Session Item

HyperArc is a relatively new VMAT non-coplanar technique that is used for stereotactic radiotherapy in brain metastasis treatment. Due to the challenges associated with high doses per fraction equal 21Gy and small-field dosimetry, it is extremely important to select an adequate radiation detector to perform dosimetric verification of treatment plan. The main aim was to consider film dosimetry (EBT-XD) as a verification method for  and patient specific QA for HyperArc treatment plans using film dosimetry. 

A group of 15 patients with one, two or three intracranial lesions were selected. Gafchromic EBT-XD (Ashland, USA) dosimetry films were used to perform patient specific QA for multimetastasis lesions with a single isocenter for the HyperArc (HA) stereotactic irradiation technique (Varian Medical Systems, USA). In order to properly prepare, detectors were calibrated (dose range 0-40 Gy) and characterized in 6 MV FFF beam. Radiochromic films were placed in a PMMA phantom, in coronal plane, in the middle of each lession. As the reference detector, the Semiflex ionization chamber (PTW, Freiburg), was selected. The films were scanned on the Epson perfection v850 PRO flatbed scanner (Seiko Epson Corporation, Japan) with 72 dpi, single scan, in red-green-blue (rbg) mode with no color or sharpness correction and consistent orientation and analyzed using gamma evaluation criteria (2%/2mm, local normalization, threshold 30%, gamma passing rates – GPR – tolerance limits> 90%, action limits>85%) in the VeriSoft program (PTW, Freiburg). 

Gafchromic EBT-XD films do not show significant dose rate dependence for 6 FFF MV beam and the obtained calibration curve for the red channel was used for the analysis. The patient specific QA results were shown in the Figure 1. Using dosimetry films, the most favorable results (GPR) were obtained for patients with one intracranial lesion (mean 98,84%, standard deviation 1,15%), three lesions (mean 98,04%, standard deviation 2,12%) and two lesions (mean 94,95%, standard deviation 4,99%) for the 2%/2mm acceptance criteria.

Figure 1. Summary of result (GPS) for gamma evaluation method for patient specific QA for the HyperArc technique using radiochromic films. The black line shows the tolerance limit > 90%.

As a result of performed verification of the feasibility of stereotactic treatment plans, acceptable results were obtained for selected dosimetric method according to the adopted analysis criteria. After proper characterization and calibration Gafchromic EBT-XD films, are a suitable tool for HyperArc verification method.