Session Item

Octavius SRS (PTW) phantom is specially designed and dedicated to using with Octavius SRS 1000 matrix (PTW). Some hospitals were equipped with a new type of matrix for pre-treatment verification of stereotactic (SRS) plans but did not purchase a new phantom. The study aimed to estimate the difference between the results of gamma analysis for stereotactic plans measured with the matrix in the case of using a different phantom than the phantom dedicated to this matrix. As part of the study, the impact of phantom selection on the results of gamma analysis of SRS plans measured with the Octavius SRS 1000 matrix was assessed using the OCTAVIUS phantom in the standard and SRS version.

The modular phantom of the OCTAVIUS 4D system (PTW) may be used in two, independent configurations: as a standard phantom with 32 cm diameter or SRS phantom with 17 cm diameter. The Octavius 4D phantom in the SRS configuration is dedicated to the Octavius SRS 1000 (PTW) matrix. Both phantom configurations are made of 1.05 g/cm3 polystyrene. The stereotactic VMAT plans (6X photon beam) for five patients (12 arcs) were prepared. Verification measurements were performed using Versa HD accelerator (Elekta). For each arc, the Octavius SRS 1000 (PTW) matrix measurement was performed twice: in the standard and dedicated phantom for the 1000 SRS matrix. The measured dose distributions for all arcs were compared in the Verisoft v.7.1 (PTW) software with the TPS Monaco v.5.2 (Elekta) calculations. The Verisoft program has The volume analysis option.for gamma analysis parameters: 3mm / 3% and 2mm / 2% (max. and local dose), with 5% threshold was used.

The results of gamma analysis expressed in gamma passing rate (GPR) for measurements performed in the OCTAVIUS phantom in the standard and SRS configuration for parameters 3mm / 3% (max. and local dose) are not lower than 98.1%. For 2mm / 2% gamma analysis parameters (max. and local dose), GPR results do not reach value lower than 93.1% (Tab. 1). Maximum GPR differences for individual distributions do not exceed 0.8 percentage point (pp) for 3mm / 3% (max. and local dose) and 4.3 pp for 2mm / 2% (max. and local lose).

The results of gamma analysis for 3mm / 3% (both max. and local dose) parameters do not show a significant difference between the measurement performed in standard and dedicated to measuring stereotactic plans phantoms. In the absence of a phantom dedicated to measurements with the Octavius SRS 1000 matrix, measurements may be performed in a standard configuration or in older (not modular) Octavius phantom, which shape, dimensions, and type of material identical with the modular phantom in standard configuration