Dosimetric comparison of the VMAT and IMRT planning in rectal carcinoma

Abstract

Objective To compare the plan quality and dose precision of volumetric modulated arc therapy(VMAT) plans with intensity modulated radiation therapy (IMRT) plans in rectal carcinoma. Methods In 10 patients, five-field IMRT plans were compared to two dual arcs VMAT plans. All the patients underwent surgery and CT simulation orientation, target volumes and normal tissues were drawn in the images. Plans were optimized by Oncentra 4.3 planning system and designed with the same optimize parameters. Plans were normalized so that 95% of PTV would receive the prescription dose of 50 Gy in 25 fractions. Dose-volume histograms and isodose distributions were evaluated. Conformity indices (CI), homogeneous indices (HI), total segments, monitor units (MUs), delivery time were also compared. Measurements of the plan dose distribution were performed and analyzed with Delta4 detector array by γ method. Results Both plans could meet target dose specifications and normal tissue constraints. VMAT plan achieved better CI than IMRT plans, CIVMAT=0.614, CIIMRT=0.737, the differences were statistically significant (t=4.94, P 0.05). In the normal tissue including bladder and femurs, there was an increase in the volume receiving low dose radiation in VMAT. The number of MU was reduced from 599 in VMAT plans to 515 in IMRT plans(t=4.72, P<0.05), but VMAT plan had a significantly shorter delivery time (201 s) compared with 304 s in IMRT plans(t=5.83, P<0.05). In the dose verification, an average of 93.13% of the detector points passed the 3%/3 mm γ criterion for VMAT plans, while in IMRT plans the dose verification was 96%(t=3.75, P<0.05). Conclusions VMAT and IMRT techniques can both achieve a good PTV coverage, HI and CI. VMAT offers shorter treatment delivery time than IMRT for rectal carcinoma, enhances the efficiency of treatment. Further clinical evaluation of VMAT would be warranted. Key words: Rectal carcinoma; Intensity-modulated radiotherapy(IMRT); Volumetric modulated arc therapy(VMRT); Dose verification; Organs at risk; Delta4 detector array