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Effect of X-Ray Source Spot Size Modeling in AcurosXB Dose Calculation Algorithm On VMAT Based SBRT Treatments

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M Kathirvel

Kathirvel M.1*, Sai Subramanian V.S.1, Thirumalai Swamy S.1, Arun G.1, Jagadheeskumar N.1, Arivarasan I.1, Subramani V.2, (1) Yashoda Hospital, Research & Development Centre, Bharathiar University, Hyderabad, Andhra Pradesh, (2) All India Institute of Medical Sciences, New Delhi, New Delhi

SU-E-T-423 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Aim of this study is to evaluate AcurosXB(AXB) dose calculation algorithm dosimetrically with different X-ray source spot size modeling and its clinical effect on Volumetric Modulated Arc Therapy(VMAT) based Stereotactic Body Radiotherapy(SBRT).

10 SBRT cases were purposely selected to analyze the effect of source spot size for smaller field size. All the cases were planned in Varian Eclipse(V10.0) treatment planning system(TPS) which uses AXB algorithm for VMAT dose computation. For this study source spot sizes varied from 0.5mm, 1.0mm, 1.5mm and 2.0mm in X as well as Y-direction in the beam data configuration. SBRT plans were optimized using Progressive Resolution Optimizer III(PRO-III) and dose was calculated by AXB with different source spot sizes. Plan quality index such as conformity index(CI) and gradient measure for PTV and for normal tissues volume of 90% and 25% prescribed dose were used for analysis. VMAT plans were delivered with Varian Clinac-iX and 2D-planner dose was measured using iba iMatrixx 2D array in Multicube phantom. Gamma (3mm/3%) analyses were performed between measured and predicted doses for different spot sizes.

Conformity index value for PTV were found to be 1.16±0.21, 1.14±0.20, 1.13±0.2 & 1.08±0.17 and gradient measure were 1.45±0.26, 1.46±0.26, 1.47±0.25 & 1.52±0.27 for spot size of 0.5mm, 1.0mm, 1.5mm & 2.0mm respectively. Volume of normal tissue receiving 90% of prescribed dose for 2mm spot size was 2.7% more in compare to 0.5mm spot size. Gamma analysis(3mm/3%) were found to be 98.77±1.03%, 98.96±0.91%, 99.18±0.76% & 99.43±0.60% for spot size 0.5mm, 1.0mm, 1.5mm & 2.0mm respectively.

The result showed that spot size configuration of 1.5-2.0mm were matching very closely with our measurements. Based on the variation in the PTV coverage and dose to normal tissues it is highly recommended to fine tune spot size parameters for VMAT based SBRT.

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