Assessment of variation in Elekta® plastic spherical-calibration phantom and its impact on the Leksell Gamma Knife® calibration

Purpose: Traditionally, the dose-rate calibration (output) of the Leksell Gamma Knife® (LGK) unit is performed using a 160 mm diameter plastic spherical phantom provided by the vendor of the LGK, Elekta Instrument AB. The purpose of this study was to evaluate variations in the Elekta spherical phantom and to assess its impact and use for the LGK calibration. Methods: Altogether, 13 phantoms from six different centers were acquired, 10 of these phantoms were manufactured within the past 10 years and the last 3 approximately 15–20 years ago. To assess variation in phantoms, the diameter and mass densities were measured. To assess the impact on LGK calibration, the output of two models of LGK (LGK Perfexion™ and LGK 4C) were measured under identical irradiation conditions using all 13 phantoms for each LGK model. Results: The mean measured deviation in diameter from expected nominal 160 mm for 13 phantoms was 0.51 mm (range of 0.09–1.51 mm). The mean measured phantom mass density for 13 phantoms was 1.066 ± 0.019 g / cm 3 (range of 1.046 – 1.102 g / cm 3 ). The percentage deviation of output for individual phantom from mean of 13 phantom outputs ranged from −0.37% to 0.55% for LGK Perfexion™. Similarly, the percentage deviation of output for individual phantom from mean of 13 phantom outputs ranged from −0.72% to 0.47% for LGK 4C. Conclusions: This study demonstrated that small variations in terms of phantom size and mass density of the phantom material do not have a significant impact on dose-rate measurements of the Leksell Gamma Knife®. Also, date of manufacture of the phantom did not show up to be a significant factor in this study.