Dosimetric Study Of Volumetric Modulated Arc Therapy With Static Intensity-modulated Radiotherapy In The Short-term Preoperative Radiotherapy Of Rectal Cancer

Objective: To investigate the dosimetric characteristics of volumetric modulated arc therapy(VMAT) and static intensity-modulated radiotherapy(s IMRT) in the short-term preoperative radiotherapy of rectal cancer in order to understand the potential advantages of volumetric modulated arc therapy and to provide references for clinic work.Methods: 14 patients with the clinical stage â…¡/â…¢ rectal cancer previously treated with preoperative radiotherapy in our hospital were selected for this study. Using the treatment planning system, each CT positioning image was respectively re-planned with VMAT and s IMRT modality. Prescription dose was set 95 percent of PTV1 and PTV2 acceptable dose. The treatment plan was designed to deliver in a single phase process a dose of 25 Gy to the PTV1 in 5 fractions and at the same time 22.5 Gy to the PTV2. Differences of target conformities, homogeneity index and dose-volume of the organs at risk(OARs) are compared to assess the two sets of plans.Results: two kinds of treatment plan could meet the requirements of the prescription dose. Conformity index of PTV1 in VMAT and s IMRT plans were 0.889 and 0.838 respectively(P = 0.000), homogeneity index were 0.050 and 0.082(P = 0.000); Conformity index of PTV2 in VMAT and s IMRT plans were 0.917 and 0.917 respectively(P = 0.000), homogeneity index were 0.155 and 0.192(P = 0.000). VMAT plans showed significant superior in the dose conformity and homogeneity over s IMRT. There were significant statistical differences in CI and HI. Compared with VMAT, s IMRT plans achieved the higher minimum PTV1 and PTV2 dose and the lower maximal PTV2 dose, s IMRT also yielded the lower maximal PTV1 dose, although the difference was not statistically significant. The control of the target cold spots and hot spots, VMAT plans had more advantages than s IMRT. In terms of organ at risk sparing, VMAT plans reduced the V15 of small bowel nearly 33% compared with s IMRT. For the small bowel, bladder and femoral heads, VMAT plans could significantly reduce the volume of high dose irradiation and increase the volume of low dose irradiation, VMAT achieved significantly higher dose in the lowest dose of organ at risk, this may be associated with VMAT using arc rotating irradiation. Compared with s IMRT, VMAT plans have more beam Angle and beam number, in terms to reduce the volume of the normal tissue around the target by high dose irradiation, by improving the target dose distribution at the same time increasing the low dose irradiation range and the minimum dose.Conclusion: VMAT and s IMRT plans can satisfy the requirements of target prescription dose; VMAT can further improve the dose Conformity and homogeneity. The control of the target cold spots and hot spots, VMAT plans have more advantages than s IMRT. For the organ at risk, VMAT plans can significantly reduce the volume of high dose irradiation and are likely to further reduce the highest dose of the organ at risk, so VMAR will reduce the toxicity of the radiotherapy related reaction.