Absorbed dose calorimetry.

This article reviews the development and summarizes the state-of-the-art in absorbed dose calorimetry for all the common clinical beam modalities covered in reference dosimetry codes of practice, as well as for small and nonstandard fields, and brachytherapy. It focuses primarily on work performed in the last ten years by national laboratories and research institutions and is not restricted to primary standard instruments. The most recent absorbed dose calorimetry review article was published over twenty years ago by Ross and Klassen (1996 Phys. Med. Biol. 41 1-29), and even then, its scope was limited to water calorimeters. Since the application of calorimetry to the measurement of radiation has a long and often overlooked history, a brief introduction into its origins is provided, along with a summary of some of the landmark research that have shaped the current landscape of absorbed dose calorimeters. Technical descriptions of water and graphite calorimetry are kept general, as these have been detailed extensively in relatively recent review articles (e.g. McEwen and DuSautoy (2009 Metrologia 46 S59-79) and Seuntjens and Duane (2009 Metrologia 46 S39-58). The review categorizes calorimeters by the radiation type for which they are applied; from the widely established standards for Co-60 and high-energy x-rays, to the prototype calorimeters used in high-energy electrons and hadron therapy. In each case, focus is placed on the issues and constraints affecting dose measurement in that beam type, and the innovations developed to meet these requirements. For photons, electrons, proton and carbon ion beams, a summary of the ionization chamber beam quality conversion factors (k Q ) determined using said calorimeters is also provided. The article closes with a look forward to some of the most promising new techniques and areas of research and speculates about the future clinical role of absorbed dose calorimetry.

Development and application of a water calorimeter for the absolute dosimetry of short-range particle beams.

In this work, we describe a new design of water calorimeter built to measure absorbed dose in non-standard radiation fields with reference depths in the range of 6-20 mm, and its initial testing in clinical electron and proton beams. A functioning calorimeter prototype with a total water equivalent thickness of less than 30 mm was constructed in-house and used to obtain measurements in clinical accelerator-based 6 MeV and 8 MeV electron beams and cyclotron-based 60 MeV monoenergetic and modulated proton beams. Corrections for the conductive heat transfer due to dose gradients and non-water materials was also accounted for using a commercial finite element method software package. Absorbed dose to water was measured with an associated type A standard uncertainty of approximately 0.4% and 0.2% for the electron and proton beam experiments, respectively. In terms of thermal stability, drifts were on the order of a couple of hundred µK min-1, with a short-term variation of 5-10 µK. Heat transfer correction factors ranged between 1.021 and 1.049. The overall combined standard uncertainty on the absorbed dose to water was estimated to be 0.6% for the 6 MeV and 8 MeV electron beams, as well as for the 60 MeV monoenergetic protons, and 0.7% for the modulated 60 MeV proton beam. This study establishes the feasibility of developing an absorbed dose transfer standard for short-range clinical electrons and protons and forms the basis for a transportable dose standard for direct calibration of ionization chambers in the user's beam. The largest contributions to the combined standard uncertainty were the positioning (⩽0.5%) and the correction due to conductive heat transfer (⩽0.4%). This is the first time that water calorimetry has been used in such a low energy proton beam.

Improving the energy response of external beam therapy (EBT) GafChromicTM dosimetry films at low energies (≤ 100 keV).

PURPOSE: Purpose of this work is to investigate the effects of varying the active layer composition of external beam therapy (EBT) GafChromic(TM) films on the energy dependence of the film, as well as try to develop a new prototype with more uniform energy response at low photon energies (⩽ 100 keV). METHODS: First, the overall energy response (S(AD, W)(Q)) of different commercial EBT type film models that represent the three different generations produced to date, i.e., EBT, EBT2, and EBT3, was investigated. Pieces of each film model were irradiated to a fixed dose of 2 Gy to water for a wide range of beam qualities and the corresponding S(AD, W)(Q) was measured using a flatbed document scanner. Furthermore, the DOSRZnrc Monte Carlo code was used to determine the absorbed dose to water energy dependence of the film, f(Q). Moreover, the intrinsic energy dependence, kbq(Q), for each film model was evaluated using the corresponding S(AD, W)(Q) and f(Q). In the second part of this study, the authors investigated the effects of changing the chemical composition of the active layer on SAD, W(Q). Finally, based on these results, the film manufacturer fabricated several film prototypes and the authors evaluated their S(AD, W)(Q). RESULTS: The commercial EBT film model shows an under response at all energies below 100 keV reaching 39% ± 4% at about 20 keV. The commercial EBT2 and EBT3 film models show an under response of about 27% ± 4% at 20 keV and an over response of about 16% ± 4% at 40 keV.S(AD, W)(Q) of the three commercial film models at low energies show strong correlation with the corresponding f(-) (1)(Q) curves. The commercial EBT3 model with 4% Cl in the active layer shows under response of 22% ± 4% at 20 keV and 6% ± 4% at about 40 keV. However, increasing the mass percent of chlorine makes the film more hygroscopic which may affect the stability of the film's readout. The EBT3 film prototype with 7.5% Si shows a significant improvement in the energy response at very low energies compared to the commercial EBT3 films with 4% Cl. It shows under response of 15% ± 5% at about 20 keV to 2% ± 5% at about 40 keV. However, according to the manufacturer, the addition of 7.5% Si as SiO2 adversely affected the viscosity of the active fluid and therefore affected the potential use in commercial machine coating. The latest commercial EBT3 film model with 7% Al as Al2O3 shows an overall improvement in SAD, W(Q) compared to previous commercial EBT3 films. It shows under response at all energies <100 keV, varying from 20% ± 4% at 20 keV to 6% ± 4% at 40 keV. CONCLUSIONS: The energy response of films in the energy range <100 keV can be improved by adjusting the active layer chemical composition. Removing bromine eliminated the over response at about 40 keV. The under response at energies ≤ 30 keV is improved by adding 7% Al to the active layer in the latest commercial EBT3 film models.

Sci-Thur PM: YIS - 09: Development of a graphite probe calorimeter for absolute clinical dosimetry: Numerical design optimization, prototyping and experimental proof-of-concept.

In this work, the feasibility of absolute dose to water measurements using a small-scale graphite probe calorimeter (GPC) in a clinical environment is established. A numerical design optimization study was conducted by simulating the heat transfer in the GPC resulting from irradiation using a finite element method software package. The choice of device shape, dimensions and materials was made to minimize the heat loss in the sensitive volume of the GPC. The resulting design, which incorporates a novel aerogel-based thermal insulator, was built in-house. Absorbed dose to water measurements were made under standard conditions in a 6 MV 1000 MU/min photon beam and subsequently compared against TG-51 derived values. The average measured dose to water was 95.7 ±1.4 cGy/100 MU, as compared to an expected value of 96.6 cGy/100 MU. The Monte Carlo-calculated graphite to water dose conversion factor was 1.099, while the derived heat loss correction factors varied between 1.005 and 1.013. The most significant sources of uncertainty were the repeatability (type A, 1.4%) and thermistor calibration (type B, 2.1%). The contribution of these factors to the overall uncertainty is expected to decrease significantly upon the implementation of active thermal stabilization provided by a temperature controller and direct electrical calibration, respectively. This work demonstrates the feasibility of using the GPC as a practical clinical absolute photon dosimeter and will serve as the basis for a miniaturized version applicable to small and composite fields.

Poster - Thur Eve - 45: Commissioning of the Varian ECLIPSE eMC algorithm for clinical electron treatment planning.

Fast electron Monte Carlo systems have been developed commercially, and implemented for clinical practice in radiation therapy clinics. In this work the Varian eMC (electron Monte Carlo) algorithm was commissioned for clinical electron beams of energies between 6 MeV and 20 MeV. Beam outputs, PDDs and profiles were measured for 29 regular and irregular cutouts using the IC-10 (Wellhöfer) ionization chamber. Detailed percentage depth dose comparisons showed that the agreement between measurement and eMC for different characteristic points on the PDD are generally less than 1 mm and always less than 2 mm, with the eMC calculated values being lower than the measured values. Of the 145 measured output factors, 19 cases fail a ±2% agreement but only 8 cases fail a ±3% agreement between calculation and measurement. Comparison of central axis dose distributions for two electron energies (9, and 20 MeV) for a 10 × 10 cm2 field, centrally shielded with Pb of width 0 cm (open), 1, 2 and 3 cm, shows agreement to within 3% except near the surface. Comparison of central axis dose distributions for 9 MeV in heterogeneous phantoms including bone and lung inserts showed agreement of 1 mm and 3 mm respectively with measured TLD data. The overall agreement between measurement and eMC calculation has enabled us to begin implementing this calculation model for clinical use.

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

PURPOSE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: This work demonstrates that a primary absorbed dose standard based on water calorimetry is feasible in scattered and scanned proton beams.

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.