J Lah

SU‐FF‐T‐219: Evaluation of the Long‐Term Stability for the Cylindrical Ionization Chambers

Purpose: To analyze the long‐term stability of Farmer‐type cylindrical ionization chambers using the calibration factor provided by the KFDA (Korea Food and Drug Administration). KFDA is the national secondary standard laboratory (SSDL) in Korea and approve as a member of the IAEA SSDL network. Materials and Methods: The cylindrical ionization chambers used in this study include PTW 30001(30006), 30013, 30002, 30004, 23333, Capintec PR06C, NE 2571, Exradin A12 and Wellhofer FC65G(IC70). The Nk and ND,W calibration factors for the cylindrical chambers were analyzed and the measured ND,W was compared with the calculated ND,W calibration factor. Results: The long‐term stability of PTW 30013(30006), Wellhofer FC65G(IC70) and NE 2571 varied within 0.2 %, meaning that they are more stable than other ionization chambers. This result is different from previous ones showing that the long term stability of ionization chambers with graphite wall material was superior to ionization chambers made of PMMA material. It turns out that almost the same levels of the stability of calibration factor were shown between PTW 30013(30006) ionization chamber made of PMMA wall material and Wellhofer FC65G(IC70) ionization chambers made of graphite material, and thus the former is shown to be superior from the aspect of long‐term safety, and PMMA wall material is firmer, more convenient to use, and it is more effective from the financial aspect. Therefore, it may be considered to be more appropriate to use for the regular dose measurements in comparison with graphite type ionization chamber. The measured ND,W calibration factor was approximately 1.0% higher than the calculated ND,W that was determined using the Nk calibration factor. Conclusion: The long‐term stability of the cylindrical chambers was evaluated using the Nk and ND,W calibration factor. It should help to improve clinical electron dosimetry in radiotherapy.

SU-D-BRC-02: Application of Six Sigma Approach to Improve the Efficiency of Patient-Specific QA in Proton Therapy.

PURPOSE To show how the Six Sigma DMAIC (Define-Measure-Analyze-Improve-Control) can be used for improving and optimizing the efficiency of patient-specific QA process by designing site-specific range tolerances. METHODS The Six Sigma tools (process flow diagram, cause and effect, capability analysis, Pareto chart, and control chart) were utilized to determine the steps that need focus for improving the patient-specific QA process. The patient-specific range QA plans were selected according to 7 treatment site groups, a total of 1437 cases. The process capability index, Cpm was used to guide the tolerance design of patient site-specific range. We also analyzed the financial impact of this project. RESULTS Our results suggested that the patient range measurements were non-capable at the current tolerance level of ±1 mm in clinical proton plans. The optimized tolerances were calculated for treatment sites. Control charts for the patient QA time were constructed to compare QA time before and after the new tolerances were implemented. It is found that overall processing time was decreased by 24.3% after establishing new site-specific range tolerances. The QA failure for whole process in proton therapy would lead up to a 46% increase in total cost. This result can also predict how costs are affected by changes in adopting the tolerance design. CONCLUSION We often believe that the quality and performance of proton therapy can easily be improved by merely tightening some or all of its tolerance requirements. This can become costly, however, and it is not necessarily a guarantee of better performance. The tolerance design is not a task to be undertaken without careful thought. The Six Sigma DMAIC can be used to improve the QA process by setting optimized tolerances. When tolerance design is optimized, the quality is reasonably balanced with time and cost demands.

TU-FG-201-11: Evaluating the Validity of Prospective Risk Analysis Methods: A Comparison of Traditional FMEA and Modified Healthcare FMEA.

PURPOSE To examine the ability of traditional Failure mode and effects analysis (FMEA) and a light version of Healthcare FMEA (HFMEA), called Scenario analysis of FMEA (SAFER) by comparing their outputs in terms of the risks identified and their severity rankings. METHODS We applied two prospective methods of the quality management to surface image guided, linac-based radiosurgery (SIG-RS). For the traditional FMEA, decisions on how to improve an operation are based on risk priority number (RPN). RPN is a product of three indices: occurrence, severity and detectability. The SAFER approach; utilized two indices-frequency and severity-which were defined by a multidisciplinary team. A criticality matrix was divided into 4 categories; very low, low, high and very high. For high risk events, an additional evaluation was performed. Based upon the criticality of the process, it was decided if additional safety measures were needed and what they comprise. RESULTS Two methods were independently compared to determine if the results and rated risks were matching or not. Our results showed an agreement of 67% between FMEA and SAFER approaches for the 15 riskiest SIG-specific failure modes. The main differences between the two approaches were the distribution of the values and the failure modes (No.52, 54, 154) that have high SAFER scores do not necessarily have high FMEA RPN scores. In our results, there were additional risks identified by both methods with little correspondence. In the SAFER, when the risk score is determined, the basis of the established decision tree or the failure mode should be more investigated. CONCLUSION The FMEA method takes into account the probability that an error passes without being detected. SAFER is inductive because it requires the identification of the consequences from causes, and semi-quantitative since it allow the prioritization of risks and mitigation measures, and thus is perfectly applicable to clinical parts of radiotherapy.

SU-G-TeP2-15: Feasibility Study of Fiber-Optic Cerenkov Radiation Sensors for in Vivo Measurement: Dosimetric Characterization and Clinical Application in Proton Beams

PURPOSE To evaluate the possibility of a fiber-optic Cerenkov radiation sensor (FCRS) for in vivo dose verification in proton therapy. METHODS The Cerenkov radiation due to the proton beam was measured using a homemade phantom, consisting of a plastic optical fiber (POF, PGSCD1001-13-E, Toray, Tokyo, Japan) connected to each channel of a multianode photomultiplier tube (MAPMT:H7546, Hamamatsu Photonics, Shizuoka, Japan). Data were acquired using a multi-anode photomultiplier tube with the NI-DAQ system (National Instruments Texas, USA). The real-time monitoring graphic user interface was programmed using Labview. The FCRS was analyzed for its dosimetrics characteristic in proton beam. To determine the accuracy of the FCRS in proton dose measurements, we compared the ionization chamber dose measurements using a water phantom. We investigated the feasibility of the FCRS for the measurement of dose distributions near the superficial region for proton plans with a varying separation between the target volume and the surface of 3 patients using a humanoid phantom. RESULTS The dose-response has good linearity. Dose-rate and energy dependence were found to be within 1%. Depth-dose distributions in non-modulated proton beams obtained with the FCRS was in good agreement with the depth-dose measurements from the ionization chamber. To evaluate the dosimetric accuracy of the FCRS, the difference of isocenter dose between the delivery dose calculated by the treatment planning system and that measured by the FCRS was within 3%. With in vivo dosimetry using the humanoid phantom, the calculated surface doses overestimated measurements by 4%-8% using FCRS. CONCLUSION In previous study, our results indicate that the performance of the array-type FCRS was comparable to that of the currently used a multi-layer ion chamber system. In this study, we also believe that the fiber-optic Cerenkov radiation sensor has considerable potential for use with in vivo patient proton dosimetry.

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

PURPOSE To 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. METHODS We 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). RESULTS The 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. CONCLUSION SPC 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‐139: Feasibility Study of Glass Dosimeter for in Vivo Measurement: Dosimetric Characterization and Clinical Application in Proton Beams

PURPOSE To evaluate the suitability of the GD-301 glass dosimeter for use in in vivo dose verification in proton therapy. METHODS The glass dosimeter was analyzed for its dosimetric characteristic in proton beam. Dosimeters were calibrated in a water phantom using a stair-like holder specially designed for this study. To determine the accuracy of the glass dosimeter in proton dose measurements, we compared the glass dosimeter and TLD dose measurements of plan delivery using a cylindrical phantom. We investigated the feasibility of the glass dosimeter for the measurement of dose distributions near the superficial region for proton therapy plans with a varying separation between the target volume and the surface of 6 patients. RESULTS Uniformity was within 1.5%. The dose-response has a good linear. Dose-rate, fading, and energy dependence were found to be within 3%. The beam profile measured using the glass dosimeter was in good agreement with the profile obtained from the ionization chamber. Depth-dose distributions in non-modulated and modulated proton beams obtained with the glass dosimeter were estimated to be within 3%, which was lower than those with the ionization chamber. In the phantom study, the difference of isocenter dose between the delivery dose calculated by the Eclipse and that of the measured by the glass dosimeter was within 5%. In vivo dosimetry of patients, given the results of the glass dosimeter and TLD measurements, calculated doses on the surface of the patient are typically overestimated between 4% and 16%. CONCLUSIONS 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 proton dosimetry.

SU‐FF‐T‐187: Dosimetric Accuracy Analysis of Dose Calculation Algorithms Using Inhomogeneity Phantom for IMRT Treatment Plan

Purpose: We evaluated the dosimetric accuracy of dose calculation with a convolution/superposition (C/S) and a pencil‐beam algorithm with the use of the multiple inhomogeneous head and neck phantom for intensity modulated radiation therapy(IMRT)treatment plan. Method and Materials: The inhomogeneous head and neck phantom for IMRTquality assurance (QA) was housed in a custom‐designed package for efficient evaluation of the measured doses using the different materials and various detectors. Comparisons of measured and deconvolved beam profile obtained from a Gaussian fitting approaches were performed using the real penumbra width to remove ionization chamber size effect. The relative dosimetry obtained with the single‐beam dose calculation algorithm was also compared to the MapCheck measurements for a head and neck IMRTtreatment plans. The accuracy of IMRTdose calculations with the C/S and pencil‐beam algorithms were also investigated with respect to measurements used in head and neck phantom in the presence of inhomogeneities. Results: The deconvolved penumbra was 2.2 mm and calculated value was 2.6 mm with pencil‐beam algorithm. The differences between calculated dose and measured dose with the MapCheck were less than 4.0% for the horizontal profile of IMRT head and neck treatment plan. The differences for the C/S and pencil‐beam dose calculation algorithms were within 5.0% in average target doses.Ionization chamber experiments showed approximately 2.5% better agreement than the glass rod detector. Conclusion: Our results show that accurate measurements of the penumbral region with consideration of inhomogeneities improve the accuracy of the dose calculation algorithms predicted by the treatment planning system. Therefore, it is important to choose an appropriate detector

SU‐GG‐T‐269: Experimental Determination of Detector Volume Effect Using a Gaussian Function On Output Factor Measurements in the 5 Mm Small Field

Purpose: We investigated the effect of detector volume for output factor measurement in the 5 mm collimator. Comparisons of measured and calculated output factors from a Gaussian fitting approaches the real doses to remove detector volume effect were performed. Method and Materials: A 6 MV photon beam from a third‐generation CyberKnife was used in this study. We have used a Gaussian function to correct for the spatial response of finite‐sized detectors and to obtain the real beam profiles from measurement. The beam profiles were measured with a PTW 31006 pinpoint ionization chamber and a PTW 60008 diode. The calculated output factors were obtained from a Gaussian fitting for the beam profile with the same detector, which were compared to measured value with radiochromic film. Results: The 5 mm collimator output factor measured with the ionization chamber was 0.615±0.009, the diode was 0.702±0.015 and radiochromic film was 0.695±0.022. Ionization chamber show significant differences of more than 12.4% between measured and calculated output factor by Gaussian function for the 5 mm collimator. The main reason for an underestimation of the output factor is the increase of lateral electron disequilibrium with an increase of the ionization chamber measuring volume. The agreement between output factors measured and calculated value with the diode was within 1.5%. A diode was found to be suitable for output factor measurements of small beam because of its high spatial resolution. Also, we found good agreement between the measured output factor obtained with radiochromic EBT film and the calculation values of another detector.Conclusion: The volume effect of the detector could lead to inaccurate in small field dosimetry. The results of this study indicated that the choice of a suitable detector is an important problem for dose measurement in high dose gradient region such as the 5 mm collimator.

SU-GG-T-232: Investigation of Dosimetric Characteristics of Glass Dosimeter and Thermoluminescent Dosimeter for a Mailed Dosimetry

Purpose: The purpose of this study was to investigate the dosimetric characteristics of glass dosimeter with respect to reproducibility, linearity, fading, angular dependence and energy dependence. The results of the glass dosimeter were also compared with those of the thermoluminescent dosimeter(TLD) in order to examine the possibility as a mailed dosimetry for a quality assurance (QA) audit program. Method and Materials: In this study, the model GD‐301 glass dosimeter and powder type TLD‐700 were used. All measurements with the exception of angular dependence were performed in a water phantom using an in‐house custom designed holder stand. The angular dependence was measured with 6 MV photon beam from the Varian CL 2100 linear accelerator using a spherical polystyrene phantom. The phantom is to use for routine QA and calibration in Gamma Knife and can accommodate a number of interchangeable holders. The dosimeters were irradiated in the SSDLs in Korea to achieve a reliable reference condition at a known dose. Results: The glass dosimeter has better reproducibility than the TLD for the Co‐60 beam as well as for the clinical photon beam. The glass dosimeter signal was linear as a function of applied dose in the range from 0.5 to 50 Gy for the Co‐60 gamma rays. The fading of the glass dosimeter after a received dose of 2 Gy initially was found to be within 1.7% for five months. The angular dependence of the glass dosimeter was measured about 1.4% for angles ranging ±90° from the beam axis using a spherical polystyrene phantom. Conclusion: The results of the glass dosimeter and TLD measurements comparing the dosimetric characteristics showed that the glass dosimeter is suitable for a mailed dosimetry in a QA audit program. For electron beam, the energy dependence of the glass dosimeter needs to be considered and corrected.