Aldecoa, Kim Abbegail Tan

e23419 Background: Current risk assessment for cancer-associated thrombosis remain suboptimal in NSCLC, likely due to unaccounted tumor-specific biology. Although ALK and ROS1-positive NSCLC share closely related structure and elevated thrombotic risk, real-world data on thromboembolic outcomes remain limited. Methods: This multicenter retrospective study was conducted using TriNetX, a global federated health research network;14 of 113 healthcare organizations contributed to analysis. Adults (≥18 years) with NSCLC ROS1 or ALK from 2012-2025 were included. Index event was defined as the first documented NSCLC diagnosis. Outcomes included cumulative incidence, Kaplan–Meier survival analysis (KMS), and event recurrence for arterial thromboembolism (ATE), deep vein thrombosis (DVT), and pulmonary embolism (PE). Analyses were conducted pre and post-propensity score matching (PSM) adjusted for age and sex. Subgroup analyses evaluated outcomes at 1, 3, 6, and ≥12 months. Results: 3,310 NSCLC patients were identified (ALK n=3,155; ROS1 n=155). Mean age was 73 years, with 52.8% female and 46.2% male; 72.5% White, 12.7% Black, 9.5% Asian; South (49%), Northeast (29%). ALK NSCLC were older than those with ROS1 (mean 68.9 vs 64.6 years, p<0.0001), with similar sex distribution. ROS1 had higher proportion of White (85% vs 72%), while ALK included more Asian and Black. Median follow-up prior to matching was 482 days for ROS1 (post-PSM:482) and 546 days for ALK (post-PSM:601) Incidence proportions in ROS1 NSCLC were 12.3% for ATE, 9.7% for DVT, and 12.9% for PE (vs. ALK: 22.6% for ATE, 12.5% for DVT, 11.8% for PE) For ATE, prior to PSM, ROS1 NSCLC demonstrated a significantly lower risk compared with ALK (16.6% vs 25.0%), with an absolute risk difference of −8.4% (95% CI −14.7 to −2.2; p=0.02) and relative risk of 0.66 (95% CI 0.46–0.96). KMS showed longer ATE-free survival in ROS1 compared to ALK (48.2% vs 44.5%, p=0.01; HR 0.60, 95% CI 0.40–0.91, p=0.04). After PSM, overall ATE risk difference did not reach statistical significance, although ROS1 continued to demonstrate lower risk (16.6% vs 25.8%; risk difference −9.2%, p=0.06). Notably, in the subgroup analyses, this significant risk difference emerged ≥12 months, with improved late ATE-free survival in ROS1(91.1% vs 80.3%, p=0.01). For VTE and PE, no significant differences in risk, survival analyses or recurrence, were observed between ROS1 and ALK before or after PSM (DVT risk: 9.2% vs 9.7% pre-PSM; 9.2% vs 10.1% post-PSM; PE: 11.2% vs 9.9% pre-PSM; 11.2% vs 8.6% post-PSM; all p>0.05). Conclusions: Arterial thromboembolism differed by oncogenic subtype, with higher ATE risk in ALK compared with ROS1-positive NSCLC, particularly ≥12-months. In contrast, no significant difference in venous thromboembolism and pulmonary embolism were observed; further validation in prospective studies is warranted.

e23537 Background: Follicular Dendritic Cell Sarcoma (FDCS) is a rare, indolent, low-grade sarcoma known for local recurrences managed primarily by surgery. The role for adjuvant therapy has not been established for either radiation or chemotherapy. We aimed to comprehensively analyze clinical and epidemiological profiles and responses to treatment in these patients. Methods: This is a retrospective study using the National Cancer Institute (NCI) Surveillance, Epidemiology, and End Result (SEER) database. Cases of FDCS between 2001 to 2022 were identified utilizing the International Classification of Diseases for Oncology edition 3 (ICD-O-3) code 9758/3. Only adults > 18 years of age and diagnosed by positive histology or cytology were included. Descriptive statistics were used to describe demographic and tumor characteristics. Logistic regression was used to identify the factors influencing survival. Kaplan-Meier method was used to plot survival based on different treatment regimens and stages. Results: A total of 168 patients with FDCS were identified, with more than 50% of the patients ≥ 50 years of age. The mean age at diagnosis was 52.6 years, 56% were female, and 74.4% were white. Most of the patients had localized disease (63.7%), 17.9% had distant disease, and the remaining had unknown stages. About 90% of the patients underwent surgery, with 41% receiving local tumor excision. In Univariate analysis, older age, and distant disease were found to have poor outcomes. Surgery, irrespective of stage, was found to improve overall Survival (Table 1). While chemotherapy and distant disease had poor outcomes in univariate analysis, they were not statistically significant in multivariate analysis. The majority of the patients who received chemotherapy alone without surgical intervention had distant disease, explaining the poor outcome seen in univariate analysis. Among 107 patients with localized disease, 41 patients (41.1%) underwent only surgery and 43 patients (40.1%) received adjuvant therapy (either chemotherapy or radiation vs both) along with surgery. 5-year overall survival was significantly lower in patients who received adjuvant therapy. (88.4% vs 68.4%, p = 0.001). Conclusions: In conclusion, FDCS has a fair overall 5-year survival with surgery alone. Patients with localized disease, who received adjuvant therapies had poor survival outcomes as compared to the surgery-only group owing to disease burden. Large prospective studies are required to properly evaluate the benefit of adding chemotherapy or radiotherapy in resectable disease.