Origins of the changing detector response in small megavoltage photon radiation fields

Dose area product primary standards established by graphite calorimetry at the LNE-LNHB for small radiation fields in radiotherapy

PURPOSE

To present primary standards establishment in terms of Dose Area Product (DAP) for small field sizes.

METHODS

A large section graphite calorimeter and two plane-parallel ionization chambers were designed and built in-house. These chambers were calibrated in a 6MV FFF beam at the maximum dose rate of 1400 UM/min for fields defined by specifically designed circular collimators of 5, 7.5, 10, 13 and 15 mm diameter and jaws of 5, 7, 10, 13 and 15 mm side length on a Varian TrueBeam linac.

RESULTS

The two chambers show the same behaviour regardless of field shape and size. From 5 to 15 mm, calibration coefficients slightly increase with the field size with a magnitude of 1.8% and 1.1% respectively for the two chambers, and are independent of the field shape. This tendency was confirmed by Monte Carlo calculations. The average associated uncertainty of the calibration coefficients is around 0.6% at k=1.

CONCLUSIONS

For the first time, primary standards in terms of DAP were established by graphite calorimetry for an extended range of small field sizes. These promising results open the door for an alternative approach in small fields dosimetry.

Field output correction factors and electron fluence perturbation of the microSilicon and microSilicon X detectors

Objective. The aim of this study was to determine field output correction factors k Q clin , Q ref f clin , f ref and electron fluence perturbation for new PTW unshielded microSilicon and shielded microSilicon X detectors. Approach. k Q clin , Q ref f clin , f ref factors were calculated for 6 and 10 MV with and without flattening filter beams delivered by a TrueBeam STx. Correction factors were determined for field sizes ranging from 0.5 × 0.5 cm2 to 3 × 3 cm2 using both experimental and numerical methods. To better understand the underlying physics of their response, total electron (+positron) fluence spectra were scored in the sensitive volume considering the various component-dependent perturbations. Main results. The microSilicon and microSilicon X detectors can be used down to the smallest studied field size by applying corrections factors fulfilling the tolerance of 5% recommended by the IAEA TRS483. Electron fluence perturbation in both microSilicon detectors was greater than that in water but to a lesser extent than their predecessors. The main contribution of the overall perturbation of the detectors comes from the materials surrounding their sensitive volume, especially the epoxy in the case of unshielded diodes and the shielding for shielded diodes. This work demonstrated that the decrease in the density of the epoxy for the microSilicon led to a decrease in the electron fluence perturbation. Significance. A real improvement was observed regarding the design of the microSilicon and microSilicon X detectors compared to their predecessors.

The PTW microSilicon diode: Performance in small 6 and 15 MV photon fields and utility of density compensation

PURPOSE

We have experimentally and computationally characterized the PTW microSilicon 60023-type diode's performance in 6 and 15 MV photon fields ≥5 × 5 mm² projected to isocenter. We tested the detector on- and off-axis at 5 and 15 cm depths in water, and investigated whether its response could be improved by including within it a thin airgap.

METHODS

Experimentally, detector readings were taken in fields generated by a Varian TrueBeam linac and compared with doses-to-water measured using Gafchromic film and ionization chambers. An unmodified 60023-type diode was tested along with detectors modified to include 0.6, 0.8, and 1.0 mm thick airgaps. Computationally, doses absorbed by water and detectors' sensitive volumes were calculated using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Detector response was characterized using k Q c l i n , 4 cm f c l i n , 4 cm , a factor that corrects for differences in the ratio of dose-to-water to detector reading between small fields and the reference condition, in this study 5 cm deep on-axis in a 4 × 4 cm² field.

RESULTS

The greatest errors in measurements of small field doses made using uncorrected readings from the unmodified 60023-type detector were over-responses of 2.6% ± 0.5% and 5.3% ± 2.0% determined computationally and experimentally, relative to the reading-per-dose in the reference field. Corresponding largest errors for the earlier 60017-type detector were 11.9% ± 0.6% and 11.7% ± 1.4% over-responses. Adding even the thinnest, 0.6 mm, airgap to the 60023-type detector over-corrected it, leading to under-responses of up to 4.8% ± 0.6% and 5.0% ± 1.8% determined computationally and experimentally. Further, Monte Carlo calculations indicate that a detector with a 0.3 mm airgap would read correctly to within 1.3% on-axis. The ratio of doses at 15 and 5 cm depths in water in a 6 MV 4 × 4 cm² field was measured more accurately using the unmodified 60023-type detector than using the 60017-type detector, and was within 0.3% of the ratio measured using an ion chamber. The 60023-type diode's sensitivity also varied negligibly as dose-rate was reduced from 13 to 4 Gy min^-1 by decreasing the linac pulse repetition frequency, whereas the sensitivity of the 60017-type detector fell by 1.5%.

CONCLUSIONS

The 60023-type detector performed well in small fields across a wide range of beam energies, field sizes, depths, and off-axis positions. Its response can potentially be further improved by adding a thin, 0.3 mm, airgap.

Cema-based formalism for the determination of absorbed dose for high-energy photon beams

PURPOSE

Determination of absorbed dose is well established in many dosimetry protocols and considered to be highly reliable using ionization chambers under reference conditions. If dosimetry is performed under other conditions or using other detectors, however, open questions still remain. Such questions frequently refer to appropriate correction factors. A converted energy per mass (cema)-based approach to formulate such correction factors offers a good understanding of the specific response of a detector for dosimetry under various measuring conditions and thus an estimate of pros and cons of its application.

METHODS

Determination of absorbed dose requires the knowledge of the beam quality correction factor k_Q,Qo , where Q denotes the quality of a user beam and Qo is the quality of the radiation used for calibration. In modern Monte Carlo (MC)-based methods, k_Q,Qo is directly derived from the MC-calculated dose conversion factor, which is the ratio between the absorbed dose at a point of interest in water and the mean absorbed dose in the sensitive volume of an ion chamber. In this work, absorbed dose is approximated by the fundamental quantity cema. This approximation allows the dose conversion factor to be substituted by the cema conversion factor. Subsequently, this factor is decomposed into a product of cema ratios. They are identified as the stopping power ratio water to the material in the sensitive detector volume, and as the correction factor for the fluence perturbation of the secondary charged particles in the detector cavity caused by the presence of the detector. This correction factor is further decomposed with respect to the perturbation caused by the detector cavity and that caused by external detector properties. The cema-based formalism was subsequently tested by MC calculations of the spectral fluence of the secondary charged particles (electrons and positrons) under various conditions.

RESULTS

MC calculations demonstrate that considerable fluence perturbation may occur particularly under non-reference conditions. Cema-based correction factors to be applied in a 6-MV beam were obtained for a number of ionization chambers and for three solid-state detectors. Feasibility was shown at field sizes of 4 × 4 and 0.5 cm × 0.5 cm. Values of the cema ratios resulting from the decomposition of the dose conversion factor can be well correlated with detector response. Under the small field conditions, the internal fluence correction factor of ionization chambers is considerably dependent on volume averaging and thus on the shape and size of the cavity volume.

CONCLUSIONS

The cema approach is particularly useful at non-reference conditions including when solid-state detectors are used. Perturbation correction factors can be expressed and evaluated by cema ratios in a comprehensive manner. The cema approach can serve to understand the specific response of a detector for dosimetry to be dependent on (a) radiation quality, (b) detector properties, and (c) electron fluence changes caused by the detector. This understanding may also help to decide which detector is best suited for a specific measurement situation.

The dose response of PTW microDiamond and microSilicon in transverse magnetic field under small field conditions

The aim of the present work is to investigate the behavior of two diode-type detectors (PTW microDiamond 60019 and PTW microSilicon 60023) in transverse magnetic field under small field conditions. A formalism based on TRS 483 has been proposed serving as the framework for the application of these high-resolution detectors under these conditions. Measurements were performed at the National Metrology Institute of Germany (PTB, Braunschweig) using a research clinical linear accelerator facility. Quadratic fields corresponding to equivalent square field sizesSbetween 0.63 and 4.27 cm at the depth of measurement were used. The magnetic field strength was varied up to 1.4 T. Experimental results have been complemented with Monte Carlo simulations up to 1.5 T. Detailed simulations were performed to quantify the small field perturbation effects and the influence of detector components on the dose response. The does response of both detectors decreases by up to 10% at 1.5 T in the largest field size investigated. InS = 0.63 cm, this reduction at 1.5 T is only about half of that observed in field sizesS > 2 cm for both detectors. The results of the Monte Carlo simulations show agreement better than 1% for all investigated conditions. Due to normalization at the machine specific reference field, the resulting small field output correction factors for both detectors in magnetic fieldkQclin,QmsrBare smaller than those in the magnetic field-free case, where correction up to 6.2% at 1.5 T is required for the microSilicon in the smallest field size investigated. The volume-averaging effect of both detectors was shown to be nearly independent of the magnetic field. The influence of the enhanced-density components within the detectors has been identified as the major contributors to their behaviors in magnetic field. Nevertheless, the effect becomes weaker with decreasing field size that may be partially attributed to the deficiency of low energy secondary electrons originated from distant locations in small fields.

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.

Density compensated diodes for small field dosimetry: comprehensive testing and implications for design

PURPOSE

In small megavoltage photon fields, the accuracies of an unmodified PTW 60017-type diode dosimeter and six diodes modified by adding airgaps of thickness 0.6-1.6 mm and diameter 3.6 mm have been comprehensively characterized experimentally and computationally. The optimally thick airgap for density compensation was determined, and detectors were micro-CT imaged to investigate differences between experimentally measured radiation responses and those predicted computationally.

METHODS

Detectors were tested on- and off-axis, at 5 and 15 cm depths in 6 and 15 MV fields ≥ 0.5 × 0.5 cm². Computational studies were carried out using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Experimentally, radiation was delivered using a Varian TrueBeam linac and doses absorbed by water were measured using Gafchromic EBT3 film and ionization chambers, and compared with diode readings. Detector response was characterized via the [Formula: see text] formalism, choosing a 4 × 4 cm² reference field.

RESULTS

For the unmodified 60017 diode, the maximum error in small field doses obtained from diode readings uncorrected by [Formula: see text] factors was determined as 11.9% computationally at +0.25 mm off-axis and 5 cm depth in a 15 MV 0.5 × 0.5 cm² field, and 11.7% experimentally at -0.30 mm off-axis and 5 cm depth in the same field. A detector modified to include a 1.6 mm thick airgap performed best, with maximum computationally and experimentally determined errors of 2.2% and 4.1%. The 1.6 mm airgap deepened the modified dosimeter's effective point of measurement by 0.5 mm. For some detectors significant differences existed between responses in small fields determined computationally and experimentally, micro-CT imaging indicating that these differences were due to within-tolerance variations in the thickness of an epoxy resin layer.

CONCLUSIONS

The dosimetric performance of a 60017 diode detector was comprehensively improved throughout 6 and 15 MV small photon fields via density compensation. For this approach to work well with good detector-to-detector reproducibility, tolerances on dense component dimensions should be reduced to limit associated variations of response in small fields, or these components should be modified to have more water-like densities.

Small field output correction factors of the microSilicon detector and a deeper understanding of their origin by quantifying perturbation factors

PURPOSE

The aim of this study is the experimental and Monte Carlo-based determination of small field correction factors for the unshielded silicon detector microSilicon for a standard linear accelerator as well as the Cyberknife System. In addition, a detailed Monte Carlo analysis has been performed by modifying the detector models stepwise to study the influences of the detector's components.

METHODS

Small field output correction factors have been determined for the new unshielded silicon diode detector, microSilicon (type 60023, PTW Freiburg, Germany) as well as for the predecessors Diode E (type 60017, PTW Freiburg, Germany) and Diode SRS (type 60018, PTW Freiburg, Germany) for a Varian TrueBeam linear accelerator at 6 MV and a Cyberknife system. For the experimental determination, an Exradin W1 scintillation detector (Standard Imaging, Middleton, USA) has been used as reference. The Monte Carlo simulations have been performed with EGSnrc and phase space files from IAEA as well as detector models according to manufacturer blueprints. To investigate the influence of the detector's components, the detector models have been modified stepwise.

RESULTS

The correction factors for the smallest field size investigated at the TrueBeam linear accelerator (equivalent dosimetric square field side length Sclin  = 6.3 mm) are 0.983 and 0.939 for the microSilicon and Diode E, respectively. At the Cyberknife system, the correction factors of the microSilicon are 0.967 at the smallest 5-mm collimator compared to 0.928 for the Diode SRS. Monte Carlo simulations show comparable results from the measurements and literature.

CONCLUSION

The microSilicon (type 60023) detector requires less correction than its predecessors, Diode E (type 60017) and Diode SRS (type 60018). The detector housing has been demonstrated to cause the largest perturbation, mainly due to the enhanced density of the epoxy encapsulation surrounding the silicon chip. This density has been rendered more water equivalent in case of the microSilicon detector to minimize the associated perturbation. The sensitive volume itself has been shown not to cause observable field size-dependent perturbation except for the volume-averaging effect, where the slightly larger diameter of the sensitive volume of the microSilicon (1.5 mm) is still small at the smallest field size investigated with corrections <2%. The new microSilicon fulfils the 5% correction limit recommended by the TRS 483 for output factor measurements at all conditions investigated in this work.

Technical Note: Characterization of the new microSilicon diode detector

Purpose

Dosimetric properties of the new microSilicon diode detector (60023) have been studied with focus on application in small‐field dosimetry. The influences of the dimensions of the sensitive volume and the density of the epoxy layer surrounding the silicon chip of microSilicon have been quantified and compared to its predecessor (Diode E 60017) and the microDiamond (60019, all PTW‐Freiburg, Germany).

Methods

Dose linearity has been studied in the range from 0.01 to 8.55 Gy and dose‐per‐pulse dependence from 0.13 to 0.86 mGy/pulse. The effective point of measurement (EPOM) was determined by comparing measured percentage depth dose curves with a reference curve (Roos chamber). Output ratios were measured for nominal field sizes from 0.5 × 0.5  cm² to 4 × 4 cm². The corresponding small‐field output correction factors, k, were derived with a plastic scintillation detector as reference. The lateral dose–response function, K(x), was determined using a slit beam geometry.

Results

MicroSilicon shows linear dose response (R² = 1.000) in both low and high dose range up to 8.55 Gy with deviations of only up to 1% within the dose‐per‐pulse values investigated. The EPOM was found to lie (0.7 ± 0.2) mm below the front detector’s surface. The derived k for microSilicon (0.960 at s_eff = 0.55 cm) is similar to that of microDiamond (0.956), while Diode E requires larger corrections (0.929). This improved behavior of microSilicon in small‐fields is reflected in the slightly wider K(x) compared to Diode E. Furthermore, the amplitude of the negative values in K(x) at the borders of the sensitive volume has been reduced.

Conclusions

Compared to its predecessor, microSilicon shows improved dosimetric behavior with higher sensitivity and smaller dose‐per‐pulse dependence. Profile measurements demonstrated that microSilicon causes less perturbation in off‐axis measurements. It is especially suitable for the applications in small‐field output factors and profile measurements.

Three-dimensional characterization of the active volumes of PTW microDiamond, microSilicon, and Diode E dosimetry detectors using a proton microbeam

PURPOSE

The purpose of this work is the three-dimensional characterization of the active volumes of commercial solid-state dosimetry detectors. Detailed knowledge of the dimensions of the detector's active volume as well as the detector housing is of particular interest for small-field photon dosimetry. As shown in previous publications from different groups, the design of the detector housing influences the detector signal for small photon fields. Therefore, detailed knowledge of the active volume dimension and the surrounding materials form the basis for accurate Monte Carlo simulations of the detector.

METHODS

A 10 MeV proton beam focused by the microbeam system of the Physikalisch-Technische Bundesanstalt was used to measure two-dimensional response maps of a synthetic diamond detector (microDiamond, type 60019, PTW Freiburg) and two silicon detectors (microSilicon, type 60023, PTW Freiburg and Diode E, type 60017, PTW Freiburg). In addition, the thickness of the active volume of the new microSilicon was measured using the method developed in a previous study.

RESULTS

The analysis of the response maps leads to active area of 1.18 mm² for the Diode E, 1.75 mm² for the microSilicon, and 3.91 mm² for the microDiamond detector. The thickness of the active volume of the microSilicon detector was determined to be (17.8 ± 2) µm.

CONCLUSIONS

This study provides detailed geometrical data of the dosimetric active volume of three different solid-state detector types.

The role of radiation-induced charge imbalance on the dose-response of a commercial synthetic diamond detector in small field dosimetry

PURPOSE

Discrepancy between experimental and Monte Carlo simulated dose-response of the microDiamond (mD) detector (type 60019, PTW Freiburg, Germany) at small field sizes has been reported. In this work, the radiation-induced charge imbalance in the structural components of the detector has been investigated as the possible cause of this discrepancy.

MATERIALS AND METHODS

Output ratio (OR) measurements have been performed using standard and modified versions of the mD detector at nominal field sizes from 6 mm × 6 mm to 40 mm × 40 mm. In the first modified mD detector (mD_reversed), the type of charge carriers collected is reversed by connecting the opposite contact to the electrometer. In the second modified mD detector (mD_shortened), the detector's contacts have been shortened. The third modified mD detector (mD_noChip) is the same as the standard version but the diamond chip with the sensitive volume has been removed. Output correction factors were calculated from the measured OR and simulated using the EGSnrc package. An adapted Monte Carlo user-code has been used to study the underlying mechanisms of the field size-dependent charge imbalance and to identify the detector's structural components contributing to this effect.

RESULTS

At the smallest field size investigated, the OR measured using the standard mD detector is >3% higher than the OR obtained using the modified mD detector with reversed contact (mD_reversed). Combining the results obtained with the different versions of the detector, the OR have been corrected for the effect of radiation imbalance. The OR obtained using the modified mD detector with shortened contacts (mD_shortened) agree with the corrected OR, all showing an over-response of less than 2% at the field sizes investigated. The discrepancy between the experimental and simulated output correction factors has been eliminated after accounting for the effect of charge imbalance.

DISCUSSIONS AND CONCLUSIONS

The role of radiation-induced charge imbalance on the dose-response of mD detector in small field dosimetry has been studied and quantified. It has been demonstrated that the effect is significant at small field sizes. Multiple methods were used to quantify the effect of charge imbalance with good agreement between Monte Carlo simulations and experimental results obtained with modified detectors. When this correction is applied to the Monte Carlo data, the discrepancy from experimental data is eliminated. Based on the detailed component analysis using an adapted Monte Carlo user-code, it has been demonstrated that the effect of charge imbalance can be minimized by modifying the design of the detector's contacts.