This study was conducted with the primary objective of evaluating and contrasting the acute toxicity outcomes associated with two radiotherapy modalities—3D-CRT and SIB-IMRT—administered in the preoperative setting for patients diagnosed with locally advanced rectal cancer (LARC). A total of 40 individuals who met the diagnostic criteria for LARC were enrolled and randomly allocated into two equal cohorts. Group A underwent 3D-CRT in combination with concurrent capecitabine, while group B received SIB-IMRT, also paired with capecitabine, as part of their neoadjuvant regimen. During the combined chemo-radiotherapy treatment, patients were evaluated weekly, and any instances of acute toxicity were documented. A significant reduction in grade 3 genitourinary toxicities was observed in group B compared to group A, which was statistically significant with a P-value of 0.048. Additionally, when analyzing gastrointestinal toxicities, both grade 2 and grade 3 adverse events were significantly more prevalent in the 3D-CRT cohort compared to those receiving SIB-IMRT, with P-values of 0.043 and 0.021, respectively. Dosimetric comparisons showed no significant changes in Dmean or Dmax values for the urinary bladder (UB) and small bowel (SB) between the two treatment techniques. However, the V45—the volume that receives at least 45 Gy—of the UB was significantly higher in patients treated with 3D-CRT compared to those receiving SIB-IMRT (P = 0.003), and a similar trend was observed for the SB (P = 0.001). Overall, the reduced V45 exposure for both UB and SB in the SIB-IMRT group appeared to correlate with the improved toxicity profile, as the frequency of grade 2 genitourinary and both grade 2 and grade 3 gastrointestinal toxicities were significantly lower in this cohort relative to the 3D-CRT group.
