Water Calorimetry Research Articles

Measuring the Gy:preventing convection in water calorimetry using a low duty cycle modulated beam

Abstract The most fundamental method for determining the absorbed dose to water by radiation beams is by measuring the consequent temperature increase of the water. Water calorimeters used for this purpose are not irradiated uniformly; thus spatial dose gradients within the calorimeter give rise to significant heat transport effects that interfere with the measurement of the small temperature rise associated with radiotherapy doses (e.g. 240 μK/Gy at a typical dose rate of 1 Gy/min). When subjected to periodic exposure to radiation, the calorimeter response (registered as temperature at a point in the water) reaches steady-state oscillation in which effects due to heat conduction and convection may be studied as a function of modulation frequency, so that appropriate correction factors can be derived for desired operating conditions. Theoretical treatment of the behavior is greatly simplified if experimental conditions can be arranged so that convection is negligible. While this is usually done by refrigerating the calorimeter to 4 °C, where the thermal expansion coefficient of water is zero (thus, buoyancy forces due to density fluctuations effectively vanish), we present evidence suggesting that such conditions can be achieved at room temperature by reducing the duty cycle, or effective “on” time of the radiation at a given shutter frequency. Results are presented here in the form of system transfer functions obtained under conditions of 50 and 3.5% duty cycle. Comparison of these measured transfer functions with the output of a three dimensional finite element model indicates that convection is greatly suppressed in the lower duty cycle case. Experimental uncertainty in these measurements is too large to conclude that convection is eliminated altogether, but more extended runs using this technique should enable a more definitive judgment as to whether the simpler conduction corrections alone can be applied.

Poster - Thur Eve - 64: A Water Calorimetry-Based Dosimetry Standard for Direct Measurement of Absolute Absorbed Dose in Scanning Proton Beam Delivery

The aim of this work is to develop a novel water calorimetry‐based primary standard in scanned proton beams. A relatively homogeneous dose distribution was painted with 15 layers of proton energies ranging between 128–150 MeV. A 4°C Domen‐type stagnant water calorimeter with a parallel‐plate vessel was used to directly measure the absolute absorbed dose to water D w on the central axis of the beam, at a point of minimal dose non‐uniformity (peak‐to‐trough variation < 0.25%). With the water calorimeter, the dose contributions of the individual layers to the point of measurement as well the total dose delivered were analyzed. In order to validate the calorimeter, absolute dosimetry in scattered beams (250 MeV) was performed and all calorimetric results were compared with D w determined using an Exradin T1 Mini Shonka chamber following TRS‐398 protocol. The overall 1‐sigma uncertainty on the absorbed dose determination was 0.38% (scattered) and 0.64% (scanned), compared to an estimated 1.9% uncertainty obtained with TRS‐398. The agreement between the calorimetric results and TRS‐398‐based ion chamber results was better than 0.14% (scattered) and 0.32% (scanned). The feasibility of water calorimetry in proton therapy in general and scanned beam delivery in particular has been shown both experimentally and numerically. Relative to current protondosimetry protocols (such as TRS‐398), not only would a water calorimetry‐based primary standard measure the absolute D w directly, but it would also improve the uncertainty on the overall dose measurement by a factor of 3.

Direct absorbed dose to water determination based on water calorimetry in scanning proton beam delivery

The aim of this manuscript is to describe the direct measurement of absolute absorbed dose to water in a scanned proton radiotherapy beam using a water calorimeter primary standard. The McGill water calorimeter, which has been validated in photon and electron beams as well as in HDR 192Ir brachytherapy, was used to measure the absorbed dose to water in double scattering and scanning proton irradiations. The measurements were made at the Massachusetts General Hospital proton radiotherapy facility. The correction factors in water calorimetry were numerically calculated and various parameters affecting their magnitude and uncertainty were studied. The absorbed dose to water was compared to that obtained using an Exradin T1 Chamber based on the IAEA TRS-398 protocol. The overall 1-sigma uncertainty on absorbed dose to water amounts to 0.4% and 0.6% in scattered and scanned proton water calorimetry, respectively. This compares to an overall uncertainty of 1.9% for currently accepted IAEA TRS-398 reference absorbed dose measurement protocol. The absorbed dose from water calorimetry agrees with the results from TRS-398 well to within 1-sigma uncertainty. This work demonstrates that a primary absorbed dose standard based on water calorimetry is feasible in scattered and scanned proton beams.

SU‐EE‐A2‐01: Direct Measurement of Absolute Absorbed Dose in Scanning Proton Beams Based on Water Calorimetry

Purpose: To develop a novel primary standard for absorbed dose to water DW measurement in scanning proton beams based on 4°C stagnant water calorimetry. Method and Materials: An in‐house built Domen‐type water calorimeter was used to measure the absolute DW directly in water at isocenter in a relatively flat SOBP region (peak‐to‐trough variation &lt; 0.25%) of a scattered (250 MeV proton) and a scanned (15 layers painted with proton energies between 128–150 MeV) proton beam. The heat loss correction factor, defined as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions, was numerically calculated using COMSOL MULTIPHYSICS™. The calorimeter was modeled, proton beam scanning was simulated, and a time‐dependent heat‐transport module was used. The calorimetry measurements were compared to dose results from an Exradin T1 Mini Shonka following TRS‐398 protocol. Results: The heat loss correction factor was calculated to be 0.4% in scattered and 4.7% in scanned beams. We calculated the total 1‐sigma uncertainty on dose measurements to be 0.38% (scattered) and 0.64% (scanned). This was in contrast to a 1.86% uncertainty on the dose measured using the ionization chamber. The chamber dose results agreed with the absolute dose to water measurements made using water calorimetry well to within uncertainty: A difference of 0.14% in scattered beams and 0.32% in scanned beams was observed between the two techniques. Conclusions: Feasibility of performing water calorimetry in proton therapy and specifically in scanned beam delivery has been shown both numerically and experimentally. By adopting a water calorimetry‐based primary standard and calibrating user ionization chambers directly in proton beams, national dosimetry laboratories can significantly reduce the uncertainty on dose determination in proton beams from the current 1.5%–2% level down to sub‐percent regime similar to the current standard of practice in high energy photon beams.

SU‐EE‐A2‐03: Evaluating AAPM TG‐43 In‐Water HDR 192Ir Brachytherapy Reference Dosimetry: A Comparison Study

Purpose: To investigate the accuracy of AAPM TG‐43 in HDR 192Ir brachytherapy in water reference dosimetry by comparing the protocol against ionometric and Gafchromic film calibration procedures introduced as well as a water calorimetry‐based primary standard. Methods and Materials: Dose to water Dwater was measured directly in water using an Exradin A1SL farmer‐type chamber and EBT‐1 Gafchromic films. The chamber had a NIST‐traceable 60Co calibration factor while the films were calibrated under 6 MV photons. Accurate Monte Carlo modeling and simulation of the chamber (egs++) and EBT Gafchromic films (DOSRZnrc) were performed to convert calibration factors of the two detectors from their respective conditions into 192Ir brachytherapy. The Dwater results were compared to measurements made using a Standard Imaging well‐type chamber following AAPM TG‐43 protocol and water calorimetry primary standard measurements. Results: By calculating the ratio of dose‐to‐water to dose‐to‐gas for the A1SL chamber under reference 60Co conditions and 192Ir setup conditions, the ionization measurements in 192Ir were converted to dose to water. The Monte Carlo calculations in film dosimetry revealed that if the intrinsic energy dependence of the film is negligible, a sensitometric curve obtained with 6 MV can be used in 192Ir measurements, with the energy dependence correction being 0.9971 (1σ=0.1%). The overall one‐sigma uncertainty on ionization chamber, Gafchromic film, and water calorimetry dose rate measurement amounts to 1.44%, 1.78%, and 1.96%, respectively. The indirect Dwater measurements from TG‐43 agreed to within 1.4% with ionometric measurements, 0.3% with Gafchromic measurements, and 0.6% with Calorimetric absolute dose measurements. Conclusions: Accurate ionometric and Gafchromic film based calibration protocols are introduced. For 192Ir brachytherapy, the 1‐sigma uncertainty of TG‐43 reference dosimetry was found to be better than 1.4%.

Implementation of water calorimetry in a 180 MeV scanned pulsed proton beam including an experimental determination of kQ for a Farmer chamber

Water calorimetric measurements have been performed in a 180 MeV scanned pulsed proton beam and the absorbed dose determined has been compared with the results obtained using two NE2571 Farmer chambers and the IAEA TRS-398 code of practice. The depth of measurement in water corresponded to a residual range of Rres = 16.5 cm, corresponding to a mean energy of about 150 MeV. Ionization chambers were calibrated in terms of the absorbed dose to water in 60Co at the Swedish Secondary Standard Dosimetry Laboratory, directly traceable to Bureau International des Poids et Mesures. The present experimental investigation has shown that water calorimetry is feasible in a high-energy scanned pulsed proton beam. When comparing the results obtained with water calorimetry and ionometry, the beam quality correction factor, kQ, could be determined for the two NE2571 ionization chambers used. The kQ-factor was found to be 1.032 ± 0.013, which is in good agreement with the factor tabulated in IAEA TRS-398 for this chamber type (1.039 ± 0.018). The present result has also been compared with a previously obtained result in a passively scattered proton beam having similar energy. This comparison yielded a 1.1% deviation, which is not significant considering the combined uncertainties of the two experimental determinations of kQ. The dominating contribution to the combined uncertainty stems from the correction factor for ion recombination in the scanned proton beam (1%), and further studies are required in order to reduce this uncertainty and reveal any possible differences in the kQ-factor between these two proton beam delivery techniques.

Direct measurement of absorbed dose to water in HDR I192r brachytherapy: Water calorimetry, ionization chamber, Gafchromic film, and TG-43

Gafchromic film and ionometric calibration procedures for HDR 192Ir brachytherapy sources in terms of dose rate to water are presented and the experimental results are compared to the TG-43 protocol as well as with the absolute dose measurement results from a water calorimetry-based primary standard. EBT-1 Gafchromic films, an A1SL Exradin miniature Shonka thimble type chamber, and an SI HDR 1000 Plus well-type chamber (Standard Imaging, Inc., Middleton, WI) with an ADCL traceable Sk calibration coefficient (following the AAPM TG-43 protocol) were used. The Farmer chamber and Gafchromic film measurements were performed directly in water. All results were compared to direct and absolute absorbed dose to water measurements from a 4 degrees C stagnant water calorimeter. Based on water calorimetry, the authors measured the dose rate to water to be 361 +/- 7 microGy/(h U) at a 55 mm source-to-detector separation. The dose rate normalized to air-kerma strength for all the techniques agree with the water calorimetry results to within 0.83%. The overall 1-sigma uncertainty on water calorimetry, ionization chamber, Gafchromic film, and TG-43 dose rate measurement amounts to 1.90%, 1.44%, 1.78%, and 2.50%, respectively. This work allows us to build a more realistic uncertainty estimate for absorbed dose to water determination using the TG-43 protocol. Furthermore, it provides the framework necessary for a shift from indirect HDR 192Ir brachytherapy dosimetry to a more accurate, direct, and absolute measurement of absorbed dose to water.

Development of a water calorimetry-based standard for absorbed dose to water in HDR I192r brachytherapy

The aim of this article is to develop and evaluate a primary standard for HDR 192Ir brachytherapy based on 4 degrees C stagnant water calorimetry. The absolute absorbed dose to water was directly measured for several different Nucletron microSelectron 192Ir sources of air kerma strength ranging between 21,000 and 38,000 U and for source-to-detector separations ranging between 25 and 70 mm. The COMSOL MULTIPHYSICS software was used to accurately calculate the heat transport in a detailed model geometry. Through a coupling of the "conduction and convection" module with the "Navier-Stokes incompressible fluid" module in the software, both the conductive and convective effects were modeled. A detailed uncertainty analysis resulted in an overall uncertainty in the absorbed dose of 1.90% (1 sigma). However, this includes a 1.5% uncertainty associated with a nonlinear predrift correction which can be substantially reduced if sufficient time is provided for the system to come to a new equilibrium in between successive calorimetric runs, an opportunity not available to the authors in their clinical setting due to time constraints on the machine. An average normalized dose rate of 361 +/- 7 microGy/(h U) at a source-to-detector separation of 55 mm was measured for the microSelectron 192Ir source based on water calorimetry. The measured absorbed dose per air kerma strength agreed to better than 0.8% (1 sigma) with independent ionization chamber and EBT-1 Gafchromic film reference dosimetry as well as with the currently accepted AAPM TG-43 protocol measurements. This work paves the way toward a primary absorbed dose to water standard in 192Ir brachytherapy.

Sci-Fri AM(2): Brachy-02: Realizing Absorbed Dose Directly for HDR Ir-192 Brachytherapy: Water Calorimetry and comparison to Ion Chamber, Gafchromic Film, and TG-43

A new standard based on direct measurement of absolute absorbed dose to water in HDR 192 Ir brachytherapy using water calorimetry is discussed. The calorimetry results are compared with the results from the in‐water measurement of absorbed dose using ionization chamber and EBT Gafchromic film reference dosimetry, as well as TG‐43. The water calorimetry measurements were performed using a spring‐loaded catheter holder system that mounts onto a parallel‐plate calorimetric vessel. The COMSOL MULTIPHYSICS™ software was used to perform the numerical heat‐transport calculations. The calorimetric measurements were performed with three different 192 Ir sources with nominal S k ranging from 31600–36940 U (i.e. 7.6–9.2 Ci activity) over a one‐year period. A source‐to‐detector separation d src ‐ det ranging between 51–67 mm, and an irradiation time ranging between 200–300 s were used. Based on water calorimetry, an average dose rate to water of 361±11 μGy/(h.U) at d src ‐ det =55 mm was measured. Chamber reference dosimetry resulted in a dose rate measurement of 366±9 μGy/(h.U), while EBT Gafchromic film reference dosimetry resulted in 368±11 μGy/(h.U). From TG‐43 protocol, we obtain a value of 364±15 μGy/(h.U), a 0.8% difference from water calorimetry. All Results agree well to within uncertainty. Our work paves the way to successful primary absorbed dose determination for 192 Ir using calorimetric techniques.

Sci-Fri AM(2): Brachy-06: The NRC Electron Beam Primary Standard Water Calorimeter

Introduction Electron dosimetry, for the majority of clinical dose measurements worldwide, is currently based on an ion chamber calibrated at the standards laboratory in a beam. Conversion factors obtained from a protocol such as AAPM TG-51 are then required to derive the dose in a linac electron beam. Electron beam water calorimetry offers a direct method to calibrate ion chambers in the clinical beams that they will be used in. This presentation details the development of a primary standard water calorimeter at the National Research Council in Ottawa and the method to calibrate ion chambers. Materials and Method The calorimeter is based on the NRC absorbed dose primary standard but the design is optimized for use in electron beams down to 8 MeV. After validation of the design the calorimeter was used to calibrate a set of cylindrical and parallel-plate chambers (NE2571, NACP-02, PTW Roos) in 12, 18 and 22 MeV electron beams from an Elekta Precise linac. Results The values obtained were compared to those obtained using TG-51 and found to agree at the 1% level. The measured energy dependence of ion chamber calibrations was compared with data from three other laboratories that have developed electron beam standards and agreement generally better than 1% was obtained there. The standard uncertainty in the calibration of an ion chamber is estimated to be 0.35%, which will give significant improvement in the measured dose uncertainty compared to using protocol-based values.

SU‐FF‐T‐378: Accurate Water Calorimetry Without Corrections for Heat Transport?

Purpose: To use the phase transform of the output‐time waveform from a water calorimeter subject to periodic radiation exposure for purposes of identifying shutter periods at which corrections for heat transport are unnecessary. Method and Materials: The response of a Domen‐type calorimeter of cylindrical geometry to radiation exposure in a 60Co beam is studied analytically, numerically and experimentally under conditions in which the beam is chopped by a shutter. Previous studies in our laboratory have shown an oscillatory dependence of the apparent dose rate upon shutter period which is the signature in the frequency domain of the effects of heat conduction and convection. However, because the apparent dose rate oscillates about the actual dose rate, it should be possible to identify one or more shutter periods at which the effects cancel and the resultant Fourier amplitude provides an accurate measure of the dose rate despite the presence of distortions in the time waveform. Identifying these nodes in the magnitude frequency response of the calorimeter is possible with accurate analytical and numerical models, but we are also exploring the relationship between real and imaginary components of the calorimeter frequency response to identify such points from the phase transform. Using COMSOL Multiphysics, models of our calorimeter have been run with heat sources that mimic a 10 cm × 10 cm 60Co beam. Output of these models is compared to series solutions of the heat transport equations and experimental data. Results: Computational and experimental data exhibit correlated oscillations in the phase and magnitude transforms that suggest a simple analytical relationship between them that would allow the identification of shutter periods at which the effects of heat transport are negligible. Conclusion: The existence of distinct shutter periods where heat transport distortions cancel is demonstrated, suggesting that water calorimetry might be done without such corrections.

SU‐FF‐T‐379: Procedure for Dose Rate Determination Via System Response of A Water Calorimeter

Purpose: To extract a consistent dose rate from water calorimetry measurements for all irradiation times. Method and Materials: Conventionally, the dose rate is determined at a length of time short compared to the heat conduction time in the system with the correction made by a model calculation for the given length of time (denoted t_open). We advocate a systematic approach by tracing the response of the calorimeter beyond the small t_open (10s of seconds) to 100s of seconds where conduction effect contributes significantly, requiring conduction correction to be valid over the entire time scale. This is made practical by modulating the radiation shutter at various t_open and by frequency domain analysis. The output from a 3D finite element model simulating the experimental protocol in a Co‐60 beam with a 20 mm radius vessel undergoes the same analysis as the experimental data to extract the response to unity radiation dose which can be scaled to any dose rate. Results: We scale the calculated curve to match the data by chi‐square minimization; the scaling factor represents the dose rate for all t_open values. Two sets of data were analyzed; the results for t_open from 60 s to 240 s are compared to Domen's transfer value (t_open=70 s) at standard geometries (SSD=100 cm), yielding a dose rate 1.0% and 0.9% below the transfer values (1 sigma statistical uncertainty 0.44% and 0.17%), respectively. A set of data taken at SSD of 77.4 cm gives a dose rate 3.4% below the transferred value. Ion chamber measurements will be made to resolve this discrepancy. Conclusion: A procedure to derive the dose rate by requiring self‐consistency between the modeled and the measured system response of the calorimeter over large time scales is a step closer to re‐establish a primary standard using a water calorimeter.

Revisão sistemática sobre métodos de determinação de gasto e consumo energéticos em crianças e adolescentes

The objective of this systematic review was to identify, describe and analyze methods used for determining energy expenditure and energy intake in clinical and population-based epidemiological studies. The study population was made up of children and adolescents aged from 6 to 18 years. The articles were identifi ed by means of searching the Medline database. A total of 141 articles were identifi ed, 32 of which were selected. From these, 10 articles were chosen, 6 of which were clinical epidemiological studies and 4 were populational epidemiological studies. The methods used to determine energy expenditure in the clinical studies were doubly labeled water and indirect calorimetry. In the populational studies, questionnaires on physical activity were used to determine the energy expended during physical activity. The methods used to evaluate nutritional intake, both in the clinical and populational epidemiological studies, were 24-hour dietary recalls, food records and food frequency questionnaires. The review goes on to discuss questions such as the manner in which these methods are employed and their validation and adaptation for populations of children and adolescents. It is concluded that care must be taken with the methodological aspects of epidemiological study design in order to avoid introducing bias to results and limiting conclusions.

Sci-Sat AM(2): Brachy-01: A novel HDR Ir-192 brachytherapy water calorimeter standard.

Parameters influencing the accuracy of absorbed dose measurements for HDR 192Ir brachytherapy using water calorimetry were investigated with the goal to develop a novel primary absorbed dose to water standard. To provide greater stability, flexibility, and accuracy in the source-detector distance dsrc-det positioning and measurement, a new spring-loaded catheter holder composed of two concentric cylindrical sleeves with multiple orthogonal adjusting screws was developed. The absorbed dose from Nucletron microSelectron-HDR 192Ir brachytherapy sources with air kerma strengths ranging between 21000-38000 U was studied. dsrc-det is optimized so as to balance signal-to-noise ratio (decreasing with increasing dsrc-det ) and temperature drift effects resulting from source self-heating. The irradiation times were adjusted to yield a minimum 1 Gy of dose at the measurement point. Successful measurements at dsrc-det ranging between 25-50 mm were performed. COMSOL MULTIPHYSICS™ software was used to determine the heat loss correction due to conduction defined as the ratio between temperature rise at a point under ideal conditions to realistic conditions (i.e., no conduction). An agreement of better than 6.5% was observed between TG-43 calculated and calorimetrically measured absorbed dose rates. The effects of convection where calculated to be negligible as the glass vessel provides a convective barrier significantly decoupling the water velocity in the interior and exterior of the vessel (water velocities were 1-2 orders of magnitude different). Our work paves the way to successful primary absorbed dose determination for radioactive sources using calorimetric techniques.

The new NMi orthovolt x-rays absorbed dose to water primary standard based on water calorimetry

A new water calorimeter for orthovolt x-rays has been developed at NMi. The purpose of this calorimeter is to provide calibrations in terms of absorbed dose to water. Four internationally accepted CCRI qualities (100–250 kV) have been characterized. Correction factors have been determined with measurements, Monte Carlo calculations and heat transport models. The results of the calculations have been validated against measurements. The absorbed dose to water calibration coefficient, NDw,WCM, with the new calorimeter has been compared to the NDw,NCS based on an air-kerma calibration coefficient, using the current NCS-10 dosimetry protocol. A good agreement is found for all beam qualities where the total uncertainty (1 SD) has decreased from 2.5% for NDw,NCS to better than 1.5% for NDw,WCM, using the new water calorimeter.

SU‐GG‐T‐291: Comparison Studies of Thermistor‐Based and Ultrasonic Water Calorimeters

Purpose: High‐precision ultrasonic thermometry offers a compelling technology for use in water calorimetry, since existing prototypes are able to register temperature changes corresponding to tens of mGy without the confounding difficulty of excess heat. In addition, its potential for providing three‐dimensional dose distributions in real time via CT may offer attractive options for reference dosimetry of nonstandard beams. The present study compares ultrasonic thermometry to thermistor‐based devices with respect to sensitivity to thermal signals, noise and susceptibility to systematic errors, with the objective of motivating further developments. Method and Materials: Single‐channel ultrasonic thermometer systems — one, a pulsed, phase‐locked loop; the other, a pulsed digital phase monitor — are compared to thermistor‐based water calorimeter cells monitored via a dual‐phase lock‐in amplifier. Radiation from incandescent sources, 60Co, and a Clinac 2100C are used to evaluate performance. Estimated signal‐detection limits, obtained from overall system resolution and characteristic noise, are compared with experimental output and analyzed with regard to frequency content and heat transport effects. Results: RMS noise in the ultrasonic systems exceeded both theoretical estimates and that observed with thermistor —based systems over comparable integration times; however, thermistor‐based systems are dominated by 1/f noise. Spatial averaging of temperature along the acoustic path reduces heat‐conduction effects in ultrasonic measurements, but the latter are relatively more susceptible to convection because of the open‐phantom design. At reduced exposure times, ultrasonic dose measurements are within a few percent of expected values. Conclusion: Noise reduction for both technologies is needed for rapid quantification of heat transport effects. For ultrasonic systems, improvements to low‐pass filter circuits and algorithms are recommended; for thermistor‐based systems, proper phasing of the shutter with respect to the bridge‐excitation signal may alleviate flicker noise.

TU‐D‐AUD C‐05: Development of a Novel Absorbed Dose to Water Calorimeter‐Based Standard for HDR Ir‐192 Brachytherapy

Purpose: To study parameters affecting the design of a new standard for absorbed dose to water for high dose rate iridium‐192 brachytherapy sources based on 4 °C stagnant water calorimetry. Method and Materials: The absorbed dose rate of a Nucleotron microSelectron‐HDR brachytherapy source was measured for sources with nominal air kerma strength Sk,air 21000 and 38000 U at several source‐detector separations dsrc‐det ranging between 25–30 mm. The irradiation time was adjusted to ensure a minimum dose of 1 Gy delivered to the measurement point. The heat‐loss correction was calculated using COMSOL MULTIPHYSICS™ and is defined as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions. Results: The heat‐loss corrected dose rate measurements had a standard deviation of 1.5%, while the agreement between our measurements and the calculated dose rate results obtained using the TG‐43 formalism was within 6.5%. The effects of convection were calculated to be negligible as the glass vessel serves as a convective barrier significantly decoupling the water velocity in the interior and exterior of the vessel (differences in velocity reaching one to two orders of magnitude). It was found that small uncertainties on dsrc‐det significantly affects the shape of the drift curve, while varying the exact value of SK,air, irradiation time, and differences in initial source temperature at the time of insertion into calorimeter have minimal effects. Differences between the shape of experimental and calculated drift curves can be used to optimize the calculation parameters. Conclusion: Feasibility of water calorimetry for absolute dosimetry for HDR brachytherapy sources has been shown both numerically and experimentally. We believe absolute absorbed dose rate measurements with uncertainties significantly better than 5% are possible. Revisions to experimental designs are underway to improve the robustness of setup and improve the repeatability and uncertainty on measurements.

A flat-plate calorimeter for concentrated solar flux evaluation

In order to evaluate the energy arriving at the focus of a solar concentrator named DEFRAC (from its Spanish acronym, Device for the Study of Concentrated Radiative Fluxes) a calorimetric study was carried out. We studied the heat transfer in the receiving plate of a calorimeter; the distribution of temperatures during the transient and steady states were obtained both theoretically and experimentally. We show that the calorimeter is close to ambient temperature which validates the use of the cold water calorimetry technique. Since the heat losses are reduced it is possible to increase the exactitude and precision during the thermal evaluation. Based on an energy balance, the solar energy absorbed by the receiver of the DEFRAC and the global heat losses to the environment are quantified.

An absorbed dose to water standard for HDR brachytherapy sources based on water calorimetry: Numerical and experimental proof‐of‐principle

Water calorimetry is an established technique for absorbed dose to water measurements in external beams. In this paper, the feasibility of direct absorbed dose measurements for high dose rate (HDR) iridium-192 (192Ir) sources using water calorimetry is established. Feasibility is determined primarily by a balance between the need to obtain sufficient signal to perform a reproducible measurement, the effect of heat loss on the measured signal, and the positioning uncertainty affecting the source-detector distance. The heat conduction pattern generated in water by the Nucletron microSelectron-HDR 192Ir brachytherapy source was simulated using COMSOL MULTIPHYSICS software. Source heating due to radiation self-absorption was calculated using EGSnrcMP. A heat-loss correction k(c) was calculated as the ratio of the temperature rise under ideal conditions to temperature rise under realistic conditions. The calorimeter setup used a parallel-plate calorimeter vessel of 79 mm diameter and 1.12 mm thick front and rear glass windows located 24 mm apart. Absorbed dose was measured with two sources with nominal air kerma strengths of 38 000 and 21 000 U, at source-detector separations ranging from 24.7 to 27.6 mm and irradiation times of 36.0 to 80.0 s. The preliminary measured dose rate per unit air kerma strength of (0.502 +/- 0.007) microGy/(s U) compares well with the TG-43 derived 0.505 microGy/(s U). This work shows that combined dose uncertainties of significantly less than 5% can be achieved with only modest modifications of current water calorimetry techniques and instruments. This work forms the basis of a potential future absolute dose to water standard for HDR 192Ir brachytherapy.

TH‐C‐M100E‐01: Numerical Feasibility Study of a Novel Absorbed Dose to Water Calorimeter‐Based Standard for 192Ir HDR Brachytherapy

Purpose: To study the feasibility of developing a new standard for absorbed dose to water based on water calorimetry for high dose rate iridium‐192 brachytherapy sources. Method and Materials: The heat conduction pattern generated in water by the Nucleotron microSelectron‐HDR brachytherapy source was simulated using Comsol Multiphysics™ software. Source self‐heating due to self‐attenuation of photons was calculated with GEANT4. A smooth, well‐behaved three‐dimensional function was fit to the entire dose distribution data using TableCurve3D™. The heat‐loss correction Kc was calculated as the ratio of the temperature in the calorimeter under ideal conditions to realistic conditions. Results: The feasibility of a water calorimeter based absorbed dose standard is determined by a balance between the requirements to obtain sufficient signal to perform a reproducible measurement, the effects of heat loss on the measured signal, and the positioning uncertainties. Due to self‐absorption, the source equilibrium temperature was found to be above the ambient temperature by a constant amount that depends only on setup conditions and source activity. For the source inside its nylon‐12 catheter inserted into water, the steady state excess temperature per unit source activity was found to be 0.5671 K/Ci. The source temperature reached 96% of its steady state temperature after 60 s. Conduction correction factors Kc were calculated for several exposure times and at various measurement points away from the source inside the calorimeter. A total exposure time between 140 s and 240 s at a distance that receives a minimum of 1 Gy of dose was found to allow reduction of Kc to below 0.1% of unity. Conclusions: Water calorimetry for HDR brachytherapy is feasible and total uncertainties of significantly better than 5% on the dose can be achieved with current water calorimetry techniques and instruments.