Abstract: Thirty cases of computed tomography (CT) localization images of patients with colorectal cancer undergoing radiotherapy were selected. The CT values of air cavities within the planning target volume (PTV) were modified in the treatment planning system (TPS), and models with different volume ratios were constructed. The treatment plans were transferred to the modified images, and the dose distributions were calculated. The dose parameters (D_max, D_mean, D_98%, D_50%, D_2%, conformity index CI, homogeneity index HI) of the PTV and the dose changes of organs at risk were evaluated. Paired sample t test or Wilcoxon signed rank test was used to analyze the differences between groups, quantify the influence of changes in the CT values and volume ratios of intestinal air cavities on dose distribution, and establish risk warning thresholds. The results showed that an increase in the CT values of air cavities led to a significant decrease in D_98% of the PTV, with the average relative deviation expanding to -1.215%, while D_2% remained relatively stable. CI and HI increased significantly (P<0.05). Except for the Dmean of the small intestine, there were no statistically significant changes in the doses of other organs at risk (P>0.05). There was a critical threshold for the synergistic effect of CT values and volume ratios: when the CT value of the air cavity was ≥250 HU and the volume ratio was >6%, the maximum relative deviation of D_98% increased to -4.927%. Therefore, when both reached the thresholds simultaneously, the coupling of dose algorithm distortion and volume effect could result in clinically unacceptable cold spots. Risk stratified management should be established: for high-risk patients (those reaching the thresholds), comprehensive interventions (diet control, electron density correction) should be implemented, and the changes in air cavities should be dynamically monitored through cone beam CT (CBCT). When the volume ratio of air cavities within the PTV was >6% and the CT value was ≥250 HU, adaptive radiotherapy should be initiated.

Key words: dose calculation, colorectal cancer, intensity-modulated radiotherapy, air cavity volume threshold, plan evaluation