Comparison of Dosimetric Performance among Commercial Quality Assurance Systems for Verifying Pretreatment Plans of Stereotactic Body Radiotherapy Using Flattening-Filter-Free Beams

The purpose of this study was to compare the performance of different commercial quality assurance (QA) systems for the pretreatment verification plan of stereotactic body radiotherapy (SBRT) with volumetric arc therapy (VMAT) technique using a flattening-filter-free beam. The verification for 20 pretreatment cancer patients (seven lung, six spine, and seven prostate cancers) were tested using three QA systems (EBT3 film, I’mRT MatriXX array, and MapCHECK). All the SBRT-VMAT plans were optimized in the Eclipse (version 11.0.34) treatment planning system (TPS) using the Acuros XB dose calculation algorithm and were delivered to the Varian TrueBeam® accelerator equipped with a high-definition multileaf collimator. Gamma agreement evaluation was analyzed with the criteria of 2% dose difference and 2 mm distance to agreement (2%/2 mm) or 3%/3 mm. The highest passing rate (99.1% for 3%/3 mm) was observed on the MapCHECK system while the lowest passing rate was obtained on the film. The pretreatment verification results depend on the QA systems, treatment sites, and delivery beam energies. However, the delivery QA results for all QA systems based on the TPS calculation showed a good agreement of more than 90% for both the criteria. It is concluded that the three 2D QA systems have sufficient potential for pretreatment verification of the SBRT-VMAT plan.

Efficiency Study of 2D Diode Array Detector for IMRT Quality Assurance / 의학물리

In this study, we evaluated the effect of grid size on dose calculation accuracy using 2 head & neck and 2 prostate IMRT cases and based on this study's findings, we also evaluated the efficiency of a 2D diode array detector for IMRT quality assurance. Dose distributions of four IMRT plan data were calculated at four calculation grid sizes (1.25, 2.5, 5, and 10 mm) and the calculated dose distributions were compared with measured dose distributions using 2D diode array detector. Although there was no obvious difference in pass rate of gamma analysis with 3 mm/3% acceptance criteria for the others except 10 mm grid size, we found that the pass rates of 2.5, 5 and 10 mm grid size were decreased 5%, 20% and 31.53% respectively according to the application of the fine acceptance criteria, 3 mm/3%, 2 mm/2% and 1 mm/1%. The calculation time were about 11.5 min, 4.77 min, 2.95 min, and 11.5 min at 1.25, 2.5, 5, and 10 mm, respectively and as the grid size increased to double, the calculation time decreased about one-half. The grid size effect was observed more clearly in the high gradient area than the low gradient area. In conclusion, 2.5 mm grid size is considered acceptable for most IMRT plans but at least in the high gradient area, 1.25 mm grid size is required to accurately predict the dose distribution. These results are exactly same as the precious studies' results and theory. So we confirmed that 2D array diode detector was suitable for the IMRT QA.

PTV Margins for Prostate Treatments with an Endorectal Balloon / 대한방사선종양학회지

PURPOSE: To determine the appropriate prostate planning target volume (PTV) margins for 3-dimensitional (3D) conformal radiotherapy (CRT) and intensity-modulated radiation therapy (IMRT) patients treated with an endorectal balloon (ERB) under our institutional treatment condition. MATERIALS AND METHODS: Patients were treated in the supine position. An ERB was inserted into the rectum with 70 cc air prior to planning a CT scan and then each treatment fraction. Electronic portal images (EPIs) and digital reconstructed radiographs (DRR) of planning CT images were used to evaluate inter-fractional patient's setup and ERB errors. To register both image sets, we developed an in-house program written in visual C++. A new method to determine prostate PTV margins with an ERB was developed by using the common method. RESULTS: The mean value of patient setup errors was within 1 mm in all directions. The ERB inter-fractional errors in the superior-inferior (SI) and anterior-posterior (AP) directions were larger than in the left-right (LR) direction. The calculated 1D symmetric PTV margins were 3.0 mm, 8.2 mm, and 8.5 mm for 3D CRT and 4.1 mm, 7.9 mm, and 10.3 mm for IMRT in LR, SI, and AP, respectively according to the new method including ERB random errors. CONCLUSION: The ERB random error contributes to the deformation of the prostate, which affects the original treatment planning. Thus, a new PTV margin method includes dose blurring effects of ERB. The correction of ERB systematic error is a prerequisite since the new method only accounts for ERB random error.

Development of Phantom and Comparison Analysis for Performance Characteristics of MOSFET Dosimeter / 의학물리

This study is to develope a phantom for MOSFET (Metal Oxide Semiconductors Field Effect Transistors) dosimetry and compare the dosimetric properties of standard MOSFET and microMOSFET with the phantom. In this study, the developed phantom have two shape: one is the shape of semi-sphere with 10 cm diameters and the other one is the flat slab of 30 cm x 30 cm with 1 cm thickness. The slab phantom was used for calibration and characterization measurements of reproducibility, linearity and dose rate dependency. The semi-sphere phantom was used for angular and directional dependence on the types of MOSFETs. The measurements were conducted under 10 x 10 cm2 fields at 100 cm SSD with 6 MV photon of Clinac (21EX, Varian, USA). For calibration and reproducibility, five standard MOSFETs and microMOSFETs were repeatedly irradiated by 200 cGy five times. The average calibration factor was a range of 1.09+/-0.01~1.12+/-0.02 mV/cGy for standard MOSFETs and 2.81+/-0.03~2.85+/-0.04 mV/cGy for microMOSFETs. The response of reproducibility in the two types of MOSFETs was found to be maximum 2% variation. Dose linearity was evaluated in the range of 5 to 600 cGy and showed good linear response with R2 value of 0.997 and 0.999. The dose rate dependence of standard MOSFET and microMOSFET was within 1% for 200 cGy from 100 to 600 MU/min. For linearity, reproducibility and calibration factor, two types of MOSFETs showed similar results. On the other hand, the standard MOSFET and microMOSFET were found to be remarkable difference in angular and directional dependence. The measured angular dependence of standard MOSFET and microMOSFET was also found to be the variation of 13%, 10% and standard deviation of +/-4.4%, +/-2.1%. The directional dependence was found to be the variation of 5%, 2% and standard deviation of +/-2.1%, +/-1.5%. Therefore, dose verification of radiation therapy used multidirectional X-ray beam treatments allows for better the use of microMOSFET which has a reduced angular and directional dependence than that of standard MOSFET.

Comparison of Enhanced Dynamic Wedge with Physical Metal Wedge based on the Basic Dosimetric Parameters / 의학물리

For clinical implementation of Enhanced Dynamic Wedge (EDW), it is necessary to adequately analyze and commission its dosimetric properties in comparison to common physical metal wedge (MTW). This study was implemented with the essential measurements of parameters for clinical application, such as percentage depth dose, peripheral dose, surface dose, effective wedge factor, and wedge profile. In addition, through the comparison study of EDW with open and MTW, the analysis was performed to characterize the EDW. We also compared EDW dose profiles of measured values using chamber array 24 (CA24) with calculated values using radiation treatment planning system. PDDs of EDW showed good agreements between 0.2~0.5% of open beam, but 2% differences with MTW. In the result of the measurements of peripheral dose, it was shown that MTW was about 1% higher than open field and EDW. The surface doses of 60degrees MTW showed 10% lower than the others. We found that effective wedge factor of EDW had linear relationships according to Y jaw sizes and was independent of X jaw sizes and was independent of X jaw sizes and asymmetric Y jaw opening. In comparison with measured values and calculate values from Golden-STT based radiation treatment planning system (RTP system), it showed very good agreement within difference of 1%. It could be concluded that EDW is a very reliable and useful tool as a beam modification substitute for conventional MTW.