Clinical study on splenic radiation dose constraints.

e16138 Background: Lymphocytes, known for their high sensitivity to radiation, are vital for the body's anti-tumor mechanisms. Radiation-induced lymphopenia have been linked to worse prognosis across various cancer types. Despite being the largest secondary lymphoid organ, the spleen is not currently designated as an organ at risk in radiation therapy protocols. This study seeks to provide a reference for splenic dose-volume constraints in clinical radiotherapy practice. Methods: To exclude the influence of tumor factors on the radiation dose to the spleen and given the spleen's anatomical proximity to the target area for gastric cancer radiotherapy, we selected patients with R0 gastrectomy who underwent adjuvant radiotherapy. A retrospective group of 159 patients with locally advanced gastric cancer who had undergone postoperative chemoradiotherapy (CRT) between Jan 2010 and Jan 2020 was analyzed. Complete blood counts were collected before, during, and after CRT. Logistic regression analyses were used to evaluate the effects of variables on predicting grade 4 lymphopenia. Cox proportional hazard modeling and Kaplan–Meier method with log-rank tests were used to analyze the correlation of each variable with survival outcomes. Receiver Operating Characteristic (ROC) curves were utilized to determine the ideal threshold of splenic V 5 , lymphocyte recovery index (LRI) and changes in ALC (ΔALC). Results: The median follow-up period was 48 months. The 5-year OS and DFS were 56.6% and 40.2%, respectively. The median of ALCs at baseline was (× 10 9 /L) 1.4, and 0.8, 0.5, 0.4, 0.3, 0.25, 0.2 for week1–6 during CRT, respectively. The median values of ALC decreased by 85.71% after postoperative CRT. Patients with grade 4 lymphopenia (G4) had a poorer 5-year OS rate (44.9 vs. 62.9 months, P = 0.029) than those without. Logistic regression analysis confirmed that the spleen V 5 were an independent predictor of G4. Further analysis revealed that limiting V 5 to < 272.2cc decreased death risk by 60.9% ( P = 0.005). LRI and ΔALC were identified as independent predictors of OS and DFS, respectively. Conclusions: Constraining the spleen V 5 to < 272.2cc may indirectly improve outcomes through lymphocyte preservation. Additionally, LRI and ΔALC can function as prognostic indicators for patients undergoing radiotherapy. Consequently, we propose limiting spleen V 5 in the design of radiotherapy treatments.