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Calculation Grid Size Effects On Dose Under GRID "mini" Beam in TPS

M Xu1*, (1) Northwestern Medicine Cancer Center, Geneva, IL


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: The objective of this study is to evaluate the effect of grid-size (GS) on dose and output factor (OF) calculations under GRID “mini� beam in Eclipse TPS.

Methods: The AAA algorithm in Eclipse-v13.7 was used for GRID dose calculation on a virtual water phantom. Plans with various square field size (FS) for open and GRID beams were generated for volume dose calculations with GS of 1.00, 1.75, and 2.50-mm in the FS range of 2.5 to 20-cm with 2.5-cm interval. 100-MU was delivered in each plan with the prescription dose to D�max₎ in SSD of 100-cm setup for 6 MV photon energy. Output factor was obtained by the ratio of the dose under GRID “mini� beam over the dose in 10x10 cm² open field. The OF’s in different GS calculations were compared to carefully measured OF’s.

Results: For open field, the calculated doses were not affected by GS changes within 0.21% dose variation resulting in the insensitivity of OF’s to GS changes. For GRID field, the calculated doses were varied up to 5% with different GS from 1.00 to 2.50-mm. The OF’s showed a good agreement to the measurement OF values in the FS range of 5.0 to 20.0-cm. Moreover, calculations in 1.00-mm GS showed better agreement in the measured OF’s with 2% deviation. The range of FS in 5.0 – 22.0-cm was typically used to a bulky GTV for clinical delivery with GRID “mini� beam. The “mini� beam with the hole-diameter of 1.43-cm in GRID block can behavior a small field effect without charge particle equilibrium (CPE) established.

Conclusion: This work suggests the lower GS is reliable to perform accurate planning calculation for GRID “mini� beam. The calculations with the GS of 1.00-mm tend to provide better agreement to doses and OF’s in comparison with measurements.


Absolute Dosimetry, Grids, Treatment Planning


TH- External beam- photons: Small/nonstandard field experimental dosimetry

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