Preoperative volumetric modulated arc therapy and helical tomotherapy for rectal cancer: A dosimetric study

Objective To compare the dose distribution of volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT) in preoperative radiotherapy for rectal cancer. Methods Ten patients with rectal cancer at stage Ⅱ-Ⅲ who underwent preoperative chemo-radiotherapy were enrolled in this study. VMAT and HT plans were designed for each patient. The prescribed dose was 56.25 Gy for pGTV and 50 Gy for PTV, and delivered over 25 fractions daily using a simultaneous integrated boost approach. The dose distribution in target volumes and organs at risk was evaluated according to the dose-volume histogram. Results The HT plan provided a better conformity index for pGTV than the VMAT plan (t=-2.803, P=0.005). The PTV D₅₀ and D₉₈ were higher for the VMAT plan than for the HT plan (t=8.895 vs t=3.663, P＜ 0.05). The VMAT plan was better than the HT plan for protecting the small bowel. The V₁₅ of small bowel was about 19% lower in the VMAT plan than in the HT plan (t=-3.802, P＜ 0.05). The V₁₀ of pelvis was lower whereas the V₃₀ of pelvis was higher in the VMAT plan than in the HT plan (t=-7.047, t=4.704, P＜ 0.05). The VMAT plan was better than the HT plan for protecting bladder. The number of monitor units (MU) were about 89% lower in the VMAT plan than in the HT plan (t=-2.666, P=0.008). Conclusion The dose distribution of VMAT and HT is similar in preoperative radiotherapy for rectal cancer. VMAT is advantageous over HT in protecting the small bowel. However, the number of MU is signifcantly lower in VMAT than in HT. The curative effect of HT needs to be further assessed.