(Saturday, 4/7/2018)
Room: Foyer
Purpose: Single-isocenter stereotactic radiosurgery (SRS) technique for multiple brain metastases is gaining popularity due to its efficiency. It is prudent to apply the IMRT-style pre-treatment dosimetric QA to all of these complex plans. The challenge is the small targets with sometimes substantial distance between them. This study is aimed to validate a commercial hybrid measurement/calculation EPID – based system (PerFraction from Sun Nuclear Corp.) for dose verification of single-isocenter treatments.
Methods: The system combines MLC apertures captured by an EPID with all other time-resolved accelerator parameters extracted from the log files. The fluence is fed into a GPU-based superposition algorithm for dose calculation on a patient CT. Three SRS plans contained 3 to 5 targets ranging from 0.75 to 3.6 cm in diameter. The non-coplanar VMAT plans were optimized using a standard SRS protocol on a cylindrical PMMA phantom with a film insert, and delivered on a Varaian TrueBeam accelerator equipped with a standard Millennium 120-leaf MLC. The phantom was aligned by CBCT and at least two planes per plan were measured using calibrated EBT-XD radiochromic films. Film measurements were scaled to absolute dose using the ion chamber measurement in one of the PTVs strategically placed at the center of the phantom. The films were processed in RIT113 v 6.6 software and digital γ analysis was performed with 3%/1mm and 2%/2mm criteria (global normalization and 10% of max low-dose cutoff), and filter level 3.
Results: The PerFraction calculations were in good agreement with the film measurements. The average γ-analysis passing rates were 97.45% ± 4.28% and 96.67% ± 4.29% for 3%/1mm and 2%/2mm respectively. Equally importantly, the distance to agreement between the planned and reconstructed dose profiles in the target regions was sub-millimeter.
Conclusion: PerFraction is a feasible instrument for the pre-treatment QA of single isocenter multi-metastasis SRS treatments.
Funding Support, Disclosures, and Conflict of Interest: The work was supported in part by a grant from Sun Nuclear Corporation. SA is a graduate student, JK is an employee at SNC and VF is the PI. GZ and EM has no disclosure and conflict of interest.
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