Investigation of a synthetic diamond detector response in kilovoltage photon beams

Determination of intrinsic energy dependence of point-like inorganic scintillation detector in brachytherapy

BACKGROUND

Inorganic scintillation detectors (ISDs) are promising for in vivo dosimetry in brachytherapy (BT). ISDs have fast response, providing time resolved dose rate information, and high sensitivity, attributed to high atomic numbers. However, the conversion of the detector signal to absorbed dose-to-water is highly dependent on the energy spectrum of the incident radiation. This dependence is comprised of absorbed dose energy dependence, obtainable with Monte Carlo (MC) simulation, and the absorbed dose-to-signal conversion efficiency or intrinsic energy dependence requiring measurements. Studies have indicated negligible intrinsic energy dependence of ZnSe:O-based ISDs in Ir-192 BT. A full characterization has not been performed earlier.

PURPOSE

This study characterizes the intrinsic energy dependence of ZnSe:O-based ISDs for kV X-ray radiation qualities, with energies relevant for BT.

METHODS

Three point-like ISDs made from fiber-coupled cuboid ZnSe:O-based scintillators were calibrated at the Swedish National Metrology Laboratory for ionizing radiation. The calibration was done in terms of air kerma free-in-air,K air ${K}_{{\mathrm{air}}}$ , in 13 X-ray radiation qualities, Q $Q$ , from 25 to 300 kVp (CCRI 25-250 kV and ISO 4037 N-series), and in terms of absorbed dose to water,D w ${D}_{\mathrm{w}}$ , in a Co-60 beam,Q 0 ${Q}_0$ . The mean absorbed dose to the ISDs, relative toK air ${K}_{{\mathrm{air}}}$ andD w ${D}_{\mathrm{w}}$ , were obtained with the MC code TOPAS (Geant4) using X-ray spectra obtained with SpekPy software and laboratory filtration data and a generic Co-60 source. The intrinsic energy dependence was determined as a function of effective photon energy,E e f f ${E}_{eff}$ , (relative to Co-60). The angular dependence of the ISD signal was measured in a 25 kVp (0.20 mm Al HVL) and 135 kVp beam (0.48 mm Cu HVL), by rotating the ISDs 180° around the fiber's longitudinal axis (perpendicular to the beam). A full 360° was not performed due to setup limitations. The impact of detector design was quantified with MC simulation.

RESULTS

Above 30 keVE e f f ${E}_{eff}$ the intrinsic energy dependence varied with less than 5 ± 4% from unity for all detectors (with the uncertainty expressed as the mean of all expanded measurement uncertainties for individualE e f f ${E}_{eff}$ above 30 keV, k = 2). Below 30 keV, it decreased with up to 17% and inter-detector variations of 13% were observed, likely due to differences in detector geometry not captured by the simulations using nominal geometry. In the 25 kVp radiation quality, the ISD signal varied with 24% over a ∼45° rotation. For 135 kVp, the corresponding variation was below 3%. Assuming a 0.05 mm thicker layer of reflective paint around the sensitive volume changed the absorbed dose with 6.3% at the lowestE e f f ${E}_{eff}$ , and with less than 2% at higher energies.

CONCLUSION

The study suggests that the ISDs have an intrinsic energy dependence relative to Co-60 lower than 5 ± 4% in radiation qualities withE e f f ${E}_{eff}\ $ > 30 keV. Therefore, they could in principle be calibrated in a Co-60 beam quality and transferred to such radiation qualities with correction factors determined only by the absorbed dose energy dependence obtained from MC simulations. This encourages exploration of the ISDs' applications in intensity modulated BT with Yb-169 or other novel intermediate energy isotopes.

A simple calibration routine for small inorganic scintillation detectors for in vivo dosimetry during brachytherapy

BACKGROUND

In vivo dosimetry (IVD) is rarely performed in brachytherapy (BT), allowing potential dose misadministration to go unnoticed. This study presents a clinical routine-calibration method of detectors for IVD in high (HDR) and pulsed dose rate (PDR) Ir-192 BT.

PURPOSE

To evaluate the dosimetric precision and feasibility of an in-clinic calibration routine of detectors for IVD in afterloading BT.

METHODS

Calibrations were performed in a PMMA phantom with two needles inserted 20 mm apart. The source was loaded in one of the needles at 15 dwells for 10 s. The detector was placed in the other needle, and its signal was recorded. The mean signal at each dwell position was fitted to the expected dose rate with the calibration factor and the detector's longitudinal position being free parameters. The method was tested with an inorganic scintillation detector using one Ir-192 FlexiSource HDR and two Ir-192 GammaMedPlus PDR sources and followed by validation measurements in water.

RESULTS

The standard measurement uncertainty (k = 1) of the calibration factor in absolute terms (Gy/s) was 3.2/3.4% for the HDR/PDR source. The uncertainty was dominated by source strength uncertainty, and the precision of the method was <1%. The mean ± 1SD of the difference in measured and expected dose rate during validation was 1.5 ± 4.7% (HDR) and 0.0 ± 4.1% (PDR) with a positional uncertainty in the setup of 0.33/0.23 mm (HDR/PDR) (k = 1).

CONCLUSION

A precise and feasible in-clinic calibration method for IVD and source strength consistency tests in BT was presented.

Monte Carlo characterization of high atomic number inorganic scintillators for in vivo dosimetry in 192 Ir brachytherapy

BACKGROUND

There is increased interest in vivo dosimetry for ¹⁹² Ir brachytherapy (BT) treatments using high atomic number (Z) inorganic scintillators. Their high light output enables construction of small detectors with negligible stem effect and simple readout electronics. Experimental determination of absorbed-dose energy dependence of detectors relative to water is prevalent, but it can be prone to high detector positioning uncertainties and does not allow for decoupling of absorbed-dose energy dependence from other factors affecting detector response.

PURPOSE

To investigate which measurement conditions and detector properties could affect their absorbed-dose energy dependence in BT in vivo dosimetry.

METHODS

We used a general-purpose MC code penelope for the characterization of high-Z inorganic scintillators with the focus on ZnSe (Z¯=32). Two other promising media CsI (Z¯=54) and Al₂ O₃ (Z¯=11) were included for comparison in selected scenarios. We determined absorbed-dose energy dependence of crystals relative to water under different scatter conditions (calibration phantom 12 × 12 × 30 cm³ , characterization phantoms 20 × 20 × 20 cm³ , 30 × 30 × 30 cm³ , 40 × 40 × 40 cm³ , and patient-like elliptic phantom 40 × 30 × 25 cm³ ). To mimic irradiation conditions during prostate treatments, we evaluated whether the presence of pelvic bones and calcifications affect ZnSe response. ZnSe detector design influence was also investigated.

RESULTS

In contrast to low-Z organic and medium-Z inorganic scintillators, ZnSe and CsI media have substantially greater absorbed-dose energy dependence relative to water. The response was phantom-size dependent and changed by 11 % between limited- and full-scatter conditions for ZnSe, but not for Al₂ O₃ . For a given phantom size, a part of the absorbed-dose energy dependence of ZnSe is caused not due to in-phantom scatter but due to source anisotropy. Thus, the absorbed-dose energy dependence of high-Z scintillators is a function of not only the radial distance but also the polar angle. Pelvic bones did not affect ZnSe response, whereas large and intermediate size calcifications reduced it by 9 % and 5 %, respectively, when placed midway between the source and the detector.

CONCLUSIONS

Unlike currently prevalent low- and medium-Z scintillators, high-Z crystals are sensitive to characterization and in vivo measurement conditions. However, good agreement between MC data for ZnSe in the present study and experimental data for ZnSe:O by Jørgensen et al (2021) suggest that detector signal is proportional to the average absorbed dose to the detector cavity. This enables an easy correction for non-TG43-like scenarios (e.g., patient sizes and calcifications) through MC simulations. Information that should be provided to the clinic by the detector vendors. This article is protected by copyright. All rights reserved.

Electronic stopping power of diamond for electrons and positrons

We assess a new data set of mass electronic stopping powers of diamond for electrons and positrons. To this end, an optical energy-loss function (OELF) for this material is generated from available experimental dielectric properties of the valence band and photoabsorption mass attenuation coefficients, supplemented with a Drude-type analytical expression to interpolate at intermediate energies. From this synthetic OELF, a mean excitation energy equal to (88.5 ± 2.0) eV is extracted and the density-effect correction is evaluated using Fano's simplified method. These quantities then allow calculating the mass electronic stopping power of diamond for electrons and positrons by means of the relativistic Bethe formula. The present results for 1 keV-1 GeV electrons in diamond are 0.9%-3.3% smaller than those corresponding to graphite as tabulated in ICRU Report 90.

Impact of theI-value of diamond on the energy deposition in different beam qualities

Diamond detectors are increasingly employed in dosimetry. Their response has been investigated by means of Monte Carlo (MC) methods, but there is no consensus on what mass densityρ, mean excitation energyIand number of conduction electrons per atomn_ceto use in the simulations. The ambiguity occurs due to its seeming similarity with graphite (both are carbon allotropes). Except for the difference inρbetween crystalline graphite (2.265 g cm^-3) and diamond (3.515 g cm^-3), their dielectric properties are assumed to be identical. This is incorrect, and the two materials should be distinguished: (ρ= 2.265 g cm^-3,I= 81.0 eV,n_ce= 1) for graphite and (ρ= 3.515 g cm^-3,I= 88.5 eV,n_ce= 0) for diamond. Simulations done with the MC codepenelopeshow that the energy imparted in diamond decreases by up to 1% with respect to 'pseudo-diamond' (ρ= 3.515 g cm^-3,I= 81.0 eV,n_ce= 0) depending on the beam quality and cavity thickness. The energy imparted changed the most in cavities that are small compared with the range of electrons. The difference in the density-effect term relative to graphite was the smallest for diamond owing to an interplay effect thatρ,Iandn_cehave on this term, in contrast to pseudo-diamond media when eitherρorIalone were adjusted. The study also presents a parameterized density-effect correction function for diamond that may be used by MC codes like EGSnrc. Theestarprogram assumes thatn_ce= 2 for all carbon-based materials, hence it delivers an erroneous density-effect correction term for graphite and diamond. Despite the small changes of the energy imparted in diamond simulated with two differentIvalues and expected close-to-negligible deviation from the published small-field output correction data, it is important to pay attention to material properties and model the medium faithfully.

Monte Carlo calculation of the relative TG-43 dosimetry parameters for the INTRABEAM electronic brachytherapy source

The INTRABEAM system (Carl Zeiss Meditec AG, Jena, Germany) is an electronic brachytherapy (eBT) device designed for intraoperative radiotherapy applications. To date, the INTRABEAM x-ray source has not been characterized according to the AAPM TG-43 specifications for brachytherapy sources. This restricts its modelling in commercial treatment planning systems (TPSs), with the consequence that the doses to organs at risk are unknown. The aim of this work is to characterize the INTRABEAM source according to the TG-43 brachytherapy dosimetry protocol. The dose distribution in water around the source was determined with Monte Carlo (MC) calculations. For the validation of the MC model, depth dose calculations along the source longitudinal axis were compared with measurements using a soft x-ray ionization chamber (PTW 34013) and two synthetic diamond detectors (microDiamond PTW TN60019). In our results, the measurements in water agreed with the MC model calculations within uncertainties. The use of the microDiamond detector yielded better agreement with MC calculations, within estimated uncertainties, compared to the ionization chamber at points of steeper dose gradients. The radial dose function showed a steep fall-off close to the INTRABEAM source ([Formula: see text]10 mm) with a gradient higher than that of commonly used brachytherapy radionuclides (192Ir, 125I and 103Pd), with values of 2.510, 1.645 and 1.232 at 4, 6 and 8 mm, respectively. The radial dose function partially flattens at larger distances with a fall-off comparable to that of the Xoft Axxent® (iCAD, Inc., Nashua, NH) eBT system. The simulated 2D polar anisotropy close to the bare probe walls showed deviations from unity of up to 55% at 10 mm and 155°. This work presents the MC calculated TG-43 parameters for the INTRABEAM, which constitute the necessary data for the characterization of the source as required by a TPS used in clinical dose calculations.

Suitability of the microDiamond detector for experimental determination of the anisotropy function of High Dose Rate 192 Ir brachytherapy sources

PURPOSE

To investigate the suitability of the microDiamond detector (mDD) type 60019 (PTW-Freiburg, Germany) to measure the anisotropy function F(r,θ) of High Dose Rate (HDR) ¹⁹² Ir brachytherapy sources.

METHODS

The HDR ¹⁹² Ir brachytherapy source, model mHDR-v2r (Elekta AB, Sweden), was placed inside a water tank within a 4F plastic needle. Four mDDs (mDD1, mDD2, mDD3, and mDD4) were investigated. Each mDD was placed laterally with respect to the source, and measurements were performed at radial distances r = 1 cm, 3 and 5 cm, and polar angles θ from 0° to 168°. The Monte Carlo (MC) system EGSnrc was used to simulate the measurements and to calculate phantom effect, energy dependence and volume-averaging correction factors. F(r,θ) was determined according to TG-43 formalism from the detector reading corrected with the MC-based factors and compared to the consensus anisotropy function _CON F(r,θ).

RESULTS

At 1 cm, the differences between measurements and MC simulations ranged from -0.8% to +0.8% for θ = 0° and from -2.1% to + 2.3% for θ ≠ 0°. At 3 and 5 cm, the differences ranged from +1.4% to +3.9% for θ = 0°, and from -0.4% to +2.9% for θ ≠ 0°. All differences were within the uncertainties (k = 2). At small angles, the phantom effect correction was up to -1.9%. This effect was mainly caused by the air between source and needle tip. The energy correction was angle-independent everywhere. For small angles at 1 cm, the volume-averaging correction was up to -2.9% and became less important for larger angles and distances. The differences of the measured F(r,θ) corrected with the MC-based factors to _CON F(r,θ) ranged from -1.0% to +3.4% for mDD1, -2.2% to +4.2% for mDD2, -2.5% to +4.0% for mDD3, and -2.6% to +3.4% for mDD4. All differences were within the uncertainties (k = 2) except one at (3 cm, 0°). For all the mDDs, F(r,0°) was always higher than _CON F(r,0°), with average differences of +3.1% (1 cm), +3.6% (3 cm), and +1.9% (5 cm). The inter-detector variability was within 2.9% (1 cm), 1.8% (3 cm), and 3.4% (5 cm).

CONCLUSIONS

A reproducible method and experimental setup were presented for measuring and validating F(r,θ) of an HDR ¹⁹² Ir brachytherapy source in a water phantom using the mDD. The phantom effect and the volume-averaging need to be taken into account, especially for the smaller distances and angles. Good agreement to _CON F(r,θ) was obtained. The discrepancies at (1 cm, 0°), accurately predicted by the MC results, may suggest a reconsideration of _CON F(r,θ), at least for this position. The slight overestimations at (3 cm,0°) and (5 cm,0°), both in comparison to _CON F(r,θ) and MC results, may be due to an underestimation of the air volume between source and needle tip, dark current, intrinsic over-response of the mDDs, or radiation-induced charge imbalance in the detector's components. The results indicate that the mDD is a valuable tool for measurements with HDR ¹⁹² Ir brachytherapy sources and support its employment for the determination and validation of TG-43 parameters of such sources.