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.

Keywords: TRS-483; diode detectors; effective point of measurement; relative output factors; small field dosimetry.

Fig. 1

Setup for the measurement of relative output factors in the Gamma Knife Icon at the Centre of Advanced Medicine of Northwell Health. The diode detectors were inserted into the Leksell Solid Water (SW) phantom to measure the ionization at the center of the detector for the 16 mm, 8‐mm and 4‐mm shots.

Fig. 2

Illustrations of the four detectors used in this study: (a) PTW‐TN60008 diode detector, (b) PTW‐TN60016 diode detector, (b) PTW‐TN60017 diode detector, and (d) PTW‐60019 diamond detector. These four detectors have the‐ same outer geometry (diameter 7 mm) but different internal structures. Also shown in (e) is the PTW‐31010 ionization chamber used for the reference dosimetry, which has almost the same outer geometry (diameter 6.9 mm). Therefore, the holder drilled for PTW‐31010 ionization chamber can potentially be used for the relative dosimetry measurements using the above four detectors if the effective point of measurement (EPM) of each detector can be correctly identified.

Fig. 3

Axial (left), sagittal (right upper), and coronal (right left) views of (a) the CBCT scan of the PTW‐TN60017 diode detector. The yellow line in each panel marks the most superior part of the detector that can be identified from the scan. Also shown in (b)–(d) are the sagittal (upper) and coronal (lower) views of the other three diode detector.

Fig. 4

Relative output of the 4‐mm shot at different location along its z‐axis, measured using the PTW‐TN60017 diode detector. The 50% relative output (green box) occurs at z = 95.2 mm and 100.2 mm, so the center of the measured profile is located at (95.2 + 100.2)/2 = 97.7 mm, which is 0.1 mm different from the identified location (z_eff = 97.8 mm, red line) of the sensitive volume for this diode.

Fig. 5

Illustration of the identification of effective point of measurement (EPM) using the CBCT images of the PTW‐60017 diode detector: (a) sagittal and (b) coronal view. The yellow lines mark the Leksell coordinator z = 96.5 (mm) of the tip of detector in the z‐ or superior‐inferior direction. The red lines indicate the location of EPM. Since the EPM is 1.3 mm inferior from the tip for this diode (Table 1), z_eff = 96.5 + 1.3 = 97.8 (mm) is the z‐ Leksell coordinate of the sensitive volume in the z direction.

Fig. 6

Relative out factors (ROFs) measured with different diodes on a Leksell Gamma Knife® Icon™. ROF is equal to 1 for the 16‐mm collimator size as all measured outputs were normalized to that of the 16‐mm collimator.