Objective:To investigate the effect of abdominal circumference on intestinal radiation dose volume and acute intestinal toxicity in pelvic intensity modulated radiation therapy for rectal cancer.Methods:A total of 150 patients with locally advanced rectal cancer (LARC) who received adjuvant and neoadjuvant concurrent chemoradiotherapy at the Affiliated Cancer Hospital of Guizhou Medical University from March 2023 to January 2025 were enrolled, including 82 cases of adjuvant radiotherapy and 68 cases of neoadjuvant radiotherapy. All patients underwent radiotherapy CT simulation positioning in the standard mode of prone position with abdominal board padding and bladder filling. Intestinal toxicity was categorized as a binary variable based on the occurrence of ≥2 grade acute intestinal toxicity. Linear and logistic regression models were used to analyze the factors influencing intestinal radiation dose volumes (V 10, V 20, V 30, V 40) and acute intestinal toxicity in LARC patients. Generalized additive models and piecewise linear and logistic regression analyses were employed to examine the threshold effects of abdominal circumference on intestinal radiation dose volumes and acute intestinal toxicity. The threshold value for abdominal circumference was determined based on the upper limit of the 95% CI for the threshold. A difference test was used to validate the differences in intestinal radiation dose volume and acute intestinal toxicity between small and medium-to-large abdominal circumferences. Results:Univariate analysis showed that, gender, body mass, abdominal circumference, planning target volume (PTV), intestinal volume were all influencing factors for the radiation dose volumes (V 10, V 20, V 30, V 40) of each intestinal segment of patients with LARC undergoing adjuvant radiotherapy (all P<0.05). Body mass, abdominal circumference, intestinal volume were all influencing factors for the radiation dose volumes (V 10, V 20, V 30, V 40) of each intestinal segment of patients with LARC undergoing neoadjuvant radiotherapy (all P<0.05). Body mass index (BMI), abdominal circumference, intestinal volume and individual intestinal radiation volumes (V 10, V 20, V 30, V 40) were all influencing factors for the acute intestinal toxicity of patients with LARC undergoing adjuvant radiotherapy (all P<0.05). Body mass, BMI, abdominal circumference, multiple intestinal radiation dose volumes (V 20, V 30, V 40) were all influencing factors for the acute intestinal toxicity of patients with LARC undergoing neoadjuvant radiotherapy (all P<0.05). Multivariate analysis showed that, abdominal circumference (V 10: β=-1.01, 95% CI: -1.68--0.33, P=0.004; V 20: β=-0.94, 95% CI: -1.28--0.60, P<0.001; V 30: β=-0.58, 95% CI: -0.82--0.34, P<0.001; V 40: β=-0.41, 95% CI: -0.60--0.23, P<0.001) was an independent influencing factor for the radiation dose volume of each intestinal segment of patients with LARC undergoing adjuvant radiotherapy. Abdominal circumference (V 10: β=-0.92, 95% CI: -1.62--0.22, P=0.010; V 20: β=-0.84, 95% CI: -1.11--0.57, P<0.001; V 30: β=-0.42, 95% CI: -0.57--0.28, P<0.001; V 40: β=-0.30, 95% CI: -0.41--0.19, P<0.001) was an independent influencing factor for the radiation dose volume of each intestinal segment of patients with LARC undergoing neoadjuvant radiotherapy. Abdominal circumference ( OR=0.86, 95% CI: 0.78-0.95, P=0.002) was an independent influencing factor for the acute intestinal toxicity of patients with LARC undergoing adjuvant radiotherapy. Abdominal circumference ( OR=0.87, 95% CI: 0.79-0.96, P=0.004) was an independent influencing factor for the acute intestinal toxicity of patients with LARC undergoing neoadjuvant radiotherapy. The generalized additive model revealed a nonlinear relationship between abdominal circumference and intestinal radiation dose volume and acute intestinal toxicity of adjuvant radiotherapy patients. Further segmented regression analysis results showed that there was a threshold effect between abdominal circumference and intestinal radiation dose volume (V 10, V 20, V 30, V 40) and acute intestinal toxicity. The inflection point values between abdominal circumference and intestinal radiation dose volume V 10, V 20, V 30, V 40 in LARC patients undergoing adjuvant radiotherapy were all 71.9 cm; the inflection point values between abdominal circumference and the intestinal radiation dose volume V 10, V 20, V 30, V 40 in LARC patients undergoing neoadjuvant radiotherapy were 69.0, 69.0, 69.0, 68.6 cm, respectively; The inflection point values between abdominal circumference and acute intestinal toxicity in LARC patients undergoing adjuvant radiotherapy and neoadjuvant radiotherapy were 71.9, 69.0 cm, respectively. Based on the upper limit of the 95% CI threshold, the cutoff values for small and medium-to-large abdominal circumferences for patients undergoing adjuvant and neoadjuvant radiotherapy were set at 76.1, 71.9 cm, respectively. In patients undergoing adjuvant radiotherapy, the levels of intestinal radiation dose volume V 10 [ (7.65±2.29) cm 3vs. (5.88±2.68) cm 3, t=2.76, P=0.007], V 20 [ (4.28±1.27) cm 3vs. (2.72±1.31) cm 3, t=4.81, P<0.001], V 30 [ (2.42±1.07) cm 3vs. (1.37±0.76) cm 3, t=4.95, P<0.001], V 40 [ (1.69±0.74) cm 3vs. (0.92±0.58) cm 3, t=4.93, P<0.001] in the small abdominal circumference group ( n=22) were significantly higher than those in patients with medium-to-large abdominal circumferences ( n=60) ; In patients undergoing neoadjuvant radiotherapy, patients with small abdominal circumferences ( n=11) had significantly higher V 20 [ (3.09±0.84) cm 3vs. (2.28±1.17) cm 3, t=2.17, P=0.033], V 30 [1.44 (1.22, 1.53) cm 3vs. 0.91 (0.56, 1.22) cm 3, Z=-3.04, P=0.002], V 40 [0.93 (0.84, 1.09) cm 3vs. 0.44 (0.30, 0.81) cm 3, Z=-3.19, P=0.001] than patients with medium-to-large abdominal circumferences ( n=57). In patients receiving adjuvant radiotherapy and neoadjuvant radiotherapy, there were statistically significant differences in acute intestinal toxicity between patients with small abdominal circumferences and with medium-to-large abdominal circumferences ( χ2=10.46, P=0.001; χ2=8.13, P=0.004) . Conclusions:In the standard mode (prone position with abdominal board padding and bladder filling), abdominal circumference is an independent factor influencing the intestinal radiation dose volume and acute intestinal toxicity in rectal cancer radiotherapy patients. There is a significant non-linear threshold effect between abdominal circumference and different levels of intestinal radiation dose volume and acute intestinal toxicity. The impact of abdominal circumference on intestinal radiation dose volume and toxicity differs significantly before and after the inflection point value. Patients with smaller abdominal circumferences not only fail to achieve the expected benefits under the current standard radiotherapy regimen but also face higher risks of intestinal radiation dose volume and toxicity.