Evaluation of influence of different photon beam energy and number of arcs to dose distribution in volumetric modulated arc therapy for cervical cancer


Purpose: To compare the dosimetric impact of different photon beam energy and number of arcs to dose distribution in Volumetric modulated arc therapy (VMAT) for cervical cancer.

Materials and methods: From Oct 2018 to May 2020, Fifteen patients with carcinoma cervix underwent radiotherapy in Radiotherapy Department - Vinmec International Hospital Times City. VMAT plans with 6 MV and 15 MV photon energies using single arc (SA), double arc (DA) and Triple Arc (TA) were generated. The Eclipse Planning System Version 13.0 (Varian Medical System) was adopted to design all these plans. For a fair comparison, the planning target volume (PTV) coverage of all VMAT plans were normalized to the same level. All VMAT plans were compared with each other to evaluate coverage, homogeneity index (HI) and conformity index (CI) of PTV, sparing of OARs, delivery time and monitor units (MUs).

Results: Comparable dose coverage to PTV was observed for all the energies and arcs. V98 of PTV in TA6MV plans (97.93) are better than DA6MV(97.84), SA6MV(97.63) plans and in TA15MV plans (97.98) better than DA15MV(97.87), SA15MV(97.68) plans (p <0.05). The dose D2 of PTV in SA6MV plans (56.31) and SA15MV(56.07) are the largest (p <0.05). TA6MV plans have CI (0.979), HI (0.112) better than DA6MV(CI = 0.978; HI = 0.117) and SA6MV(CI = 0.976; HI = 0.132) plans (p <0.05), similar CI (0.979) and HI (0.111) of TA15MV plans are better than DA15MV(CI = 0.978; HI = 0.117) and SA15MV(CI = 0.976; HI = 0.127) plans (p <0.05). Distribution of dose to PTV between TA6MV and TA15MV; DA6MV and DA15MV are nonsignificant (p> 0.05). Dmean, Dmax, V40Gy to the Bowel bag and Dmean to the bladder of TA plans are better than SA, DA plans at both levels energy 6MV and 15MV (p <0.05). At level 6MV, Dmean to rectum is better at TA6MV(41.48) than DA6MV (41.66), SA6MV(42.17) (p <0.05). The dose of femur heads (Right & Left) are not much different (p> 0.05). Dmean, Dmax to Illiac bone in TA plans are better than DA and SA at both levels 6MV and 15MV (p <0.05); at level 15MV, the dose V10Gy in TA15MV plans (93.01 ± 4.11, p <0.05) are lower than in the other plans; at level 6MV, the dose of V40Gy in TA6MV plans (15.4 ± 5.35, p <0.05) are the lowest. SA plans have treatment time and number of MUs lower compare with DA and TA (p<0.05).

Conclusions: The study showed no greater advantage of higher energy on dose distribution to PTV and OARs. The TA plans with 6 MV photon energy was a good choice of treatment for carcinoma cervix as it delivered a highly homogeneous and conformal plan with superior target coverage and better OAR sparing.

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