173 Background: Microsatellite instability (MSI) localized colorectal cancer (CRC) constitutes a subgroup of patients (pts) with different clinical and molecular characteristics and prognosis compared to microsatellite stable (MSS) disease. Here, we present our real world data regarding the management and clinical outcomes of a cohort of MSI-h localized CRC patients included in the Spanish Group of Treatment of Digestive Tumors (TTD) Registry (RETUD). Methods: RETUD is a national, multicenter registry for gastrointestinal tumors from the Spanish TTD Group. In this cross-sectional analysis we evaluated a real-world cohort of MSI-h localized CRC pts diagnosed from 1 st January 2017 to 29 th April 2024. Baseline characteristics and neo/adjuvant treatments are descriptively presented. Disease-free survival (DFS) and overall survival (OS) analyzed by Kaplan-Meier method are presented along with 95% confidence intervals (CI). Results: Five hundred and sixty-one (561) evaluable pts out of 679 MSI-CRC included in RETUD had a localized disease as initial diagnosis. Pts had a median age of 73.1 years and were predominantly Caucasian (97.7%) and female (52.6%). Fifty (12.4%) pts had Lynch Syndrome. Main tumor baseline characteristics are described (Table). Local-locoregional therapeutic procedures were: primary tumor resection in 531 (99.1%) pts and radiotherapy in 18 (3.2%) pts. A total of 198 (35.3%) pts received systemic therapy, mostly chemotherapy (n=165 83.3%) but 18 (9.1%) were treated with pembrolizumab due to unresectable disease. At database cut-off, 123 (21.9%) patients died, mostly due to not related intercurrent illness, and 96 (17.1%) pts developed metastasis. After a median follow up period of 28.5 months, the median (95% CI) OS was not reached and DFS was 78.2 (55.4-NA) months. The impact of adjuvant systemic therapy (chemotherapy vs. non-chemotherapy) for both stage II and stage III pts is under evaluation. Conclusions: Our work provides valuable real-world data from a cohort of MSI-h localized CRC pts treated under clinical practice conditions in Spain, reiterating the good disease prognosis exhibited by this subset of pts in contrast to MSS pts. Main tumor characteristics. Stage at initial diagnosis, n (%) I/II n (%) 55 (9.8) / 258 (46.0) III, n (%) 248 (44.2) Location of primary tumor a , n (%) right colon /left colon /rectum 447 (78.4) / 82 (14.4) / 41 (7.2) Type histological b , n (%) Intestinal 271 (48.9) Mucinous (colloid) adenocarcinoma (>50% mucinous) 115 (20.8) Signet ring cell carcinoma (>50% signet ring) 14 (2.5) Medullary carcinoma 18 (3.2) a Pts with more than 1 primary tumor; the percentage may be over 100%. b Unknown histology in 132 (23.8%) pts and missing data in 7 pts.

Colorectal cancer (CRC) is one of the most common tumours worldwide, and 70% of CRC patients are over 65 years of age. However, the scientific evidence available for these patients is poor, as they are underrepresented in clinical trials. Therefore, a group of experts from the Oncogeriatrics Section of the Spanish Society of Medical Oncology (SEOM), the Spanish Cooperative Group for the Treatment of Digestive Tumours, (TTD) and the Multidisciplinary Spanish Group of Digestive Cancer (GEMCAD) have reviewed the scientific evidence available in older patients with CRC. This group of experts recommends a multidisciplinary approach and geriatric assessment (GA) before making a therapeutic decision because GA predicts the risk of toxicity and survival and helps to individualize treatment. In addition, elderly patients with localized CRC should undergo standard cancer resection, preferably laparoscopically. The indication for adjuvant chemotherapy (CT) should be considered based on the potential benefit, the risk of recurrence, the life expectancy and patient comorbidities. When the disease is metastatic, the possibility of radical treatment with surgery, radiofrequency (RF) or stereotactic body radiation therapy (SBRT) should be considered. The efficacy of palliative CT is similar to that seen in younger patients, but elderly patients are at increased risk of toxicity. Clinical trials should be conducted with the elderly population and include GAs and specific treatment plans.