Commercial Diode Array Research Articles

Dose-rate dependence and IMRT QA suitability of EBT3 radiochromic films for pulse reduced dose-rate radiotherapy (PRDR) dosimetry.

Pulsed reduced dose rate (PRDR) is an emerging radiotherapy technique for recurrent diseases. It is pertinent that the linac beam characteristics are evaluated for PRDR dose rates and a suitable dosimeter is employed for IMRT QA. This study sought to investigate the pulse characteristics of a 6 MV photon beam during PRDR irradiations on a commercial linac. The feasibility of using EBT3 radiochromic film for use in IMRT QA was also investigated by comparing its response to a commercial diode array phantom. A plastic scintillator detector was employed to measure the photon pulse characteristics across nominal repetition rates (NRRs) in the 5-600 MU/min range. Film was irradiated with dose rates in the 0.033-4Gy/min range to study the dose rate dependence. Five clinical PRDR treatment plans were selected for IMRT QA with the Delta4 phantom and EBT3 film sheets. The planned and measured dose were compared using gamma analysis with a criterion of 3%/3mm. EBT3 film QA was performed using a cumulative technique and a weighting factor technique. Negligible differences were observed in the pulse width and height data between the investigated NRRs. The pulse width was measured to be 3.15±0.01 and the PRF was calculated to be 3-357Hz for the 5-600 MU/min NRRs. The EBT3 film was found to be dose rate independent within 3%. The gamma pass rates (GPRs) were above 99% and 90% for the Delta4 phantom and the EBT3 film using the cumulative QA method, respectively. GPRs as low as 80% were noted for the weighting factor EBT3 QA method. Altering the NRRs changes the mean dose rate while the instantaneous dose rate remains constant. The EBT3 film was found to be suitable for PRDR dosimetry and IMRT QA with minimal dose rate dependence.

Technical Note: Dosimetric feasibility of lattice radiotherapy for breast cancer using GammaPod.

Studies on Lattice radiotherapy (LRT) for breast cancer have been largely lacking. This study investigates the dosimetric feasibility of using Gamma Pod, a stereotactic radiotherapy apparatus originally designed for breast SBRT, to deliver LRT to large, bulky breast tumor as a noninvasive treatment option. The GammaPod-based LRT was simulated using Geant4 Gate Monte Carlo software. The simulated GammaPod was equipped with 5mm diameter non-coplanar circular beams that span 28° latitudinally from 18° to 43° off the horizontal plane. Two degrees longitudinal intervals were used to simulate rotating sources. To simulate the treatments to different breast sizes, three water-equivalent hemisphere volumes with diameters of 10, 15, and 20cm were analyzed. The lattice was planned by spacing focal points 2cm apart in the transverse and sagittal planes and 2.5cm in the coronal plane. This resulted in 22-172 shots for full breast treatment. The maximum dose for each individual shot was 20Gy. The peak-to-valley dose differences and skin dose were analyzed. To verify the feasibility of delivering LRT, a test plan was created and delivered to a commercial diode array dose verification device using a clinical GammaPod system with 15mm collimators. The dose profiles showed the average peak-to-valley dose percent differences of 94.10% in the 10cm hemispherical volume, 88.95% in the 15cm hemispherical volume, and 83.60% in the 20cm hemispherical volume. Average skin dose was 1.27, 1.72, and 2.13Gy for the 10, 15, and 20cm irradiation volumes, respectively. The LRT plan delivered using a clinical GammaPod system with larger collimators verified the feasibility of LRT plan delivery. GammaPod-based lattice radiotherapy is a viable treatment option and its application can be extended to treating large bulky breast tumors.

Positive Temperature Coefficient Compensating Heating for Analytical Devices.

Positive temperature coefficient thermistors acting as heating devices are quickly growing in popularity and are being adapted into critical applications in many sectors from medical to space discovery. Positive temperature coefficient heating offers substantial benefits for miniaturized and portable analytical devices in key aspects such as energy efficiency, safety in overheating, size, scalability, and in discovering new thermal management strategies. These heaters can reach 230 °C without additional requirements for regulating electronics. By incorporating positive temperature coefficient technology into a commercial diode array photometric detector, the detector is made suitable for coupling with gas chromatography. The detector cartridge flow cell is heated to a specific target temperature within the range of 70 to 150 °C without impacting the detector's construction material or imparting any negative effect to the surrounding detector system electronics. Applying a temperature of 150 °C to the cell permits analysis of volatile and semivolatile compounds with a boiling point equivalent to that of n-hexadecene (285 °C). Model compounds of alkene homologues from C8 to C16 showed a maximum peak asymmetry of 1.10 with the heated cell design. A high degree of repeatability was observed with RSD of less than 0.01% in retention time and 3% in peak area ( n = 10).

Patient specific QA for RapidArc

Abstract: RapidArc is a volumetric arc therapy which was recently introduced to Ireland by St. Lukes Radiation Oncology Network. RapidArc is characterised by continuous MLC and gantry motion in combination with varying gantry speed, dose rate and MLC speed. This approach leads to significantly shortened treatment times and dose conformity which is comparable to that achieved by static IMRT. Because of the complexity of RapidArc treatment plans, it is vital to implement an efficient and accurate protocol for routine patient specific QA. We decided to use the ArcCHECK phantom for this purpose. ArcCHECK is a commercial diode array designed specifically for rotational measurements. Our ArcCHECK measurements were validated by comparison with Gafchromic film measurements as this is a standard high resolution patient verification method. Specifically, our work involved delivering a number of prostate RapidArc treatment plans to ArcCHECK. Independent validation of ArcCHECK measurements with Gafchromic film, in two orientations was performed for a subset of these plans. Gafchromic film analysis was performed using the ‘triple channel dosimetry’ approach. This is a new method of evaluating radiochromic film dosimetry data which improves the integrity of the dose information in the film by removing disturbances in scanned images. As a result of following this method, a high level dosimetric accuracy was evident in our film measurements. Dose distributions were analysed using gamma criteria of 3% dose and 3 mm distance to agreement. A pass rate of at least 95% was achieved using data from ArcCHECK and Gafchromic film respectively. This work combines the construction of an efficient protocol for using ArcCHECK for patient specific QA of RapidArc treatment plans with an implementation of the ‘triple channel dosimetry’ approach for Gafchromic film analysis. Thus we have formulated a protocol for patient specific QA of RapidArc treatment plans and incorporated Gafchromic film as a suitable backup system.

SU-E-T-162: Rapid Assessment of Cyberknife IRIS Variable Collimator Performance Using a Commercial Diode Array.

To describe an efficient method for verifying the size and centering of the radiation field from the Cyberknife IRIS variable collimator with sub-millimeter accuracy using a general purpose commercially available diode array. We present a technique using a conventional diode array (Sun Nuclear Profiler) with the array at an extended distance of 320 cm. The projection of the 4 mm diode spacing back to the 80 cm field definition distance gives an effective spacing of 1 mm, sufficient to confirm proper operation of the IRIS. We describe the data acquisition process and present data comparing the Profiler measurements to scanned measurements for both profile and FWHM analysis and reproducibility of the technique over repeated measurements. Average difference between original water scanner measurements and diode array measurements over the 12 aperture sizes) from 5 mm to 60 mm) were - 0.14 mm (range 0.03 mm to 0.83 mm). Reproducibility and centering measurements had a similar range of accuracy. A general purpose commercially available diode array can be used to quickly and accurately characterize the field size and centering of the Cyberknife IRIS variable collimator system with sub-millimeter accuracy subsequent to service, software recalibration, software upgrades or associated with routine QA. This technique avoids the time consuming and cumbersome water tank scanning with a diode and the difficulties associated with image based measurements (CR or radiochromic film) that require time consuming and careful calibration and choice of threshold values.

Feasibility of conventional and Roundup Ready® soybeans discrimination by different near infrared reflectance technologies

Identification and proper labelling of genetically modified organisms is required and increasingly demanded by legislation and consumers worldwide. In this study, the feasibility of three near infrared reflectance technologies (a chemical imaging unit, a commercial diode array instrument, and a light tube non-commercial instrument) were compared for discriminating Roundup Ready® and not genetically modified soybean seeds. Over 200 seeds of each class (Roundup Ready® and conventional) were used. Principal Component Analysis with Artificial Neural Networks (PCA-ANN) and Locally Weighted Principal Component Regression (LW-PCR) were used for creating the discrimination models. Discrimination accuracies when new tested seeds belonged to samples included in the training sets achieved accuracies over 90% of correctly classified seeds for LW-PCR models. The light tube performed the best, while the imaging unit showed the worse accuracies overall. Models validated with new seeds from samples not included in the training set had accuracies of 72-79%.

A prototype quantitative film scanner for radiochromic film dosimetry

We have developed a high resolution, quantitative, two-dimensional optical film scanner for use with a commercial high sensitivity radiochromic film (RCF) for measuring single fraction external-beam radiotherapy dose distributions. The film scanner was designed to eliminate artifacts commonly observed in RCF dosimetry. The scanner employed a stationary light source and detector with a moving antireflective glass film platen attached to a high precision computerized X-Y translation stage. An ultrabright red light emitting diode (LED) with a peak output at 633 nm and full width at half maximum (FWHM) of 16 nm was selected as the scanner light source to match the RCF absorption peak. A dual detector system was created using two silicon photodiode detectors to simultaneously measure incident and transmitted light. The LED light output was focused to a submillimeter (FWHM 0.67 mm) spot size, which was determined from a scanning knife-edge technique for measuring Gaussian optical beams. Data acquisition was performed with a 16-bit A/D card in conjunction with commercial software. The linearity of the measured densities on the scanner was tested using a calibrated neutral-density step filter. Sensitometric curves and three IMRT field scans were acquired with a spatial resolution of 1 mm for both radiographic film and RCF. The results were compared with measurements taken with a commercial diode array under identical delivery conditions. The RCF was rotated by 90 deg and rescanned to study orientation effects. Comparison between the RCF and the diode array measurements using percent dose difference and distance-to-agreement criteria produced average passing rates of 99.0% using 3%/3 mm criteria and 96.7% using 2%/2 mm criteria. The same comparison between the radiographic film and diode array measurements resulted in average passing rates 96.6% and 91.6% for the above two criteria, respectively. No measurable light-scatter or interference scanner artifacts were observed. The RCF rotated by 90 deg showed no measurable orientation effect. A scan of a 15 x 15 cm2 area with 1 mm resolution required 22 min to acquire. The LED densitometer provides an accurate film dosimetry system with 1 mm or better resolution. The scanner eliminates the orientation dependence of RCF dosimetry that was previously reported with commercial flatbed scanners.

Evaluation of a diode array for QA measurements on a helical tomotherapy unit

A helical tomotherapy system is used in our clinic to deliver intensity-modulated radiation therapy (IMRT) treatments. Since this machine is designed to deliver IMRT treatments, the traditional field flatness requirements are no longer applicable. This allows the unit to operate without a field flatness filter and consequently the 400 mm wide fan beam is highly inhomogeneous in intensity. The shape of this beam profile is mapped during machine commissioning and for quality assurance purposes the shape of the beam profile needs to be monitored. The use of a commercial diode array for quality assurance measurements is investigated. Central axis beam profiles were acquired at different depths using solid water built-up material. These profiles were compared with ion chamber scans taken in a water tank to test the accuracy of the diode array measurements. The sensitivity of the diode array to variations in the beam profile was checked. Over a seven week period, beam profiles were repeatedly measured. The observed variations are compared with those observed with an on-board beam profile monitor. The diode measurements were in agreement with the ion chamber scans. In the high dose, low gradient region the average ratio between the diode and ion chamber readings was 1.000 +/- 0.005 (+/- 1 standard deviation). In the penumbra region the agreement was poorer but all diodes passed the distance to agreement (DTA) requirement of 2 mm. The trend in the beam profile variations that was measured with the diode array device was in agreement with the on-board monitor. While the calculated amount of variation differs between the devices, both were sensitive to subtle variations in the beam profile. The diode array is a valuable tool to quickly and accurately monitor the beam profile on a helical tomotherapy unit.

Performance of a photothermal detector with turbid liquids

A closed-cell photothermal detector for aqueous analytes has been evaluated at 254 and 678 nm. We used a detector with a water meniscus as a pressure sensor, whose periodic deflection was measured using a low-finesse optical fiber Fabry-Perot interferometer. Performance was compared with a commercial diode array spectrometer and found to be similar for absorption measurements in nonturbid samples, but the results were affected up to 60 times less by scattered light. Finally the photothermal cell was converted into an integrating cavity using ceramic inserts, showing freedom from scattering-related errors at 678 nm but a degradation in performance at 254 nm.

SU‐FF‐T‐194: Evaluation of a Commercial Diode Array (Tomodose) for Tomotherapy Beam Profile Measurements

Purpose: TomoDose, a commercial diode array (Sun Nuclear Corporation, Melbourne, FL) is used to measure the radiation field in a TomoTherapy Hi* Art linear accelerator. The performance of TomoDose is evaluated by comparing the diode measurements with (i) ion chamber based water tank measurements and (ii) data from an on‐board ion chamber array mounted opposite the linac. Method and Materials: TomoDose has an active area that is rectangular (530 mm by 96 mm) to cover to the maximum field size (50 mm by 400 mm) of the tomotherapy beam. A total of 223 diodes allow a measure of the lateral and longitudinal beam profiles on the central axis and longitudinal profiles 50, 100, and 150 mm off‐axis. Both central axis profiles were compared with ion chamber data obtained in a water tank during an annual quality assurance test. Over a period of about seven weeks, the central axis lateral profile was monitored and compared with profile measurements obtained using the treatment machine's on‐board detector system. Results: TomoDose measurements agreed well with the ion chamber data. A repeated measurement of the lateral profile revealed subtle changes in this profile. Identical trends in profile change were independently measured using an on‐board detector array system. These changes were used for diagnosis of the system's components and can be used to provide early signs of component wear and tear. Conclusion: TomoDose is accurate and can be used for regular, e.g. monthly, monitoring of the beam profiles. It is sensitive to subtle changes in the profile that may be used to provide early warning signs of developing problems. Conflict of Interest: Two of the co‐authors are employees of TomoTherapy, Inc.

SU‐FF‐T‐154: Long‐Term Reproducibility of IMRT Planning and Dosimetry Using Clinical Fields

Purpose: To augment our patient‐specific IMRT QA program by checking the individual and overall reproducibility of the DMLC planning and delivery systems periodically using standardized patient data sets and a commercial diode array. Method and Materials: Clinical fields were chosen from IMRT treatment plans using 6MV and 15MV, respectively. It is our intent to completely re‐plan the same patients, generate leaf sequence files, and use the diode array to measure the absolute dose distributions for at least one different linac/MLC combination each month, thereby performing the evaluation for all MLC‐equipped linacs every six months. The dose distributions are recalculated in a flat homogeneous phantom using the clinical treatment planning system. Data are analyzed using dose‐difference and distance to agreement (DTA) methods. The supplied analysis program also provided a score for each field, which is defined as the percentage of the number of detectors satisfying the user defined dose‐difference and dose threshold constraints and DTA constraint. Results: The scores for the fields used in this study range from 75–100% of an average ∼200 points for each field for the 3% dose, 10% dose threshold, and 3 mm constraints that we specified. The diode array is able to identify the dose discrepancies relative to treatment planning calculations and earlier measurements and provides a quantitative score to grade each comparison, which along with the relative distribution of the failed points will serve as means for comparisons of measurements and/or calculations over time. Conclusion: The protocol that we present here enables us to provide a quick, accurate and reproducible evaluation of the overall DMLC planning and delivery process. With periodical evaluation using the same patients' data sets, we should be able to detect differences in performance among similar MLCs and linacs, and very small variations introduced over time for the same components.

Photonic crystal optrode sensor for detection of Pb2+ in high ionic strength environments.

We developed an optrode sensing device that utilizes a polymerized colloidal array (PCCA) photonic crystal material. This array diffracts light in the visible spectral region due to the periodic spacing of colloidal particles. The PCCA changes diffraction wavelength due to binding of Pb2+ to an 18-crown-6 ether molecular recognition agent. This optrode consists of a probe assembly that contains the PCCA Pb2+ sensing film. An inexpensive, commercial diode array spectrometer and a fiber-optic reflectance probe monitors the wavelength of light back diffracted by the PCCA. Liquid inlet and outlet connections are provided to introduce the sample solution and to exchange out nonbinding ions. In low ionic strength solutions, diffraction wavelength shifts are actuated by the binding of the Pb2+ to the crown ether to immobilize the Pb2+ counterions. In these low ionic strength solutions, a Donnan potential forms to cause an osmotic pressure, which swells the PCCA in proportion to the number density of bound Pb2+. This Donnan potential disappears at high ionic strengths. Thus, no response of the PCCA occurs. Our optrode design allows for the fast removal of nonbound ions from the PCCA by washing with pure water. Since the bound Pb2+ ions have a slow off rate from the crown ether, the bound Pb2+ PCCA diffraction transiently red shifts during washing, directly in proportion to the sample Pb2+ concentration. This transient diffraction red-shift can be used to quantitatively determine Pb2+ concentrations in high ionic strength solutions such as bodily fluids.

Micro-liquid chromatography with diode array detection

Commercial diode array detectors can be adapted to micro-liquid chromatography (LC) by replacing their detector cell with a miniaturized detector cell. In appropriate conditions, micro-LC can then generate an useful UV spectrum with 10 times less sample than needed with conventional LC. Some aspects of detector cell miniaturization are discussed. Results with two commercial diode array detectors, modified with a miniaturized detector cell, are presented.

A simple device for automated spectrophotometric kinetics using a diode array spectrophotometer

In this report we describe an automated system that rapidly and automatically mixes reagents and records results, such as spectrophotometric changes. It employs a commercial diode array spectrophotometer and a novel dilution chamber in a flow stream that allows repetitive spectrophotometric rate measurements at accurately measured incremental substrate concentrations. When applied to enzyme kinetic studies, initial velocities at 15 different substrate or inhibitor concentrations, or pH values, can be recorded in a few minutes with high reproducibility, i.e., standard deviations less than 1%, and high sensitivity. Reactions occur in an 8-μl flow cell and the reagent consumption is minimal. The concentration of incrementally diluted reagent in the cell is measured directly by means of an indicator dye added to the substrate. Michaelis-Menten parameters, inhibition constants, and pH profiles are determined for several enzymes including dehydrogenases producing NADH, a kinase requiring a coupled assay, and a hydrolase, carboxypeptidase A, in a reaction that produces a small decrease in absorbance.