2682 - Volumetric Modulated Arc Therapy Integrating Static Angle Modulated Ports and Dynamic Collimator Rotation Improves Dosimetry and Efficiency of Stereotactic Radiosurgery for Spine Metastasis

08:00am - 09:00am PT

Hall F

Screen: 24

POSTER

Presenter(s)

Shu-Fan Wang, MS - National Taiwan University Cancer Center, Taipei, Taipei

S. F. Wang¹, C. J. Wu¹, and F. M. Hsu^1,2; ¹Department of Radiation Oncology, National Taiwan University Cancer Center, Taipei, Taiwan, ²Division of Radiation Oncology, Department of Oncology, National Taiwan University Hospital, Taipei, Taiwan

Purpose/Objective(s): Stereotactic spine radiosurgery (SSRS) allows escalating radiation dose while sparing the surrounding organ at risks (OARs). Volumetric arc radiotherapy (VMAT) continuously modulates the beam intensity during the gantry rotation, which can achieve high dose conformity to the target and rapid dose fall-off from OARs. In conventional VMAT technique, each arc can only set up one static multileaf collimator angle, while the novel solution, volumetric modulated arc therapy (RAD), provides dynamic collimator angles during arc delivery (VMAT_P) to unlock the new degree of freedom. The aim of this study is to compare the quality and efficiency of treatment planning and delivery between VMAT and VMAT_P.

Materials/Methods: Ten patients eligible for SSRS for cervical spine metastases in a prospective clinical trial were replanned with 20 Gy in single fraction for evaluation. Two plans were generated for each case using a technology company's treatment planning system. The VMAT plan used 2 static collimator arcs, and the VMATp plan used the same number of arcs with dynamic collimator incorporating one default static modulated port. All plans were optimized to meet the criteria recommended by RTOG 0631 and normalized to 90 % of PTV receiving the prescription dose for dosimetric comparison, including target coverage, Paddick conformity index (PCI), and gradient index (GI). The treatment time, plan optimization time, and the total number of monitor units were calculated for efficiency evaluation. A two-tailed paired t-test was used to determine if dosimetric metrics or efficiency parameters were significantly (p=0.05) different between VMAT and VMAT_P.

Results: The VMAT_P plan improves the target coverage and other dosimetric metrics compared to the VMAT plan. The VMAT_P showed a trend toward an improvement in the mean dose of 95% of PTV and CTV received by 2.5% (p=0.09) and 1.3% (p=0.06), respectively. The VMAT_P plan also non-significantly reduced the mean maximal dose to 0.03 cc of the spinal cord by 3%. The VMAT_P plan provided superior PCI (VMAT_P 0.78±0.02 versus VMAT 0.75±0.04, p=0.02) and GI (VMAT_P3.26±0.21 versus VMAT 3.63±0.38, p=0.02) compared to the VMAT plan. Furthermore, the VMAT_P plan profoundly improved the efficiency of treatment planning and delivery. There is a 30% reduction in treatment delivery time (VMAT_P 5.4±0.5 mins versus VMAT 7.7±1 mins, p<0.001) and total monitor units (MUs) delivered (VMAT_P 7491±665 MUs versus VMAT 10720±1343 MUs, p<0.001) by using the VMAT_P plan and more than 80% reduction in plan optimization time (VMAT_P 1.6±0.5 mins versus VMAT 9.5±2.3 mins, p<0.001) by the RAD algorithm.

Conclusion: The RAD solution incorporates dynamic collimator angles that significantly improves the plan quality, planning efficiency, and delivery efficiency of SSRS for spine metastasis. It provided better dose conformity and fall-off for sparing normal structures without compromising the target coverage.