancer and 1 with bilateral disease were included in the analysis. Forty-three patients (38.4%) used gender-affirming hormone therapy before diagnosis. Among the entire cohort, 58 patients (51.8%) detected their own breast cancer, 68 (60.7%) had a familial breast cancer history, and 16 (14.3%) had a pathogenic germline variant. Tumors were predominantly hormone receptor positive (96 [85.7%]) and early stage (29 [25.7%] ductal carcinoma in situ [DCIS]; 51 [45.1%] stage I). The TGD cohort, compared with patients with breast cancer in the SEER dataset (N = 480 915), was younger (median [IQR] age, 42.5 [36.5-51.0] years vs 62.0 [52.0-72.0] years), had a higher proportion of persons assigned male at birth (8 [7.1%] vs 3522 [0.7%]), more often had progesterone receptor–positive disease (89 [79.5%] vs 335 491 [69.8%]), and presented with earlier-stage disease (eg, DCIS: 29 [25.9%] vs 79 604 [16.6%]). With a median (IQR) follow-up time for the TGD cohort of 38 (24-74) months, 5-year BCSS was favorable at 96.2% (95% CI, 85.3%-99.0%). Conclusions and Relevance This cohort study of TGD patients with breast cancer found that this population was young and often detected their own tumors via self-examination, suggesting opportunities to improve early detection through screening for individuals at elevated risk. Although short-term BCSS appeared favorable, prospective research is essential to define evidence-based guidelines and elucidate long-term outcomes.

603 Background: Accurate prediction of pathologic complete response (pCR) after preoperative therapy for HER2-positive (HER2+) breast cancer can help triage patients to immediate surgery vs additional preoperative therapy. Pre-surgery (i.e., after neoadjuvant therapy) imaging studies may help estimate the likelihood of pCR. In EA1181, patients (pts) with stage II-IIIA HER2+ breast cancer received four cycles of preoperative taxane, trastuzumab, and pertuzumab (THP×4). Pre-surgery imaging was required, though the specific imaging modality was not mandated. Here, we report the accuracy of pre-surgery breast MRI to predict pCR (ypT0/Tis, ypN0). Methods: Among 2,141 pts treated with THPx4, 1,378 had pre-surgery breast MRI and were representative of the overall cohort, although T3 tumors were more common (16% vs 9%). MRI reports were available for 1,351 pts; all reports were centrally reviewed and classified as radiologic complete response (rCR; no residual tumor enhancement and normalization of lymph nodes, if applicable) or radiologic residual disease (rRD). Reports with indeterminate radiologic response were independently reviewed by two study radiologists blinded to pathology outcomes. Among pts with rCR who had pathologic residual disease (pRD), we also assessed the extent of residual disease by residual cancer burden (RCB) score. Among pts with rRD who had a pCR, we determined the proportion who only had residual ductal carcinoma in situ (DCIS). Results: See Table. Conclusions: After neoadjuvant therapy, preoperative MRI accurately predicted pCR in 86% of patients with HER2+/ER− disease and may help identify patients who can proceed directly to surgery. MRI is insufficient to predict pCR for pts with HER2+/ER+ disease, as only 55% with complete response on MRI had a pCR at surgery. A pre-surgery MRI that indicated residual disease was accurate in 80% of pts with HER2+/ER+ disease, but in only 45% of pts with HER2+/ER- cancer. Accurately predicting residual disease after THPx4 allows for consideration of additional therapy that may increase the chance of pCR. Combining imaging like MRI with other biomarkers, such as tissue-based markers or circulating tumor DNA, may improve prediction of pCR and facilitate personalization of treatment decisions in HER2+ breast cancer. All pts with pre-surgery MRI (n=1351)pCR rate overall in this cohort = 47% MRI result Pathology pCR pRD rCR 70.6% (341/483) 29.4%(142/483) 78% had RCB1 rRD 34%(295/868) 21% of 295 had DCIS only 66.01% (573/868) ER- (n=589)*Overall pCR rate in ER- subset = 68% MRI result Pathology pCR pRD rCR 86.1% (205/238) 13.9% (33/238) 85% had RCB1 rRD 55.3% (194/351) 19% of 194 had only DCIS 44.7% (157/351) ER+ (n= 762)*Overall pCR rate in ER+ subset = 31% MRI result Pathology pCR pRD rCR 55.5% (136/245) 44.5% (109/245) 76% had RCB1 rRD 19.5% (101/517) 26% of 101 had only DCIS 80.5% (416/517)

12010 Background: BWEL (Alliance A011401; NCT02750826) is a Phase III trial that randomizes patients (pts) with stage II-III breast cancer and body mass index (BMI) ≥27 kg/m 2 1:1 to a 24-month (mo) lifestyle-based WLI + health education (HE) or HE alone. Here we report prespecified secondary outcomes of the impact of the WLI on patient-reported QOL and symptoms. Methods: The first 540 BWEL pts (randomized between 9/2016 and 7/2017) were included in this substudy that collected PROMIS 29 Profile 2.0 and Global Health scores at enrollment, 6 and 24 mos. Adjusting for baseline scores, mean physical function (predefined substudy primary outcome), global physical and mental health, fatigue and other scores were compared at 6 and 24 mos using analysis of covariance (ANCOVA). P-values <0.05 were considered significant. Positive differences in positive attributes (e.g., physical function) and negative differences in negative attributes (e.g., anxiety) represented better outcomes in the WLI arm. Results: At baseline, median BMI was 32.9 kg/m 2 (26.5 – 69.1), median age was 53 (25 - 78) years; 83.9% of pts were non-Hispanic White, 10.7% were Black and 5.9% were Hispanic. At 6 mos, the WLI (vs HE) arm had significantly better physical function (ANCOVA-based mean difference between arms 1.9 [95% CI 0.8, 3.0]; p<0.001), global physical health (2.0 [95% CI 0.9, 3.0]; p<0.001), global mental health (1.30 [95% CI 0.2, 2.4], p=0.03), social roles and activities (2.3 [95% CI 1.1, 3.6]; p<0.001), and fatigue (-1.7 [95% CI -3.1, -0.4], p=0.01) (Table). Improvements were generally maintained at 24 mos. Similar findings were seen in analyses using longitudinal mixed modeling. Conclusions: The BWEL WLI demonstrated QOL benefits for patients with breast cancer, resulting in significantly better physical function, global physical and mental health, and symptoms. Future analyses will evaluate which populations experienced the most benefit, as well as the relationships among QOL, symptoms, and weight loss. Support: U10CA180821, U10CA180882, UG1CA189823; U10CA180820; U10CA180868; U10CA180863, CCS 707213; U10CA180888; UG1CA189858; https://acknowledgments.alliancefound.org. Clinical trial information: NCT02750826 . Analysis of covariance-adjusted least squares means values at 6-mos adjusted for baseline scores. QOL/Symptom Attributes HE WLI Between arm difference (95% CI)* P value Physical Function 47.1 49.0 1.9 (0.8, 3.0) <0.001 Global Physical Health 46.2 48.1 2.0 (0.9, 3.0) <0.001 Global Mental Health 48.6 49.9 1.3 (0.2, 2.4) 0.03 Social Roles and Activities 52.7 55.0 2.3 (1.1, 3.6) <0.001 Anxiety 50.2 50.0 -0.2 (-1.6, 1.2) 0.77 Depression 47.5 47.5 -0.0 (11.2, 1.2) 0.96 Fatigue 50.6 48.9 -1.7 (-3.1, -0.4) 0.01 Sleep Disturbance 51.0 51.1 0.1 (-0.6, 0.8) 0.80 Pain Interference 51.3 50.3 -1.0 (-2.4, 0.4) 0.15 *Positive differences in positive attributes and negative differences in negative attributes favor WLI arm.

PURPOSE Remote symptom monitoring using patient-reported outcomes (PROs) has been shown to improve symptom control and physical function among patients with advanced cancer. However, it is unclear whether these benefits are similar across demographic groups. This exploratory analysis of the PRO-TECT trial examined whether the effects of electronic symptom monitoring varied by race, age, sex, and educational attainment. METHODS PRO-TECT was a cluster randomized trial conducted across 52 US community oncology practices comparing weekly electronic symptom monitoring with usual care (UC). Adult patients with metastatic solid tumors receiving systemic therapy were enrolled (N = 1,191). Participants in the PRO arm completed weekly symptom surveys including the PRO-CTCAE, triggering alerts to the care team for severe or worsening symptoms. Patients chose whether to complete weekly PROs online or via telephone-based interactive voice response not requiring Internet access. Outcomes were measured using the European Organisation for Research and Treatment of Cancer QLQ-C30 symptom control and physical function scales. Subgroup analyses were performed by race (Black v White), age (<65 v ≥65 years), sex (male v female), and educational attainment (≤high school v >high school). RESULTS From baseline to 3 months, PRO participants showed greater improvements in symptom control (+2.37 v −0.20, P = .002) and physical function (+1.54 v −0.93, P = .02) versus UC. Benefits were most pronounced among younger, female, Black, and less educated participants. Black participants in the PRO arm were more likely to report that symptom reporting made them feel more in control of their care compared with White participants. CONCLUSION Electronic symptom monitoring improved quality-of-life outcomes overall, including among groups who have historically experienced higher symptom burden or barriers to communication. Remote PRO systems may represent an equitable strategy to enhance cancer care delivery.

Abstract Background: While there are abundant data on the surgical management of BC after neoadjuvant chemotherapy (NAC), there is less guidance regarding surgical management of BC after NET, particularly with respect to lymph node (LN) surgery. While pathologic complete response (pCR) is common with NAC, it is a rare event with NET. In this analysis we assess the surgeons’ approach in the context of NET. Methods: The ALTERNATE trial is a phase III study that randomized postmenopausal patients (pts) with clinical stage II/III ER+ HER2- BC to receive neoadjuvant anastrozole (A), fulvestrant (F), or both for 6 mos. before surgery. The primary objective was to determine if neoadjuvant F or A+F increases the rate of pCR or ypT1-2N0/N1mic/Ki67 &lt;2.7% residual disease over A alone. Breast surgery was per standard of care. Sentinel lymph node biopsy (SLNB) with or without axillary lymph node dissection (ALND) was required to determine preoperative endocrine prognostic index PEPI score and residual cancer burden (RCB). Surgeons were required to indicate if the patient would be eligible for breast- conserving surgery (BCS) prior to NET. Patient accrual occurred from 2014-2018. The surgical outcomes are reported for the entire group. Results: Among the 3 treatment arms, 933 patients completed 6 cycles of NET. The clinical T stage at diagnosis was 73.6% T2, 22.6% T3and 3.8% T4. Initial tumor size was assessed by mammogram and ultrasound. Primary tumor histology was ductal in 64.8% and lobular in 25.4%.568 pts (60.9%) were clinically node negative (cN0), 36.2% cN1, 2.6% cN2 with 0.3% cN3. LNs were palpable in 25.2%. 383 had LN needle biopsy pre NET, of which 80.9% were positive. The surgeons’ initial impression of the patient’s eligibility for BCS was 74% while 0.6% were considered inoperable.75.5% of pts had post-NET imaging performed. Of those with tumor size recorded, 52.9% were T1 or not visible. Overall, 69.9% of the 933 patients had BCS. Of those 330 pts without post-NET imaging or who had post-NET imaging without a size noted, 64.8% underwent BCS. For pts undergoing BCS, 80.2% required only one surgery for margin clearance. Of the 690 pts deemed eligible for BCS pre NET, 79% had BCS. Of the 240 pts deemed ineligible for BCS pre-NET, 43.8% had BCS. Of the pre-NET cT2 tumors, 79% had BCS while only 45.5% of T3 tumors and 37.1% of T4 tumors had BCS. On surgical pathology, 474 (50.8%) pts were LN+ compared to pre-NET cN+ 39.1% and needle biopsy-proven 33.2%. For pts with pN0, 93.3% had SLNB alone. 6.7 % of pN0 had ALND alone or SLNB+ALND. For those with pN1-3 , 36.3% had SLN only; 37.3% SLN+ ALND; 26.4% ALND alone. Only 0.8% of the patients had no ALND or SLNB performed. Pts undergoing BCS were more likely to have SLNB alone for a +SLN compared to mastectomy pts (62.9% vs. 37.6%; chi-square p&lt;0.0001). For pts with pre-NET +LN on needle biopsy, only 26.8% underwent SLNB alone, while 37.4% underwent ALND alone. Of the 204 LN-positive pts having SLNB alone, 55.9% had only 1 + LN and 27% had only 2+ LNs. For pN+ pts who underwent SLNB +ALND, 42.1% had 1-2 + LNs. Conclusion: With NET, 69.9% of pts achieved BCS, including 43.8% deemed ineligible pre-NET. Given the low rate of pCR to NET, there should not be the expectation of pCR in +LNs. Based on the procedures performed in pts with +SLNs, it appears that surgeons were applying ACOSOG Z0011 criteria for pts having BCS, but were less likely to omit ALND for +LN in pts having mastectomy. The finding of only 1-2 +LNs in many of the LN+ pts suggests that omission of ALND may be a reasonable approach post-NET. Longer follow-up is required to assess local-regional recurrence. Support: U10CA180821, U10CA180882; https://acknowledgments.alliancefound.org. Clinical Trials.gov Identifier: NCT01953588 Citation Format: A. M. Leitch, T. Dockter, V. suman, A. Weiss, G. W. Unzeitig, J. Guenther, S. Sanati, K. Vij, J. Hoog, A. Caudle, A. Tiersten, M. Mita, W. Razaq, T. J. Hieken, Y. Wang, M. Rimawi, M. J. Ellis, E. Winer, K. K. Hunt, A. H. Partridge, L. A. Carey. Surgical outcomes in the ALTERNATE trial (Alliance A011106) -a randomized phase 3 neoadjuvant endocrine therapy (NET) trial in postmenopausal women with clinical stage II/III estrogen receptor positive (ER+) and HER2 negative (HER2-) breast cancer (BC) [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr RF1-02.

Abstract Background: Despite its widespread adoption, there is a lack of data as to whether breast MRI for local staging and surgical planning of newly diagnosed breast cancer patients improves oncologic outcomes. The Alliance A011104 phase III trial was designed to test the hypothesis that MRI detection of mammographically occult disease helps to tailor surgical intervention thereby reducing local recurrence and improving local regional control. Here we report on the primary outcome of 5-year local regional recurrence. Methods: Patients with newly diagnosed clinical stage I-II, ER/PR- and HER2- negative (triple- negative) or ER/PR-negative and HER2-positive breast cancer, deemed eligible for breast- conserving surgery (BCS) based on clinical exam, mammography +/- ultrasound were randomized to undergo staging breast MRI or no further local staging. Patients with multifocal tumors that could be encompassed in a single operative resection were eligible. Patients who were carriers of gBRCA 1/2, or who had bilateral breast cancer, or a prior history of breast cancer were excluded. Initial eligibility was limited to patients not scheduled to receive neoadjuvant chemotherapy; however, a subsequent amendment allowed for enrollment after completion of neoadjuvant chemotherapy. Categorical variables were compared between arms with a chi-square or Fisher’s exact test. LRR, distant recurrence and overall survival (OS) were estimated with Kaplan-Meier point estimates and 95% confidence intervals (CIs). Hazard ratios (HRs) and 95% CIs were obtained from Cox proportional hazards models. Results: Of 319 patients enrolled from 2/2014-12/2019,161 were randomized to undergo breast MRI and 158 to no breast MRI evaluation. The mean age at enrollment was 58.9 years (range 29-85). Most patients (72.1%) had T1 tumors, 93.4% were cN0 and 19.7% were HER2 positive. Systemic chemotherapy was utilized in 85% with 17.6% receiving treatment neoadjuvantly; receipt of systemic therapy was unknown for 15 patients (4.7%). Two hundred and ninety-eight patients (93.4%) underwent surgical intervention; 91.9% underwent BCS as initial surgical procedure with no significant differences noted between groups (92.7% in the no MRI group and 91.9% in the MRI group [p=0.62]). Of patients who underwent breast surgery, 293 underwent axillary evaluation with the majority (270 patients, 92%) undergoing sentinel lymph node biopsy as the only axillary intervention. Among patients who received neoadjuvant chemotherapy, the overall pathologic complete response rate was 39.3%, 29.0%in the MRI arm vs. 52.0% no MRI arm (p=0.10). Receipt of adjuvant radiation was comparable between arms, 85.4% and 85.0% receiving radiation in the no MRI and MRI arms, respectively. Among 298 patients evaluable for the primary endpoint, with median follow-up of 61.1 months (0.2, 68.1), there were no differences in rates of LRR; 5-year local regional control was 93.2% (89.0-97.6%) in the MRI group and 95.7% (92.3-99.1%) in the no MRI group (HR: 1.1, 95% CI: 0.3, 3.9). For the entire cohort, 5-year distant recurrence-free rate and OS were 94.3% (91.7, 97.1) and 92.2% (89.1, 95.4), respectively, with no differences between groups. Conclusions: With multimodality therapy, rates of 5-year LRR in patients with early stage triple- negative and HER2-positive breast cancer are very low. Breast MRI for local staging and surgical planning in this setting does not result in improved local regional control. Support: U10CA180821, U10CA180882; UG1CA189823; U10CA180868 (NRG Oncology); ClinicalTrials.gov Identifier: NCT01805076; https://acknowledgments.alliancefound.org Citation Format: I. Bedrosian, K. Ballman, L. M. McCall, C. E. Comstock, G. W. Unzeitig, T. Yen, A. Weiss, B. G. Haffty, J. M. Bensenhaver, S. M. DeSnyder, J. J. Liu, A. H. Partridge, L. A. Carey, K. K. Hunt.Effect of Preoperative Breast MRI Staging on Local Regional Recurrence (LRR) in Early Stage Breast cancer: Alliance A011104/ACRIN 6694 [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr GS2-07.

Importance Obesity is associated with a higher risk of recurrence, mortality, comorbidities, treatment-related adverse effects, and poor quality of life in patients with breast cancer. Scalable interventions are needed to promote weight loss in this population. Objective To evaluate the impact of a remotely delivered weight loss intervention (WLI) on weight change at 1 year in patients with breast cancer and obesity and to explore factors associated with weight change. Design, Setting, and Participants The Breast Cancer Weight Loss trial is a phase 3, randomized clinical trial evaluating the impact of a telephone-based WLI on invasive disease–free survival and other outcomes in women with obesity and early breast cancer at 637 sites across the US and Canada. Participants were enrolled to the study between August 2016 and February 2021. Participants included women with stage II to III, ERBB2 -negative breast cancer and a body mass index (BMI) of 27 or higher. Interventions Participants were randomized to a 2-year, telephone-based WLI plus health education or health education alone control group. Main Outcome and Measures The primary end point for this prespecified secondary analysis was weight change at 1 year. Weight was measured at baseline and 1 year, and changes in weight were compared between groups. Weight change was evaluated with a linear mixed-effects model including treatment group, weight over time, a time-by-group interaction, menopausal status, race and ethnicity, and hormone receptor status. Results A total of 3180 women with breast cancer and BMI of 27 and higher were included in the study; 1591 were randomized to the WLI and 1589 to the control group. At baseline, the mean (SD) age of participants was 53.4 (10.6), and the mean (SD) BMI was 34.4 (5.6). The racial and ethnic breakdown included 406 (12.8%) Black, 231 (7.3%) Hispanic or Latino, 2906 (91.4%) non-Hispanic, and 2555 (80.3%) White participants. WLI participants lost a mean of 4.3 kg (95% CI 3.9-4.6 kg), or 4.7% (95% CI, 4.3%-5.0%) of baseline body weight at 1 year vs control participants, who gained 0.9 kg (95% CI, 0.5-1.3 kg), or 1.0% (95% CI 0.1%-1.4%) of baseline body weight ( P &amp;lt; .001). Participants randomized to WLI experienced significant weight loss (vs control group participants) across demographic and tumor factors. WLI effect differed significantly by menopausal status, with postmenopausal participants having greater weight loss than premenopausal participants, and by race and ethnicity, with Black and Hispanic participants having less weight loss compared to other races and ethnicities. Conclusions and Relevance In this secondary analysis of a randomized clinical trial, a telephone-based WLI induced significant weight loss in patients with breast cancer with overweight and obesity across demographic and treatment factors. Further follow-up of the Breast Cancer Weight Loss trial will evaluate whether the WLI improves disease outcomes. Trial Registration ClinicalTrials.gov Identifier: NCT02750826

PURPOSE Patients with cancer take different types of medications with varying schedules and settings. They are also sometimes instructed to stop medications due to toxicity. To measure self-reported nonadherence in this heterogeneous population, we modified and evaluated a measure originally developed to assess nonadherence to daily oral antihypertensives, the Domains of Subjective Extent of Nonadherence (DOSE-Nonadherence). METHODS The measure was refined in an iterative process incorporating feedback from patient investigators and participant interviews in English and Spanish. Branching logic was added for participant selection of medication administration setting: (1) only home, (2) only clinic, or (3) partly home/partly clinic. Participants reported their adherence to medications taken over a setting-specific reference period (1 week for home, 1 month for clinic medications). Participants who missed medications then reported on reasons for nonadherence. Adherence was dichotomized for analysis (complete adherence v any nonadherence). For participants who received clinic-administered medications, concordance between chart-abstracted and self-reported adherence was evaluated. RESULTS Seventy-three participants completed the measure (68% English; 32% Spanish; 86% female; 44% age ≥60 years). The majority had breast cancer; 64% had metastatic disease. Twenty-six percent (15/58) of participants who received medication in clinic and 24% (11/46) of those who took medication at home reported nonadherence. Participants felt able to respond accurately to both reference periods and perceived all reasons for nonadherence to be relevant. Among participants who completed the final version of the measure for clinic-administered medication, 96% (26/27) accurately reported their adherence compared with chart-abstracted data. CONCLUSION Our results support the validity of the DOSE-Nonadherence-Cancer for assessing cancer treatment nonadherence. This measure can be used to assess nonadherence in patients with cancer receiving a broad array of systemic therapies.