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 â€œminiâ€? 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 Dâ‚?maxâ‚Ž in SSD of 100-cm setup for 6 MV photon energy. Output factor was obtained by the ratio of the dose under GRID â€œminiâ€? beam over the dose in 10x10 cmÂ² open field. The OFâ€™s in different GS calculations were compared to carefully measured OFâ€™s.
Results: For open field, the calculated doses were not affected by GS changes within 0.21% dose variation resulting in the insensitivity of OFâ€™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 OFâ€™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 OFâ€™s with 2% deviation. The range of FS in 5.0 â€“ 22.0-cm was typically used to a bulky GTV for clinical delivery with GRID â€œminiâ€? beam. The â€œminiâ€? 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 â€œminiâ€? beam. The calculations with the GS of 1.00-mm tend to provide better agreement to doses and OFâ€™s in comparison with measurements.