Can lymphocytic toxicity be induced by radiation dose to the bowel bag in whole pelvic radiotherapy?
Article information
Abstract
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.
Introduction
Whole pelvic radiotherapy (RT) plays a crucial role in cancer treatment, and is mainly used to treat cervical, prostate, and bladder cancers. According to the National Comprehensive Cancer Network guidelines, definitive RT is a standard treatment option for patients with locally advanced cervical cancer and is often combined with chemotherapy. In prostate cancer, RT is a key component of curative-intent treatment across various risk groups, including as an alternative to surgery. In bladder cancer, RT is used as part of bladder preservation strategies in selected patients. Given the wide use of RT across these malignancies, understanding and managing radiation-induced toxicities such as lymphocytopenia is becoming increasingly important.
Various studies have emphasized the need to minimize organs-at-risk (OARs) toxicity during pelvic RT, with particular attention to lymphocytopenia as a significant adverse effect [1]. Although the transition from conformal techniques to intensity-modulated radiotherapy (IMRT) has reduced many treatment-related toxicities, lymphocyte depletion remains a persistent issue [2,3]. Given the growing recognition of the role of lymphocyte preservation in maintaining immune function and enabling potential future use of immunotherapy, pelvic RT-associated lymphocytopenia should not be overlooked.
In patients with cervical cancer treated with pelvic RT, a correlation between higher radiation dose to the pelvic bone marrow (BM) and lymphocytopenia has been reported [4]. However, due to the abundance of blood vessels and proximity to mesenteric lymph nodes, irradiation of the bowel bag may also influence lymphocyte counts [5]. Previously reported articles have shown a relationship between the irradiated dose to the bowel bag and the development of acute diarrhea [6]. However, studies on the hematological effects of the radiation dose on the bowel bag are limited. Therefore, this study aimed to investigate how lymphocyte counts decrease based on the amount of irradiation to the bowel bags during pelvic RT.
Although RT planning may differ depending on the cancer type and target location, bowel bag delineation within the pelvic region follows a generally consistent approach. The bowel bag typically includes all bowel loops and mesenteric structures encompassed by the RT field. Previous studies have shown a relationship between pelvic BM dose and lymphocytic toxicity following concurrent chemoradiotherapy (CCRT) [7]. These studies suggest that lower radiation dose to pelvic BM is associated with reduced risk of lymphocytopenia. Given the rich vascularization and immunologic relevance of the bowel bag and adjacent mesenteric lymph nodes, it is plausible that radiation to this region may similarly influence lymphocyte counts. Therefore, this study aimed to investigate the relationship between bowel bag dosimetric parameters and the incidence of grade ≥3 lymphocytopenia during pelvic RT.
Materials and Methods
1. Patients
This was a retrospective study that included patients who underwent RT in the pelvic area at Seoul St. Mary’s Hospital between October 2016 and November 2022. All patients were pathologically diagnosed with cancer. The inclusion criteria included: (1) patients undergoing whole pelvic RT of ≥45 Gy, (2) patients with a Karnofsky Performance Status score of ≥70, and (3) blood tests routinely conducted before treatment, during treatment at weekly intervals, and within 1 month after treatment completion. The exclusion criteria comprised: (1) patients who received chemotherapy or RT within 3 months prior to or following RT, (2) patients who had interruptions in RT exceeding 1 week for any reason, (3) patients with secondary primary tumors, and (4) patients whose baseline (pre-RT) absolute lymphocyte count (ALC) was <1,000/μL were excluded.
2. Radiotherapy
RT was delivered using a conventional fractionation scheme with a total dose ranging from 45 to 60 Gy, administered over 20 to 31 fractions. The fraction size varied between 1.8 Gy and 2.5 Gy per day, delivered once daily, five times per week. Treatment planning was performed using simulation computed tomography, and all patients were treated using the IMRT technique.
All patients were treated with whole pelvic RT. The clinical target volume (CTV) encompassed the gross tumor (if applicable) and regional lymphatic areas, depending on tumor type. The planning target volume was defined by adding appropriate margins to the CTV. The superior border of the radiation field was generally set at the L4–L5 junction, and the inferior border was placed at or 2–3 cm below the level of the symphysis pubis, adjusted slightly based on anatomical and target volume considerations. OARs, including the bowel bag, femoral heads, and pelvic BM, were contoured in all cases.
Among the 40 patients with gynecologic malignancies who received whole pelvic RT, three patients (7.5%) also received intracavitary brachytherapy following external beam RT. Brachytherapy was delivered at 25 Gy in 5 fractions for two patients, and 30 Gy in 6 fractions for one patient. As the brachytherapy component was limited in both number and dose contribution, and was not included in the treatment planning system used for dose-volume histogram analysis, only external beam RT plans were used for dosimetric evaluation in this study.
3. Dosimetric parameters of the BM and bowel bag
BM contouring included the lower lumbar level (L4–5), sacrum, and ilium. The contouring of the bowel bag involves the superior border at the L3–4 junction and the inferior border extending to the bladder and coccyx, encompassing all bowel and mesenteric structures in between. Volume dosimetric parameters (Vx) represent the percentage of a given radiation dose (x Gy) occupying a certain volume (V) relative to the total volume. For instance, V5 indicates the percentage of the total volume that is received in a dose of 5 Gy. Similarly, V10, V15, V20, V25, and V30 represent the corresponding percentages of the total volume that received doses of 10, 15, 20, 25, and 30 Gy, respectively. In addition to that, the mean radiation doses received by the bowel bags (Dmean) were also calculated.
4. Lymphocyte count
Lymphocyte counts were determined using a complete blood count. The ALC measured at 3 weeks after initiation of RT was used to define the lowest ALC and assess lymphocyte toxicity. Toxicity grades were assigned based on the Common Terminology Criteria for Adverse Events (CTCAE) version 5.0: grade 1 (800–1,000/μL), grade 2 (500–800/μL), grade 3 (200–500/μL), and grade 4 (≤200/μL). Grade 0 represents the absence of an adverse event, defined as normal lymphocyte count (>1,000/μL).
5. Statistical analysis
We compared the mean dose of each dosimetric factor between groups using independent sample t-tests. Cutoff values for each dosimetric variable (e.g., Dmean, Vx) were determined using the Youden index derived from the receiver operating characteristic (ROC) curve. After dichotomizing patients based on these cutoff values, we performed logistic regression analysis to assess the association between each dosimetric parameter and the risk of developing grade 3 lymphocytopenia. Odds ratios (ORs) represent the odds of developing grade 3 lymphocytopenia in patients with dosimetric values above the cutoff compared to those below the cutoff. The 95% confidence intervals were calculated using the standard error of the log OR. Data analysis was conducted using the SPSS software version 23.0 (SPSS IBM Corp., Armonk, NY, USA). Statistical significance was attributed to all values with a significance level of p < 0.05.
Results
1. Patient characteristics
Based on these criteria, 168 patients were included in this study with a median age of 70 years (range, 29 to 91). The median pre-RT ALC was 1360.8, and all patients exhibited a baseline ALC grade of 0, defined as ALC >1,000/μL according to CTCAE version 5.0. The overall lymphocytic toxicity rate was 92.3%; among them, the grade 3 lymphocytic toxicity rate was 23.8%. There was no patient with grade 4 lymphocytic toxicity. Table 1 shows the patients characteristics. For the purpose of comparative analysis, patients were stratified into two groups based on the lowest ALC measured 3 weeks after initiating RT: those with grade 0–2 lymphocytopenia and those with grade ≥3.
Patients characteristics
2. Comparison of dosimetric factors by lymphocytopenia severity
Patients were grouped based on the severity of lymphocytopenia, as determined by the ALC measured 3 weeks after the start of RT: those exhibiting toxicity of grade 3 and those showing toxicity of grade 2 or lower. When conducting independent t-tests based on the variables Dmean, V5, V10, V15, V20, V25, and V30, we found significant differences in all continuous variables except for V25. The following Table 2 shows the results of independent t-tests for dosimetric parameters of bowel bag between ALC grades 0–2 and grade 3 patients.
Independent t-tests for dosimetric parameters of bowel bag between ALC grades 0–2 and grade 3 patients
3. Area under the curve and cutoff determination
ROC curves were generated for dosimetric parameters, and the endpoint was the presence of grade 3 ALC. Table 3 and Fig. 1 show the area under the curve and corresponding cutoff values for Dmean, V5, V10, V15, V20, V25, and V30 in patients grouped by ALC grade 3 and those grouped by ALC grade 2 or lower. Significant results were observed when the dose exceeded the cutoff value for Dmean, V5, V10, and V15 (p < 0.05).
ROC curves and cutoff values
ROC curves for the dosimetry parameters. ROC for Dmean (A), V5 (B), V10 (C), V15 (D), V20 (E), V25 (F), and V30 (G). ROC, receiver operating characteristic; Dmean, mean dose; Vx, volume dosimetric parameters (Vx), the percentage of a given radiation dose (x Gy) occupying a certain volume (V) relative to the total volume; AUC, area under the curve.
4. Association between dosimetric parameters and grade 3 lymphocytopenia
Using these cutoff values, multivariate logistic regression analysis was then performed to evaluate the association between each dosimetric parameter and the risk of developing grade 3 lymphocytopenia. Table 4 summarizes the results that higher values of Dmean, V5, V10, V15, and V25 were significantly associated with an increased risk of grade 3 lymphocytopenia. ORs for these parameters ranged from 3.0 to 5.9, all with p-values < 0.01. These results suggest that specific bowel bag dose-volume thresholds may serve as potential predictors of grade 3 lymphocytopenia during pelvic RT.
Logistic regression analysis of bowel bag dosimetric parameters based on ROC-derived cutoff values for predicting grade 3 lymphocytopenia
5. Summary
Independent t-tests for dosimetric parameters of bowel bag between ALC grades 0–2 and grade 3 patients revealed significant differences between the two groups for Dmean, V5, V10, V15, V20, and V30, with p-values less than 0.05. When deriving ROC curves and cutoff values, significant differences between the two groups were observed for Dmean, V5, V10, and V15. Using the cutoff values obtained from the ROC curves, we divided the data for Dmean, V5, V10, V15, V20, V25, and V30 into two groups and compared their ORs. This comparison yielded significant results for Dmean, V5, V10, V15, and V25 (p < 0.05). Consequently, we confirmed a significant association between the reduction in ALC and the dosimetric parameters of the bowel bag for Dmean, V5, V10, and V15, as these parameters showed significant results across all three statistical analyses.
Discussion and Conclusion
Radiotherapy for tumors located within the pelvis can induce hematological side effects. Studies have shown the potential hematologic challenges posed by irradiation to the BM [8]. Particularly, a significant reduction in the number of lymphocytes can affect the intensity and duration of RT, potentially undermining the intended therapeutic effect [9]. Lymphocytes are pivotal in the human immune system and are essential for combating tumor cells [10,11]. Their participation in inducing cell apoptosis, cytotoxicity, and antitumor responses, notably through CD8+ and CD4+ T cells and cytokine secretion, highlights the importance of maintaining normal lymphocyte counts for effective tumor treatment [12-15].
Consequently, preserving patients’ lymphocyte levels is critical, underscoring the need to modulate the irradiated doses to the normal tissues. Because a substantial portion of the active BM resides in the pelvis and lumbar spine, regulating the irradiated dose to the pelvis is an effective approach for managing the hematologic side effects potentially induced by BM irradiation [16]. Considering such aspects, it can be inferred that exposure to radiation in the blood within the bowel bag or mesenteric lymph nodes may also lead to a decrease in lymphocyte counts, such as the radiation investigated in the pelvic bones.
Recent studies investigating the link between pelvic BM and lymphocyte toxicity during CCRT in patients with pelvic tumor have shown that adjusting the radiation dose to the bowel bag may alleviate lymphocytic toxicity. However, specific findings regarding the effect of RT alone on lymphocyte toxicity when directed to the bowel bag remain uncertain. Investigating this is important for understanding the minimal irradiated dose to the normal tissues that is necessary to maintain lymphocyte counts. Against this backdrop, we examined the correlation between dosimetric parameters of the bowel bag and lymphocyte toxicity during pelvic tumor RT. Our findings suggest that dosimetric modulation of RT may help mitigate lymphocyte toxicity, although further studies are warranted.
Previous research has claimed an association between the radiation dose administered to the BM and lymphocytopenia. This is likely attributed to the meticulous exclusion of pelvic bone from the contour, possibly owing to the increased utilization of IMRT in recent times for planning RT. Similarly to the outcomes of CCRT for cervical cancer, this suggests that even when RT is administered without concurrent chemotherapy, careful attention should be paid to minimizing unnecessary low-to-intermediate dose exposure to the BM.
In our study, BM was contoured and included in the dosimetric analysis to evaluate its potential association with lymphocytopenia. However, the independent t-test results showed no statistically significant differences in BM dosimetric parameters (Dmean, V5–V30) between patients with grade 0–2 and grade 3 ALC toxicity (Supplementary Table S1). This may be attributed to the relatively low BM radiation dose in our cohort, likely resulting from the use of modern IMRT planning techniques that effectively spare the pelvic bones. These findings suggest that, in settings where chemotherapy is not administered and IMRT is utilized, the contribution of BM irradiation to lymphocytopenia may be minimal. Based on the lack of significant association between BM dose and lymphocytopenia in our cohort, the potential for confounding bias between bowel bag dose and BM dose is unlikely. Nevertheless, future prospective studies incorporating multivariate analyses are warranted to further evaluate potential interactions between bowel bag and BM doses.
Our study had certain limitations. Given that this was a retrospective study, prospective validation is needed to confirm the findings. In addition, we did not adjust for pelvic BM radiation dose, which is a well-established predictor of lymphocyte depletion. This confounding factor may have influenced the observed association between bowel bag dose and lymphocytic toxicity. Furthermore, our estimation of bowel bag blood volume and the number of mesenteric lymph nodes may not precisely reflect physiological conditions.
In conclusion, during pelvic RT, low-dose radiation exposure to the bowel bag may be associated with the development of high-grade lymphocytopenia. However, given the retrospective design and the lack of BM dosimetry adjustment, further prospective studies are needed to clarify the independent impact of bowel bag irradiation.
Notes
Statement of Ethics
This retrospective study was reviewed and approved by the Institutional Review Board (IRB) of Seoul St. Mary's Hospital, The Catholic University of Korea (No. KC22RISI0715). The requirement for informed consent was waived by the IRB due to the retrospective nature of the study.
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Funding
This study was supported by the Research Fund of Seoul St. Mary’s Hospital, The Catholic University of Korea.
Author Contributions
Conceptualization, MKK; Investigation and methodology, MKK, KHC, SHS; Resources, MKK, KHC, WS, JHH, YNK; Supervision, KHC, WS, BOC, YSK, HJ; Writing of the original draft, MKK; Writing of the review and editing, MKK, KHC, WS; Formal analysis, MKK, KHC, WS, JHH. Data curation, MKK, KHC. All the authors have proofread the final version.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Supplementary Materials
Supplementary materials can be found via https://doi.org/10.3857/roj.2025.00220.
Independent t-tests for dosimetric parameters of bone marrow between ALC grades 0–2 and grade 3 patients