Robustness quantification methods comparison in volumetric modulated arc therapy to treat head and neck cancer

Abstract

Background: To compare plan robustness of volumetric modulated arc therapy (VMAT) with intensity modulated radiation therapy (IMRT) and to compare the effectiveness of 3 plan robustness quantification methods.

Methods and materials: The VMAT and IMRT plans were created for 9 head and neck cancer patients. For each plan, 6 new perturbed dose distributions were computed using ±3 mm setup deviations along each of the 3 orientations. Worst-case analysis (WCA), dose-volume histogram (DVH) band (DVHB), and root-mean-square dose-volume histogram (RVH) were used to quantify plan robustness. In WCA, a shaded area in the DVH plot bounded by the DVHs from the lowest and highest dose per voxel was displayed. In DVHB, we displayed the envelope of all DVHs in band graphs of all the 7 dose distributions. The RVH represents the relative volume on the vertical axis and the root-mean-square-dose on the horizontal axis. The width from the first 2 methods at different target DVH indices (such as D_95% and D_5%) and the area under the RVH curve for the target were used to indicate plan robustness. Results were compared using Wilcoxon signed-rank test.

Results: The DVHB showed that the width at D_95% of IMRT was larger than that of VMAT (unit Gy) (1.59 vs 1.18) and the width at D_5% of IMRT was comparable to that of VMAT (0.59 vs 0.54). The WCA showed similar results between IMRT and VMAT plans (D_95%: 3.28 vs 3.00; D_5%: 1.68 vs 1.95). The RVH showed the area under the RVH curve of IMRT was comparable to that of VMAT (1.13 vs 1.15). No statistical significance was found in plan robustness between IMRT and VMAT.

Conclusions: The VMAT is comparable to IMRT in terms of plan robustness. For the 3 quantification methods, WCA and DVHB are DVH parameter-dependent, whereas RVH captures the overall effect of uncertainties.

Figure 1

Comparison of plan robustness of the CTV_high for patient 1 using 3 robustness quantification methods: (a) WCA, (b) DVHB, and (c) RVH.

Figure 2

Comparison of plan robustness of the CTV_high for 9 patients using 3 robustness quantification methods at different DVH parameters: (a) width at D_95% from WCA, (b) width at D_5% from WCA, (c) width at D_95% from DVHB, (d) width at D_5% from DVHB, (e) AUC of the CTV_high RVH curves.

Figure 3

(a) Comparison of plan robustness of the CTV_high averaged over 9 patients using (a) width at D_95% from WCA and DVHB, width at D_5% from WCA and DVHB and (b) AUC from RVH curves. Numbers at the top of the columns are P values.

Figure 4

Comparison of plan robustness of (a) spinal cord, (b) brainstem, (c) left parotid, (d) right parotid and (e) mandible for 9 patients using the corresponding AUCs.

Figure 5

Comparison of plan robustness of spinal cord, brainstem, left parotid, right parotid, and mandible averaged over 9 patients using the corresponding AUCs. P values are shown on the top of the columns.