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Dosimetric Evaluation of Small Fields for SBRT Treatment

R Gupta

R Gupta*, A Eldib , B Wang , C Ma , J Li , Fox Chase Cancer Center, Philadelphia, PA


SU-E-T-323 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:Stereotactic body radiation therapy (SBRT) is commonly employed to treat small targets for effective tumor control with radiation beams of small field sizes. The goal of this work was to evaluate dosimetrically a treatment planning system (TPS) by comparing the calculated dose for SBRT treatment with ion-chamber measurements.

Methods:3D images of a solid-water phantom with a pinpoint ion-chamber (0.015cm3) inside were acquired with a CT scanner. Active volume of the ion-chamber was delineated on CT images. Targets with a diameter of 1.5cm, 2cm, 3cm, 4cm and 5cm were drawn around the chamber. 3DCRT plans were generated for each target size with centrally opened 6MV beams and off-axis beams by changing the isocenter location, respectively, using a TPS with the Analytical Anisotropic Algorithm. A 21iX linear accelerator was employed for plan delivery. The measured and calculated doses were compared. To evaluate the dose calculations in heterogeneity for small fields SBRT treatment, similar plans were also generated and delivered on a heterogeneous thoracic phantom for 5 different size targets in the lung.

Results:Dose comparisons between measurements and calculations showed 5.2%, 1.88%, 1.34%, 1.01% and 0.85% difference for SBRT plans with small central axis beams and 0.96%, 0.15%, 0.58%, 0.22% and 0.77% difference for plans with off-axis beams for five different size targets. For the thoracic phantom, the differences on dose between measurements and calculations are bigger, which are 8%, 5.9%, 4.5%, 3.9% and 4.5%, respectively.

Conclusion:Dose verification for small fields used in the SBRT treatment has been performed based on ion-chamber measurements in both homogenous and heterogeneous phantoms. More than a 5% difference has been observed in the heterogeneous phantom, especially for very small fields. To meet the ICRU recommendation on a dose difference of no more than 5%, some corrections on the commissioning parameters of the TPS are needed.

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