Cavity Theory Research Articles

DESIGN OF H-PLANE INDUCTANCE DIAPHRAGM WAVEGUIDE BAND-PASS FILTER FOR MILLIMETER IMAGING FRONTEND

This study presents an equivalent circuit and a design of an H-plane waveguide bandpass fllter (BPF) with chamfer. Traditionally, only thin inductive diaphragm with no chamfers considered in the direct-coupled cavity theory, but this will lead to di-culties in the BPF manufacturing. During manufacturing process the chamfer cannot be avoided, and its equivalent circuit and efiects on frequency shifting are investigated in this paper. A new design method is proposed in order to compensate the efiect of chamfer in the half-wavelength resonator connection between the inductance diaphragm and the waveguide. A modifled empirical formula and corresponding procedure are provided for designing such fllters. The working center frequency and 3dB bandwidths (BW) are simulated considering difierent chamfer radius. The simulated center frequencies are 18GHz, 26GHz, 34GHz and 42GHz, and BWs are 2.265%, 2.5%, 10%, 15% and 20%. Results show that the modifled formula, which conforms better with the simulated results, is superior to the traditional formula. Two H-plane waveguide BPFs are manufactured with center frequency 26GHz with 2.5% BW and 34GHz with 2.265% BW. The results of the modifled formula are in good agreement with measured ones.

English

This study investigates the possibility of achieving unity energy response using the very sensitive CaSO4:Dy/Teflon thermoluminescent detector at photon energies below 100 keV. Accurate measurements for the energy responses were carried-out using ultra thin, 3 micron thick, TLD discs with average grain size of about 3μm. The present work shows that the experimental reduction factor for the 15 % phosphor loading with average grain size of 3 μm is in good agreement with the calculation based on the cavity theory but at grain size of 1.0 μm. This indicates that the cavity theory is underestimating the real experimental reduction factor in the energy response. The present work expect that unity energy response can be achieved with grain size of about 1 μm.

TE<sub>0</sub> Mode Resonant Properties of the Rubidium Clock Microwave Cavity Loaded with Ceramic Material

Resonant properties of a Microwave cavity with ceramic material for a new type of rubidium clock is studied by mode matching method. In order to study main factors influencing resonant characteristics, resonant frequencies of TE011 mode are calculated. In addition, the effect of the ceramic material on resonant frequency are also analyzed. This work is of great significance for cavity design and theory perfection in atomic clock.

Implementation and validation of a fluence pencil kernels model for GaN-based dosimetry in photon beam radiotherapy

Gallium nitride (GaN), a direct-gap semiconductor that is radioluminescent, can be used as a transducer yielding a high signal from a small detecting volume and thus potentially suitable for use in small fields and for high dose gradients. A common drawback of semiconductor dosimeters with effective atomic numbers higher than soft tissues is that their responses depend on the presence of low energy photons for which the photoelectric cross section varies strongly with atomic number, which may affect the accuracy of dosimetric measurements. To tackle this ‘over-response’ issue, we propose a model for GaN-based dosimetry with readout correction. The local photon spectrum is calculated by convolving fluence pencil kernel spectra with the beam aperture fluence distribution. The response of a GaN detector is modelled by combining large cavity theory and small cavity theory for the low and high energy components of the local spectrum. Monte Carlo simulations are employed for determination of specific correction factors for different GaN transducer sizes and irradiation conditions. Some model parameters such as the cut-off energy and partitioning energy are discussed. The accuracy of the GaN dosimetric response model has been evaluated for tissue phantom ratio experiments along the central axis. These experiments have shown that calculated and measured GaN responses stay within ±3% at all depths beyond the build-up depth. The calculated GaN response factor is also in good agreement with measured data (±2.5%). The validated model with response compensation improves significantly the accuracy of dosimetric measurements: below 2.5% deviation as compared to 13% without compensation, for a 10 × 10 cm2 field, at depth from 1.5 to 22 cm.

SU-E-T-82: Change of Ionization Chamber Correction Factors (Ppol , Pion , KWall ) with Chamber Walls of Different Materials in Continuous and Pulsed Beams

Purpose: To study the effect of wall material on correction factors for polarity effect Ppol , collection efficiency Pion and scatter and attenuation in the chamber wall/central electrode Kwall , in continuous and pulsed beams. Methods: An Exadin A12 ionization chamber was modified to have geometrically identical chamber walls built from aluminum and copper. Measurements were performed using the different walls in both 60Co, and Varian‐Clinac 21EX 6 MV beams. Ppol was measured using AAPM TG51 protocol. Pion was obtained form measurement data, where the collected charge was measured at 10 voltage settings and used to form Jaffe‐plots. Measurements were compared against Monte Carlo simulated data (egs++/egs_chamber). Kwall values were also calculated using CAVRZnrc. Results: For all beams and all wall materials, Ppol was found to be less than the 0.3 % limit recommended in TG‐51. Saturation charges (extrapolated from the Jaffe‐plots) were observed to increase with wall materials of increasing atomic number Z. The breakdown of the Boag‐predicted linearity of Jaffe‐plots in the near‐saturation region was observed for all beam types and wall materials. The ratio of the saturation charges (relative to the C552 wall) were predicted by cavity theory, and agreed with simulations to within 9.8 % for 60Co and 5.2 % for 6 MV. Pion measured by the two‐voltage technique and from Jaffe‐plots were all found to be less than the 1.05 accepted upper limit recommended by TG‐51. Kwall showed that attenuation and scatter for the Cu wall influences the measured signal by 1.9 % for 60Co. Conclusion: Ppol and Pion were measured for an Exradin A12 chamber with C552, Al, and Cu wall materials. The basic behavior of Jaffe‐plots is independent of wall material. As expected, Kwall increases drastically for high Z walls with decreasing beam energies.

Modeling gan dosimetric response to therapeutic photon beam radiation

Introduction The use of Gallium Nitride (GaN), a direct-gap semiconductor, has been recently pro-posed as a radioluminescent transducer for real-time in-vivo dosimetric applications [1]. GaN has the following advantages: a lower e–h pair creation energy than conventional scintillators, a high prompt radioluminescence yield resulting from the high probability of band edge radiative recombination. Nevertheless, it shares the common drawback of silicon diodes or MOSFET dosimeters with an over response relative to water for low energy scattered photons. The aim of our study is to compensate for this dependence through modelling, which may lead us to improve the accuracy of GaN-based dosimetric systems. Our modeling work presented here is focused on GaN dosimetric response for therapeutic photo beam irradiation from medical linacs. Experimental The dosimetric response of a GaN transducer was first modeled by combining large cavity theory and the Spencer– Attix small cavity theory respectively for the low and high energy components of the local spectrum according the approach proposed in [2]. The local spectra were calculated from fluence pencil kernels, and Monte Carlo simulations were performed to determine some key parameters included into the model. Results and discussion The developed model was used to compute TMR curves of a implantable GaN-based probe. A good agreement with measurements is achieved which validates the modeling work. Furthermore, we used the model to calculate GaN response factor at different depths in water for 6 MV photon beam. The obtained results (not shown here) confirm that GaN response factor is proportional to the square field aperture (with a proportionality coefficient which varies with depth). This linear relationship was experimentally observed in clinical conditions [3].

Using cavity theory to describe the dependence on detector density of dosimeter response in non-equilibrium small fields

The dose imparted by a small non-equilibrium photon radiation field to the sensitive volume of a detector located within a water phantom depends on the density of the sensitive volume. Here this effect is explained using cavity theory, and analysed using Monte Carlo data calculated for schematically modelled diamond and Pinpoint-type detectors. The combined impact of the density and atomic composition of the sensitive volume on its response is represented as a ratio, Fw,det, of doses absorbed by equal volumes of unit density water and detector material co-located within a unit density water phantom. The impact of density alone is characterized through a similar ratio, Pρ −, of doses absorbed by equal volumes of unit and modified density water. The cavity theory is developed by splitting the dose absorbed by the sensitive volume into two components, imparted by electrons liberated in photon interactions occurring inside and outside the volume. Using this theory a simple model is obtained that links Pρ − to the degree of electronic equilibrium, see, at the centre of a field via a parameter Icav determined by the density and geometry of the sensitive volume. Following the scheme of Bouchard et al (2009 Med. Phys. 36 4654–63) Fw,det can be written as the product of Pρ −, the water-to-detector stopping power ratio , and an additional factor Pfl −. In small fields changes little with field-size; and for the schematic diamond and Pinpoint detectors Pfl − takes values close to one. Consequently most of the field-size variation in Fw,det originates from the Pρ − factor. Relative changes in see and in the phantom scatter factor sp are similar in small fields. For the diamond detector, the variation of Pρ − with see (and thus field-size) is described well by the simple cavity model using an Icav parameter in line with independent Monte Carlo estimates. The model also captures the overall field-size dependence of Pρ − for the schematic Pinpoint detector, again using an Icav value consistent with independent estimates.

Shielding Effectiveness of Carbon–Fiber Composite Aircraft Using Large Cavity Theory

This paper extends reverberation chamber theory to include chambers constructed out of non-metallic composite materials. This extension allows reverberation chamber theory to predict the shielding effectiveness (SE) of modern aluminum and composite aircraft. Existing theory is based on a power balance approach for aperture-excited cavities, and this paper extends it to include leakage through the cavity walls. Cavity excitation and power dissipation mechanisms are examined in detail, and the cavity SE is related to cavity energy loss in terms of the “quality factor.” SE measurements were made on a partially assembled Uncrewed Aerial System constructed with a carbon-fiber composite skin. The test-analysis agreement shows a high degree of correlation.

Design of compact planar crossover using Sierpinski carpet microstrip patch

The design of a planar four-port microstrip crossover is presented. The design starts by using a half-wavelength square patch and two sets of orthogonal feeding lines. Based on the cavity theory, operation of the patch crossover is analysed. Full-wave simulations and measurements are used to support the analysis. By etching the patch as a Sierpinski carpet, resonant frequency can be lowered significantly, and this property can be employed to reduce the size of the conventional patch crossover. It is shown that the size of the crossover can be reduced by 23% with even a better performance when a second-order Sierpinski carpet microstrip patch is used. Full-wave simulations and measurements are used to validate the proposed design method. The measured data show less than 1 dB insertion loss, more than 10 dB return loss, more than 17 dB isolation and about 0.1 ns group delay deviation across 10% fractional bandwidth centred at 2.4 GHz.

IMPROVED ANALYTICAL METHOD FOR PLASMA ELECTRON DENSITY MEASUREMENT BY RESONANT CAVITY PERTURBATION THEORY

A method of plasma density measurement based on microwave resonant cavity perturbation (by Kornegay (14)) is described. Resonant cavity theory was analyzed and a resonant cavity with special structure was designed for measuring the low density plasma. In the middle of the closed cavity, there were cut-ofi tubes which were extended a little into the cavity to get through the plasma. It was found that the distribution of the electrical fleld intensity was the densest near the cut-ofi tubes when the cylindrical cavity operating with TM010 mode. By using the method of resonant equivalent circuit analysis, both the amplitudes and phases of the Scattering matrices (S matrices) were obtained before the plasma came and at the time of the plasma passing through. Then the electron line density (Ne) and the electron-molecule collision frequency for momentum transfer (vm) were calculated. A modifled formula was proposed based on our simulation which was conducted in HFSS and experimental results. With the comparison of our results and Kornegay's, it was found that the accuracy of the plasma dielectric constant calculation was improved about 5 percent.

電子に対する制限質量衝突阻止能の計算・検索ソフトウェアの開発

In external radiotherapy, the absorbed doses are measured using an ionization process in a gas-filled ionization chamber and estimated by the extended cavity theory. The calculation requires both the W-value of cavity gas and the restricted mass collision stopping powers (L/ρ) for the gas and medium. ICRU Report 37 gives us the data regarding the mass collision stopping powers (S/ρ) for several elements and chemical compounds or mixtures. However, there are no detailed data for L/ρ. In this study, we developed an in-house program to calculate the L/ρ for arbitrary substances by the use of the equation described in ICRU Report 37. With this program, we can search the calculated L/ρ easily. When we calculated L/ρ for chemical compounds and mixtures with implementation of Bragg's additivity rule, a large error of density effect corrections was observed. Therefore, our program adopted both the mean excitation energy and density effect correction obtained by ESTAR, which was developed by Berger et al. With adoption of these values, the calculation accuracy of our program was improved. Our program is useful to search L/ρ for radiation dosimetry in radiotherapy.

Study on TE<sub>011</sub> Mode’s Resonant Characteristic of Complicated Microwave Cavity with Ceramics Layer in Rubidium Frequency Standard

Complicated microwave cavity loaded with ceramics is first investigated by mode matching method in rubidium frequency standard. In order to study main factors influencing resonant characteristics, resonant frequencies of TE011 mode are calculated and compared with commercial software (CST) results. The results show good agreement between theoretical computations and CST simulations. This work is of great significance for cavity design and theory perfection in atomic frequency standard.

Validating plastic scintillation detectors for photon dosimetry in the radiologic energy range

Photon dosimetry in the kilovolt (kV) energy range represents a major challenge for diagnostic and interventional radiology and superficial therapy. Plastic scintillation detectors (PSDs) are potentially good candidates for this task. This study proposes a simple way to obtain accurate correction factors to compensate for the response of PSDs to photon energies between 80 and 150 kVp. The performance of PSDs is also investigated to determine their potential usefulness in the diagnostic energy range. A 1-mm-diameter, 10-mm-long PSD was irradiated by a Therapax SXT 150 unit using five different beam qualities made of tube potentials ranging from 80 to 150 kVp and filtration thickness ranging from 0.8 to 0.2 mmAl + 1.0 mmCu. The light emitted by the detector was collected using an 8-m-long optical fiber and a polychromatic photodiode, which converted the scintillation photons to an electrical current. The PSD response was compared with the reference free air dose rate measured with a calibrated Farmer NE2571 ionization chamber. PSD measurements were corrected using spectra-weighted corrections, accounting for mass energy-absorption coefficient differences between the sensitive volumes of the ionization chamber and the PSD, as suggested by large cavity theory (LCT). Beam spectra were obtained from x-ray simulation software and validated experimentally using a CdTe spectrometer. Correction factors were also obtained using Monte Carlo (MC) simulations. Percent depth dose (PDD) measurements were compensated for beam hardening using the LCT correction method. These PDD measurements were compared with uncorrected PSD data, PDD measurements obtained using Gafchromic films, Monte Carlo simulations, and previous data. For each beam quality used, the authors observed an increase of the energy response with effective energy when no correction was applied to the PSD response. Using the LCT correction, the PSD response was almost energy independent, with a residual 2.1% coefficient of variation (COV) over the 80-150-kVp energy range. Monte Carlo corrections reduced the COV to 1.4% over this energy range. All PDD measurements were in good agreement with one another except for the uncorrected PSD data, in which an over-response was observed with depth (13% at 10 cm with a 100 kVp beam), showing that beam hardening had a non-negligible effect on the PSD response. A correction based on LCT compensated very well for this effect, reducing the over-response to 3%. In the diagnostic energy range, PSDs show high-energy dependence, which can be corrected using spectra-weighted mass energy-absorption coefficients, showing no considerable sign of quenching between these energies. Correction factors obtained by Monte Carlo simulations confirm that the approximations made by LCT corrections are valid. Thus, PSDs could be useful for real-time dosimetry in radiology applications.

Sci-Sat AM: Brachy - 07: Plastic scintillation detector validation for kV dosimetry.

To characterize the plastic scintillation detectors (PSDs) response in the diagnostic energy range. A fast and adaptable method for real-time dosimetry in superficial x-ray therapy and interventional radiology is proposed. A PSD (1 mm diameter and 10 mm long) is coupled to a 5 m long optical fiber. Scintillation photons are guided to a polychromatic photodiode which provides an electrical current proportional to the input light signal. If the incident energy spectrum is known, the dose measured in the PSD's polystyrene sensitive volume can be converted to score dose in any other media such as air, water or soft tissues using the large cavity theory (LCT). A software simulating x-ray tube spectra and filtration has been benchmarked and is used for analysis. The method is confirmed by Monte Carlo simulations. PSDs cannot be assumed energy independent with low-energy photons as a factor 2 has been observed in the energy response between 80 kVp and 150 kVp. When the dose is converted to the desired medium, the PSD's energy dependence is compensated and a 2.1% standard deviation was observed upon the studied energy ranges, which is inside the measurement and calculation uncertainties. Percent depth dose (PDD) measurements are in good agreement with Monte Carlo simulations and results can be improved if the proposed method is applied to compensate beam hardening. PSDs present great potential for real-time dose measurements with radiologic photon energy.

Dose to tissue medium or water cavities as surrogate for the dose to cell nuclei at brachytherapy photon energies

It has been suggested that modern dose calculation algorithms should be able to report absorbed dose both as dose to the local medium, Dm,m, and as dose to a water cavity embedded in the medium, Dw,m, using conversion factors from cavity theory. Assuming that the cell nucleus with its DNA content is the most important target for biological response, the aim of this study is to investigate, by means of Monte Carlo (MC) simulations, the relationship of the dose to a cell nucleus in a medium, Dn,m, to Dm,m and Dw,m, for different combinations of cell nucleus compositions and tissue media for different photon energies used in brachytherapy. As Dn,m is very impractical to calculate directly for routine treatment planning, while Dm,m and Dw,m are much easier to obtain, the questions arise which one of these quantities is the best surrogate for Dn,m and which cavity theory assumptions should one use for its estimate. The Geant4.9.4 MC code was used to calculate Dm,m, Dw,m and Dn,m for photon energies from 20 (representing the lower energy end of brachytherapy for 103Pd or125I) to 300 keV (close to the mean energy of 192Ir) and for the tissue media adipose, breast, prostate and muscle. To simulate the cell and its nucleus, concentric spherical cavities were placed inside a cubic phantom (10 × 10 × 10 mm3). The diameter of the simulated nuclei was set to 14 µm. For each tissue medium, three different setups were simulated; (a) Dn,m was calculated with nuclei embedded in tissues (MC-Dn,m). Four different published elemental compositions of cell nuclei were used. (b) Dw,m was calculated with MC (MC-Dw,m) and compared with large cavity theory calculated Dw,m (LCT-Dw,m), and small cavity theory calculated Dw,m (SCT-Dw,m). (c) Dm,m was calculated with MC (MC-Dm,m). MC-Dw,m is a good substitute for MC-Dn,m for all photon energies and for all simulated nucleus compositions and tissue types. SCT-Dw,m can be used for most energies in brachytherapy, while LCT-Dw,m should only be considered for source spectra well below 50 keV, since contributions to the absorbed dose inside the nucleus to a large degree stem from electrons released in the surrounding medium. MC-Dm,m is not an appropriate substitute for MC-Dn,m for the lowest photon energies for adipose and breast tissues. The ratio of MC-Dm,m to MC-Dn,m for adipose and breast tissue deviates from unity by 34% and 15% respectively for the lowest photon energy (20 keV), whereas the ratio is close to unity for higher energies. For prostate and muscle tissue MC-Dm,m is a good substitute for MC-Dn,m. However, for all photon energies and tissue types the nucleus composition with the highest hydrogen content behaves differently than other compositions. Elemental compositions of the tissue and nuclei affect considerably the absorbed dose to the cell nuclei for brachytherapy sources, in particular those at the low-energy end of the spectrum. Thus, there is a need for more accurate data for the elemental compositions of tumours and healthy cells. For the nucleus compositions and tissue types investigated, MC-Dw,m is a good substitute to MC-Dn,m for all simulated photon energies. Whether other studied surrogates are good approximations to MC-Dn,m depends on the target size, target composition, composition of the surrounding tissue and photon energy.

TH-E-BRB-10: Examination of General Cavity Theory for Lithium Fluoride TLDs in Bone and Lung Heterogeneities

Purpose: To compare the measured and theoretical dose response of Lithium Fluoride Thermoluminescent Detectors (TLD-100) of different thickness in bone and lung heterogeneities. Methods: After determining individual TLD sensitivities, a total of 45 TLD chips (3.2×3.2 mm and depths of 0.15mm -thin, 0.38mm -medium and 0.89mm -thick TLDs) were placed in bone and lung slab phantoms and irradiated with a 6MV photon beam. According to the Burlin cavity theory, the proportion of the dose in the cavity that is due to secondary electrons generated in the medium is given by the parameter d which depends on β and g. Four different methods of calculating β and three formulas to calculate g were tested in this study which resulted in twelve different versions of the d parameter for both lung and bone. The mean stopping power ratios and mean mass energy absorption coefficients of the 6 MV photon spectrum in bone and lung heterogeneities were calculated with BEAMnrc Monte Carlo simulations. Then the ratio of dose in TLD to dose in medium (f) was calculated and compared against the measured values. Results: The Burlin cavity theory resulted in a good agreement with measurements for the TLDs in lung, where it overestimated the f factor by 1.9%, 2.2% and 3.0%, for thin, medium and thick TLDs respectively. However, the discrepancy between the calculated and measured f factors for bone were higher and the f factors were underestimated by 5.6%, 5.4% and 3.5% with the Burlin cavity theory for thin, medium and thick TLDs respectively. Conclusions: Better agreement of the theoretical and measured response of LiF TLDs was observed in lung than the bone heterogeneities. Furthermore all twelve versions of the Burlin cavity theory used in this study agreed to within 0.5% of each other in both lung and bone.

Characterisation of energy response of Al2O3:C optically stimulated luminescent dosemeters (OSLDs) using cavity theory

Aluminium oxide (Al(2)O(3):C) is a common material used in optically stimulated luminescent dosemeters (OSLDs). OSLDs have a known energy dependence, which can impact on the accuracy of dose measurements, especially for lower photon energies, where the dosemeter can overrespond by a factor of 3-4. The purpose of this work was to characterise the response of Al(2)O(3):C using cavity theory and to evaluate the applicability of this approach for polyenergetic photon beams. The cavity theory energy response showed good agreement (within 2 %) with the corresponding measured values. A comparison with measured values reported in the literature for low-energy polyenergetic spectra showed more varied agreement (within 6 % on average). The discrepancy between these results is attributed to differences in the raw photon energy spectra used to calculate the energy response. Analysis of the impact of the photon energy spectra versus the mean photon energy showed improved accuracy if the energy response was determined using the entire photon spectrum rather than the mean photon energy. If not accounted for, the overresponse due to photon energy could introduce substantial inaccuracy in dose measurement using OSLDs, and the results of this study indicate that cavity theory may be used to determine the response with reasonable accuracy.

A theoretical re-examination of Spencer–Attix cavity theory

The aim of radiation dosimetry is to evaluate, under specific conditions, absorbed dose in a medium of interest using a detection device. In comparison to what is meant to be evaluated, the distinctive composition of the detector causes particle fluence perturbation and shifted absorbed-dose response, both effects depending on beam quality. For electron and megavoltage photon beams, Spencer–Attix cavity theory further adapted by Nahum remains the accepted standard method used to convert absorbed dose in a wall-less detector to absorbed dose in the medium of interest. For several decades, the approach has been widely used in protocols to generate data for ionization chamber dosimetry. As a considerable effort was made towards accurate Monte Carlo methods, computation techniques are nowadays available to determine absorbed dose accurately in complex geometries, including radiation detectors. In the development of nonstandard beam protocols, direct Monte Carlo dose calculations using realistic models are being suggested and used to generate data for ionization chamber dosimetry. This indicates that for a general dosimetric context, including nonstandard beams, a more general cavity theory in agreement with what is currently being done could be adopted. Not only this could be of interest in the dosimetry standards community, but also for educational purposes. This paper re-examines Spencer–Attix theory from first principles, using a new general cavity theory rigorously derived from radiation transport equations. The approach is based on the same schematization as for Spencer–Attix’s (i.e. groups of slow and fast electrons) and yields a general expression of absorbed dose for suitably implemented Monte Carlo methods. The Spencer–Attix–Nahum formulation is shown to be a special case of the presented model, outlining specific issues of the standard method. By providing an expression of absorbed dose which reflects the gold standard calculation method (i.e. Monte Carlo), the proposed theory could be adopted by the radiation dosimetry community.

Evaluations of absorbed dose ratio factor of Al2O3 dosemeter in radiotherapy photon beams using cavity theory

The aim of the work was to evaluate the absorbed dose ratio factor f(md) of an Al(2)O(3) dosemeter to water in photon radiotherapy beams using cavity theory. Burlin theory was used for calculating of this ratio. The effective mass attenuation coefficient β was obtained by comparing Monte Carlo simulations in monoenergetic photon beams. The evaluations of the absorbed dose ratio factor f(md) were studied for Al(2)O(3) dosemeters of different sizes, which were placed at various depths of the water phantom in different radiation field sizes of Mohan's 6, 10 and 15-MV X-rays. Beyond the build-up region, the variation of f(md) increases by 0.25 % as the depth increases from 4 to 10 cm. The maximum variation due to different dosemeter sizes is 8.3 %. The difference in the f(md) due to different radiation field sizes is 1.5 %. The effect of the dosemeter size cannot be neglected. The difference in the f(md) due to the radiation field sizes of different beams would increase as the dosemeter size increases.

Energy response of optically stimulated luminescent dosimeters for non-reference measurement locations in a 6 MV photon beam

Optically stimulated luminescent dosimeters (OSLDs) are becoming increasingly popular for measuring an absorbed dose in clinical radiotherapy. OSLDs have known energy dependence, and this is accounted for by either calibrating the OSLD with a specific nominal energy, or using a standard energy correction factor to account for differences between the experimental beam photon energy and the photon energy used to establish the OSLD's sensitivity (e.g., 60Co). This work is typically done under reference conditions (e.g., at dmax). The impact of variations in photon spectra on the OSLD response is typically ignored for measurement positions that are different than the reference position. We determined that it is generally necessary to apply an additional non-reference energy correction factor to OSLD measurements made at locations that do not correspond to the reference position, particularly for OSLD measurements made out-of-field, where the photon spectra are softer. We determined this energy correction factor for a range of 6 MV photon spectra using two independent methods: Burlin cavity theory and measurements. The non-reference energy correction factor was found to range from 0.97 to 1.00 for in-field measurement locations and from 0.69 to 0.95 for out-of-field measurement locations. The use of a non-reference energy correction factor can improve the accuracy of OSLDs, especially when used out-of-field.