Room: Exhibit Hall | Forum 4
Purpose: To verify that planned and delivered dose from HyperArcâ„¢ (Varian Medical Systems) treatment agrees in cases where target volume consists of multiple small parts distributed on a large area.
Methods: Nine sphere targets (diameter 1.0 cm) are placed on the central frontal plane of the plastic water phantom (IEC61217: X=30 cm x Y=30 cm x Z=10 cm). Individual spheres are set on the diagonal++ or the diagonal-+ axis with 4 cm offsets while the central target ball is at the isocentre. HyperArcâ„¢ plan is optimized to prescription dose 10 Gy (PTV(V98%)) using FFF 6 MV beam from TrueBeam treatment unit equipped with 120 leaf MLC. Two other plans are prepared for the sphere size 0.5 cm and 2.0 cm, respectively. On each plan four static MLC fields, with 5 x 5 mmÂ² openings, are added to enable digitized film image registration to collimator coordinates during image post-processing. Two-dimensional dose distribution is detected using Gafchromic EBT3 -film and Vidar Dosimetry Pro Advantage(red) -densitometer. To control random errors measurements are repeated three times. Dose calculation is made using Anisotropic Analytical Algorithm (AAA) with 1 mm grid size. Measured and calculated doses are compared on a cross shaped region of interest (bars width 6 cm, bars height 20 cm) using gamma and absolute difference analyses.
Results: For HyperArcâ„¢ plans gamma (2 mm / 2 %) pass rates are: 99.1 % (diameter 2 cm), 99.4 % (diameter 1 cm) and 99.7 % (diameter 0.5 cm) and average absolute differences are: 0.18 Gy (Ã˜ 2 cm), 0.16 Gy (Ã˜ 1 cm) and 0.12 Gy (Ã˜ 0.5 cm).
Conclusion: The agreement between planned and delivered dose distributions has been found clinically acceptable. The size of target spheres diameter down to 0.5 cm seems not to impact the level of agreement.
Funding Support, Disclosures, and Conflict of Interest: The presenting author holds a research physicist position financially supported by Varian Medical Systems Finland.
Not Applicable / None Entered.
TH- External beam- photons: Small/nonstandard field experimental dosimetry