Small Ionization Chamber Research Articles

Quality assurance for radiotherapy in prostate cancer: Point dose measurements in intensity modulated fields with large dose gradients

Purpose: We aimed to evaluate an optimization algorithm designed to find the most favorable points to position an ionization chamber (IC) for quality assurance dose measurements of patients treated for prostate cancer with intensity-modulated radiotherapy (IMRT) and fields up to 10 cm × 10 cm. Methods and Materials: Three cylindrical ICs (PTW, Freiburg, Germany) were used with volumes of 0.6 cc, 0.125 cc, and 0.015 cc. Dose measurements were made in a plastic phantom (PMMA) at 287 optimized points. An algorithm was designed to search for points with the lowest dose gradient. Measurements were made also at 39 nonoptimized points. Results were normalized to a reference homogeneous field introducing a dose ratio factor, which allowed us to compare measured vs. calculated values as percentile dose ratio factor deviations ΔF (%). A tolerance range of ΔF (%) of ±3% was considered. Results: Half of the ΔF (%) values obtained at nonoptimized points were outside the acceptable range. Values at optimized points were widely spread for the largest IC (i.e., 60% of the results outside the tolerance range), whereas for the two small-volume ICs, only 14.6% of the results were outside the tolerance interval. No differences were observed when comparing the two small ICs. Conclusions: The presented optimization algorithm is a useful tool to determine the best IC in-field position for optimal dose measurement conditions. A good agreement between calculated and measured doses can be obtained by positioning small volume chambers at carefully selected points in the field. Large chambers may be unreliable even in optimized points for IMRT fields ≤10 cm × 10 cm.

Sci-Sat AM (1) General-07: Water equivalent dosimeter array for small fields external beam radiotherapy

Scintillation dosimeters consisting of a scintillating fiber coupled to an optical fiber and read with a CCD camera have shown accurate dose measurement in external beam radiotherapy. This work presents the next step, which is to investigate the development of a multi-channel dosimeter array in conjunction with a CCD camera for dose measurement that includes small fields. The light collection of the CCD camera was studied to evaluate the number of detectors that can be read simultaneously. We also looked at possible sources of dose perturbation with a single-fiber detector surrounded by other optical fibers. We then constructed a prototype array with 10 detectors and compared with measurements taken with small ion chambers. No dose perturbations were seen when a plastic optical fiber was used to transmit the scintillation light to the CCD. Depth dose curves measured in water with a scintillation dosimeter with up to 75 plastic optical fibers in the beam showed no discrepancy to within 0.3% when compared to the same curve taken with an ion chamber and without the plastic fibers. Agreement within 0.6% was seen for output factor measured with the scintillator and the A16 Exradin chamber for fields of 1×1 cm$^{2}$ and 5×0.5 cm$^{2}$. The ten-fiber prototype allowed precise evaluation of profile and depth dose curves in a single irradiation. This work has shown that use of a multi-channel scintillation dosimeter is feasible. The prototype of 10 detectors produced excellent results and could be extended up to 3000 detectors in the near future.

Calibration of an x-ray cabinet unit for radiobiology use

A Faxitron sealed x-ray cabinet, operated at 100 kV, was modified to irradiate monkey testicles, to a uniform, accurately calibrated dose, for work aimed at investigating spermatogenesis in children undergoing radiotherapy. An aluminium filter was added to increase the beam quality and a lead collimating system manufactured to reduce the beam size to between 1 and 4 cm diameter. Percentage depth doses and profiles were analysed and relative in-air outputs measured with a selection of small (0.2 cc, 0.015 cc) ion chambers. The absolute calibration of the unit was carried out in a 10 × 10 cm2 beam with a 0.6 cc chamber. Backscatter factors were based on standard tables, but then modified according to experimental results with thermoluminescent dosimeters (TLD) in a phantom to account for reduced scatter in the irradiation situations. A suitable irradiation set-up was devised for the monkeys, to ensure accuracy of delivered dose to the target volume and minimize the dose to the surrounding healthy tissue. The homogeneity throughout the testes was calculated to be well within ±5%, using a parallel-opposed irradiation technique. The TLD measured doses to the testes on three monkeys were lower than the calculated doses by 3 to 6%. Following modifications to the standard percentage depth doses to account for changes in scatter conditions, these differences became ±3%. The uncertainties on both calculated and measured dose were estimated to be approximately ±3.2% at 1 SD.

Monte Carlo dose calculations for a 6-MV photon beam in a thorax phantom

In this study we evaluated the accuracy of the Monte Carlo (MC) and effective path length (EPL) methods for dose calculations in the inhomogeneous thorax phantom. The Philips SL 75/5 linear accelerator head was modeled using the MCNP4C Monte Carlo code. An anatomic inhomogeneous thorax phantom was irradiated with a 6-MV photon beam, and the doses along points of the central axis of the beam were measured by a small ionization chamber. The central axis relative dose was calculated by the MCNP4C code and the EPL method in a conventional treatment planning system. The results of calculations and measurements were compared. For all measured points on the thorax phantom the results of the MC method were in agreement with the actual measurement (local difference was less than 2%). For the EPL method, the amount of error was dependent on the field size and the point location in the phantom. The maximum error was +19.5 and +26.8 for field sizes of 10 x 10 and 5 x 5 cm2 for lateral irradiation. Our study showed large, unacceptable errors for EPL calculations in the lung for both field sizes. The accuracy of the MC method was better than the recommended value of 3%. Thus, application of this method is strongly recommended for lung dose calculations, especially for small field sizes.

TU‐E‐224A‐04: Determination of Total Scatter Factors for Stereotactic Radiation Fields by Optimized Fitting of Readings From a Small Ion Chamber and a Mini‐Diode

Purpose: To determine the total scatter factors (TSF) for small radiation fields on the BrainLab micro‐MLC (M3) and cones by fitting the readings from two detectors: a small ion chamber and a mini‐diode. Method and Materials: Outputs were measured for 6MV beams using a small ion chamber (Wellhofer IC3, active volume 28mm3) and a stereotactic diode (Scanditronix DEB050, active volume 0.017mm3). It has been reported that ion chambers (IC) underestimate output for small fields (<20 mm) and diodes overestimate output for large fields (>50mm). Herein outputs at intermediate field sizes (20–50 mm) were used to combine data from the two detectors. The output from IC was normalized to a reference 10×10cm field; the output from diode was scaled to match the output from IC at intermediate field sizes. The scaling was determined with modified least square optimization. The final TSF was composed of scaled diode output for small fields, IC output for large fields, and their averages for intermediate fields. Results: For square fields formed by M3 and secondary jaws, the scaled TSF from diode matched the TSF from IC to within ±0.6% for field sizes 24–60 mm. The diode overestimated TSF of the reference field by 3.2% at depth of 5cm. The IC underestimated TSF by 23%, 5%, 1.4% for 6, 12, 18 mm fields, respectively. Similar results were found for the cones. TSF from diode was 0.702 for the 5mm cone, an improvement of 29% from IC measurement. Conclusion: We have demonstrated that neither IC or diode alone provides accurate TSF for stereotactic fields of all sizes. The diode is more accurate than IC for fields <20 mm, however, we show that the diode should be cross‐compared with an IC using radiation fields of intermediate sizes. A modified least square optimization method is presented for this purpose.

WE-D-224C-01: Water Equivalent Multichannel Dosimeter Arrays for External Beam Radiotherapy

Introduction: Scintillation dosimeters consisting of a single scintillating fiber coupled to an optical fiber and read with a CCD camera have shown accurate dose measurement in high-energy external beam radiotherapy. This work presents the next step, which is to investigate the development of a multi-channel dosimeter array in conjunction with a CCD camera for photodetection. Methods and Materials: The light collection of the CCD camera (Apogee Alta U2000c) was first studied to evaluate the maximum number of detectors that can be read simultaneously. We then looked at possible sources of dose perturbation with a single-fiber detector surrounded by other optical fibers. We constructed a prototype array with 10 detectors. Finally, depth-dose and cross-plane profiles were compared with measurements taken with small ion chambers (Exradin models A14 and A16). Results: We found up to 3000 individual fibers could be detected simultaneously with the full CCD chip. No dose perturbations were seen when a plastic optical fiber was used to transmit the light from the scintillating fiber to the CCD. Depth dose curves measured in water with a single scintillation dosimeter with up to 75 plastic optical fiber in the beam showed no discrepancy to within 0.3% when compared to the same curve taken with an ion chamber and without the plastic fibers. The ten-fiber prototype allowed precise evaluation of profile and depth dose curves in a single irradiation. Conclusion: This work has shown that use of a multi-channel scintillation dosimeter is feasible. The prototype of 10 detectors produced excellent results and could be extended up to 3000 detectors in the near future.

SU‐DD‐A1‐03: IMRT Quality Assurance: Dosimetric Assessment of Three Current Methods

Purpose: To compare traditional IMRT quality assurance using film dosimetry and small volume ionization chamber measurements with two new commercial products, the Wellhofer® MatriXX ionization chamber array and Varian® Portal Dosimetry. Available analysis software, hardware requirements and approximate operator times for data preparation, measurement and analysis will also be examined. Method and Materials: Fluence patterns from several 6X and 18X IMRT treatment plans for pelvis and head and neck radiotherapy patients were measured using radiographic film, the MatriXX array, and the Portal Dosimetry array. In each case the fluence pattern predicted by the treatment planning system was compared to the measured fluence pattern using ordinary γ‐analysis. Absolute dose at a point in a low‐gradient region of the fluence was also measured in the solid water phantom with an ionization chamber and compared to the dose prediction of the TPS. The absolute dose measured at the same point by the MatriXX array was also compared. Results: The absolute dose measurements made in a region of low‐gradient using an ionization chamber were, on the average, within 3% of the TPS predicted dose. The absolute dose measurements made using the MatriXX were, on the average, within 5% of the predicted dose. The ion chamber and MatriXX agreed to within 3%. An average of about 4% of pixels failed an ordinary γ‐analysis using 5% dose agreement and 3mm DTA criteria for both film and MatriXX measurements. A smaller percentage of pixels measured using Portal Dosimetry failed. The time spent preparing the data was comparable for all methods. Data measurement and analysis times were significantly reduced using the MatriXX and Portal Dosimeter procedures. Conclusions: This work indicates significant time savings for the new methods. In addition, the MatriXX system measures absolute dose at each chamber position.

SU‐FF‐T‐286: Inhomogeneity Correction in Brachytherapy

Purpose: Dose calculation accuracy at interfaces and in medium with inhomogeneities is still not well established in brachytherapy. Commercial planning systems do not account for the presence of inhomogeneity, while theoretical calculations in inhomogeneities have poor agreement with measurements. The wide use of brachytherapy for breast, prostate, brain and lung treatment in recent development necessitates the need for improvement in brachytherapy dose calculations. In this study, we present an experimental approach to investigate the effect of inhomogeneity for Ir‐192 and Cs‐137. Method and Materials: To study the effect of lung in brachytherapy dose calculation, measurements are performed in a polystyrene phantom embedded with cork sheets of density 0.25 g/cm3. Radioactive source used in the study are Ir‐192 (HDR source) and Cs‐137 tube. Two conditions are studied, one mimics the partial breast treatment with the Mammosite technique, and the other brachytherapy in lung after wedge resection. Dose is measured with a small volume parallel plate ion chamber, embedded in polystyrene or in cork by varying the position of the chamber from the source. Inhomogeneity correction factor (ICF), defined as dose in inhomogenerity / dose in water at the same distance (Di/Dw) is calculated. Results: The calculated ICF values qualitatively agree with our measured values. However there are significant differences that could be due to inaccuracies in build up factors and other assumptions made in the calculations. The dose in medium falls exponentially with distance. The measured ICF is strongly dependent on the position and energy. For example, at 2 cm, ICF for the full inhomogeneity condition is approximately 3.5 and 8.5 for Cs‐137 and Ir‐192 respectively. Conclusions: The magnitude of correction is significantly dependent on the distance and energy as well as the condition of partial or full inhomogeneity. TPS vendors should provide ICF for all commonly used brachytherapy sources.

Absorbed dose measurements of a handheld 50 kVP X-ray source in water with thermoluminescence dosemeters

Absorbed dose rate measurements of a 50 kV(p) handheld X-ray probe source in a water phantom are described. The X-ray generator is capable of currents of up to 40 microA, and is designed for cranial brachytherapy and intraoperative applications with applicators. The measurements were performed in a computer-controlled water phantom in which both the source and the detectors are mounted. Two different LiF thermoluminescence dosemeter (TLD) phosphors were employed for the measurements, MTS-N (LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P). Two small ionisation chambers (0.02 and 0.0053 cm(3)) were also employed. The TLDs and chambers were positioned in watertight mounts made of water-equivalent plastic. The chambers were calibrated in terms of air-kerma rate, and conventional protocols were used to convert the measurements to absorbed dose rate. The TLDs were calibrated at National Institute of Standards and Technology (NIST) in terms of absorbed dose rate using a (60)Co teletherapy beam and narrow-spectrum X-ray beams. For the latter, absorbed dose was inferred from air-kerma rate using calculated air-kerma-to-dose conversion factors. The reference points of the various detectors were taken as the center of the TLD volumes and the entrance windows of the ionisation chambers. Measurements were made at distances of 3-45 mm from the detector reference point to the source center. In addition, energy dependence of response measurements of the TLDs used was made using NIST reference narrow spectrum X-ray beams. Measurement results showed reasonable agreement in absorbed dose rate determined from the energy dependence corrected TLD readings and from the ionisation chambers. Volume averaging effects of the TLDs at very close distances to the source were also evident.

Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods

The purpose of this investigation is to study the feasibility of using an alternative method to commission stereotactic radiosurgery beams shaped by micro multi-leaf collimators by using Monte Carlo simulations to obtain beam characteristics of small photon beams, such as incident beam particle fluence and energy distributions, scatter ratios, depth–dose curves and dose profiles where measurements are impossible or difficult. Ionization chambers and diode detectors with different sensitive volumes were used in the measurements in a water phantom and the Monte Carlo codes BEAMnrc/DOSXYZnrc were used in the simulation. The Monte Carlo calculated data were benchmarked against measured data for photon beams with energies of 6 MV and 10 MV produced from a Varian Trilogy accelerator. The measured scatter ratios and cross-beam dose profiles for very small fields are shown to be not only dependent on the size of the sensitive volume of the detector used but also on the type of detectors. It is known that the response of some detectors changes at small field sizes. Excellent agreement was seen between scatter ratios measured with a small ion chamber and those calculated from Monte Carlo simulations. The values of scatter ratios, for field sizes from 6 × 6 mm2 to 98 × 98 mm2, range from 0.67 to 1.0 and from 0.59 to 1.0 for 6 and 10 MV, respectively. The Monte Carlo calculations predicted that the incident beam particle fluence is strongly affected by the X–Y-jaw openings, especially for small fields due to the finite size of the radiation source. Our measurement confirmed this prediction. This study demonstrates that Monte Carlo calculations not only provide accurate dose distributions for small fields where measurements are difficult but also provide additional beam characteristics that cannot be obtained from experimental methods. Detailed beam characteristics such as incident photon fluence distribution, energy spectra, including composition of primary and scattered photons, can be independently used in dose calculation models and to improve the accuracy of measurements with detectors with an energy-dependent response. Furthermore, when there are discrepancies between results measured with different detectors, the Monte Carlo calculated values can indicate the most correct result. The data set presented in this study can be used as a reference in commissioning stereotactic radiosurgery beams shaped by a BrainLAB m3 on a Varian 2100EX or 600C accelerator.

Integral system for detecting evaporation residues in fusion reactions

We have designed and constructed an integral system that involves two electrostatic deflectors followed by a small ionization chamber with a multi-wire-proportional-counter in its entrance. The function of the former part is to separate evaporation residues from beam ions using their different electric rigidities while the latter is to detect these evaporation residues being distinguished from the beam-like particles originated in slits. Up to now, both devices have been tested and evaluated performing different experiments: the double-electrostatic deflector through scattering studies and the ionization chamber by means of scattering and fusion reactions. We present a brief description of the two devices and some results obtained in different tests.

High-intensity positive beams extracted from a compact double-chamber ion source

This work presents the design and development of a simple ion source, the associated ion extraction optics, and the beam transport of a low-energy and high-current proton accelerator. In its actual version, the ion source can deliver positive proton currents up to 100mA. This rather high beam current is achieved by adding a small ionization chamber between the discharge chamber containing the filament and the extraction electrode of the ion source. Different parameters of the ion source and the injection beam line are evaluated by means of computer simulations to optimize the beam production and transmission.

Microionization chamber for reference dosimetry in IMRT verification: clinical implications on OAR dosimetric errors

Intensity modulated radiotherapy (IMRT) has become a treatment of choice in many oncological institutions. Small fields or beamlets with sizes of 1 to 5 cm2 are now routinely used in IMRT delivery. Therefore small ionization chambers (IC) with sensitive volumes ⩽0.1 cm3are generally used for dose verification of an IMRT treatment. The measurement conditions during verification may be quite different from reference conditions normally encountered in clinical beam calibration, so dosimetry of these narrow photon beams pertains to the so-called non-reference conditions for beam calibration. This work aims at estimating the error made when measuring the organ at risk's (OAR) absolute dose by a micro ion chamber (μIC) in a typical IMRT treatment. The dose error comes from the assumption that the dosimetric parameters determining the absolute dose are the same as for the reference conditions. We have selected two clinical cases, treated by IMRT, for our dose error evaluations. Detailed geometrical simulation of the μIC and the dose verification set-up was performed. The Monte Carlo (MC) simulation allows us to calculate the dose measured by the chamber as a dose averaged over the air cavity within the ion-chamber active volume (Dair). The absorbed dose to water (Dwater) is derived as the dose deposited inside the same volume, in the same geometrical position, filled and surrounded by water in the absence of the ion chamber. Therefore, the Dwater/Dair dose ratio is the MC estimator of the total correction factor needed to convert the absorbed dose in air into the absorbed dose in water. The dose ratio was calculated for the μIC located at the isocentre within the OARs for both clinical cases. The clinical impact of the calculated dose error was found to be negligible for the studied IMRT treatments.

Low-dose measurement with a MOSFET in high-energy radiotherapy applications

The ability of a MOSFET dosimetry system to measure low therapeutic doses has been evaluated for accuracy for high-energy X-ray radiotherapy applications. The MOSFET system in high sensitivity mode produces a dose measurement reproducibility of within 10%, 4% and 2.5% for 2, 5 and 10 cGy dose assessment, respectively. This is compared to 7%, 4% and 2% for an Attix parallel plate ionisation chamber and 20%, 7% and 3.5% for a Wellhofer IC4 small volume ionisation chamber. Results for our dose standard thimble ionisation chamber and low-noise farmer dosimeter were 2%, 0.5% and 0.25%, respectively for these measurements. The quoted accuracy of the MOSFET dosimetry system is partially due to the slight non-linear dose response (reduced response) with age of the detector but mainly due to the intrinsic variations in measured voltage differential per applied dose. Results have shown that the MOSFET dosimetry system provides an adequate measure of dose at low dose levels and is comparable in accuracy to the Attix parallel plate ionisation chambers for relative dose assessment at levels of 2– 10 cGy . The use of the MOSFET dosimeter at low doses can extend the life expectancy of the device and may provide useful information for areas where low dose assessment is required.

Miniature ionization chamber detector developed for X-ray microprobe measurements.

A windowless small ionization chamber detector has been developed for monitoring the intensity of the microbeam at the ID18F microprobe end-station of the European Synchrotron Radiation Facility. The small dimensions of the ionization chamber (10 mm along the beam direction and 5 mm perpendicular to it) make it possible to place it very close to the sample. A pinhole of diameter 50 microm was used for defining the entrance window of the ionization chamber; thus the small counter can be used as an order-selecting aperture while measuring simultaneously the intensity after the aperture. In the present work the technical characteristics, such as the current-voltage curve, stability and linearity, of the small monitor have been tested.

128 Oral Is there any benefit in using small ion chambers for verification of target dose delivered by IMRT?

128 Oral Is there any benefit in using small ion chambers for verification of target dose delivered by IMRT?

Construction and operation of an ion chamber consisting of lead plates immersed in 2,2,4,4 tetramethylpentane

A small test ion chamber has been constructed using 2,2,4,4 Tetramethylpentane as the ionizing liquid in direct contact with an array of lead plates used as the high voltage and ground planes. The transient response resulting from the passage of cosmic ray muons through the ion chamber was monitored for a period of 54 weeks to measure possible degradation of the signal due to contamination of the ionizing liquid that may have come from the lead plates. Over the first 17 weeks of the experiment, the loss of pulse height was measured to be less than 11% per year at 95 confidence. During the following 37 weeks, a more sensitive measurement was consistent with zero loss, or a limit of less than 2.9% per year at 95% confidence.

Dosimetry of pulsed clinical proton beams by a small ionization chamber.

Response of a micro volume (0.01 ml) ionization chamber has been studied with pulsed proton beams which are used for clinical purposes and has been compared with those of some JARP ionization chambers (0.6 ml). All chambers used had been calibrated by standard 60Co beams at the Electrotechnical Laboratory (ETL) and exposure calibration factors, Nx, were obtained on advance. Two methods are used to compensate the general recombination which occurs during pulsed beam irradiations: theoretical correction by a Boag's formulation and a modified two-voltage technique. An evaluation of absolute absorbed dose-to-water is performed on the basis of the protocol provided by ICRU report 59. The results imply that, to a first approximation, both chambers indicate the almost same result within 2% when unknown chamber-dependent parameters of the micro chamber are tentatively assumed to be identical to those of the JARP chamber for the calibration with 60Co beams. The about 1.5% discrepancy observed in the response of both chambers is not discussible due to presumably 1-2% uncertainty of the protocol of ICRU report 59 which does not include any chamber-dependent corrections for the perturbation effects in proton beams.

Assessment of new small-field detectors against standard-field detectors for practical stereotactic beam data acquisition

Two new detectors (0.015 cm3 ion chamber from PTW, 0.6 mm diameter diode from Scanditronix AB) designed specifically for use in small stereotactic fields were compared against similar, more routine, detectors (0.125 cm3 ion chamber, parallel plate chamber, shielded and unshielded diodes and film). Percentage depth doses, tissue maximum ratios, off-axis ratios and relative output factors were compared for circular fields in the 40-12.5 mm diameter range, with a view to identifying the optimum detector for stereotactic beam data acquisition. Practical suggestions for beam data collection and analysis are made, with an emphasis on what is achievable practically in radiotherapy departments where the primary demand is to provide a routine service. No single detector was found to be ideal, and neither of the two new measurement devices had any significant advantages over more routine devices, in the situations measured. Although the new 0.015 cm3 ion chamber was an improvement on a 0.125 cm3 ion chamber in the measurement of profiles, it was still too large when compared with a diode. The new small diode had a low signal to noise ratio which made reliable data difficult to extract and its only advantage is possibly improved resolution in fields smaller than the range tested. The use of a larger unshielded diode is recommended for all measurements, with the additional cross-checking of data against at least one small ion chamber and film. A simple method of obtaining reliable output data from the detectors used is explained.

Operation of an ion chamber consisting of lead plates immersed in 2,2,4,4 tetramethylpentane

A small test ion chamber has been constructed using 2,2,4,4 tetramethylpentane as the ionizing liquid and an array of lead plates as the high voltage and ground planes. Signals resulting from ionization deposited in the ion chamber by cosmic-ray muons were monitored for possible degradation from contamination that may have come from the lead plates. At the 95% confidence level, the loss of pulse height was measured over the first 17 weeks of the experiment to be less than 11% per year. A more sensitive measurement over the following 37 weeks was consistent with zero loss, or a limit of less than 2.9% per year at 95% confidence.