A dosimetric comparison of non-coplanar volumetric modulated arc therapy and non-coplanar fixed field intensity modulated radiation therapy in hippocampus-avoidance whole-brain radiation therapy with a simultaneous integrated boost for brain metastases

Abstract

Objective: The aim of this study was to investigate the dosimetric differences between non-coplanar volumetric modulated arc therapy (VMAT) and non-coplanar fixed-field intensity-modulated radiotherapy (IMRT) in hippocampus-avoidance whole-brain radiation therapy with a simultaneous integrated boost (HA-WBRT+SIB) for brain metastases using the Monaco treatment planning system (TPS).

Method: A total of 22 patients with brain metastases were retrospectively enrolled. Two radiotherapy treatment plans were designed for each patient: non-coplanar VMAT and non-coplanar fixed field IMRT. The dose distribution of targets and organs at risk (OAR), the number of monitor units (MUs), and pre-treatment plan verification were compared between the two plans while meeting the prescribed dose requirements of the target volume.

Results: There were no significant differences in V₅₀, V₅₅, D_max, heterogeneity index (HI) and conformity index (CI) of target PGTV between the two plans (p>0.05). For PTV-brain-SIB, there was no significant difference in D_98% between IMRT and VMAT (p=0.103). VMAT significantly improved the V₃₀ of PTV-brain-SIB (p<0.001), decreased HI (p=0.003), and increased CI (p<0.001). There were no significant differences in the D_max to the brain stem, left and right lens, optic chiasm, pituitary gland, and left and right hippocampus between the two plans (p>0.05). Compared with IMRT, VMAT significantly reduced the D_max to the left and right eyes (p<0.001) and significantly increased the D_max to the right inner ear (p=0.010). There was no significant difference in the D_max to the left inner ear between VMAT and IMRT (p=0.458). Compared with IMRT, VMAT significantly reduced the D_max to the left optic nerve (p=0.006), but significantly increased the D_max to the right optic nerve (p=0.001). There was no significant difference in the D_max to the left and right hippocampus between VMAT and IMRT (p>0.05), but VMAT significantly increased the D_100% (p<0.05) compared with IMRT. Compared with VMAT, IMRT significantly reduced the MU (p<0.001) but VMAT has a higher treatment efficiency than IMRT, with an average reduction of 41 seconds (294.1 ± 16.4 s for VMAT, 335.8 ± 34.9 s for IMRT, p<0.001). Under the conditions of 3%/2 mm, and 2%/2 mm, the gamma passing rate of the IMRT QA was improved compared to VMAT, with an average increase of 0.6%, p=0.013, and 1.7%, p<0.001, respectively.

Conclusion: Both non-coplanar VMAT and non-coplanar fixed field IMRT based on the Monaco TPS produce clinically acceptable results for HA-WBRT+SIB in patients with brain metastases. Compared with IMRT, VMAT has better dose distribution in the target volume and treatment efficiency, but IMRT can better protect the hippocampus and reduce the number of MUs.

Keywords: brain metastases; hippocampus sparing; intensity modulated radiotherapy; simultaneous integrated boost; volumetric modulated arc therapy; whole brain radiotherapy.

Figure 1

Plan setting parameters. (A–D) are the sequencing parameters of the VMAT plan, sequencing parameters of the IMRT plan, the calculation properties of the planned dose deposition, and the prescription dose parameters, respectively.

Figure 2

The dose distribution of the VMAT (upper) and IMRT (down) plans in cross-section (left), coronal (middle), and sagittal plane (right) for a typical patient. The red area represents the coverage with an isodose of 50 Gy and the blue area represents the coverage with an isodose level at 30 Gy.

Figure 3

DVH of the VMAT and IMRT plans for a representative patient. The solid line indicates the VMAT plan and the dotted line indicates the IMRT plan.