Flexible real-time skin dosimeter based on a thin-film copper indium gallium selenide solar cell for electron radiation therapy.

PURPOSE: Various dosimeters have been proposed for skin dosimetry in electron radiotherapy. However, one main drawback of these skin dosimeters is their lack of flexibility, which could make accurate dose measurements challenging due to air gaps between a curved patient surface and dosimeter. Therefore, the purpose of this study is to suggest a novel flexible skin dosimeter based on a thin-film copper indium gallium selenide (CIGS) solar cell, and to evaluate its dosimetric characteristics. METHODS: The CIGS solar cell dosimeter consisted of (a) a customized thin-film CIGS solar cell and (b) a data acquisition (DAQ) system. The CIGS solar cell with a thickness of 0.33 mm was customized to a size of 10 × 10 mm2 . This customized solar cell plays a role in converting therapeutic electron radiation into electrical signals. The DAQ system was composed of a voltage amplifier with a gain of 1000, a voltage input module, a DAQ chassis, and an in-house software. This system converted the electrical analog signals (from solar cell) to digital signals with a sampling rate of ≤50 kHz and then quantified/visualized the digital signals in real time. We quantified the linearity/ sampling rate effect/dose rate dependence/energy dependence/field size output factor/reproducibility/curvature/bending recoverability/angular dependence of the CIGS solar cell dosimeter in therapeutic electron beams. To evaluate clinical feasibility, we measured the skin point doses by attaching the CIGS solar cell to an anthropomorphic phantom surface (for forehead, mouth, and thorax). The CIGS-measured doses were compared with calculated doses (by treatment planning system) and measured doses (by optically stimulated luminescent dosimeter). RESULTS: The normalized signals of the solar cell dosimeter increased linearly as the delivered dose increased. The gradient of the linearly fitted line was 1.00 with an R-square of 0.9999. The sampling rates (2, 10, and 50 kHz) of the solar cell dosimeter showed good performance even at low doses (<50 cGy). The solar cell dosimeter exhibited dose rate independence within 1% and energy independence within 3% error margins. The signals of the solar cell dosimeter were similar (<1%) when penetrating the same side of the CIGS cell regardless of the rotation angle of the solar cell. The field size output factor measured by the solar cell dosimeter was comparable to that measured by the ion chamber. The solar cell signals were similar between the baseline (week 1) and the last time point (week 4). Our detector showed curvature independence within 1.8% (curvatures of <0.10 mm- ) and bending recovery (curvature of 0.10 mm-1 ). The differences between measured doses (CIGS solar cell dosimeter vs. optically stimulated luminescent dosimeter) were 7.1%, 9.6%, and 1.0% for forehead, mouth, and thorax, respectively. CONCLUSION: We present the construction of a flexible skin dosimeter based on a CIGS solar cell. Our findings demonstrate that the CIGS solar cell has a potential to be a novel flexible skin dosimeter for electron radiotherapy. Moreover, this dosimeter is manufactured with low cost and can be easily customized to various size/shape, which represents advantages over other dosimeters.

Assessment of a Therapeutic X-ray Radiation Dose Measurement System Based on a Flexible Copper Indium Gallium Selenide Solar Cell.

Several detectors have been developed to measure radiation doses during radiotherapy. However, most detectors are not flexible. Consequently, the airgaps between the patient surface and detector could reduce the measurement accuracy. Thus, this study proposes a dose measurement system based on a flexible copper indium gallium selenide (CIGS) solar cell. Our system comprises a customized CIGS solar cell (with a size 10 × 10 cm2 and thickness 0.33 mm), voltage amplifier, data acquisition module, and laptop with in-house software. In the study, the dosimetric characteristics, such as dose linearity, dose rate independence, energy independence, and field size output, of the dose measurement system in therapeutic X-ray radiation were quantified. For dose linearity, the slope of the linear fitted curve and the R-square value were 1.00 and 0.9999, respectively. The differences in the measured signals according to changes in the dose rates and photon energies were <2% and <3%, respectively. The field size output measured using our system exhibited a substantial increase as the field size increased, contrary to that measured using the ion chamber/film. Our findings demonstrate that our system has good dosimetric characteristics as a flexible in vivo dosimeter. Furthermore, the size and shape of the solar cell can be easily customized, which is an advantage over other flexible dosimeters based on an a-Si solar cell.

Feasibility study of patient-specific energy verification using a multilayer acrylic-disk radiation sensor.

PURPOSE: Obtaining an integral depth-dose (IDD) curve using a recently developed acrylic-disk radiation sensor (ADRS) is time-consuming because its single structure requires point-by-point measurements in a water phantom. The goal of this study was to verify the ability of a newly designed multilayer ADRS, composed of 20 layers, to measure the energy of proton pencil beam scanning (PBS) in patient-specific quality assurance (QA). MATERIALS AND METHODS: The multilayer ADRS consisted of a disk-type transmitter, with a diameter of 15 cm and with a thickness of 1 mm, surrounded by a thin optical fiber; this ADRS provided a higher spatial resolution than the single ADRS, which was 2 mm. The dosimetric characteristics of the multilayer ADRS were determined to accurately measure the energy delivered layer-by-layer. We selected five patients to verify the energy measured using the multilayer ADRS from the actual clinical proton therapy plans. The accuracy of the results measured using the multilayer ADRS was compared with that of measurements by a Bragg peak ionization chamber (IC) and that calculated by a Monte Carlo TOPAS simulation. RESULTS: The difference between the multilayer ADRS measurements and those of the TOPAS simulation was within 1% for all patients. The ranges, corresponding to the beam energies for each patient, measured using the multilayer ADRS were closer to those calculated using the TOPAS simulation than those measured using the Bragg peak IC. CONCLUSIONS: The multilayer ADRS is well suited to verifying the energy of a pencil beam. The acrylic materials used in its configuration make this device easier to use and more cost-effective than conventional detectors. This device, with its high extensibility and stability, may be applicable as a new dosimetry tool for PBS.

Tolerance design of patient-specific range QA using the DMAIC framework in proton therapy.

PURPOSE: To implement the DMAIC (Define-Measure-Analyze-Improve-Control) can be used for customizing the patient-specific QA by designing site-specific range tolerances. METHODS: The DMAIC framework (process flow diagram, cause and effect, Pareto chart, control chart, and capability analysis) were utilized to determine the steps that need focus for improving the patient-specific QA. The patient-specific range QA plans were selected according to seven 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. RESULTS: For prostate field, our results suggested that the patient range measurements were capable at the current tolerance level of ±1 mm in clinical proton plans. For other site-specific ranges, we analyzed that the tolerance tends to be overdesigned to insufficient process capability calculated by the patient-specific QA data. The customized tolerances were calculated for treatment sites. Control charts were constructed to simulate the patient QA time before and after the new tolerances were implemented. It is found that the total simulation QA time was decreased on average of approximately 20% after establishing new site-specific range tolerances. We simulated the financial impact of this project. The QA failure for whole process in proton therapy would lead up to approximately 30% increase in total cost. CONCLUSION: DMAIC framework can be used to provide an effective QA by setting customized tolerances. When tolerance design is customized, the quality is reasonably balanced with time and cost demands.

A comparison of two prospective risk analysis methods: Traditional FMEA and a modified healthcare FMEA.

PURPOSE: To examine the abilities of a traditional failure mode and effects analysis (FMEA) and modified healthcare FMEA (m-HFMEA) scoring methods by comparing the degree of congruence in identifying high risk failures. METHODS: The authors 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 were based on the risk priority number (RPN). The RPN is a product of three indices: occurrence, severity, and detectability. The m-HFMEA approach utilized two indices, severity and frequency. A risk inventory matrix was divided into four 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: The two methods were independently compared to determine if the results and rated risks matched. The authors' results showed an agreement of 85% between FMEA and m-HFMEA approaches for top 20 risks of SIG-RS-specific failure modes. The main differences between the two approaches were the distribution of the values and the observation that failure modes (52, 54, 154) with high m-HFMEA scores do not necessarily have high FMEA-RPN scores. In the m-HFMEA analysis, when the risk score is determined, the basis of the established HFMEA Decision Tree™ or the failure mode should be more thoroughly investigated. CONCLUSIONS: m-HFMEA is inductive because it requires the identification of the consequences from causes, and semi-quantitative since it allows the prioritization of high risks and mitigation measures. It is therefore a useful tool for the prospective risk analysis method to radiotherapy.

A treatment planning study of proton arc therapy for para-aortic lymph node tumors: dosimetric evaluation of conventional proton therapy, proton arc therapy, and intensity modulated radiotherapy.

BACKGROUND: The purpose of this study is to evaluate the dosimetric benefits of a proton arc technique for treating tumors of the para-aortic lymph nodes (PALN). METHOD: In nine patients, a proton arc therapy (PAT) technique was compared with intensity modulated radiation therapy (IMRT) and proton beam therapy (PBT) techniques with respect to the planning target volume (PTV) and organs at risk (OAR). PTV coverage, conformity index (CI), homogeneity index (HI) and OAR doses were compared. Organ-specific radiation induced cancer risks were estimated by applying organ equivalent dose (OED) and normal tissue complication probability (NTCP). RESULTS: The PAT techniques showed better PTV coverage than IMRT and PBT plans. The CI obtained with PAT was 1.19 ± 0.02, which was significantly better than that for the IMRT techniques. The HI was lowest for the PAT plan and highest for IMRT. The dose to the OARs was always below the acceptable limits and comparable for all three techniques. OED results calculated based on a plateau dose-response model showed that the risk of secondary cancers in organs was much higher when IMRT or PBT were employed than when PAT was used. NTCPs of PAT to the stomach (0.29 %), small bowel (0.69 %) and liver (0.38 %) were substantially lower than those of IMRT and PBT. CONCLUSION: This study demonstrates that there is a potential role for PAT as a commercialized instrument in the future to proton therapy.

Feasibility study of using statistical process control to customized quality assurance in proton therapy.

PURPOSE: To evaluate and improve the reliability of proton quality assurance (QA) processes and, to provide an optimal customized tolerance level using the statistical process control (SPC) methodology. METHODS: The authors investigated the consistency check of dose per monitor unit (D/MU) and range in proton beams to see whether it was within the tolerance level of the daily QA process. This study analyzed the difference between the measured and calculated ranges along the central axis to improve the patient-specific QA process in proton beams by using process capability indices. RESULTS: The authors established a customized tolerance level of ±2% for D/MU and ±0.5 mm for beam range in the daily proton QA process. In the authors' analysis of the process capability indices, the patient-specific range measurements were capable of a specification limit of ±2% in clinical plans. CONCLUSIONS: SPC methodology is a useful tool for customizing the optimal QA tolerance levels and improving the quality of proton machine maintenance, treatment delivery, and ultimately patient safety.

Dosimetric evaluation of a glass dosimeter for proton beam measurements.

PURPOSE: The GD-301 radiophotoluminescent glass dosimeter system has recently become commercially available. The purpose of this study was to investigate the dosimetric characteristics (reproducibility, linearity, dose rate, fading, angular dependence, and depth-dose distribution) of this system for clinical dosimetry in a high-energy proton beam and to compare it with lithium fluoride TLD-100. MATERIALS AND METHODS: The depth-dose distribution measured with the glass dosimeter was compared to those from GEANT4 Monte-Carlo simulation. 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-shaped holder specially designed for this study. Maximum height was 100mm with 1mm steps in each of ten-tiers. 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 dose rate dependence of both dosimeters was within 1.5%. The fading effect of the glass dosimeter was found to be within 1.6% for 6 months. Angular dependence of the glass dosimeter was measured to be approximately 1.3% for angles that were 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 measured with the ionization chamber and simulation model using GEANT4 code. The Bragg peak depths determined from the ionization chamber, the glass dosimeter and GEANT4 simulation were 84.8mm, 84.2mm and 85.0mm, respectively. For the modulated proton beam, the SOBP width between the 90% proximal and the distal dose levels as obtained from the glass dosimeter was 48.1mm. The SOBP width measured with the ionization chamber was 52.2mm. CONCLUSIONS: Measurements comparing the glass dosimeter and TLD-100 dosimetric characteristics demonstrated the suitability of use of the glass dosimeter for dose measurement in high-energy proton beam therapy.

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 in vivo dose verification in proton therapy. METHODS AND MATERIALS: The glass dosimeter was analyzed for its dosimetrics characteristic in proton beam. Dosimeters were calibrated in a water phantom using a stairlike holder specially designed for this study. To determine the accuracy of the glass dosimeter in proton dose measurements, we compared the glass dosimeter and thermoluminescent dosimeter (TLD) dose measurements 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 AND DISCUSSION: Uniformity was within 1.5%. The dose-response has good linearity. 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 nonmodulated 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 treatment planning system and that measured by the glass dosimeter was within 5%. With in vivo dosimetry, the calculated surface doses overestimated measurements by 4%-16% using glass dosimeter and TLD. CONCLUSION: It is recommended that bolus be added for these clinical cases. We also believe that the glass dosimeter has considerable potential for use with in vivo patient proton dosimetry.

Dose response of commercially available optically stimulated luminescent detector, Al2O3:C for megavoltage photons and electrons.

This study examined the dose response of an optically stimulated luminescence dosemeter (OSLD) to megavoltage photon and electron beams. A nanoDot™ dosemeter was used to measure the dose response of the OSLD. Photons of 6-15 MV and electrons of 9-20 MeV were delivered by a Varian 21iX machine (Varian Medical System, Inc. Milpitas, CA, USA). The energy dependency was <1 %. For the 6-MV photons, the dose was linear until 200 cGy. The superficial dose measurements revealed photon irradiation to have an angular dependency. The nanoDot™ dosemeter has potential use as an in vivo dosimetric tool that is independent of the energy, has dose linearity and a rapid response compared with normal in vivo dosimetric tools, such as thermoluminescence detectors. However, the OSLD must be treated very carefully due to the high angular dependency of the photon beam.

Estimation of the secondary cancer risk induced by diagnostic imaging radiation during proton therapy.

We have estimated the secondary cancer risk (SCR) introduced by image-guided procedures during proton therapy. The physical dose from imaging radiation and the corresponding organ equivalent dose were calculated for the case of a lumbar spine patient. The maximum physical dose delivered to the patient during the imaging procedure was estimated to be ~0.35% of the prescribed dose of 46 Gy. However, this small imaging dose substantially raised the radiation-induced SCR by ~8%. In addition, the clinical benefit (improved accuracy during the procedure) and costs (extra SCR) associated with image-guided procedures were quantitatively modelled by systematically investigating the changes in SCR as a function of the prescribed dose, treatment target volume and imaging field size. The results showed that the SCR varied sensitively with the volume receiving the imaging and the therapeutic radiation, whereas the SCR depended to a lesser extent on the magnitude of the applied therapeutic radiation. These results showed that the additional SCR introduced by imaging radiation could be efficiently reduced by minimizing the imaging field size during image-guided procedures.

The effect of a contrast agent on proton beam range in radiotherapy planning using computed tomography for patients with locoregionally advanced lung cancer.

PURPOSE: We evaluated the effect of a contrast agent (CA) on proton beam range in a treatment planning system (TPS) for patients with locoregionally advanced lung cancer. METHODS AND MATERIALS: Two sets of computed tomography (CT) images (with and without CA) were obtained from 20 patients with lung cancer. Because the increase in Hounsfield unit (∆HU) value of the heart and great vessels due to the effect of CA is most prominent among thoracic structures, to evaluate the effect of CA on proton beam range in the TPS, we compared the calculated distal ranges in the plan with CA-enhanced CT with those with corrected CT, in which the HU values of the heart and great vessels in the CA-enhanced CT were replaced by average HU values obtained from the unenhanced CT. RESULTS: The mean ∆HU value and the longest length of the heart and great vessels within the proton beam path in the field that passed through these structures were 189 ± 29 HU (range, 110-250 HU) and 7.1 ± 1.1 cm (range, 2.6-11.2 cm), respectively. The mean distal range error in the TPS because of the presence of CA was 1.0 ± 0.7 cm (range, 0.2-2.6 cm). CONCLUSION: If CA-enhanced CT images are used for radiotherapy planning using a proton beam for the treatment of lung cancer, our results suggest that the HU values of the heart and great vessels should be replaced by the average HU values of soft tissue to avoid discrepancies between planned and delivered doses.

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.

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.