Session Item

The purpose of this work is to compare the dose distribution for whole brain radiotherapy (WBRT) with hippocampal sparing using HyperArc technology and the non-coplanar VMAT method.

A total of 25 patients treated earlier with WBRT in the radiotherapy department were selected for the study, and 50 treatment plans were calculated for them: 25 HyperArc plans and 25 non-coplanar VMAT plans. A comparative analysis of dose distribution for both methods was performed in the Eclipse 16.1 radiotherapy planning system (Varian Medical Systems). The analysis was conducted according to the following criteria: HI homogeneity index, CI conformity index, GI gradient index, dose to the hippocampus at D(Mean), D(95%), D(Max) and D(0,5cm3), time of plan creation, time of plan calculation and optimization, total number of MUs. A verification analysis QA was also performed with various normalizations (Local and Global) and high detectors density using the ArcCHECK dosimetry system with SNC Patient and 3DVH software (SunNuclear Corporation).

Dosimetric analysis:

The conformity and homogeneity of the dose distribution in the target is almost the same for both methods. Outside PTV dose decreases faster with ncVMAT (an average difference between methods is 0.41). The maximum lens dose is best reduced when planning ncVMAT (difference is about 0.45 Gy). The dose to the hippocampus is reduced  much better when planning HyperArc (Dmax difference 0.8 Gy). Equipment wear, proportional to the number of monitor units, is the same for both methods. The time of plan calculation and optimization when using the HyperArc technology increases by 9.8 minutes.

Gamma analysis: 

At threshold TH10% ncVMAT gamma passing rate GPR<95% for 2%/2mm and 3%/2mm are 91.7 and 94.4, respectively. Verification at ТН10% GPR> 95%: HyperArc passes 2%/2mm and 3%/2mm, and ncVMAT passes 2%/3mm and 3%/3mm.

Analysis with Local and Global normalization:

For Hyperarc with Global normalization, GPR regardless of the comparison criteria (2%/2mm - 3%/3mm, TH5-20%) is 100%. For ncVMAT with Global normalization,  GPR is 99.7% on average.

Analysis with high detectors density:

On average, ncVMAT GPR increases by 0.7%: at thresholds ТН5-10% (2%/2mm - 3%/3mm) and ТН5-20% (2%/2mm). With HyperArc, the difference is only 0.1%.

The methods satisfy the protocols conditions:

HyperArc: TG-119 (3%, 3mm, TH10%, Global), TG-218 (3%, 2mm, TH10%, Global ), TG-244 (2%, 2mm, TH10%, Local).

ncVMAT: TG-119 (3%, 3mm, TH10%, Global), TG-218 (3%, 2mm, TH10%, Global).

In the case of WBRT with hippocampal sparing both methods work equally well and satisfy the RTOG-933 recommendations. The MUs and dose in PTV and critical structures are almost identical. The difference for methods in time of plan calculation and optimization is 10 minutes (HyperArc - 28.5 minutes, ncVMAT - 18.7 minutes). HyperArc passes verification much better even on the most stringent TG-244 requirements (2%, 2mm, TH10%, local).