Dosimetric Comparison and Feasibility of Simultaneous Integrated Boost (SIB) in Treatment of Malignant Gliomas Using Intensity Modulated Radiotherapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT).

Objective: To evaluate the dosimetric parameters of Simultaneous Integrated Boost in the treatment of malignant gliomas and compare the SIB plans of VMAT and IMRT. Methodology: CT and MRI of 28 patients were used for generating SIB plans with VMAT and IMRT. A dose of 2Gy per fraction was prescribed to the CPTV and 2.4Gy to the GPTV for a total of 25 fractions. The plans were accepted only if they met the set of planning objectives defined in the protocol. Results: We could achieve the planning objectives in all the SIB plans. Although GPTV coverage was statistically better in VMAT (98.67% vs 98.19% ;p=0.024) the difference is not clinically meaningful. The conformity index for GPTV was higher in IMRT (0.83 vs 0.76; p=0.001). The coverage of CPTV was better in IMRT (97.88% vs 96.87%; p=0.021). But the conformity index of CPTVannulus was higher in VMAT (0.72 vs 0.67; p=0.01). There was no difference in homogeneity index of GPTV and CPTV annulus between the plans. The mean dose received by normal brain was higher in IMRT (28Gy vs 24.2Gy; p<0.001). Ipsilateral optic nerve has received lesser Dmax in IMRT (44.2Gy vs 46.95Gy; p=0.02). No difference was seen in Dmax of brainstem, optic chiasm, contralateral optic nerve. The treatment times and monitor units were significantly less in VMAT. Conclusion: SIB is dosimetrically feasible for hypofractionation in malignant gliomas using IMRT and VMAT. IMRT plans had better boost conformity, lower ipsilateral optic nerve and brainstem maximum doses compared to VMAT. Whereas, VMAT had better coverage, better overall PTV conformity, lower normal brain mean dose, lower monitor units and lesser treatment times. Although planning of VMAT is cumbersome and time consuming, the advantage of reducing treatment time is beneficial to the patients’ comfort and better managing of patient load in high volume centres.