Use of PTW-microDiamond for relative dosimetry of unflattened photon beams.

The increasing interest in SBRT treatments encourages the use of flattening filter free (FFF) beams. Aim of this work was to evaluate the performance of the PTW60019 microDiamond detector under 6MV and 10MVFFF beams delivered with the EDGE accelerator (Varian Medical System, Palo Alto, USA). A flattened 6MV beam was also considered for comparison. Short term stability, dose linearity and dose rate dependence were evaluated. Dose per pulse dependence was investigated in the range 0.2-2.2mGy/pulse. MicroDiamond profiles and output factors (OFs) were compared to those obtained with other detectors for field sizes ranging from 40×40cm2 to 0.6×0.6cm2. In small fields, volume averaging effects were evaluated and the relevant correction factors were applied for each detector. MicroDiamond short term stability, dose linearity and dependence on monitor unit rate were less than 0.8% for all energies. Response variations with dose per pulse were found within 1.8%. MicroDiamond output factors (OF) values differed from those measured with the reference ion-chamber for less than 1% up to 40×40cm2 fields where silicon diodes overestimate the dose of ≈3%. For small fields (<3×3cm2) microDiamond and the unshielded silicon diode were in good agreement. MicroDiamond showed optimal characteristics for relative dosimetry even under high dose rate beams. The effects due to dose per pulse dependence up to 2.2mGy/pulse are negligible. Compared to other detectors, microDiamond provides accurate OF measurements in the whole range of field sizes. For fields <1cm correction factors accounting for fluence perturbation and volume averaging could be required.

Reggiori G ，Stravato A ，Pimpinella M ，Lobefalo F ，De Coste V ，Fogliata A ，Mancosu P ，De Rose F ，Palumbo V ，Scorsetti M ，Tomatis S ... -  《-》

Investigation of a synthetic diamond detector response in kilovoltage photon beams.

An important characteristic of radiation dosimetry detectors is their energy response which consists of absorbed-dose and intrinsic energy responses. The former can be characterized using Monte Carlo (MC) simulations, whereas the latter (i.e., detector signal per absorbed dose to detector) is extracted from experimental data. Such a characterization is especially relevant when detectors are used in nonrelative measurements at a beam quality that differs from the calibration beam quality. Having in mind the possible application of synthetic diamond detectors (microDiamond PTW 60019, Freiburg, Germany) for nonrelative dosimetry of low-energy brachytherapy (BT) beams, we determined their intrinsic and absorbed-dose energy responses in 25-250 kV beams relative to a 60 Co beam, which is usually the reference beam quality for detector calibration in radiotherapy. Three microDiamond detectors and, for comparison, two silicon diodes (PTW 60017) were calibrated in terms of air-kerma free in air in six x-ray beam qualities (from 25 to 250 kV) and in terms of absorbed dose to water in a 60 Co beam at the national metrology laboratory in Sweden. The PENELOPE/penEasy MC radiation transport code was used to calculate the absorbed-dose energy response of the detectors (modeled based on blueprints) relative to air and water depending on calibration conditions. The MC results were used to extract the relative intrinsic energy response of the detectors from the overall energy response. Measurements using an independent setup with a single ophthalmic BEBIG I25.S16 125 I BT seed (effective photon energy of 28 keV) were used as a qualitative check of the extracted intrinsic energy response correction factors. Additionally, the impact of the thickness of the active volume as well as the presence of extra-cameral components on the absorbed-dose energy response of a microDiamond detector was studied using MC simulations. The relative intrinsic energy response of the microDiamond detectors was higher by a factor of 2 in 25 and 50 kV beams compared to the 60 Co beam. The variation in the relative intrinsic energy response of silicon diodes was within 10% over the investigated photon energy range. The use of relative intrinsic energy response correction factors improved the agreement among the absorbed dose to water values determined using microDiamond detectors and silicon diodes, as well as with the TG-43 formalism-based calculations for the 125 I seed. MC study of microDiamond detector design features provided a possible explanation for inter-detector response variation at low-energy photon beams by differences in the effective thickness of the active volume. MicroDiamond detectors had a non-negligible variation in the relative intrinsic energy response (factor of 2) which was comparable to that in the absorbed-dose energy response relative to water at low-energy photon beams. Silicon diodes, in contrast, had an absorbed-dose energy dependence on photon energy that varied by a factor of 6, whereas the intrinsic energy dependence on beam quality was within 10%. It is important to decouple these two responses for a full characterization of detector energy response especially when the user and reference beam qualities differ significantly, and MC alone is not enough.

Kaveckyte V ，Persson L ，Malusek A ，Benmakhlouf H ，Alm Carlsson G ，Carlsson Tedgren Å ... -  《-》

A novel method for the determination of field output factors and output correction factors for small static fields for six diodes and a microdiamond detector in megavoltage photon beams.

The goal of this work is to provide a large and consistent set of data for detector-specific output correction factors, k Q clin , Q ref f clin , f ref , for small static fields for seven solid-state detectors and to determine field output factors, Ω Q clin , Q ref f clin , f ref , using EBT3 radiochromic films and W1 plastic scintillator as reference detectors on two different linear accelerators and four megavoltage photon beams. Consistent measurement conditions and recommendations given in the International Code of Practice TRS-483 for small-field dosimetry were followed throughout the study. Ω Q clin , Q ref f clin , f ref were determined on two linacs, Elekta Versa HD and Varian TrueBeam, for 6 and 10 MV beams with and without flattening filter and for nine fields ranging from 0.5 × 0.5 cm2 to 10 × 10 cm2 . Signal readings obtained with EBT3 radiochromic films and W1 plastic scintillator were fitted by an analytical function. Volume averaging correction factors, determined from two-dimensional (2D) dose matrices obtained with EBT3 films and fitted to bivariate Gaussian function, were used to correct measured signals. k Q clin , Q ref f clin , f ref were determined empirically for six diodes, IBA SFD, IBA Razor, PTW 60008 P, PTW 60012 E, PTW 60018 SRS, and SN EDGE, and a PTW 60019 microDiamond detector. Field output factors and detector-specific k Q clin , Q ref f clin , f ref are presented in the form of analytical functions as well as in the form of discrete values. It is found that in general, for a given linac, small-field output factors need to be determined for every combination of beam energy and filtration (WFF or FFF) and field size as the differences between them can be statistically significant (P < 0.05). For different beam energies, the present data for k Q clin , Q ref f clin , f ref are found to differ significantly (P < 0.05) from the corresponding data published in TRS-483 mostly for the smallest fields (<1.5 cm). For the PTW microDiamond detector, statistically significant differences (P < 0.05) between k Q clin , Q ref f clin , f ref values were found for all investigated beams on an Elekta Versa HD linac for field sizes 0.5 × 0.5 cm2 and 0.8 × 0.8 cm2 . Significant differences in k Q clin , Q ref f clin , f ref between beams of a given energy but with and without flattening filters are found for measurements made in small fields (<1.5 cm) at a given linac. Differences in k Q clin , Q ref f clin , f ref are also found when measurements are made at different linacs using the same beam energy filtration combination; for the PTW microDiamond detector, these differences were found to be around 6% and were considered as significant. Selection of two reference detectors, EBT3 films and W1 plastic scintillator, and use of an analytical function, is a novel approach for the determination of Ω Q clin , Q ref f clin , f ref for small static fields in megavoltage photon beams. Large set of k Q clin , Q ref f clin , f ref data for seven solid-state detectors and four beam energies determined on two linacs by a single group of researchers can be considered a valuable supplement to the literature and the TRS-483 dataset.

Casar B ，Gershkevitsh E ，Mendez I ，Jurković S ，Huq MS ... -  《-》

Characterization of a microDiamond detector in high-dose-per-pulse electron beams for intra operative radiation therapy.

To characterize a synthetic diamond dosimeter (PTW Freiburg microDiamond 60019) in high dose-per-pulse electron beams produced by an Intra Operative Radiation Therapy (IORT) dedicated accelerator. The dosimetric properties of the microDiamond were assessed under 6, 8 and 9 MeV electron beams by a NOVAC11 mobile accelerator (Sordina IORT Technologies S.p.A.). The characterization was carried out with dose-per-pulse ranging from 26 to 105 mGy per pulse. The microDiamond performance was compared with an Advanced Markus ionization chamber and a PTW silicon diode E in terms of dose linearity, percentage depth dose (PDD) curves, beam profiles and output factors. A good linearity of the microDiamond response was verified in the dose range from 0.2 Gy to 28 Gy. A sensitivity of 1.29 nC/Gy was measured under IORT electron beams, resulting within 1% with respect to the one obtained in reference condition under (60)Co gamma irradiation. PDD measurements were found in agreement with the ones by the reference dosimeters, with differences in R50 values below 0.3 mm. Profile measurements evidenced a high spatial resolution of the microDiamond, slightly worse than the one of the silicon diode. The penumbra widths measured by the microDiamond resulted approximately 0.5 mm larger than the ones by the Silicon diode. Output factors measured by the microDiamond were found within 2% with those obtained by the Advanced Markus down to 3 cm diameter field sizes. The microDiamond dosimeter was demonstrated to be suitable for precise dosimetry in IORT applications under high dose-per-pulse conditions.

Di Venanzio C ，Marinelli M ，Tonnetti A ，Verona-Rinati G ，Falco MD ，Pimpinella M ，Ciccotelli A ，De Stefano S ，Felici G ，Marangoni F ... -  《-》

A Monte Carlo study on the PTW 60019 microDiamond detector.

Data on the output correction factor for small photon beam dosimetry of the microDiamond detector manufactured by the company PTW can be found in a variety of papers. Referring either to measurements or to Monte Carlo (MC) calculations, they show substantial disagreements particularly at very small fields. This work reports results of a further MC study aiming at a better understanding of how specific properties of the microDiamond detector are influencing its output correction factor and whether this can explain at least some of the disagreements. In this study the method of a fluence-based decomposition of the dose conversion factor was used which is considered as a useful tool to understand the response of a detector in nonreference conditions. This decomposition method yields the following three factors: (a) the stopping power ratio water to diamond, (b) a perturbation factor pint taking into account all fluence changes in the transition from a small water voxel at the point of dose determination to the bare diamond detector, and (c) a perturbation factor pext taking into account all additional fluence changes in the fully simulated diamond detector caused by the material and design details outside the sensitive volume. Monte Carlo calculated output correction factors were obtained for Co-60, 6 MV and 10 MV photon beams showing that the maximum variation with field size remained in the order of 2% for quadratic field sizes larger than about 0.3 cm. For field sizes smaller than about 0.5 cm a clear under-response is obtained at all three radiation qualities in agreement with all known MC calculations, however, in contrast to some measured result. The shape of the output correction factor can be well explained by an opposite mode of action between under-response expressed by the perturbation factor pint and over-response expressed by the perturbation factor pext where the first one is mainly influenced by volume averaging, and the second one by a back scatter effect of electrons from the diamond substrate into the sensitive volume. The response of microDiamond detector can be well described under various measuring conditions by the dose conversion factor and the dependency of its fluence-based subfactors on detector characteristics. Monte Carlo simulations offer an improvement in the understanding particularly of small-field effects by relating the output correction factor to spectral fluence changes in the sensitive volume of the detector. The most significant influence factors are the finite size of the active volume and the presence of the high-density diamond substrate causing a field size-dependent backscattering. These perturbations are opposite in their effects. The diamond in the sensitive volume itself and in particular its density has almost no influence. Scattering of results at very small field sizes can be explained by different gradients of dose profiles around the beam axis at identical full width half maximum (FWHM) field size parameters and by possible deviations of the radius of the sensitive volume from the nominal radius. The backscattering effect also has an influence on the determination of profiles and for very small field sizes on the response at different rotation angles.

Hartmann GH ，Zink K 《-》