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Impact and Role of Calculated and Reconstructed Dose Grid Size On Gamma Results in Octavius 3D Dosimetry System

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M Manavalan

M Muthukumaran1*, D Manigandan2 , Vikraman3 , V Murali4 , Ganapathy Krishnan5 , S Chitra6 , K Kanakavel7 , (1) Apollo Speciality Hospitals, Chennai(2) Fortis Cancer Institute, Mohali, Punjab, (3) The Brunei Cancer Center, Brunei Darussalam, (4) Apollo Speciality Hospitals, Chennai (5) Apollo Speciality Hospitals, Chennai (6) Apollo Speciality Hospitals, Chennai (7) PTW Dosimetry India Ltd, Chennai.


SU-I-GPD-T-282 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To study the impact and influence of various grid sizes in calculated and measured(reconstructed) matrixes on gamma results in Octavius 4D system.

Methods: Twenty Rapidarc patients with different treatment sites were taken for this study. PTW Octavius 4D phantom, Octaviius 729 Array detector, PTW Verisoft software were used for this study. In Verisoft the dose can be reconstructed from the measured data with defined grid size. After measurement with Octavius 4D, dose is reconstructed for two different grid sizes - 2.5mm and 5.0mm. Similarly in TPS dose is calculated with two different gird sizes - 2.5mm and 5.0 mm. Gamma analysis is done with 3%DD and 3mm DTA with global dose and suppressing the dose by 5% for various combinations of calculated and reconstructed grid sizes. 2D gamma, 3D gamma and volume gamma were analysed for axial, coronal and sagital dose planes.

Results: Always the 3D gamma was giving better gamma results when compared with 2D gamma in all the reconstructed axail, coronal and sagital planes. 2D and 3D gamma pass in sagital and coronal dose planes were always less when compared with the gamma result for axial dose planes. However we could not establish direct correlation between calculated and reconstructed grid sizes on gamma results. There is a huge variation on the gamma results for some cases with gird sizes. Again smaller grid size does not always give better gamma pass results when compared with larger grid size.

Conclusion: The gamma result is influenced by various factors like grid size of TPS, grid size used during reconstruction of dose in phantom, expression of dose gradients in the dose distribution, location of the dose gradient especially high dose gradient in the evaluated volume, the quality of interpolation algorithm used in Verisoft and phantom setup during measurements.

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