Can lymphocytic toxicity be induced by radiation dose to the bowel bag in whole pelvic radiotherapy?

Purpose: Lymphocytopenia is a known side effect of radiation exposure to the bone marrow. However, the hematological impact of irradiating the bowel bag—a highly vascularized region containing numerous mesenteric lymph nodes—remains poorly understood. This study aimed to examine whether radiation to the bowel bag leads to a decrease in absolute lymphocyte count (ALC) during pelvic radiotherapy. Materials and Methods: We retrospectively analyzed 168 patients with prostate, bladder, gynecologic, and gastrointestinal cancers who received whole pelvic radiotherapy (45–60 Gy) without concurrent chemotherapy between October 2016 and November 2022. Dose-volume parameters (V5, V10, V15, V20, and V30) representing the percentage of bowel bag volume receiving ≥x Gy were calculated. The primary endpoint was grade 3 lymphocytopenia (ALC 200–500/μL) measured at 3 weeks post-treatment initiation. Logistic regression analysis evaluated associations between bowel bag dosimetric parameters and severe lymphocytopenia. Results: The overall lymphocytopenia rate was 92.3%, with 23.8% developing grade 3 toxicity. Logistic regression analysis demonstrated that higher radiation doses to the bowel bag at V5, V10, and V15 significantly increased the risk of grade 3 lymphocytopenia (odds ratios, 3.0 to 5.9, all p < 0.01). Conclusion: Radiation dose to the bowel bag independently predicts severe lymphocytopenia during pelvic radiotherapy. These findings suggest that dose constraints to the bowel bag should be considered alongside bone marrow sparing techniques to preserve hematopoietic function. Prospective validation and development of bowel bag dose-volume guidelines are warranted.