Jeong-Eun Rah

Feasibility study of radiophotoluminescent glass rod dosimeter postal dose intercomparison for high energy photon beam

A radiophotoluminescent glass rod dosimeter (GRD) system has recently become commercially available. In this study we evaluated whether the GRD would be suitable for external dosimetric audit program in radiotherapy. For this purpose, we introduced a methodology of the absorbed dose determination with the GRD by establishing calibration coefficient and various correction factors (non-linearity dose response, fading, energy dependence and angular dependence). A feasibility test of the GRD postal dose intercomparison was also performed for eight high photon beams by considering four radiotherapy centers in Korea. In the accuracy evaluation of the GRD dosimetry established in this study, we obtained within 1.5% agreements with the ionization chamber dosimetry for the (60)Co beam. It was also observed that, in the feasibility study, all the relative deviations were smaller than 3%. Based on these results, we believe that the new GRD system has considerable potential to be used for a postal dose audit program.

Application of a glass rod detector for the output factor measurement in the CyberKnife.

A radiophotoluminescent glass rod detector has recently become commercially available. We evaluated the feasibility of the commercial glass rod as a new detector for measuring output factors in the CyberKnife. The glass rod detector was irradiated in a water phantom using a holder stand, which was specially designed for this study. The holder was composed of a PMMA tube with an attached vertical bar for the glass rod detector. The measured output factors obtained with the glass rod detector were compared with measurements made with a pinpoint ionization chamber, a diode, and a radiochromic film. The measured relative output factors obtained with the glass rod detector agreed with other detectors within 1.0% for collimator sizes larger than 20mm. However, it was observed that the differences between the output factors measured with the glass rod detector and those obtained with the pinpoint chamber increased rapidly as the collimator size decreased. The relative output factors measured with the diode were consistently higher than those obtained using other detectors for the collimators sizes less than 10mm in diameter. The glass rod detector results were in good agreement with those obtained from the radiochromic EBT film over the entire range of collimator sizes.

Overlapped seed localization in seed implant brachytherapy

A procedure for the determination of the location of prostate implant seeds that are wholly overlapped in a projection view has been developed. The procedure mainly consists of a series of image processing and an in-house developed localization software based on a three-film technique. To verify the efficacy of the procedure, a simulation phantom was built and nine sets of simulation were performed. For the assessment of the location of the seeds in the phantom, three images, one in anterior-posterior direction and two others in oblique angles, were acquired and a series of image processing was applied to the images for the removal of unnecessary background and the improvement of imaging quality. In this study, three types were considered; first, when two seeds were overlapped in one of projection images, second, more than three seeds were overlapped in one of projection images, and the third, all images contained wholly overlapped seeds. The developed software separates wholly overlapped seeds by calculating the distance between seeds in each film. This software can provide valuable information for establishing effective quality assurance in permanent prostate brachytherapy.

Measurements of the Relative Output Factors for Radiosurgical CyberKnife Collimators Using a Glass Rod Dosimeter

The purpose of this study was to evaluate the accuracy of a glass rod dosimeter (GRD) for relative output factor measurements in the CyberKnife radiosurgery system. The output factors measured with the GRD were compared to those obtained with an ionization chamber, a diode and a Gafchromic film. The GRD was irradiated in a water phantom using an in-house custom designed holder stand. The relative output factors measured with the four dosimeters showed very similar results for collimator diameters larger than 20 mm. The mean value of the output factors of the GRD for the 5 mm collimator was 0.695. The output factor measured with the ionization chamber was approximately 13.7% lower than the corresponding GRD values for the 5 mm collimator. The diode output factors were 3.1% and 1.9% higher than the GRD for the 5 mm and 7.5 mm collimators, respectively. However, the GRD results were in agreement with the measurements obtained with a Gafchromic film for all of the collimator diameters.

SU‐E‐T‐107: Dosimetric Characteristics of a Glass Dosimeter for Proton Beam Measurements

Purpose: The purpose of this study was to investigate the dosimetric characteristics of the GD‐301 glass dosimeter for clinical dosimetry in a high‐energy proton beam and to compare it with LiF TLD‐100. Materials: The dose distribution measured with the glass dosimeter was evaluated by comparing them to those from GEANT4 MC simulation. We also compared dose measured with glass dosimeter and calculated with treatment planning system, Eclipse calculated dose of plan delivery in order to evaluate its potential as a new dosimeter in clinical proton beams. All measurements were performed in a proton beam (IBA Proton Therapy System‐Proteus 235) at the National Cancer Center in Korea. Dosimeters were irradiated in a water phantom using a stair‐like holder specially designed for this study. Maximum height was 100 mm with each step of 1 mm in ten layers. Results: Reproducibility in the 200 MeV proton beam was within 1.5% for the glass dosimeter, and within 1.7% for TLD‐chip responses. The glass dosimeter signal was linear as a function of applied dose in the range of 1—10 Gy. The fading effect of the glass dosimeter after a received dose of 2 Gy was initially found to be within 1.6% for six months. Angular dependence of the glass dosimeter was measured to be approximately 1.3% for angles ranging 80° from the beam axis using a cylindrical phantom. Depth‐dose distributions in the non‐modulated and modulated proton beams obtained with the glass dosimeter were estimated to be within 3.0% lower than those with the ionization chamber and simulation model using GEANT4 code. The difference between delivery dose calculated by the Eclipse and measured by the glass dosimeter is within 5.0%. Conclusions: Measurements comparing the glass dosimeter and TLD‐100 dosimetric characteristics demonstrated the suitability of the glass dosimeter for dose measurement in high‐energy proton beam therapy.

SU‐E‐T‐491: Accuracy of Dose Calculation in Inhomogeneity Region for Proton Radiotherapy

Purpose: The purpose of this study was to evaluate the accuracy of dose calculation in inhomogeneity region provided with Eclipse treatment planning system for proton therapy using the head and neck phantom. Method and Materials: The inhomogenous head and neck phantom for proton therapyquality assurance (QA) was housed in a custom‐designed package for efficient evaluation of the measured doses using the different materials and various dosimeters (pinpoint type ionization chamber, TLD‐ 100, glass dosimeter). All measurements were performed in clinical proton beam at National Cancer Center in Korea. The accuracy of proton therapydose calculations with pencil‐beam algorithms (Varian Eclipse) were also investigated with respect to measurements used in head and neck phantom in the presence of inhomogeneity region (PMMA, air, bony structure). Results: The differences for pencil‐beam dose calculation algorithm and measured dose with dosimeters were 3.0% in average target doses (PMMA region). However, the measured results with dosimeters were approximately 7.0% higher than the calculated dose by Eclipse in air region. Conclusion: Our results show that accurate measurements of the inhomogeneity region improve the accuracy of the dose calculation algorithms predicted by the treatment planning system.

SU‐GG‐T‐401: The Effects on Parameters of Bragg Peak Curve by Inhomogeneities Location

Purpose: To describe the effect on the Bragg peak curve by inhomogeneous materials placed in entrance Plateau and Bragg peak region, respectively. Method and Materials: We evaluated Bragg peak position, range, penumbra and doses by using the Geant4, version4.9.02, simulation toolkit. The bone and adipose tissue were inserted in entrance Plateau and Bragg peak region. Positions of inhomogeneities were selected about proximal 36% (entrance region of Plateau) and 50% (entrance region of Bragg peak) point of maximum dose. The thickness of each material was varied with range from 0.1 and 1.0 cm. And also the initial energy of proton beams was changed from 108 MeV to 220 MeV with an order of 30 MeV. Physics models of Geant4 consist of electro‐magnetic process and nuclear process. The electro‐magnetic model bases on the ICRU49 and nuclear process was applied by the theory‐driven G4PreCompound model. Results: The dose perturbation was shown in Bragg peak and inhomogeneous material regions concerning the location of inhomogeneities. There were, especially, a great differences when material of high density was inserted within homogeneous structure. However, there were not significantly variations of other evaluated parameters concerned with inserted location of each material. Conclusion: Effects of location of inhomogeneous material were principally shown to the dose variation. Our study suggests that the location of heterogeneous materials mainly affects dose variation when material of high density is inserted within homogeneous structure. These highlight the need for accurate location relations between materials with high density and target within treatment path.

TH‐C‐BRB‐04: In Vivo Measurements for Proton Therapy Using a Glass Dosimeter

Purpose: The purpose of this study was to measure the dose distributions near the superficial using a glass dosimeter for proton therapy plans with a varying separation between the target volume and the surface of 3 patients. Method and Materials: All measurements were performed in clinical proton beam at National Cancer Center in Korea. The proton therapy test cases were planned using the Varian Eclipse protontreatment planning system. In vivo surface dosimetry for proton therapy was performed for 3 patients. For all patients the surface doses were measured for delivers using dosimeters (glass dosimeter TLD) placed on the patients surface. A model GD‐301 glass dosimeter and FGD‐1000 automatic reader were used. In the one case of our patient that GTV was defined from the superficial the surface dose was 189 cGy and the case that GTV was defined by the expansion from the surface by 5 mm it was 138 cGy. Results: The results of glass dosimeter were substantially higher than the TLD result. The doses measured by the glass dosimeter indicated that the proton therapy Eclipse treatment planning system overestimates superficial doses by 7.4±1.9%. The TLD dose test case was 168±2.44 cGy as compared to the calculated dose of 190±4.54 cGy. The calculated and measured doses for the clinical cases where the target volume extends to the surface of the patient indicate that the target will be underdosed without the use of bolus. Conclusion: Given the results of the glass dosimeter and TLD‐100 measurements the calculated doses on the surface of the patient are typically overestimated between 4% and 15%. As such it is recommended that bolus be added for these clinical cases. We also believe that the glass dosimeter has considerable potential to be used for in vivo patient protondosimetry.

SU‐GG‐T‐456: Dosimetric Evaluation of Heterogeneity Correction Algorithm for Proton Therapy

Purpose: The purpose of this study was to evaluate the accuracy of heterogeneity correction algorithm provided with Eclipse treatment planning system for proton therapy using the head and neck phantom. Method and Materials: The heterogeneity head and neck phantom for proton therapyquality assurance (QA) was housed in a custom‐designed package for efficient evaluation of the measured doses using the different materials and various dosimeters (pinpoint type ionization chamber, TLD‐100, glass dosimeter). Before the measurement, we performed a dosimetric test of the glass dosimeter with respect to reproducibility, linearity, fading, angular dependence and dose‐distribution (Bragg peak curve, Spread‐out Bragg curve) for proton therapy beam. All measurements were performed in clinical proton beam at National Cancer Center in Korea. The accuracy of proton therapy dose calculations with pencil‐beam algorithms (Varian Eclipse) were also investigated with respect to measurements used in head and neck phantom in the presence of heterogeneities. Results: The differences for pencil‐beam dose calculation algorithm and measured dose with glass dosimeter was 7.0% in average target doses. Ionization chamber experiments showed approximately 5.0% better agreement than the glass dosimeter. The results of glass dosimeter were substantially higher than the TLD result. Conclusion: Our results show that accurate measurements of the heterogeneity region improve the accuracy of the dose calculation algorithms predicted by the treatment planning system. We also investigate that the glass dosimeter has considerable potential to be used for protondosimetry.