Dosimetric comparison of split field and fixed jaw techniques for target volumes in the rectum cancer

Objective: To study the dosimetric comparison of split field and fixed jaw techniques for large IMRT target volumes in the rectum cancer. Methods: CT images of fifteen patients with rectum malignancies and regional target volumes were transferred into Eclipse planning system. SFT plan and FJT plan were performed on an Eclipse TPS using beam data generated for linear accelerator. A standard beam arrangement consisting of seven coplanar fields was used in both techniques.Institutional dose-volume constraints used in rectum cancer were kept the same for both techniques. Target and organs at risk were evaluated. Results: PTV₉₅ in FJT plan coverage was lower (t=-2.24, P<0.05). D_mean in FJT plan was increased (t=2.54, P<0.05), but D_max was not different. HI in FJT plan became inferior (t=3.09, P<0.05), while CI was not different. There was no difference in dose distribution among bladder, femoral head and cauda equina. The value of V₅ of small intestine increased in FJT plan (t=4.76, P<0.05), and the values of V₂₀ and V₅₀ of bone marrow were better than those in SFT plan (t=-2.66, -3.36, P<0.05), while D_max was higher than that in SFT plan (t=3.30, P<0.05). The value of V₂₀ of body was higher in FJT plan than that in SFT plan (t=2.48, P<0.05). The number of MU was significantly lower in FJT plan than that in SFT plan (t=-9.38, P<0.05). The average segments in FJT plan decreased by 39.4% compared with SFT plan (t=-6.46, P<0.05). Verification rate in FJT plan group was better than that in SFT plan (t=10.46, P<0.05), and the treatment time was shortened from 12 to 6 min. Conclusions: Compared to SFT technique, patients with rectal cancer who were treated with FJT could get better dose of target and organs, which can meet the clinical treatment requirements. The technique could shorten the treatment time and reduce the treatment MU. It also could increase the number of patients to be treated, reduce their waiting time and reduce the difficulty of QA. (authors)