S500
Clinical - Haemotology
ESTRO 2026
1 Radiation Oncology, Showa Medical University Hospital, Shinagawa, Tokyo, Japan. 2 Medical Physics, Graduate School of Health Care Sciences, Showa Medical University, Yokohama, Japan. 3 Radiation Oncology, Showa Medical University Fujigaoka Hospital, Yokohama, Japan. 4 Radiation Oncology, Showa Medical University Northern Yokohama Hospital, Yokohama, Japan. 5 Radiation Oncology, Showa Medical University Koto Toyosu Hospital, Koto, Tokyo, Japan Purpose/Objective: Treatment-related lymphopenia is a significant clinical problem associated with an increased risk of opportunistic infections. Previous studies on lymphopenia have focused on its link to spleen irradiation during chemo-radiotherapy. 1This study aims to investigate the impact of incidental spleen irradiation in patients treated with radiotherapy alone for gastric/duodenal MALT and follicular lymphoma, focusing on the relationship between dose-volume parameters and both acute and recovery phases of lymphopenia. Material/Methods: A retrospective analysis was conducted on 24 patients treated between 2011 and 2023 with 3D conformal radiotherapy (3DCRT) alone (30 Gy in 15 fractions). The spleen was contoured on planning CT scans to extract dose-volume histogram (DVH) parameters. Lymphocyte counts were collected at baseline and at three subsequent time periods, categorized as acute (median 37 days), intermediate (median 134 days), and late (median 274 days) phases. Statistical analyses included Spearman's correlation, multiple linear regression, and ROC analysis. Results: The cohort included 24 patients (15 males, 9 females; 21 MALT, 3 follicular lymphoma) with a median age of 58.5 years. The median lymphocyte ratio at nadir (acute phase) dropped to 48.8% (IQR, 30.6-66.1%) of baseline (Figure 1A). The nadir showed a significant negative correlation with the spleen mean dose ( ρ = - 0.441, p = 0.035) (Figure 1B), which was the sole independent predictor in multivariate analysis (p < 0.001). Long-term recovery was significantly predicted by the nadir value (p < 0.0001) (Figure 2C), indicating that a deeper drop was associated with poorer recovery. An exploratory ROC analysis for Grade ≥ 3 lymphopenia showed poor predictive ability (AUC = 0.593), and the difference in nadir based on a 16.3 Gy cutoff was not statistically significant (p = 0.061) (Figure 2D).
Conclusion: In patients treated with radiotherapy alone for upper GI lymphoma, incidental spleen irradiation is a critical determinant of acute lymphopenia. The nadir is strongly dependent on the spleen mean dose, and the severity of this initial drop is the primary predictor of long-term recovery. Although a significant statistical association was found, the ability of mean dose alone to predict severe lymphopenia was modest (AUC = 0.593). These findings, however, are limited by the retrospective nature of the study and the non-uniform timing of follow-up assessments. Therefore, prospective validation is required to establish definitive dose constraints. Nevertheless, our study establishes incidental spleen dose as an independent, modifiable risk factor for lymphopenia, suggesting that spleen-sparing techniques warrant consideration even in radiotherapy-alone regimens. References: 1. Alexandru M, Rodica A, Dragos-Eugen G, Mihai- Teodor G. Assessing the Spleen as an Organ at Risk in Radiation Therapy and Its Relationship with Radiation-
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