A Simplified Three-Isocenter VMAT for Craniospinal Irradiation

Abstract

Craniospinal irradiation (CSI) has become an important treatment method for Central nervous system (CNS) relapse of hematological malignancies. However, conventional CSI is technically very challenging and field edge matching is needed because of the mechanical limitations of standard linear accelerators. We have developed a simple Linac based volumetric modulated arc therapy (VMAT) technique for CSI. The aim of this study is to report initial clinical experience of CSI-VMAT for CNS relapse of hematological malignancies and compare dose homogeneity and dose to organs at risk (OAR) with conventional three-dimensional conformal radiotherapy (3D-CRT) technique. Seven patients with CNS relapse of hematological malignancies were treated with CSI using a novel VMAT technique and seven patients were previously underwent conventional 3D-CRT technique. A dose of 22.5 Gy was prescribed to be delivered in 15 fractions. Planning target volume (PTV) included the whole brain and spinal canal to the S3 vertebral level. Patient is positioned supine and immobilized using a face mask and Vac-Lock in the body. The VMAT plan used three full arcs, each with a unique isocenter (Brain, Superior Spine, Inferior Spine). Spinal two arcs turned off lateral beams (225-315o and 45-135o) to minimize lung dose. Adjacent fields were intentionally overlapped to eliminate the issues of beam edge matching, and the optimization process smoothly integrated the dose inside the overlapped junction. The 3D-CRT plan used edge matching field sets and shifted junctions three times. Comparing the VMAT plan with the 3D-CRT plan, the average simplified homogeneity index (HI=Dmax/Dmin) of PTV was 1.19 ± 0.11 and 1.46 ± 0.13, respectively. Mean doses to avoidable OAR were significantly decreased (for the heart: 5.3 Gy and 11.1 Gy, for the esophagus: 11.6 Gy and 19.7 Gy, for the parotid glands: 8.8 Gy and 20.5 Gy, using VMAT and 3D-CRT, respectively). The lung V5, V10 and Dmean were 28.3%, 5.4%, 3.4 Gy, in VMAT, 20.1%, 15.3 %, 4.6 Gy, in 3D-CRT. The doses of lung except V5 were lower in VMAT. Short term follow-up evaluations revealed no unexpected toxicity associated with this novel VMAT technique. We have developed a simplified three-isocenter Linac based VMAT approach for CSI. The dose to the brain and spine is significantly improved, compared to the conventional 3D-CRT. The single VMAT plan is easier to manage. This new technique simplifies the set up and eliminates the need for shifting junctions during course of CSI treatment and saves OAR of patients with CNS relapse of hematological malignancies.