D Shin

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.

Risk of second cancer from scattered radiation of intensity-modulated radiotherapies with lung cancer

PurposeTo compare the risk of secondary cancer from scattered and leakage doses following intensity-modulated radiotherapy (IMRT), volumetric arc therapy (VMAT) and tomotherapy (TOMO) in patients with lung cancer.MethodsIMRT, VMAT and TOMO were planned for five lung cancer patients. Organ equivalent doses (OEDs) are estimated from the measured corresponding secondary doses during irradiation at various points 20 to 80 cm from the iso-center by using radio-photoluminescence glass dosimeter (RPLGD).ResultsThe secondary dose per Gy from IMRT, VMAT and TOMO for lung cancer, measured 20 to 80 cm from the iso-center, are 0.02~2.03, 0.03~1.35 and 0.04~0.46 cGy, respectively. The mean values of relative OED of secondary dose of VMAT and TOMO, which is normalized by IMRT, ranged between 88.63% and 41.59% revealing 88.63% and 41.59% for thyroid, 82.33% and 41.85% for pancreas, 77.97% and 49.41% for bowel, 73.42% and 72.55% for rectum, 74.16% and 81.51% for prostate. The secondary dose and OED from TOMO became similar to those from IMRT and VMAT as the distance from the field edge increased.ConclusionsOED based estimation suggests that the secondary cancer risk from TOMO is less than or comparable to the risks from conventional IMRT and VMAT.

SU-E-T-208: Incidence Cancer Risk From the Radiation Treatment for Acoustic Neuroma Patient

PURPOSEnThe present study aimed to compare the incidence risk of a secondary cancer from therapeutic doses in patients receiving intensitymodulated radiotherapy (IMRT), volumetric modulated arc therapy (VMAT), and stereotactic radiosurgery (SRS).nnnMETHODSnFour acoustic neuroma patients were treated with IMRT, VMAT, or SRS. Their incidnece excess relative risk (ERR), excess absolute risk (EAR), and lifetime attributable risk (LAR) were estimated using the corresponding therapeutic doses measured at various organs by radio-photoluminescence glass dosimeters (RPLGD) placed inside a humanoid phantom.nnnRESULTSnWhen a prescription dose was delivered in the planning target volume of the 4 patients, the average organ equivalent doses (OED) at the thyroid, lung, normal liver, colon, bladder, prostate (or ovary), and rectum were measured. The OED decreased as the distance from the primary beam increased. The thyroid received the highest OED compared to other organs. A LAR were estimated that more than 0.03% of AN patients would get radiation-induced cancer.nnnCONCLUSIONnThe tyroid was highest radiation-induced cancer risk after radiation treatment for AN. We found that LAR can be increased by the transmitted dose from the primary beam. No modality-specific difference in radiation-induced cancer risk was observed in our study.

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.

Characteristics of fiber-optic radiation sensor for passive scattering proton beams

The aims of this study were to investigate the characteristics of a fiber-optic radiation sensor (FORS) that detects the fluorescence light produced by proton beam and to verify its effectiveness in proton therapy quality assurance (QA). Various characteristics of the FORS were investigated, such as the linearity of its relationships to the sensitive length of fiber for the proton beams of intermediate ranges (165.46 and 178.37 MeV) and to the measured dose, as well as its dose rate dependence. In addition, patient specific precription dose QA was conducted for five patients actually undergoing proton therapy and the results were compared with the doses measured using an ion chamber. The results show that the signal of the FORS is linearly related to the sensitive length of fiber and to the irradiated dose in the range from 1 to 500 cGy. The QA results obtained using the FORS system showed good agreement with the corresponding ion chamber results, with an average difference of 0.40% and a standard deviation of 0.35%. The FORS was dose-rate independent for proton currents up to 5 Gy/min. The profiles of various proton beams obtained using an array of FORS, which were measured as an application of the developed dosimetric system, closely agreed with the profiles acquired using EBT3 film. In summary, the experimental results of FORS demonstrated its effectiveness for use in various proton therapy QA tests.

SU-E-T-760: Tolerance Design for Site-Specific Range in Proton Patient QA Process Using the Six Sigma Model

Purpose: To show how tolerance design and tolerancing approaches can be used to predict and improve the site-specific range in patient QA process in implementing the Six Sigma. Methods: In this study, patient QA plans were selected according to 6 site-treatment groups: head &neck (94 cases), spine (76 cases), lung (89 cases), liver (53 cases), pancreas (55 cases), and prostate (121 cases), treated between 2007 and 2013. We evaluated a model of the Six Sigma that determines allowable deviations in design parameters and process variables in patient-specific QA, where possible, tolerance may be loosened, then customized if it necessary to meet the functional requirements. A Six Sigma problem-solving methodology is known as DMAIC phases, which are used stand for: Define a problem or improvement opportunity, Measure process performance, Analyze the process to determine the root causes of poor performance, Improve the process by fixing root causes, Control the improved process to hold the gains. Results: The process capability for patient-specific range QA is 0.65 with only ±1 mm of tolerance criteria. Our results suggested the tolerance level of ±2–3 mm for prostate and liver cases and ±5 mm for lung cases. We found that customized tolerance between calculated and measured range reduce that patient QA plan failure and almost all sites had failure rates less than 1%. The average QA time also improved from 2 hr to less than 1 hr for all including planning and converting process, depth-dose measurement and evaluation. Conclusion: The objective of tolerance design is to achieve optimization beyond that obtained through QA process improvement and statistical analysis function detailing to implement a Six Sigma capable design.

SU-E-CAMPUS-T-04: Statistical Process Control for Patient-Specific QA in Proton Beams

PURPOSEnTo evaluate and improve the reliability of proton QA process, to provide an optimal customized level using the statistical process control (SPC) methodology. The aim is then to suggest the suitable guidelines for patient-specific QA process.nnnMETHODSnWe investigated the constancy of the dose output and range to see whether it was within the tolerance level of daily QA process. This study analyzed the difference between the measured and calculated ranges along the central axis to suggest the suitable guidelines for patient-specific QA in proton beam by using process capability indices. In this study, patient QA plans were classified into 6 treatment sites: head & neck (41 cases), spinal cord (29 cases), lung (28 cases), liver (30 cases), pancreas (26 cases), and prostate (24 cases).nnnRESULTSnThe deviations for the dose output and range of daily QA process were ±0.84% and ±019%, respectively. Our results show that the patient-specific range measurements are capable at a specification limit of ±2% in all treatment sites except spinal cord cases. In spinal cord cases, comparison of process capability indices (Cp, Cpm, Cpk ≥1, but Cpmk ≤1) indicated that the process is capable, but not centered, the process mean deviates from its target value. The UCL (upper control limit), CL (center line) and LCL (lower control limit) for spinal cord cases were 1.37%, -0.27% and -1.89%, respectively. On the other hands, the range differences in prostate cases were good agreement between calculated and measured values. The UCL, CL and LCL for prostate cases were 0.57%, -0.11% and -0.78%, respectively.nnnCONCLUSIONnSPC methodology has potential as a useful tool to customize an optimal tolerance levels and to suggest the suitable guidelines for patient-specific QA in clinical proton beam.

SU-E-T-267: Preliminary Study of Development of the Respiratory Training System by Using the MEMS Acceleration Sensor for the Radiation Therapy

Purpose: In this study, we developed and estimated the home‐made patient‐respiration‐training‐system (PRTS) by using the micro‐electromechanical‐system (MEMS) which can help to avoid the problems for the limitation of the RGRT applicable patient cases. Methods: We compared the response of the PRTS with period of real movement and commercially introduced real time patient monitoring (RPM) system. We measured the response of the PRTS by using the respiratory gating platform (RGP) for a 2.5, 3.0 and 3.5 second of the period and a 20, 30, 40 mm of the amplitude. Under the same condition, we measured the response of the RPM system by using the RGP. Results: The period error from the PRST was measured as 0.6∼6.0% compared with period of real movement. Considering the overall shape of the graphs, the uncertainty of the signal of the RPM system measured about 1% in an amplitude for each interval, on the other hand, MEMS signals had 3∼11% of the error in amplitude. Conclusion: In term of simplicity and convenience, we found the possibility of PRTS as the patient training system and we expect more detail of study for more realistic situation can be established, in near future.

SU-E-T-459: Measurement of Gated Rapidarc Accuracy

Purpose: Recently, Kyung Hee university hospital at Gangdong installed the Varian 21iX linac which has the volumetric imrt (RapidArc) option and a real‐time position management (RPM) system. One of advantage of rapidarc is a short treatment time comparing with other treatments. Thus, the rapidarc treatment with the RPM system for the lung and liver cases, can reduce the treatment time comparing with the intensity modulation radiotherapy(IMRT) and 3 dimensional radiotherapy (3DRT) with gating system. Therefore, we estimate the accuracy of the gated rapidarc treatment. Methods: 10 patients data are used for this study. Each patient was planned by rapidarc technique using varian ECLISPE v8.6 planning machine. For the patient Quality Assurence (QA), a MATRIXX detector and IM RT software are used. At bottom and top of MATRIXX detector, 5×30×30cm3 of solid water phantoms are install for build up. A MATRIXX detector is fixed but the infrared reflecting box is periodically moved to provide the gating signal to the RPM system. Each patients dosimetric distribution is measured and the treatment summary which is provided by linac operating system is recoded to estimate the accuracy of the gated rapidarc treatment. Results: The dosimetric comparison between the measurement and the treatment plan is matched well for the 10 patients. With 3% of delta dose and 3mm of delta distance, more than 98% of area gives the gamma value as less than 1. Conclusions: We found that the patient dose measurement is matched well with the RTP plans dose distribution for the gated rapidarc treatment cases in the mechanical point of view. This study shows the gated rapidarcs dose reproducibility during the patient QA but this result has a limitation for the dose confirming for the real patients which has more soft and flexible body than the solid phantom.