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PURPOSE To compare overall survival (OS) in patients with baseline metastatic gastric adenocarcinoma (GA) with and without ovarian metastasis (OM). Furthermore, within the group that had ovarian metastases, we aimed to assess whether there was a survival benefit (SB) with palliative oophorectomy (PO). PATIENTS AND METHODS This is a single-institution retrospective analysis of the clinicopathological features of women diagnosed with metastatic GA with a comparison of outcomes based on PO status. We identified 240 women with baseline metastatic GAs who were treated at MD Anderson Cancer Center between February 2003 and September 2022. Among these women, we categorized a subgroup of 102 women who had OM from their primary GA. Patients were analyzed whether they underwent PO. RESULTS Patients who developed OM were most often non-Caucasian, had peritoneal involvement, and had tumors that were both human epidermal growth factor receptor 2–negative and PD-L1–negative, with signet ring cell features and diffuse histological type. Among patients with ovarian metastases, those who had PO had an Eastern Cooperative Oncology Group = 0, more comprehensive molecular and immunohistochemical profiling, lower percentage of family history of gastroesophageal malignancies, and lower interval between diagnosis of the GA primary and the OM. PO was associated with significantly improved OS in this subgroup (hazard ratio, 0.5 [95% CI, 0.31 to 0.81]; P = .005). CONCLUSION To our knowledge, this is the largest multiethnic population study assessing SB of PO in patients with GA with OM. Additionally, it is the largest study analyzing survival in this population according to the patient's multiethnic characteristics and metastasis timing and growth patterns. PO presents as a therapeutic option for women with GAs with OM if the patient is clinically suitable for surgical resection.

Abstract Background Magnetic Resonance‐guided Radiation Therapy (MRgRT) systems are increasingly used in clinical practice, necessitating a dedicated dosimetry protocol to address the challenges posed by the presence of strong magnetic fields. Purpose AAPM TG351 introduces a reference dosimetry protocol for external beam MRgRT systems, aligning with AAPM TG51 reports and incorporating elements from the IAEA TRS398 code of practice. Methods The protocol offers practical guidance on beam quality and dosimetry setup in the presence of strong magnetic fields. It reviews relevant literature to identify reference‐class chambers suitable for external beam MRgRT systems and determines their beam quality conversion and other relevant correction factors through a detailed analysis of available data and associated uncertainties. Results AAPM TG351 offers a comprehensive dosimetry protocol for external beam MRgRT systems, addressing the unique challenges posed by the presence of magnetic fields. It includes detailed guidance for measurement setups, a list of reference‐class chambers appropriate for reference dosimetry in MRgRT systems, and updated beam quality conversion and correction factors. Additionally, the protocol presents an in‐depth analysis of the uncertainty budget, identifying key contributors and providing strategies to minimize uncertainties, thereby enhancing clinical accuracy and reliability. Conclusions This protocol provides a practical, evidence‐based reference for medical physicists conducting reference dosimetry in external beam MRgRT systems.

10032 Background: Identifying clinical and biological factors associated with outcomes in localized Ewing sarcoma (ES) will enable risk-stratified clinical trials with the goal of improved outcomes for high-risk patients and decreased treatment toxicity for low-risk patients. The specific aims of this analysis in localized ES patients were 1) to classify extraosseous (EO) primary tumors with subdivisions into deep (viscera, glands, body cavities, muscle, nerves) and superficial (cutaneous and subcutaneous) sites, and 2) to understand the relationship between baseline clinical factors (age, sex, primary tumor site, size (maximum dimension and volume)) and event-free survival (EFS). Methods: The analytic cohort included ES patients treated with ICC on AEWS0031 and AEWS1031. Primary tumor sites were defined as pelvic, non-pelvic, and EO (deep vs. superficial). Post-enrollment EFS was the primary endpoint. Univariate analyses used the Kaplan–Meier method (logrank test). Multivariable analyses used Cox proportional hazards models. To assess the impact of tumor volume as a continuous variable, a subgroup analysis was conducted using AEWS1031 only, as tumor volumes were collected prospectively on this study. Visual exploration of effects of continuous variables on EFS event hazard used restricted cubic splines with 3 knots. Tests were performed at the 5% level. Results: AEWS0031 (n = 628) and AEWS1031 Regimen B (n = 283) yielded 911 patients. In univariate analyses, difference in risk of EFS event was observed between tumor sites ( P = 0.03). EO tumors had the highest estimated EFS compared to pelvic and non-pelvic (5 year EFS 84.7% vs. 72.4% and 75.9%). Superficial EO appeared to be a very low-risk group, albeit interpretations are limited by the small group size and singular event (one second malignant neoplasm among 15 patients). In multivariable analysis of the combined cohort, sex, race, and ethnicity were not prognostic. EO tumors may be associated with decreased hazard compared with non-pelvic bone primaries (HR 0.58, P = 0.07), and tumors ≥200 mL with increased hazard (HR 1.56, P < 0.01). In the subgroup analysis, tumor volume and age were prognostic and in visualizations age was non-linearly related to the hazard of EFS event, increasing until approximately 15 years. Tumor volume was non-linearly related to the hazard of EFS event and increased until approximately 400 mL. Conclusions: Patients with ES and primary tumors ≥200 mL continue to be at higher risk of an EFS event when treated with ICC, and EO tumors may be lower risk compared with other sites. Risk of an event appears to remain constant in ES patients ≥15 years or with primary tumors ≥400 mL. These findings should be validated in prospective trials and tumor biology integrated with clinical factors to improve risk stratification for localized ES.

TPS11585 Background: Soft tissue sarcomas (STS) with genomic complexity are often aggressive but may be sensitive to further genotoxic stress. Leiomyosarcoma (LMS), a common STS, frequently harbors genomic complexity and DNA damage response (DDR) dysregulation. Preclinical data showed hyper-dependency on DNA-PK-mediated non-homologous end joining (NHEJ) DDR, and low-dose liposomal doxorubicin (LD-LPD) synergized with the DNA-PK inhibitor peposertib to inhibit tumor growth in LMS models. We hypothesize that a low, sensitizing dose of LPD enhances DNA damage and safely synergizes with peposertib in LMS and other genomically complex STS. Methods: This phase 1, open label, multicenter dose escalation and dose expansion study (NCT05711615) evaluates LD-LPD given intravenously (IV) on day 1 of 28-day cycles with peposertib given orally (PO) twice daily (BID) continuously (Table). Up to 18 patients (pts) over 18 years-old with advanced or unresectable LMS, undifferentiated pleomorphic sarcoma, myxofibrosarcoma, dedifferentiated liposarcoma, and synovial sarcoma, ECOG performance status ≤2, who received ≥1 prior line of systemic therapy (including anthracycline ≤300 mg/m 2 ) are eligible for dose escalation. Dose expansion will include 12 pts with LMS. The primary objective is to determine the recommended phase 2 dose of LPD+peposertib based on the dose limiting toxicity rate. The Bayesian Optimal Interval (BOIN) design will inform dose escalation decisions. Secondary endpoints for the expansion cohort include adverse event rate, progression-free survival, and objective response rate per RECIST v1.1. Potential predictive biomarkers and changes in DDR biomarkers will be evaluated on biopsies during screening and at cycle 1 day 7. Circulating tumor DNA (ctDNA) collected at baseline (dose escalation and dose expansion), on treatment and at progression (dose expansion) will be correlated with disease activity and response. This trial activated on 8-May-2023 through the Experimental Therapeutics Clinical Trials Network (ETCTN) and is enrolling at select sites in the United States. Dose escalation is ongoing as of January 2025. Clinical trial information: NCT05711615 . Planned dose levels for dose escalation. Dose Escalation Schedule Dose Level Dose Liposomal doxorubicin Peposertib (tablet) -1 10 mg/m 2 IV 50 mg PO BID 0* 10 mg/m 2 IV 100 mg PO BID +1 10 mg/m 2 IV 150 mg PO BID +2 10 mg/m 2 IV 200 mg PO BID +3 15 mg/m 2 IV 200 mg PO BID +4 20 mg/m 2 IV 200 mg PO BID *Starting dose.

6008 Background: ATC patients present with advanced, often surgically unresectable disease with historically dismal prognosis. Median PFS and OS of DT without pembrolizumab were 6.7 and 13.5 months, respectively, in the ROAR trial. Neoadjuvant DTP achieved locoregional control without radical surgery in a retrospective series of BRAF m-ATC patients, providing rationale to evaluate efficacy and safety in a phase 2 prospective trial. Methods: In this single-arm multicenter phase 2 trial, patients with BRAF m-ATC stage IVB/IVC were enrolled in 5 US centers. Following a 3-6 week run-in with D (150 mg BID) and T (2 mg daily), P (200 mg Q3W) was added, with restaging every three (21-day) cycles. Post-operatively, patients continued P (or DTP) with radiotherapy or transitioned directly to adjuvant DTP (up to 26 cycles). Primary endpoints included R0/R1 resection rate (historically 5%) and overall survival (OS). Secondary endpoints included RECIST 1.1 response after DTP and progression-free survival (PFS). Results: Between 9/2021-1/2025, 42 patients were enrolled; 36 are included in the current analysis (3 not evaluable, 3 pending surgery) (Table). Patients received median 4 (range: 2-7) neoadjuvant DTP cycles, with 26 (72%) achieving radiographic PR/CR. 30 patients (83%) had surgery after neoadjuvant DTP, achieving R0/R1 resection in 29/30 (97%). Mean surgical morbidity score (0-4 scale, 4=unresectable) improved from 3.3 to 1.6 after DTP (p<0.01). Complete ATC pathologic response occurred in 20/30 patients (67%), while 10/30 (33%) had residual ATC in the surgical specimen. Postoperatively, 11/30 (37%) received adjuvant neck radiation, and 28/36 (78%) completed a median of 11 (range: 1-26) adjuvant DTP cycles. With median follow up 18 months, 15/36 (42%) patients died. Median OS was 20 months (95% CI: 12.6-NR); 1- and 2-year OS were 71% and 48%. Complete pathologic responders had better 2-year OS than those with residual ATC (69% vs. 22%, p=0.02). Median PFS was 13.9 months (95% CI, 7.5-NR); 1- and 2-year PFS were 57% and 36%. Grade 5 adverse events occurred in 8 patients (22%), including one possible (duodenal perforation), one probable (kidney injury with sepsis), and 6 unlikely/unrelated treatment-related deaths. Conclusions: Neoadjuvant DTP enables surgical resection in BRAF m-ATC compared with historical controls, and leads to improved PFS and OS. This approach should now be considered a standard of care for BRAF m-ATC. Clinical trial information: NCT04675710 . Total N=36 Age (y), median (range) 67 (46-86) Stage IVB/IVC, n (%) 15 (42%)/21 (58%) Best RECIST response neoadjuvant phase, n (%) CR 2 (6%) PR 24 (67%) SD 6 (17%) PD 4 (11%) Percent change target lesion diameter, mean (95% CI) -44% (-54%, -34%) Surgical morbidity score change, mean (95% CI) -1.7 (-2.1, -1.3)

3520 Background: Recent analyses highlight nonhierarchical outcomes using the 8 th Edition AJCC staging system for CC. For instance, the 5-year survival rate for stages I and IIIa patients (pts) closely align. Additionally, tumor deposits (TDs) have been established as significant prognostic indicators. The AJCCCCEP commissioned this study to develop an updated pathological staging system for CC focused specifically on pts without distant metastasis (M0), while retaining the existing stage IV classification. Methods: Individual patient data (IPD) from pts diagnosed with cc (2010- 2017) in the NCDB were divided into training (70%) and internal validation (30%) datasets. External validation used IPD from clinical trials. The primary endpoint was overall survival (OS). Risk classification development for M0 pts incorporated ungrouped data on pathologic T categories, the number of involved regional lymph nodes (LN+), and TD counts. Recursive partitioning and regression tree analyses were applied to construct hierarchical staging levels. Pre-specified criteria required survival probabilities to be consecutive and show clear separations using Kaplan-Meier (KM) estimates with pairwise log-rank test P of < 0.005 for the training and < 0.05 for validation analyses. Results: Data from 281,997 pts (median age 67 years, 50% male, 81% white, 55% T3, 19% T4, 44% N+, 26% M+, and 11% with ≥1 TD) were analyzed, with a median follow-up of 7.3 years. The updated staging system (Table) met pre-specified criteria, with all observed pairwise P < 0.0001 in the development and internal validation sets. KM OS curves displayed a hierarchical separation across all sub-levels after the 1 st year of diagnosis. Consistent results were seen in pts treated with adjuvant chemotherapy in 4 trials (all pair-wise P < 0.0001). Conclusions: The proposed pathological staging system for M0 pts fulfills pre-specified criteria for hierarchical risk stratification, validated both internally and externally, and provides an evidence-based update. Pending review process, the AJCCCCEP will recommend that these changes be made to the Version 9 staging protocol for colon cancer to improve prognostication for CC pts. Stage T, # of LN+, # of TD M % of pts 1y OS (CI), % 3y OS (CI), % 5y OS (CI), % I T1, 0, 0 0 5 96 (95-97) 91 (90-92) 84 (83-86) IIa T2, 0, 0 0 10 95 (94-95) 88 (88-89) 80 (79-81) IIb T1, 0, 1+T1, 1+, 0T2, 0, 1+T2, 1-4, 0T3, 0, 0 0 27 93 (92-93) 84 (84-85) 75 (75-76) IIIa T1, 1+, 1+T2, 1-4, 1+T2, 5+, 0T3, 0, 1+T3, 1-4, 0 0 14 92 (91-92) 80 (80-81) 71 (70-72) IIIb T2, 5+, 1+T3, 1-4, 1+T3, 5+, 0T4a, 0-4, 0T4b, 0-2, 0 0 13 86 (86-87) 69 (68-70) 58 (57-59) IIIc T3, 5+, 1+T4a, 0-4, 1+T4a, 5+, anyT4b, 0-2, 1+T4b, 3+, any 0 5 78 (77-80) 53 (51-54) 40 (38-41) IVa Any 1a 19 59 (58-60) 28 (28-29) 17 (16-18) IVb Any 1b 7 43 (42-44) 14 (13-14) 6 (6-7) CI: 95% confidence internal; Peritoneum involvement data were not available before 2018 in NCDB. Thus, IVa/b were based on 7 th Edition.

3569 Background: Immune checkpoint inhibitors (ICIs) are a standard treatment for gastrointestinal (GI) cancers with mismatch repair deficiency (dMMR) or microsatellite instability (MSI). However, around 50% of patients develop resistance to ICIs, during treatment or after discontinuation. In these patients, the efficacy of reuse an ICI in patients progressing during a previous ICI (rechallenge), or in patients who progressed after discontinuation (reintroduction), remains unknown. The INFLATE study evaluates the efficacy of ICI reuse in patients who progressed on or after discontinuing ICI. Methods: This is a multicenter international retrospective study from the IMMUNODIG cohort, including patients from 34 centers in France, the United States, Italy, Belgium and Spain. All patients received ICI for dMMR/MSI GI cancer. We analyzed patients who had progressed following initial ICI (ICI-1) and subsequently received a rechallenge or a reintroduction with ICI (ICI-2), either monotherapy (mono-ICI) or biotherapy (bi-ICI). Results: A total of 77 patients were included, receiving bi-ICI (N = 34) or mono-ICI (N = 43) during ICI-2. The majority (76%) had a metastatic colorectal cancer. The reason for discontinuing ICI-1 was disease progression in 53% of cases, end of treatment in 15%, toxicity in 5% and other reasons in 26%. Patients who discontinued ICI-1 due to progression received MONO-ICI-2 in 29% of cases and BI-ICI-2 in 71%, whereas those who stopped for other reasons received MONO-ICI-2 in 86% and BI-ICI-2 in 14% of cases. Efficacy results are shown in Table 1. The ORR and DCR were 26% and 79% with MONO-ICI-2, and 16% and 71% with BI-ICI-2. Among patients who discontinued ICI-1 due to progression, BI-ICI-2 (N = 29) achieved an ORR and DCR of 8% and 65%, with a median PFS of 5.5 months (95%CI 4.07-10.4). These outcomes were 17%, 67%, and 3.7 months (95%CI 2.37-NA), respectively, with MONO-ICI-2 (N = 12). For patients who discontinued ICI-1 for reasons other than progression, BI-ICI-2 (N = 5) achieved an ORR and DCR of 60% and 100%, with a median PFS not reached (95%CI 11.5-NR), while MONO-ICI-2 (N = 31) achieved 29%, 82%, and 14.2 months (95%CI 11.2–NR), respectively. Conclusions: This is the first multicenter real-world study evaluating ICI reuse in dMMR/MSI GI cancers. In patients who discontinued ICI for reasons other than progression, reintroduction of ICI therapy upon progression achieves tumor control in 85% of cases. In cases of progression on mono-ICI, Bi-ICI re-challenge achieve tumor control in two-thirds of cases and might be to consider in some patients. ORR % DCR % PFS median (95%CI) OS median (95%CI) Overall N=77 21 76 7.65 months(5.5–11.5) 27.6 months(22.2–55.7) Discontinued ICI-1 due to Progression N=41 11 68 5.03 months(3.7–8.05) 26.7 months(21.1–NR) Discontinued ICI-1 for Other Reasons N=36 33 85 14.22 months(11.24–NR) 27.6 months(22.2–NR)

TPS3644 Background: Currently, there are no biomarkers validated prospectively in randomized studies for resected colon cancer (CC) to determine need for adjuvant chemotherapy (AC). However, circulating tumor DNA (ctDNA) represents a highly specific and sensitive approach (especially with serial monitoring) for identifying minimal/molecular residual disease (MRD) post-surgery in CC patients (pts), and may outperform traditional clinical and pathological features in prognosticating risk for recurrence. CC pts who do not have detectable ctDNA (ctDNA-) are at a much lower risk of recurrence and may be spared the toxicities associated with AC. Furthermore, for CC pts with detectable ctDNA (ctDNA+) who are at a very high risk of recurrence, the optimal AC regimen has not been established. We hypothesize that for pts whose CC has been resected, ctDNA status may be used to risk-stratify for making decisions about AC. Methods: In this prospective phase II/III trial, up to 1,912 pts with resected stage IIB, IIC, and III CC will be enrolled. Based on the post-operative ctDNA status using personalized and tumor-informed assay (Signatera™, bespoke assay), those who are ctDNA- (Cohort A) will be randomized to immediate AC with fluoropyrimidine (FP)+oxaliplatin (Ox) for 3-6 mos per established guidelines v serial ctDNA monitoring. Patients who are ctDNA+ post-operatively, or with serial monitoring (Cohort B), will be randomized to FP+Ox v more intensive AC with addition of irinotecan (I) for 6 mos. One cycle of chemotherapy is allowed while awaiting ctDNA testing results for cohort assignment. The primary endpoints for Cohort A are time to ctDNA+ status (phase II) and disease-free survival (DFS) (phase III) in the immediate v delayed AC arms. The primary endpoint for Cohort B is DFS in the FP+Ox v FP+Ox+I arms for both phase II and phase III portions of the trial. Secondary endpoints include prevalence of detectable ctDNA post-operatively, time-to-event outcomes (overall survival and time to recurrence) by ctDNA status, and the assessment of compliance to adjuvant therapy. Biospecimens including archival tumor tissue, as well as post-operative plus serial matched/normal blood samples, will be collected for exploratory correlative research. Active enrollment across the NCTN started in June 2022 with CCTG sites joining in August 2023. Current accrual (as of 1-27-2025): 647/1,912. NCT: 05174169. Support: U10 CA180868, -180822; -180888; UG1 CA189867; Natera, Inc. Clinical trial information: NCT05174169 .

TPS3650 Background: A total neoadjuvant therapy (TNT) approach improves compliance with chemotherapy and increases rates of tumor response compared to neoadjuvant chemoradiation (CRT) alone in those with locally advanced rectal cancer. Recent data indicate that optimal sequencing of TNT involves consolidation (rather than induction) chemotherapy to improve complete response rates. The use of FOLFIRINOX has shown to improve response and outcomes compared to CRT and surgery alone. Data have also shown that patients with clinical complete response (cCR) after TNT may be managed with a watch and wait approach (WW) instead of preemptive total mesorectal resection (TME). However, the optimal consolidation chemotherapy regimen to improve cCR rates has not been established, and a randomized clinical trial has not robustly evaluated cCR as a primary endpoint. We designed this NCI-sponsored study of chemotherapy intensification to address this and to increase cCR rates, provide opportunity for organ preservation, and survival outcomes. Methods: In this multigroup randomized, seamless phase II/III trial (1:1), up to 760 patients with LARC, T4N0, any T with node positive disease (any T, N+) or T3N0 requiring abdominoperineal resection or coloanal anastomosis and distal margin within 12 cm of anal verge will be enrolled. Stratification factors include tumor stage (T4 vs T1-3), nodal stage (N+ vs N0) and distance from anal verge (0–4; 4–8; 8–12 cm). Patients will be randomized to receive neoadjuvant long-course chemoradiation (LCRT) followed by consolidation doublet (mFOLFOX6 or CAPOX (control arm)) or triplet chemotherapy (FOLFIRINOX (experimental arm)) for 3–4 months. LCRT in both arms involve 4500 cGy in 25 fractions over 5 weeks +900 cGy boost in 5 fractions with a fluoropyrimidine. Patients will undergo assessment 8–12 (±4) weeks post-TNT completion. The primary endpoint for the phase II portion will compare cCR between treatment arms. A total number of 312 patients (156 per arm) will provide statistical power of 90.5% to detect a 17% increase in cCR rate, at a one-sided alpha = 0.048. The primary endpoint for the phase III portion will compare disease-free survival (DFS) between arms. A total of 285 DFS events will provide 85% power to detect an effect size of hazard ratio 0.70 at a one-sided alpha of 0.025, requiring enrollment of 760 patients (380 per arm). Secondary objectives include overall survival, organ preservation time, time to distant metastasis, and adverse event rates. This study has accrued 587 patients as of January 2025, and is investigating exploratory correlatives (e.g., ctDNA). Support: U10CA180821, U10CA180882, U24 CA196171. https://acknowledgments.alliancefound.org. Clinicaltrials.gov ID: NCT05610163. Clinical trial information: NCT05610163 .