Kook Jin Chun

Determination of the Absorbed Dose Rate to Water for the 18-mm Helmet of a Gamma Knife

PURPOSE To measure the absorbed dose rate to water of (60)Co gamma rays of a Gamma Knife Model C using water-filled phantoms (WFP). METHODS AND MATERIALS Spherical WFP with an equivalent water depth of 5, 7, 8, and 9 cm were constructed. The dose rates at the center of an 18-mm helmet were measured in an 8-cm WFP (WFP-3) and two plastic phantoms. Two independent measurement systems were used: one was calibrated to an air kerma (Set I) and the other was calibrated to the absorbed dose to water (Set II). The dose rates of WFP-3 and the plastic phantoms were converted to dose rates for an 8-cm water depth using the attenuation coefficient and the equivalent water depths. RESULTS The dose rate measured at the center of WFP-3 using Set II was 2.2% and 1.0% higher than dose rates measured at the center of the two plastic phantoms. The measured effective attenuation coefficient of Gamma Knife photon beam in WFPs was 0.0621 cm(-1). After attenuation correction, the difference between the dose rate at an 8-cm water depth measured in WFP-3 and dose rates in the plastic phantoms was smaller than the uncertainty of the measurements. CONCLUSIONS Systematic errors related to the characteristics of the phantom materials in the dose rate measurement of a Gamma Knife need to be corrected for. Correction of the dose rate using an equivalent water depth and attenuation provided results that were more consistent.

Ischemia-modified albumin (IMA) is not useful for detecting myocardial ischemia during symptom-limited exercise stress tests

Background/Aims We examined the ischemia-modified albumin (IMA) level during exercise in patients with coronary artery disease (CAD). Methods Forty patients with a history of chest pain underwent both symptom-limited treadmill exercise stress testing and coronary angiography within one week. During the treadmill tests, blood samples were obtained at baseline and 5 min after exercise to measure the serum IMA level. Results Of the 40 patients, fourteen (35%, CAD group) had significant coronary artery stenosis, while the other 26 (65%, non-CAD group) did not. The baseline and post-exercise IMA levels in the two groups did not differ significantly (105.2±7.2 vs. 107.7±6.7 U/mL at baseline and 93.1±10.1 vs. 94.8±5.7 U/mL at post-exercise in the CAD and non-CAD groups, p=0.29 and 0.57, respectively). The changes in IMA after exercise did not differ either (-10.4±7.5 vs. -14.0±7.6 U/mL in the CAD and non-CAD groups, respectively, p=0.10). Similarly, the change in IMA between the exercise ECG test positive (TMT positive, n=9) and negative (TMT negative, n=20) groups did not differ (-14.63±5.19, vs -8.50±9.01 U/mL, p=0.15, in the TMT positive and negative groups, respectively). Conclusions Our results suggest that IMA has limitation in detecting myocardial ischemia during symptom-limited exercise stress tests.

Assessment of image co-registration accuracy for frameless gamma knife surgery

Image co-registration is used in frameless gamma knife radiosurgery (GKSRS) to assign a stereotactic coordinate system and verify patient setup before irradiation. The accuracy of co-registration with cone beam computed tomography (CBCT) images of a Gamma Knife IconTM (GK Icon) was assessed, and the effects of the region of co-registration (ROC) were studied. CBCT-to-CBCT co-registration is used for patient setup verification, and its accuracy was examined by co-registering CBCT images taken at various configurations with a reference CBCT series. The accuracy of stereotactic coordinate assignment was investigated by co-registering stereotactic CT images with CBCT images taken at various configurations. An anthropomorphic phantom was used, and the coordinates of fifteen landmarks inside the phantom were measured. The co-registration accuracy between stereotactic magnetic resonance (MR) and CBCT images was evaluated using images from forty-one patients. The positions of the anterior and posterior commissures were measured in both a fiducial marker-based system and a co-registered system. To assess the effects of MR image distortions, co-registration was performed with four different ranges, and the accuracy of the results was compared. Co-registration between CBCT images gave a mean three-dimensional deviation of 0.2 ± 0.1 mm. The co-registration of stereotactic CT images with CBCT images produced a mean deviation of 0.5 ± 0.2 mm. The co-registration of MR images with CBCT images resulted in the smallest three-dimensional difference (0.8 ± 0.3 mm) when a co-registration region covering the skull base area was applied. The image co-registration errors in frameless GKSRS were similar to the imaging errors of frame-based GKSRS. The lower portion of the patient’s head, including the base of the skull, is recommended for the ROC.

A Numerical Approach to Dose Optimization for Moving Targets Using Monte Carlo Simulations

Abstract Chun, K. J., Park, Y., Choi, Y. and Hyun, S. A Numerical Approach to Dose Optimization for Moving Targets Using Monte Carlo Simulations. Radiat. Res. 171, 245–253 (2009). A novel simulation model for the dose distribution of moving targets for high-energy photons was analyzed using the EGSnrc Monte Carlo simulation. We provide here a fundamental numerical framework for the calculation of doses delivered to moving tissues in respiratory systems with improved accuracy. A spherical object with periodic motions inside a water phantom irradiated with incident photons was taken into consideration. The dose distributions of the target and its surrounding region were calculated for a variety of radiation conditions such as photon energy, beam numbers, and beam orientations as well as the target motions determined by realistic respiratory patterns. To determine the optimal dose, two parameters, the average absorbed dose ratio and dose deviation, were newly defined for the moving targets in the phantom. Optimal conditions were examined for treatment planning on tumors in motions based on the defined parameters. We found that the actual doses delivered to the tumor generally were not correlated to the respiratory patterns. Our quantitative assessment suggests useful guidelines for improved clinical radiotherapy to escalate dose concentration in the tumors by using multiple photon beams.