A novel and effective method for validation and measurement of output factors for Leksell Gamma Knife® Icon™ using TRS 483 protocol

Abstract

The objective of this work was to identify the exact location of the effective point of measurement (EPM) of four different active detectors to compare the relative collimator output factors (ROF) of Leksell Gamma Knife (LGK) according to IAEA TRS‐483 recommendations. ROF was measured at the center of the spherical LGK‐Solid Water (LGK‐SW) Phantom for three (4‐, 8‐, and 16‐mm in diameter) collimators using four (PTW‐TN60008, PTW‐TN60016, PTW‐TN60017, and PTW‐60019 diode/diamond) detectors. Since diode detectors have a much smaller sensitive volume than the PTW‐31010 ion chamber used for reference dosimetry, its EPM might not be at the center of the phantom, or (100, 100, 100) of the Leksell Coordinate System, particularly in the z‐ direction. Hence for each diode detector, a CBCT image was acquired after it was inserted into the phantom, from which the z‐Leksell coordinate of EPM was determined. Relative collimator output factors was then measured by focusing GK beams on the determined EPM of each diode. Measured ROFs were compared with the vendor‐provided values in GK treatment planning system. For validation, a plan was generated to measure the output of 4‐mm collimator for PTW‐TN60017 at various couch locations along the z‐axis. For PTW‐TN60008, the percentage variations were 0.6 ± 0.4%, and −0.8 ± 0.2% for 4 and 8‐mm collimators, respectively. For PTW‐TN60016, the percentage variations were 0.8 ± 0.0%, and 0.2 ± 0.1%, respectively. The percentage variations were −3.3 ± 0.0% and −0.9 ± 0.1%, respectively, for PTW‐TN60017, and −0.5 ± 0.0% and −0.8 ± 0.2%, respectively, for PTW‐TN60019. Center of the measured profile for PTW‐TN60017 was only 0.1 mm different from that identified using the CBCT. In conclusion, we have developed a simple and effective method to determine the EPMs of diode detectors when inserted into the existing LGK‐SW phantom. With the acquired positional information and using TRS‐483 protocol, good agreements were obtained between the measured ROFs and manufacturer recommended values.

Feasibility of isodose‐shaped scintillation detectors for the measurement of gamma knife output factors

“…The OF 4 value obtained using EBT films agreed with the LGP value (Figure 8). Cyriac et al measured OF 4 using T60019 and the TRS-483 OCFs and reported a value of 0.810 (0.000), 25 which is close to the LGP value. However, this study, as well as those that used LIC and solid-state detectors, as shown in Figure 8, suggested a higher OF 4 value of approximately 0.826.…”

Replacing gamma knife beam‐profiles on film with point‐detector scans

“…The coordinate of the RFP was defined as the midpoint between the half‐maximum of the rising and of the falling edge in the respective dose profiles. The PPS center for the scintillator and microdiamond was corrected for detector positioning errors based on CBCT imaging, as suggested by a recent publication 18 …”

“…The PPS center for the scintillator and microdiamond was corrected for detector positioning errors based on CBCT imaging, as suggested by a recent publication. 18…”

Determination of volume averaging correction factors using an elliptical absorbed dose model for Gamma Knife Perfexion

“…For small field dosimetry, it is crucial to determine the field output correction factor $$k_{{Q}_{msr},\ {Q}_{clin}\ }^{{f}_{msr},\ {f}_{clin}}$$ to be able to correct the detector's reading and account for the abovementioned nonequilibrium conditions in machine‐specific reference f msr (msr) and clinical fields f clin (clin) with beam qualities Q msr and Q clin . Together with the field output factor (OF) $${{\Omega}}_{{Q}_{msr},\ {Q}_{clin}}^{{f}_{msr},{f}_{clin}}$$ determination, these were the focus of investigation performed by different groups 2–11 . OF and field output correction factor determination for different detectors can be performed by measurements or with Monte Carlo (MC) simulations.…”

“…Together with the field output factor (OF) determination, these were the focus of investigation performed by different groups. 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 OF and field output correction factor determination for different detectors can be performed by measurements or with Monte Carlo (MC) simulations. In general, field output correction factors are a product of four different perturbations in the small field; ratios of water‐to‐detector‐medium stopping powers in the clinical and machine‐specific reference field, volume averaging, fluence, and of spectral perturbations in clinical and machine‐specific reference fields.…”