Small Bowel Dose-volume Research Articles

Use of expanders for bowel protection in pediatric pelvic tumor radiation therapy: 15 years of tolerance results

PurposeBowel is often the dose-limiting organ in curative pelvic irradiation because of radiation enteritis when dose exceeds 40 to 45 Gy. To limit radiation enteritis, a silicone tissue-expander prothesis (STEP) connected to a subcutaneous self-sealing valve was prospectively used in children undergoing pelvic or abdominal radiation therapy. This study reports the 15-year long-term outcomes of this prospective series. Methods and materialsBetween 1987 and 2008, 29 children from 3 Paris institutions received pelvic radiation therapy after surgical placement of a STEP in the pelvis. The median prescribed dose was 50.4 Gy (44.1-55 Gy) using 5 daily fractions of 1.8 Gy per week, except for 1 patient receiving 4 fractions of 2.5 Gy per week. The median treatment duration was 40 days (29-49 days). After 2000, computed tomography (CT) conformal 3-dimensional radiation therapy was used, and 12 patients had CT simulation. Four had CT before and after insertion of the STEP, enabling us to compare pre- and postprosthesis insertion bowel dose-volume histograms. Acute and late toxicities were captured using the Radiation Therapy Oncology Group or the National Cancer Institute Common Terminology Criteria for Adverse Events 4.0 scale. ResultsNo patient experienced significant perioperative complications. Pre- and post-STEP insertion small bowel dose-volume histograms show significant reductions in small bowel: 51%, 45%, and 64%, respectively, in V10, V15, and V40 (Vx = irradiated volume by x Gy). Twenty-five patients (86%) completed their radiation therapy with no or minimal small bowel toxicity. One patient also treated with neutrons developed delayed grade 4 toxicities. The 15-year complication-free survival for those surviving was 70.3%. ConclusionsWe report the long-term follow-up of STEP prosthesis insertion in children to reduce the small bowel volume in the radiation field. The acute and long-term tolerances were excellent. The STEP reduced the bowel dose over 40 Gy by 64%.

Stomach Dose-Volume Parameters and Clinical Factors Predict Acute Toxicity in Pancreatic Cancer Chemoradiotherapy

Gastrointestinal (GI) toxicity impedes dose escalation and likelihood of cure in chemoradiotherapy (CRT) for hepatobiliary malignancies. The risk of toxicity can depend on clinical and radiotherapy (RT) dose-volume metrics. We aimed to identify predictive factors using data from two prospective phase-II clinical trials of locally-advanced pancreatic cancer (LAPC). 91 patients with available dose cubes from the ARCII (EudraCT 2008-006302-42, 59.4 Gy in 33 # with gemcitabine, cisplatin and nelfinavir, n=23) and SCALOP (NCT 01032057, 50.4 Gy in 28 # with capecitabine or gemcitabine, n=74) trials. Independent variables: clinical factors (age, sex, performance status (PS), baseline symptoms, tumour size, weight loss, chemo regimen) and DVH parameters (stomach, duodenum and small bowel) in 5-Gy bins. Outcome measures: grade / risk of CTCAE grade ≥2 RTOG acute upper-GI toxicity (UGIT) (anorexia, pain, nausea and/or vomiting); risk of diarrhoea grade ≥2. Statistics: Correlation/prediction of grade - Spearman’s rank/ordinal regression; risk of CTCAE grade ≥2 events - Kendall tau-b/multivariable logistic regression (MVA) with backwards stepwise selection, criteria p< 0.1, and selection by AIC and predictive accuracy; risk thresholds - ROC analysis. UGIT: CTCAE grade ≥2 symptoms occurred in 38 patients (42%) (Table). On univariate analysis increasing stomach V35-45Gy was predictive of risk (odds ratio 1.035, 95% CI 1.007- 1.063), and also grade (1.023, 1.003-1.044), of toxicity. AUC was 0.632 (0.516-0.747) with toxicity risk 33/66 (50%) above and 5/25 (20%) below the optimal discriminatory threshold (7.1 cc), sensitivity 0.892 and specificity 0.377. Using a threshold of 30cc, risk was 13/20 (65%) vs 25/71 (35%). Optimal MVA model incorporated patient sex, chemo regimen and stomach V35-45Gy and correctly classified 71.4% of patients. Concurrent chemotherapy (gemcitabine vs capecitabine, odds ratio 3.965, 95% CI 1.274- 12.342) and weight loss during induction chemotherapy (1.216, 1.043-1.419) were significant predictors in MVA for the SCALOP cohort, while age was a significant predictor of toxicity risk among ARCII patients only (1.344, 1.015-1.780). Duodenum and small-bowel dose-volume did not predict toxicity risk or severity in any cohort. Diarrhoea: Grade ≥2 diarrhoea occurred in 19 patients (21%). For the combined cohort, GTV size and the presence of diarrhoea symptoms at baseline were significant predictors of risk of grade ≥2 diarrhoea. In CRT for LAPC the volume of stomach irradiated to moderately high dose (35-45Gy) predicts incidence and severity of acute toxicity. Other predictive factors include age, sex, recent weight-loss and choice of concomitant chemotherapy.Abstract 2373UGIT grade (CTCAE)PatientsMedian Stomach V35-45Gy (IQR) [cc]017 (19.1%)9.70 (1.72-22.00)136 (39.6)12.93 (5.54-23.37)226 (29.2%)15.31 (9.95-31.51)312 (13.2%)28.02 (9.74-32.26) Open table in a new tab

Uncertainty of small bowel dose-volume and normal tissue complication probability assessment due to small bowel motility during intensity-modulated radiotherapy for rectal cancer

Objective To evaluate the uncertainty of the small bowel dose-volume and the normal tissue complication probability (NTCP) during intensity-modulated radiotherapy (IMRT) for rectal cancer, and to provide a reference for the dose limit and protection of the small bowel during IMRT for rectal cancer. Methods A total of 20 patients with rectal cancer who received postoperative adjuvant radiotherapy from March 2014 to August 2015 were enrolled in this study, including 10 patients receiving CT scan in the supine position and 10 patients in the prone position. All patients received computed tomography (CT) scan before the treatment and at weeks 1, 2, 3, and 4 of treatment, and they were defined as Plan, 1W, 2W, 3W, and 4W CT groups, respectively. The small bowel loop (BL) and peritoneal space (PS) were delineated on the images. The IMRT plan based on the Plan CT was copied to the 1W, 2W, 3W, and 4W CT groups, and then the small bowel dose-volume and NTCP were assessed for all CT groups. The paired t-test was used for comparison between groups. The Pearson method was used to analyze the correlation between NTCPC (chronic NTCP) and dose-volume. Results A total of 89 CT images of 20 patients were obtained. In all the patients, the volumes of BL and PS were 251.21 cm3 and 1324.16 cm3, respectively, and the shift% was 23.15% and 11.34%, respectively. The V15 of BL and PS was 184.86 cm3 and 792.45 cm3, respectively, and the shift% was 31.69% and 3.70%, respectively. The V30 of BL and PS was 88.01 cm3 and 645.73 cm3, respectively, and the shift% was 37.66% and 10.49%, respectively. The V15 of BL in 35% of patients and V15 of PS in 20% of patients, the Dmax of BL in 50% of patients, and the NTCP of 15% of patients in the course of treatment exceeded the safety limits. The 1-4W CT groups had a significantly higher NTCPC than the Plan CT group (4.02% vs. 3.20%, P=0.104), and their SD% was 41.68%. There was a significant correlation between NTCPC and V30-V50 of BL (R>0.400, P=0.000). The NTCPA (acute NTCP) and NTCPC in the supine position were significantly higher than those in the prone position (62.30% vs. 56.74%, P=0.061; 4.88% vs. 3.22%, P=0.145). Conclusions Small bowel motility leads to an uncertainty of the adverse event assessment during IMRT for rectal cancer. The change in BL is significantly larger than that in PS and the change in BL and PS in the supine position is significantly larger than that in the prone position. Using the prone position and minimizing V15 and V30 when designing the treatment plan can reduce the NTCPA and NTCPC in the small bowel. Key words: Rectal neoplasms/intensity-modulated radiotherapy; Small bowel; Normal tissue complication probability

Bag and loop small bowel contouring strategies differentially estimate small bowel dose for post-hysterectomy women receiving pencil beam scanning proton therapy

Background Small bowel (SB) dose-volume relationships established during initial computed tomography (CT) simulations may change throughout therapy due to organ displacement and motion. We investigated the impact of organ motion on SB dose-volume histograms (DVHs) in women with gynecologic malignancies treated with pencil beam scanning (PBS) proton therapy and compared PBS SB DVHs to intensity-modulated radiation therapy (IMRT). Material and methods Post-hysterectomy patients (n = 11) treated for gynecologic cancers were enrolled on an image-guided proton therapy protocol involving CT simulation with full (CTF) and empty (CTE) bladders and weekly/biweekly on-treatment scans. IMRT plans were generated for comparative analysis. SB was contoured as bowel loops or bowel bag. Wilcoxon signed-rank tests were used for matched-pair comparisons of SB, bladder, and rectum dose-volumes between CT scans and between PBS and IMRT plans. Results In PBS loops analysis, on-treatment DVH was significantly higher than CTF for doses <45 Gy (p < 0.05), and not significantly different than CTE. Specifically, V15 for loops was higher on-treatment (median 240 cm3) compared to CTF (median 169 cm3, p = 0.03). In PBS bag analysis, on-treatment DVH was not significantly different from CTF across all dose ranges. Bowel bag V45 was not significantly different between on-treatment (median 540 cm3) and CTF (median 499 cm3, p = 0.53). Decreasing bladder volume was associated with increasing V15 for loops and V45 for bowel bag (p < 0.005, both). Comparing PBS and IMRT, PBS resulted in significantly lower DVHs at low dose regions (<38 Gy) and higher DVHs at high dose regions (42.5–45.5 Gy) in both loops and bag analysis. IMRT plans demonstrated higher on-treatment SB loop DVHs and only minimal differences in bowel bag DVHs compared to CTF. Conclusions SB DVHs were well estimated by CTF bowel bag and underestimated by CTF loops in the setting of inconsistent bladder filling. Verifying bladder filling prior to treatment or using CTE for planning may more conservatively estimate SB dose-volume relationships.

Evaluating QUANTEC Small Bowel Dose-Volume Guidelines for Rectal Cancer Patients Treated Using a Couch Top Inclined Belly Board

PurposeThe goal of this work was to analyse small bowel (SB) dose-volume following the Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) guidelines for rectal cancer patients treated using a couch top inclined belly board (iBB). As part of this, the consistency in SB displacement was evaluated using on-treatment cone-beam computed tomographic (CBCT) imaging. MethodsTwenty-four patients with rectal cancer were treated on a commercially available iBB. All patients went through the standard radiochemotherapy protocol in either a pre- or postoperative setup. All patients underwent weekly CBCT imaging during the course of radiation treatment. The planning computed tomographic data sets were used to analyze the quality of SB displacement, and the CBCT data sets were used to assess the reproducibility in SB displacement during treatment. The SB dose volume was evaluated and compared with QUANTEC-recommended dose limitations. Similarly, the impact of body mass index on dose volume and SB displacement was evaluated. ResultsThe SB displacement was assessed respectively as “good” and “very good” by both independent evaluating radiation oncologists. The consistency of SB displacement through the course of radiation treatment was scored as “excellent” for 22 of 24 and 23 of 24 patients by both radiation oncologists, respectively. The QUANTEC recommendation was met for all patients without bowel adhesions; however, the most benefit was observed for patients with body mass index > 23 kg/m2. ConclusionsOur study has shown that QUANTEC recommendations for SB dose during rectal cancer treatment can easily be met by treating patients on a couch top iBB. This technique is robust and produces consistent SB displacement.

Evaluating Small Bowel Dose–Volume Tolerance Limits for SBRT Using a Preoperative Rectal Radiation Therapy Model: Are We Setting the Bar Too High?

Evaluating Small Bowel Dose–Volume Tolerance Limits for SBRT Using a Preoperative Rectal Radiation Therapy Model: Are We Setting the Bar Too High?

Consolidating duodenal and small bowel toxicity data via isoeffective dose calculations based on compiled clinical data

PurposeTo consolidate duodenum and small bowel toxicity data from clinical studies with different dose fractionation schedules using the modified linear quadratic (MLQ) model. A methodology of adjusting the dose–volume (D,v) parameters to different levels of normal tissue complication probability (NTCP) was presented. Methods and MaterialsA set of NTCP model parameters for duodenum toxicity were estimated by the χ2 fitting method using literature-based tolerance dose and generalized equivalent uniform dose (gEUD) data. These model parameters were then used to convert (D,v) data into the isoeffective dose in 2 Gy per fraction, (DMLQED2,v) and convert these parameters to an isoeffective dose at another NTCP (DMLQED2’,v). ResultsThe literature search yielded 5 reports useful in making estimates of duodenum and small bowel toxicity. The NTCP model parameters were found to be TD50(1)model = 60.9 ± 7.9 Gy, m = 0.21 ± 0.05, and δ = 0.09 ± 0.03 Gy-1. Isoeffective dose calculations and toxicity rates associated with hypofractionated radiation therapy reports were found to be consistent with clinical data having different fractionation schedules. Values of (DMLQED2’,v) between different NTCP levels remain consistent over a range of 5%-20%. ConclusionsMLQ-based isoeffective calculations of dose–response data corresponding to grade ≥2 duodenum toxicity were found to be consistent with one another within the calculation uncertainty. The (DMLQED2,v) data could be used to determine duodenum and small bowel dose–volume constraints for new dose escalation strategies.

Patient-Specific PTV Margins and Their Implementation in Radiation Therapy for Bladder Cancer

Background: Precision during bladder cancer radiotherapy delivery is particularly challenging due to large daily variations in bladder position and volume. The literature suggests that the optimal approach to margin calculation is a patient-specific one, because the magnitude and direction of bladder changes varies significantly between patients. There is considerable inconsistency in the strategies used to calculate a patient-specific planning target volume (psPTV), with authors reporting the use of data from 1 to 15 with a pattern of daily to weekly fractions. Therefore, the purpose of this study is to determine the optimal number and pattern of data points to be used to generate psPTV margins and to determine if implementing those margins at the earliest opportunity would result in a reduction in organ at risk (OAR) dose. Methods and Materials: This is a secondary analysis of a prospective clinical trial. All patients provided informed consent prior to participation in the clinical trial. Of the 30 patients accrued to the clinical trial, a sample of 15 (selected reverse chronologically) will be included in this secondary analysis. These patients received 46Gy in 23f to the whole pelvis and 20Gy in 10f to the bladder or partial bladder. Daily kV conebeam CTs (CBCT) were acquired for image guidance purposes. Local current practice requires bladder contours from the first 15 daily CBCT to generate a psPTV for implementation in the phase 2 treatment. This current practice will be used as a gold standard (GS) against which various experimental strategies (ES) will be compared, such as ‘‘first five daily fractions,’’ four weekly fractions, etc. Differences between the psPTV generated using GS versus all ES will be quantified using the following study metrics: volume, superior margin, posterior margin and clinical target volume (CTV) coverage. Once the optimal margin generation strategy is identified, the psPTV will be incorporated into the treatment plan for the remaining number of fractions. Rectal and small bowel dose volume histograms will be used to quantify any differences between the strategies. Results: Although data collection is not yet complete, based on this research design and the findings from similar evaluations for other pelvic organs, we hypothesize that it will be possible to generate psPTV margins with data from less than 15 daily fractions. This would allow an earlier adoption of the psPTV margin, which could reduce doses to the small bowel and rectum. Alternatively, if our findings suggest that 15 daily data points are necessary to generate an adequate psPTV, this would support our current practice as the optimal method. Conclusions: Precision radiation therapy for bladder cancer is technically challenging. A patient-specific approach to PTV margins is clearly necessary, but there is little evidence to guide the practitioner when designing such margin generation strategies. With this study we hope to establish an optimal regimen for generating psPTV that will result in excellent CTV coverage, but the lowest possible doses to the adjacent OARs.

The Relationship between Chronic Bowel Dysfunction and the Small Bowel Dose Volume Histogram of Rectal Cancer Survivors

Materials/Methods: Billing records and tumor registry were used to identify all living patients treated with RT for rectal cancer between January 1996 and January 2009. After exclusions for stomas, active metastatic disease, no 3D treatment planning, inadequate small bowel contrast, unrecoverable treatment data, repeat pelvic RT, wide local excision/no surgery, lost to follow-up, incapacitation from other disease, or declining participation, the remaining patients answered a validated 18 item questionnaire (Temple, et al. Diseases Colon Rectum 2005;48:1353-65). This resulted in a frequency score, a dietary score, an urgency/soilage score, and four scores from independent questions. These seven scores were tested using linear regression to evaluate the correlation the absolute small bowel volume for each dose at 5 Gy increments. The Student’s unpaired t test and Pearson chi-square/Fisher’s Exact test were used to compare continuous and categorical variables between preoperative and postoperative groups, respectively. A p value of 0.05 was considered statistically significant. Statistical analyses were performed with SPSS Version 17.0 (SPSS Inc., Chicago, IL), and all statistical tests were two-sided.

High-dose, Spot Scanning Based Proton Therapy For Paraspinal / Retroperitoneal Neoplasms and Small Bowel Tolerance: Dose Distribution Analysis in a Patient Cohort

Mesenchymal tumors require high-dose RT. Small bowel dose constraints have historically restricted dose delivery to paraspinal targets and moderate doses to large abdominal volumes carry the risk of long term morbidity. Pre-clinical comparisons of proton radiation therapy (PT) vs. photon RT have suggested a reduction in integral dose to normal tissues by PT. This study correlated target coverage with small bowel dose-volume histograms and clinical tolerance.

The Dose–Volume Relationship of Small Bowel Irradiation and Acute Grade 3 Diarrhea During Chemoradiotherapy for Rectal Cancer

Previous work has found a highly significant relationship between the irradiated small-bowel volume and development of Grade 3 small-bowel toxicity in patients with rectal cancer. This study tested the previously defined parameters in a much larger group of patients. A total of 96 consecutive patients receiving pelvic radiation therapy for rectal cancer had treatment planning computed tomographic scans with small-bowel contrast that allowed the small bowel to be outlined with calculation of a small-bowel dose-volume histogram for the initial intended pelvic treatment to 45 Gy. Patients with at least one parameter above the previously determined dose-volume parameters were considered high risk, whereas those with all parameters below these levels were low risk. The grade of diarrhea and presence of liquid stool was determined prospectively. There was a highly significant association with small-bowel dose-volume and Grade 3 diarrhea (p < or = 0.008). The high-risk and low-risk parameters were predictive with Grade 3 diarrhea in 16 of 51 high-risk patients and in 4 of 45 low-risk patients (p = 0.01). Patients who had undergone irradiation preoperatively had a lower incidence of Grade 3 diarrhea than those treated postoperatively (18% vs. 28%; p = 0.31); however, the predictive ability of the high-risk/low-risk parameters was better for preoperatively (p = 0.03) than for postoperatively treated patients (p = 0.15). Revised risk parameters were derived that improved the overall predictive ability (p = 0.004). The highly significant dose-volume relationship and validity of the high-risk and low-risk parameters were confirmed in a large group of patients. The risk parameters provided better modeling for the preoperative patients than for the postoperative patients.

2164: Principal Component Analysis of Small Bowel Dose Volume Histograms and Acute Diarrhea in the Treatment of Rectal Cancer

We have previously shown a significant relationship between small bowel dose/volume and the incidence of acute diarrhea during radiation therapy (RT) for rectal cancer with evidence of a threshold volume at all dose levels, especially at 15Gy (Baglan IJROBP 52:176–183, 2002). Our other work, however, has shown the considerable variability in small bowel position when followed with weekly CT scans during treatment, implying that DVH analysis would not be predictive of toxicity (Nuyttens IJROBP 51:1271–1280, 2001).

Treatment with a belly-board device significantly reduces the volume of small bowel irradiated and results in low acute toxicity in adjuvant radiotherapy for gynecologic cancer: results of a prospective study

Background and purpose To determine whether treatment prone on a belly-board significantly reduces the volume of small bowel irradiated in women receiving adjuvant radiotherapy for gynecologic cancer, and to prospectively study acute small bowel toxicity using an accepted recording instrument. Material and methods Thirty-two gynecologic patients underwent simulation with CT scanning supine and prone. Small bowel was delineated on every CT slice, and treatment was prone on the belly-board using 3–5 fields—typically Anterior, Right and Left Lateral, plus or minus Lateral Boosts. Median prescribed dose was 50.4 Gy and all treatments were delivered in 1.8 Gy fractions. Concomitant Cisplatin was administered in 13 patients with cervical carcinoma. Comparison of small bowel dose–volumes was made between supine and prone, with each subject acting as their own matched pair. Acute small bowel toxicity was prospectively measured using the Common Toxicity Criteria: Version 2.0. Results Treatment prone on the belly-board significantly reduced the volume of small bowel receiving ≥100; ≥95; ≥90; and ≥80% of the prescribed dose, but not ≥50%. This was found whether volume was defined in cubic centimeters or % of total small bowel volume. Of 29 evaluable subjects, 2 (7%) experienced 1 episode each of grade 3 diarrhoea. All other toxicity events were grade 2 or less and comprised diarrhoea (59%), abdominal pain or cramping (48%), nausea (38%), anorexia (17%), vomiting (10%). There were no Grade 4 events and no treatment days were lost due to toxicity. Conclusions Treatment prone on a belly-board device results in significant small bowel sparing, during adjuvant radiotherapy for gynecologic cancer. The absence of Grade 4 events or Treatment Days Lost compares favorably with the published literature.

Intensity-modulated radiotherapy as a means of reducing dose to bone marrow in gynecologic patients receiving whole pelvic radiotherapy

Purpose To evaluate intensity-modulated whole pelvis radiotherapy (IM-WPRT) (with bone marrow [BM] as a planning constraint) as a means to reduce the volume of pelvic BM irradiated. Methods and materials Ten women with cervical or endometrial cancer previously treated using IM-WPRT were selected for this analysis. Using the treatment planning CT scan, the clinical target volume was defined to encompass the gross tumor, parametrial tissues, uterus (if present), and regional lymph nodes. The clinical target volume was expanded by a 1-cm margin to form the planning target volume (PTV). The bladder, rectum, small bowel, and pelvic BM were delineated in each patient. Three plans were created for each patient: a standard four-field WPRT plan, an IM-WPRT treatment plan designed to conform the dose to the PTV while minimizing dose to the normal tissues (excluding BM), and a BM-sparing (BMS) IM-WPRT plan that included the BM as an additional treatment planning constraint. Dose–volume histograms for the PTV, small bowel, and BM were compared for each patient. Results For each of the 10 patients, BMS IM-WPRT treatment plans demonstrated a significant reduction of the volume of BM receiving >40% (18 Gy) of the prescription dose (45 Gy) compared with both IM-WPRT and four-field treatment. On average, BMS IM-WPRT resulted in only 60% of the BM volume irradiated to >50% of the dose compared with 87.4% ( p <0.001) of the BM volume in a four-field plan and 75.7% ( p < 0.003) of the volume in an IM-WPRT plan. Furthermore, the BMS IM-WPRT plans resulted in significant sparing of all other normal tissues that was comparable to the original IM-WPRT. In all 10 cases, the BMS IM-WPRT treatment plan did not result in any significant differences in the PTV and small bowel dose–volume histograms compared with the IM-WPRT treatment plans. Conclusion BMS IM-WPRT significantly reduces the volume of pelvic BM irradiated compared with conventional WPRT. In addition, BMS IM-WPRT did not compromise the improvements previously seen in IM-WPRT treatment plans that did not consider BM. Clinical studies are necessary to assess the significance of BMS IM-WPRT in reducing hematologic toxicity.

The dose-volume realationship of acute small bowel toxicity from concurrent 5-FU-based chemotherapy and radiation therapy for rectal cancer

Purpose: A direct relationship between the volume of small bowel irradiated and the degree of acute small bowel toxicity experienced during concurrent 5-fluorouracil (5-FU)-based chemoradiotherapy for rectal carcinoma is well recognized but poorly quantified. This study uses three-dimensional treatment-planning tools to more precisely quantify this dose-volume relationship. Methods and Materials: Forty patients receiving concurrent 5-FU-based chemotherapy and pelvic irradiation for rectal carcinoma had treatment-planning CT scans with small bowel contrast. A median isocentric dose of 50.4 Gy was delivered using a posterior-anterior and opposed lateral field arrangement. Bowel exclusion techniques were routinely used, including prone treatment position on a vacuum bag cradle to allow anterior displacement of the abdominal contents and bladder distension. Individual loops of small bowel were contoured on each slice of the planning CT scan, and a small bowel dose-volume histogram was generated for the initial pelvis field receiving 45 Gy. The volume of small bowel receiving each dose between 5 and 40 Gy was recorded at 5-Gy intervals. Results: Ten patients (25%) experienced Common Toxicity Criteria Grade 3+ acute small bowel toxicity. A highly statistically significant association between the development of Grade 3+ acute small bowel toxicity and the volume of small bowel irradiated was found at each dose level. Specific dose-volume threshold levels were found, below which no Grade 3+ toxicity occurred and above which 50–60% of patients developed Grade 3+ toxicity. The volume of small bowel receiving at least 15 Gy ( V 15) was strongly associated with the degree of toxicity. Univariate analysis of patient and treatment-related factors revealed no other significant predictors of severe toxicity. Conclusions: A strong dose-volume relationship exists for the development of Grade 3+ acute small bowel toxicity in patients receiving concurrent 5-FU-based chemoradiotherapy for rectal carcinoma.