Practical methods of electron depth-dose measurement compared to use of the NACP design chamber in water

Central axis relative dose versus depth measurements were performed using two different small volume thimble ionization chambers and a p-type silicon diode in a water phantom and with two parallel-plate ionization chambers, thermoluminescent dosimeters, and radiographic film in a popular clear polystyrene phantom. Values obtained were compared to the results of similar measurements in a water phantom performed with a plane-parallel ionization chamber designed and optimized for use in electron beams by the Nordic Association of Clinical Physicists (NACP). The NACP chamber is expected to minimally perturb the electron fluence and be least prone to point of measurement uncertainties. Its use in a water phantom closely approximates the spirit of recent international protocols. Data were obtained for the foil scattered electron beams generated by two different accelerators for field sizes from 6 cm X 6 cm to 25 cm X 25 cm and energies between 6 and 20 MeV. Easily identifiable effective points of measurements were defined for each measurement device and standard corrections were applied to the raw data to obtain depth-dose curves. The degree of agreement between the various techniques and the NACP-water standard was quantitatively analyzed through comparison of the resulting depths of 50% dose and practical range. All methods were found to yield reasonable results when carefully implemented, with average differences of less than 1 mm being easily achievable. Measurements with p-type silicon diode detectors were found to be particularly useful, as they are pointlike and appear from all practical considerations to directly represent relative dose, thus requiring little or no correction to raw readings.     

General characteristics of the use of silicon diode detectors for clinical dosimetry in proton beams

The properties of silicon diode detectors, used for dosimetry in clinical proton beams, were investigated with special regard to the measurement of relative dose distributions in water. Different types of silicon diode detector were studied, and the resulting distributions of detector signal versus depth in the water phantom were compared with the corresponding distributions obtained with a plane-parallel NACP ionization chamber. The measurements were performed in a proton beam with an initial energy of 173 MeV. It is shown that the Hi-p silicon detector gives a signal which is proportional to the ionization density in the silicon crystal in all parts of the Bragg curve, and for all levels of accumulated dose to the detector. This is in contrast to detectors based on n-type silicon, or on low resistivity p-type silicon. After pre-irradiation, these latter detectors show a stopping-power dependent recombination, yielding an increase in the detector signal per unit dose with increasing LET. This effect leads to an over-response in the Bragg peak, which increases gradually with the accumulated detector dose. Using the Hi-p silicon diode detector, the depth ionization distribution was found to be equal to the distribution obtained with the plane-parallel NACP ionization chamber at all pre-irradiation levels, within the experimental accuracy. This implies that the quotient between the ionization in the detector and the absorbed dose to the surrounding water is equal for these detectors.     

Dosimetric characterization of silicon and diamond detectors in low-energy proton beams

The dosimetric behaviour of a Scanditronix p-type silicon diode and a PTW natural diamond detector was studied in low-energy proton beams in the 8.3-21.5 MeV range. The properties investigated were linearity, reproducibility, dose rate dependence, energy and linear energy transfer (LET) dependence. The influence of detector thickness on the results of depth dose measurements was also demonstrated. A Markus parallel plate ionization chamber was used for reference dosimetry. Silicon diode and diamond detectors showed linearity at therapeutic dose level, reproducibility better than 1% (1sigma) and sensitivity variation with dose rate and proton energy.     

Monte Carlo study of si diode response in electron beams

Silicon semiconductor diodes measure almost the same depth-dose distributions in both photon and electron beams as those measured by ion chambers. A recent study in ion chamber dosimetry has suggested that the wall correction factor for a parallel-plate ion chamber in electron beams changes with depth by as much as 6%. To investigate diode detector response with respect to depth, a silicon diode model is constructed and the water/silicon dose ratio at various depths in electron beams is calculated using EGSnrc. The results indicate that, for this particular diode model, the diode response per unit water dose (or water/diode dose ratio) in both 6 and 18 MeV electron beams is flat within 2% versus depth, from near the phantom surface to the depth of R50 (with calculation uncertainty <0.3%). This suggests that there must be some other correction factors for ion chambers that counter-balance the large wall correction factor at depth in electron beams. In addition, the beam quality and field-size dependence of the diode model are also calculated. The results show that the water/diode dose ratio remains constant within 2% over the electron energy range from 6 to 18 MeV. The water/diode dose ratio does not depend on field size as long as the incident electron beam is broad and the electron energy is high. However, for a very small beam size (1 X 1 cm(2)) and low electron energy (6 MeV), the water/diode dose ratio may decrease by more than 2% compared to that of a broad beam.     

Diamond detector versus silicon diode and ion chamber in photon beams of different energy and field size

The aim of this work was to test the suitability of a PTW diamond detector for nonreference condition dosimetry in photon beams of different energy (6 and 25 MV) and field size (from 2.6 cm x 2.6 cm to 10 cm x 10 cm). Diamond behavior was compared to that of a Scanditronix p-type silicon diode and a Scanditronix RK ionization chamber. Measurements included output factors (OF). percentage depth doses (PDD) and dose profiles. OFs measured with diamond detector agreed within 1% with those measured with diode and RK chamber. Only at 25 MV, for the smallest field size, RK chamber underestimated OFs due to averaging effects in a pointed shaped beam profile. Agreement was found between PDDs measured with diamond detector and RK chamber for both 6 MV and 25 MV photons and down to 5 cm x 5 cm field size. For the 2.6 cm x 2.6 cm field size, at 25 MV, RK chamber underestimated doses at shallow depth and the difference progressively went to zero in the distal region. PDD curves measured with silicon diode and diamond detector agreed well for the 25 MV beam at all the field sizes. Conversely, the nontissue equivalence of silicon led, for the 6 MV beam, to a slight overestimation of the diode doses in the distal region, at all the field sizes. Penumbra and field width measurements gave values in agreement for all the detectors but with a systematic overestimate by RK measurements. The results obtained confirm that ion chamber is not a suitable detector when high spatial resolution is required. On the other hand, the small differences in the studied parameters, between diamond and silicon systems, do not lead to a significant advantage in the use of diamond detector for routine clinical dosimetry.     

Measurements of output factors with different detector types and Monte Carlo calculations of stopping-power ratios for degraded electron beams

The aim of the present study was to investigate three different detector types (a parallel-plate ionization chamber, a p-type silicon diode and a diamond detector) with regard to output factor measurements in degraded electron beams, such as those encountered in small-electron-field radiotherapy and intraoperative radiation therapy (IORT). The Monte Carlo method was used to calculate mass collision stopping-power ratios between water and the different detector materials for these complex electron beams (nominal energies of 6, 12 and 20 MeV). The diamond detector was shown to exhibit excellent properties for output factor measurements in degraded beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of output factors, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth (especially for lower electron energies). Application of ionization-chamber-based dosimetry, according to international dosimetry protocols, will introduce uncertainties smaller than 0.3% into the output factor determination for conventional IORT beams if the variation of the water-to-air stopping-power ratio is not taken into account. The IORT system at our department includes a 0.3 cm thin plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. This was verified by the measurements. In small-electron-beam dosimetry, the water-to-air stopping-power ratio variation with field size could mostly be ignored. For fields with flat lateral dose profiles (>3 x 3 cm2), output factors determined with the ionization chamber were found to be in close agreement with the results of the diamond detector. For smaller field sizes the lateral extension of the ionization chamber hampers its use. We therefore recommend that the readily available silicon diode detector should be used for output factor measurements in complex electron fields.     

Relative electron beam measurements: scaling depths in clear polystyrene to equivalent depths in water

For purposes of mean incident energy determination and the accrual of consistent treatment planning data, measurements of relative ionization or dose made in clear polystyrene must be scaled in depth to produce depth-ionization or depth-dose curves equivalent to what would have been measured in water. Recommendations from various protocols for clear polystyrene to water depth scaling factors differ by as much as 5%. Here, central axis measurements of relative ionization as a function of depth have been made with parallel-plate chambers both in a popular clear polystyrene phantom of density 1.045 g/cm3 and in water. Perturbation and displacement corrections were thus minimized. Both depth-ionization and depth-dose curves were formed for electron beams with nominal incident energies between 6 and 20 MeV and field sizes from 6 cm X 6 cm to 15 cm X 15 cm. Comparisons of the depths for 50% relative reading and practical range between the two phantoms yield average empirical scaling factors of 0.990 and 1.002, respectively.     

Dosimetry techniques for narrow proton beam radiosurgery

Characterization of narrow beams used in proton stereotactic radiosurgery (PSRS) requires special efforts, since the use of finite size detectors can lead to distortion of the measured dose distributions. Central axis depth doses, lateral profiles and field size dependence factors are the most important beam characteristics to be determined prior to dosimetry calculations and beam modelling for PSRS. In this paper we report recommendations for practical dosimetry techniques which were developed from a comparison of beam characteristics determined with a variety of radiation detectors for 126 and 155 MeV narrow proton beams shaped with 2-30 mm circular brass collimators. These detectors included small-volume ionization chambers, a diamond detector, an Hi-p Si diode, TLD cubes, radiographic and radiochromic films. We found that both types of film are suitable for profile measurements in narrow beams. Good agreement between depth dose distributions measured with ionization chambers, diamond and diode detectors was demonstrated in beams with diameters of 20-30 mm. The diode detector can be used in smaller beams, down to 5 mm diameter. For beams with diameters less than 5 mm, reliable depth dose data may be obtained only with radiochromic film. The tested ionization chambers are appropriate for calibration of beams with diameters of 20-30 mm. TLD cubes and diamond detectors are useful to determine relative dose in beams with diameters of 10-20 mm. Field size factors for smaller beams should be obtained with diode and radiochromic film. We conclude that dosimetry characterization of proton beams down to several millimetres in diameter can be performed using the described procedures.     

Measurements of Gamma-Knife helmet output factors using a radiophotoluminescent glass rod dosimeter and a diode detector

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume p-type silicon diode detector are used for the measurement of the output factors from Gamma-Knife fields. The GRD system consists of small rod-shaped glass chip detectors and an automatic readout device. The output factors measured with the GRD from the 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.981, 0.942 and 0.877, respectively. Similarly, the corresponding output factors measured with the p-type silicon diode detector are 0.980, 0.949 and 0.867, respectively. The output factors are corrected for the end effect for each helmet. The output factors obtained from both detectors are in good agreement with the values in a recent publication and the values recommended by Elekta, the manufacturer. The directional dependence of these detectors is also measured. For the Gamma-Knife angle ranging from 6 to 36 degrees in the y-z plane of the stereotactic space, the measured angular dependence of the GRD is approximately 1.0% at a 4 MV x-ray beam. The response of the silicon diode detector indicates approximately 3-4% directional dependence for the same angular range for a 6 MV x-ray beam. The Gamma-Knife helmet output factors measured with the silicon diode detector are corrected for angular dependence.     

Comparative dosimetry of diode and diamond detectors in electron beams for intraoperative radiation therapy

The aim of the present study is to examine the validity of using silicon semiconductor detectors in degraded electron beams with a broad energy spectrum and a wide angular distribution. A comparison is made with diamond detector measurements, which is the dosimeter considered to give the best results provided that dose rate effects are corrected for. Two-dimensional relative absorbed dose distributions in electron beams (6-20 MeV) for intraoperative radiation therapy (IORT) are measured in a water phantom. To quantify deviations between the detectors, a dose comparison tool that simultaneously examines the dose difference and distance to agreement (DTA) is used to evaluate the results in low- and high-dose gradient regions, respectively. Uncertainties of the experimental measurement setup (+/- 1% and +/- 0.5 mm) are taken into account by calculating a composite distribution that fails this dose-difference and DTA acceptance limit. Thus, the resulting area of disagreement should be related to differences in detector performance. The dose distributions obtained with the diode are generally in very good agreement with diamond detector measurements. The buildup region and the dose falloff region show good agreement with increasing electron energy, while the region outside the radiation field close to the water surface shows an increased difference with energy. The small discrepancies in the composite distributions are due to several factors: (a) variation of the silicon-to-water collision stopping-power ratio with electron energy, (b) a more pronounced directional dependence for diodes than for diamonds, and (c) variation of the electron fluence perturbation correction factor with depth. For all investigated treatment cones and energies, the deviation is within dose-difference and DTA acceptance criteria of +/- 3% and +/- 1 mm, respectively. Therefore, p-type silicon diodes are well suited, in the sense that they give results in close agreement with diamond detectors, for practical measurements of relative absorbed dose distributions in degraded electron beams used for IORT.     

Characteristics of silicon and diamond detectors in a 60 MeV proton beam

The calibration factor variation for a PTW natural diamond detector and a Scanditronix p-type stereotactic silicon diode (designed for use in photon beams) was studied in the 10-59 MeV range. Irradiations were performed in a water phantom with the 60 MeV ocular therapy beam at the CCO (UK). The diamond detector showed a sensitivity increase with energy, underestimating the dose by about 18% at the Bragg peak, by 7% at the centre and by 17% at the distal end of the SOBP region. The silicon diode did not show any significant sensitivity change with energy. However, a decrease in response of 24% was observed for an accumulated dose of 300 Gy.     

Comparative dosimetry in narrow high-energy photon beams

A comparison of the response of different dosimeters in narrow photon beams (phi > or = 4 mm) of 6 and 18 MV bremsstrahlung has been performed. The detectors used were a natural diamond detector, a liquid ionization chamber, a plastic scintillator and two dedicated silicon diodes. The diodes had a very small detection volume and one was a specially designed double diode using two parallel opposed active volumes with compensating interface perturbations. The characteristics of the detectors were investigated both for dose distribution measurements, such as depth-dose curves and lateral beam profiles, and for output factors. The dose rate and angular dependence of the diamond and the two diodes were also studied separately. The depth-dose distributions for small fields agree well for the diamond, the scintillator and the single diode, while the measured dose maximum for the double diode is about 1% higher and for the liquid chamber about 1% lower than the mean of the others when normalized at a depth of 10 cm. The plastic scintillator and the liquid ionization chamber detect a penumbra width that is slightly broadened due to the influence of their finite size, while the double diode may even underestimate the penumbra width due to its small size and high density. When corrected for the extension of the detector volume a good agreement with Monte Carlo calculated beam profiles was obtained for the plastic scintillator and the liquid ionization chamber. Profiles measured with the diamond show an asymmetry when positioned with the smallest dimension facing the beam, while the double diode, the scintillator and the liquid chamber measure symmetric profiles irrespective of positioning. Significant differences in the output factors were obtained with the different detectors. The natural diamond detector measures output factors close to those with an ionization chamber (less than 1% difference) for field sizes between 3 x 3 and 15 x 15 cm2, but overestimates the output factors for large fields and underestimates the output factors for the smallest field sizes. The single and double diodes overestimated the output factor for large field sizes by up to 7 and 12% respectively due to the high content of low-energy photons. The double diode, and to some extent the single diode, also showed a relative increase in response compared with the more water equivalent liquid chamber and plastic scintillator at the smallest fields where there is a lack of lateral electron equilibrium. Both the plastic scintillator and the liquid chamber also show responses that deviate from the ionization chamber for larger field sizes. The major deviations can be explained based on the characteristics of the sensitive materials and the construction of the detectors.     

Monte Carlo study of a Cyberknife stereotactic radiosurgery system

This study investigated small-field dosimetry for a Cyberknife stereotactic radiosurgery system using Monte Carlo simulations. The EGSnrc/BEAMnrc Monte Carlo code was used to simulate the Cyberknife treatment head, and the DOSXYZnrc code was implemented to calculate central axis depth-dose curves, off-axis dose profiles, and relative output factors for various circular collimator sizes of 5 to 60 mm. Water-to-air stopping power ratios necessary for clinical reference dosimetry of the Cyberknife system were also evaluated by Monte Carlo simulations. Additionally, a beam quality conversion factor, kQ, for the Cyberknife system was evaluated for cylindrical ion chambers with different wall material. The accuracy of the simulated beam was validated by agreement within 2% between the Monte Carlo calculated and measured central axis depth-dose curves and off-axis dose profiles. The calculated output factors were compared with those measured by a diode detector and an ion chamber in water. The diode output factors agreed within 1% with the calculated values down to a 10 mm collimator. The output factors with the ion chamber decreased rapidly for collimators below 20 mm. These results were confirmed by the comparison to those from Monte Carlo methods with voxel sizes and materials corresponding to both detectors. It was demonstrated that the discrepancy in the 5 and 7.5 mm collimators for the diode detector is due to the water non-equivalence of the silicon material, and the dose fall-off for the ion chamber is due to its large active volume against collimators below 20 mm. The calculated stopping power ratios of the 60 mm collimator from the Cyberknife system (without a flattening filter) agreed within 0.2% with those of a 10 X 10 cm2 field from a conventional linear accelerator with a heavy flattening filter and the incident electron energy, 6 MeV. The difference in the stopping power ratios between 5 and 60 mm collimators was within 0.5% at a 10 cm depth in water. Furthermore, kQ values for the Cyberknife system were in agreement within 0.3% with those of the conventional 6 MV-linear accelerator for the cylindrical ion chambers with different wall material.     

An application of GafChromic MD-55 film for 67.5 MeV clinical proton beam dosimetry

The purpose of this study is to explore the use of GafChromic MD-55 (RC) film for 67.5 MeV clinical proton beam dosimetry at the Crocker Nuclear Laboratory, University of California, Davis. Several strips of RC film 6 cm x 6 cm in dimension were irradiated at a depth of 18.2 mm corresponding to the middle of a 24 mm spread-out Bragg peak (SOBP). The films were irradiated to a proton dose in the range of 0.5 Gy to 100 Gy. The beam profiles were also measured at the middle of the 24 mm SOBP. The Bragg peak was measured by using a wedge shaped phantom made of Lucite. The Bragg peak measured with RC film was compared with diode and ionization chamber measurements. After background subtraction, the calibration of the dose response of RC film showed, to a maximum deviation of 10%, a linear increase of optical density (OD) with dose from 0.5 to 100 Gy. The uniformity of OD over a single sheet of film showed a variation of +/-6%. The distal-fall off between 90% and 20% measured with GafChromic film for the Bragg peak was 1.3 mm as compared to 1.1 mm for a diode measurement and 1.4 mm for an ionization chamber measurement. The FWHM of the Bragg peak was 7.5 mm when measured with GafChromic film, 5.3 mm when measured with a diode and 8.1 mm as measured by an ionization chamber. The peak/plateau ratio with GafChromic film was 3.3 as compared to 3.7 with a diode and 3.2 with an ionization chamber. In conclusion, GafChromic MD-55 film may be a useful and convenient detector for dose measurement and quality assurance programmes of proton beams.     

Stopping-power ratios for clinical electron beams from a scatter-foil linear accelerator.

Restricted mass collision stopping-power ratios for electron beams from a scatter-foil medical linear accelerator (Varian Clinac 2100C) were calculated for various combinations of beams, phantoms and detector materials using the Monte Carlo method. The beams were of nominal energy 6, 12 or 20 MeV, with square dimensions 1 x 1 cm2 to 10 x 10 cm2. They were incident at nominal SSDs of 100 or 120 cm and inclined at 90 degrees or 30 degrees to the surface of homogeneous water phantoms or water phantoms interspersed with layered lung or bone-like materials. The broad beam water-to-air stopping-power ratios were within 1.3% of the AAPM TG21 protocol values and consistent with the results of Ding et al to within 0.2%. On the central axis the stopping-power ratio variations for narrow beams compared with normally incident broad beams were 0.1% or less for water-to-LiF-100, graphite, ferrous sulfate dosimeter solution, polystyrene and PMMA, 0.5% for water-to-silicon and 1% for water-to-air and water-to-photographic-film materials. The transverse variations of the stopping-power ratios were up to 4% for water-to-silicon, 7% for water-to-photographic-film materials and 10% for water-to-air in the penumbral regions (where the dose was 10% of the global dose maximum) at shallow depths compared with the values at the same depths on the central axis. In the inhomogeneous phantoms studied, the stopping-power ratio correction factors varied more significantly for air, followed by photographic materials and silicon, at various depths on the central axis in the heterogeneous regions. For the simple layered phantoms studied, the estimation of the stopping-power ratio correction factors based on the relative electron-density derived effective depth approach yielded results that were within 0.5% of the Monte Carlo derived values for all the detector materials studied.     

A diamond detector in the dosimetry of high-energy electron and photon beams.

A diamond detector type 60003 (PTW Freiburg) was examined for the purpose of dosimetry with 4-20 MeV electron beams and 4-25 MV photon beams. Results were compared with those obtained by using a Markus chamber for electron beams and an ionization chamber for photon beams. Dose distributions were measured in a water phantom with the detector connected to a Unidos electrometer (PTW Freiburg). After a pre-irradiation of about 5 Gy the diamond detector shows a stability in response which is better than that of an ionization chamber. The current of the diamond detector was measured under variation of photon beam dose rate between 0.1 and 7 Gy min(-1). Different FSDs were chosen. Furthermore the pulse repetition frequency and the depth of the detector were changed. The electron beam dose rate was varied between 0.23 and 4.6 Gy min(-1) by changing the pulse-repetition frequency. The response shows no energy dependence within the covered photon-beam energy range. Between 4 MeV and 18 MeV electron beam energy it shows only a small energy dependence of about 2%, as expected from theory. For smaller electron energies the response increases significantly and an influence of the contact material used for the diamond detector can be surmised. A slight sublinearity of the current and dose rate was found. Detector current and dose rate are related by the expression i alpha Ddelta, where i is the detector current, D is the dose rate and delta is a correction factor of approximately 0.963. Depth-dose curves of photon beams, measured with the diamond detector, show a slight overestimation compared with measurements with the ionization chamber. This overestimation is compensated for by the above correction term. The superior spatial resolution of the diamond detector leads to minor deviations between depth-dose curves of electron beams measured with a Markus chamber and a diamond detector.     

Dosimetric properties of radiophotoluminescent glass rod detector in high-energy photon beams from a linear accelerator and cyber-knife

A fully automatic radiophotoluminescent glass rod dosimeter (GRD) system has recently become commercially available. This article discusses the dosimetric properties of the GRD including uniformity and reproducibility of signal, dose linearity, and energy and directional dependence in high-energy photon beams. In addition, energy response is measured in electron beams. The uniformity and reproducibility of the signal from 50 GRDs using a 60Co beam are both +/- 1.1% (one standard deviation). Good dose linearity of the GRD is maintained for doses ranging from 0.5 to 30 Gy, the lower and upper limits of this study, respectively. The GRD response is found to show little energy dependence in photon energies of a 60Co beam, 4 MV (TPR20(10)=0.617) and 10 MV (TPR(20)10=0.744) x-ray beams. However, the GRD responses for 9 MeV (mean energy, Ez = 3.6 MeV) and 16 MeV (Ez = 10.4 MeV) electron beams are 4%-5% lower than that for a 60Co beam in the beam quality dependence. The measured angular dependence of GRD, ranging from 0 degrees (along the long axis of GRD) to 120 degrees is within 1.5% for a 4 MV x-ray beam. As applications, a linear accelerator-based radiosurgery system and Cyber-Knife output factors are measured by a GRD and compared with those from various detectors including a p-type silicon diode detector, a diamond detector, and an ion chamber. It is found that the GRD is a very useful detector for small field dosimetry, in particular, below 10 mm circular fields.     

Commissioning stereotactic radiosurgery beams using both experimental and theoretical methods

The purpose of this investigation is to study the feasibility of using an alternative method to commission stereotactic radiosurgery beams shaped by micro multi-leaf collimators by using Monte Carlo simulations to obtain beam characteristics of small photon beams, such as incident beam particle fluence and energy distributions, scatter ratios, depth-dose curves and dose profiles where measurements are impossible or difficult. Ionization chambers and diode detectors with different sensitive volumes were used in the measurements in a water phantom and the Monte Carlo codes BEAMnrc/DOSXYZnrc were used in the simulation. The Monte Carlo calculated data were benchmarked against measured data for photon beams with energies of 6 MV and 10 MV produced from a Varian Trilogy accelerator. The measured scatter ratios and cross-beam dose profiles for very small fields are shown to be not only dependent on the size of the sensitive volume of the detector used but also on the type of detectors. It is known that the response of some detectors changes at small field sizes. Excellent agreement was seen between scatter ratios measured with a small ion chamber and those calculated from Monte Carlo simulations. The values of scatter ratios, for field sizes from 6 x 6 mm2 to 98 x 98 mm2, range from 0.67 to 1.0 and from 0.59 to 1.0 for 6 and 10 MV, respectively. The Monte Carlo calculations predicted that the incident beam particle fluence is strongly affected by the X-Y-jaw openings, especially for small fields due to the finite size of the radiation source. Our measurement confirmed this prediction. This study demonstrates that Monte Carlo calculations not only provide accurate dose distributions for small fields where measurements are difficult but also provide additional beam characteristics that cannot be obtained from experimental methods. Detailed beam characteristics such as incident photon fluence distribution, energy spectra, including composition of primary and scattered photons, can be independently used in dose calculation models and to improve the accuracy of measurements with detectors with an energy-dependent response. Furthermore, when there are discrepancies between results measured with different detectors, the Monte Carlo calculated values can indicate the most correct result. The data set presented in this study can be used as a reference in commissioning stereotactic radiosurgery beams shaped by a BrainLAB m3 on a Varian 2100EX or 600C accelerator.     

PTW-diamond detector: dose rate and particle type dependence

In this paper the suitability of a PTW natural diamond detector (DD) for relative and reference dosimetry of photon and electron beams, with dose per pulse between 0.068 mGy and 0.472 mGy, was studied and the results were compared with those obtained by a stereotactic silicon detector (SFD). The results show that, in the range of the examined dose per pulse the DD sensitivity changes up to 1.8% while the SFD sensitivity changes up to 4.5%. The fitting parameter, delta, used to correct the dose per pulse dependence of solid state detectors, was delta = 0.993 +/- 0.002 and delta = 1.025 +/- 0.002 for the diamond detector and for the silicon diode, respectively. The delta values were found to be independent of particle type of two conventional beams (a 10 MV x-ray beam and a 21 MeV electron beam). So if delta is determined for a radiotherapy beam, it can be used to correct relative dosimetry for other conventional radiotherapy beams. Moreover the diamond detector shows a calibration factor which is independent of beam quality and particle type, so an empirical dosimetric formalism is proposed here to obtain the reference dosimetry. This formalism is based on a dose-to-water calibration factor and on an empirical coefficient, that takes into account the reading dependence on the dose per pulse.     

A correction method for diamond detector signal dependence with proton energy

Small dosimeters as solid state detectors can be useful for the dosimetric characterization and periodic quality control of radiotherapy proton beams. The calibration of solid state detectors for proton beams is not a solved problem especially for ophthalmologic proton beams, where these detectors present a LET-dependent signal. In this work a PTW diamond detector has been selected because of its good signal reproducibility (0.3%) and stable response with accumulated dose. A method that takes into account the LET dependence of the diamond detector signal, at 62 MeV proton beam, is here proposed. In particular an empirical correction factor, kDD(Eo) (Rres), has been determined as a function of the residual range quality index, to correct the diamond detector signal for a proton beam of incident effective energy E0= 62 MeV. A dedicated software allows us to use the diamond detector as an on-line reference dosimeter, where an ionization chamber may be difficult to use, or for periodic quality control procedures. The article also reports a comparison between the signal dependence on proton energy of silicon, diamond, and radiochromic film detectors.