Development and benchmarking of a dose rate engine for raster-scanned FLASH helium ions.

BACKGROUND: Radiotherapy with charged particles at high dose and ultra-high dose rate (uHDR) is a promising technique to further increase the therapeutic index of patient treatments. Dose rate is a key quantity to predict the so-called FLASH effect at uHDR settings. However, recent works introduced varying calculation models to report dose rate, which is susceptible to the delivery method, scanning path (in active beam delivery) and beam intensity. PURPOSE: This work introduces an analytical dose rate calculation engine for raster scanned charged particle beams that is able to predict dose rate from the irradiation plan and recorded beam intensity. The importance of standardized dose rate calculation methods is explored here. METHODS: Dose is obtained with an analytical pencil beam algorithm, using pre-calculated databases for integrated depth dose distributions and lateral penumbra. Dose rate is then calculated by combining dose information with the respective particle fluence (i.e., time information) using three dose-rate-calculation models (mean, instantaneous, and threshold-based). Dose rate predictions for all three models are compared to uHDR helium ion beam (145.7 MeV/u, range in water of approximatively 14.6 cm) measurements performed at the Heidelberg Ion Beam Therapy Center (HIT) with a diamond-detector prototype. Three scanning patterns (scanned or snake-like) and four field sizes are used to investigate the dose rate differences. RESULTS: Dose rate measurements were in good agreement with in-silico generated distributions using the here introduced engine. Relative differences in dose rate were below 10% for varying depths in water, from 2.3 to 14.8 cm, as well as laterally in a near Bragg peak area. In the entrance channel of the helium ion beam, dose rates were predicted within 7% on average for varying irradiated field sizes and scanning patterns. Large differences in absolute dose rate values were observed for varying calculation methods. For raster-scanned irradiations, the deviation between mean and threshold-based dose rate at the investigated point was found to increase with the field size up to 63% for a 10 mm × 10 mm field, while no significant differences were observed for snake-like scanning paths. CONCLUSIONS: This work introduces the first dose rate calculation engine benchmarked to instantaneous dose rate, enabling dose rate predictions for physical and biophysical experiments. Dose rate is greatly affected by varying particle fluence, scanning path, and calculation method, highlighting the need for a consensus among the FLASH community on how to calculate and report dose rate in the future. The here introduced engine could help provide the necessary details for the analysis of the sparing effect and uHDR conditions.

Diamond detectors for dose and instantaneous dose-rate measurements for ultra-high dose-rate scanned helium ion beams.

BACKGROUND: The possible emergence of the FLASH effect-the sparing of normal tissue while maintaining tumor control-after irradiations at dose-rates exceeding several tens of Gy per second, has recently spurred a surge of studies attempting to characterize and rationalize the phenomenon. Investigating and reporting the dose and instantaneous dose-rate of ultra-high dose-rate (UHDR) particle radiotherapy beams is crucial for understanding and assessing the FLASH effect, towards pre-clinical application and quality assurance programs. PURPOSE: The purpose of the present work is to investigate a novel diamond-based detector system for dose and instantaneous dose-rate measurements in UHDR particle beams. METHODS: Two types of diamond detectors, a microDiamond (PTW 60019) and a diamond detector prototype specifically designed for operation in UHDR beams (flashDiamond), and two different readout electronic chains, were investigated for absorbed dose and instantaneous dose-rate measurements. The detectors were irradiated with a helium beam of 145.7 MeV/u under conventional and UHDR delivery. Dose-rate delivery records by the monitoring ionization chamber and diamond detectors were studied for single spot irradiations. Dose linearity at 5 cm depth and in-depth dose response from 2 to 16 cm were investigated for both measurement chains and both detectors in a water tank. Measurements with cylindrical and plane-parallel ionization chambers as well as Monte-Carlo simulations were performed for comparisons. RESULTS: Diamond detectors allowed for recording the temporal structure of the beam, in good agreement with the one obtained by the monitoring ionization chamber. A better time resolution of the order of few µs was observed as compared to the approximately 50 µs of the monitoring ionization chamber. Both diamonds detectors show an excellent linearity response in both delivery modalities. Dose values derived by integrating the measured instantaneous dose-rates are in very good agreement with the ones obtained by the standard electrometer readings. Bragg peak curves confirmed the consistency of the charge measurements by the two systems. CONCLUSIONS: The proposed novel dosimetric system allows for a detailed investigation of the temporal evolution of UHDR beams. As a result, reliable and accurate determinations of dose and instantaneous dose-rate are possible, both required for a comprehensive characterization of UHDR beams and relevant for FLASH effect assessment in clinical treatments.

Diamond based integrated detection system for dosimetric and microdosimetric characterization of radiotherapy ion beams.

BACKGROUND: Ion beam therapy allows for a substantial sparing of normal tissues and higher biological efficacy. Synthetic single crystal diamond is a very good material to produce high-spatial-resolution and highly radiation hard detectors for both dosimetry and microdosimetry in ion beam therapy. PURPOSE: The aim of this work is the design, fabrication and test of an integrated waterproof detector based on synthetic single crystal diamond able to simultaneously perform dosimetric and microdosimetric characterization of clinical ion beams. METHODS: The active elements of the integrated diamond device, that is, dosimeter and microdosimeter, were both realized in a Schottky diode configuration featured by different area, thickness, and shape by means of photolithography technologies for the selective growth of intrinsic and boron-doped CVD diamond. The cross-section of the sensitive volume of the dosimetric element is 4 mm2 and 1 µm-thick, while the microdosimetric one has an active cross-sectional area of 100 × 100 µm2 and a thickness of about 6.2 µm. The dosimetric and microdosimetric performance of the developed device was assessed at different depths in a water phantom at the MedAustron ion beam therapy facility using a monoenergetic uniformly scanned carbon ion beam of 284.7 MeV/u and proton beam of 148.7 MeV. The particle flux in the region of the microdosimeter was 6·107  cm2 /s for both irradiation fields. At each depth, dose and dose distributions in lineal energy were measured simultaneously and the dose mean lineal energy values were then calculated. Monte Carlo simulations were also carried out by using the GATE-Geant4 code to evaluate the relative dose, dose averaged linear energy transfer (LETd ), and microdosimetric spectra at various depths in water for the radiation fields used, by considering the contribution from the secondary particles generated in the ion interaction processes as well. RESULTS: Dosimetric and microdosimetric quantities were measured by the developed prototype with relatively low noise (∼2 keV/µm). A good agreement between the measured and simulated dose profiles was found, with discrepancies in the peak to plateau ratio of about 3% and 4% for proton and carbon ion beams respectively, showing a negligible LET dependence of the dosimetric element of the device. The microdosimetric spectra were validated with Monte Carlo simulations and a good agreement between the spectra shapes and positions was found. Dose mean lineal energy values were found to be in close agreement with those reported in the literature for clinical ion beams, showing a sharp increase along the Bragg curve, being also consistent with the calculated LETd for all depths within the experimental error of 10%. CONCLUSIONS: The experimental indicate that the proposed device can allow enhanced dosimetry in particle therapy centers, where the absorbed dose measurement is implemented by the microdosimetric characterization of the radiation field, thus providing complementary results. In addition, the proposed device allows for the reduction of the experimental uncertainties associated with detector positioning and could facilitate the partial overcoming of some drawbacks related to the low sensitivity of diamond microdosimeters to low LET radiation.

A diamond detector based dosimetric system for instantaneous dose rate measurements in FLASH electron beams.

Objective.A reliable determination of the instantaneous dose rate (I-DR) delivered in FLASH radiotherapy treatments is believed to be crucial to assess the so-called FLASH effect in preclinical and biological studies. At present, no detectors nor real-time procedures are available to do that in ultra high dose rate (UH-DR) electron beams, typically consisting ofµs pulses characterized by I-DRs of the order of MGy/s. A dosimetric system is proposed possibly overcoming the above reported limitation, based on the recently developed flashDiamond (fD) detector (model 60025, PTW-Freiburg, Germany).Approach.A dosimetric system is proposed, based on a flashDiamond detector prototype, properly modified and adapted for very fast signal transmission. It was used in combination with a fast transimpedance amplifier and a digital oscilloscope to record the temporal traces of the pulses delivered by an ElectronFlash linac (SIT S.p.A., Italy). The proposed dosimetric systems was investigated in terms of the temporal characteristics of its response and the capability to measure the absolute delivered dose and instantaneous dose rate (I-DR). A 'standard' flashDiamond was also investigated and its response compared with the one of the specifically designed prototype.Main results. Temporal traces recorded in several UH-DR irradiation conditions showed very good signal to noise ratios and rise and decay times of the order of a few tens ns, faster than the ones obtained by the current transformer embedded in the linac head. By analyzing such signals, a calibration coefficient was derived for the fD prototype and found to be in agreement within 1% with the one obtained under reference60Co irradiation. I-DRs as high as about 2 MGy s-1were detected without any undesired saturation effect. Absolute dose per pulse values extracted by integrating the I-DR signals were found to be linear up to at least 7.13 Gy and in very good agreement with the ones obtained by connecting the fD to a UNIDOS electrometer (PTW-Freiburg, Germany). A good short term reproducibility of the linac output was observed, characterized by a pulse-to-pulse variation coefficient of 0.9%. Negligible differences were observed when replacing the fD prototype with a standard one, with the only exception of a somewhat slower response time for the latter detector type.Significance.The proposed fD-based system was demonstrated to be a suitable tool for a thorough characterization of UH-DR beams, providing accurate and reliable time resolved I-DR measurements from which absolute dose values can be straightforwardly derived.

On the measurement uncertainty of microdosimetric quantities using diamond and silicon microdosimeters in carbon-ion beams.

PURPOSE: The purpose of this paper is to compare the response of two different types of solid-state microdosimeters, that is, silicon and diamond, and their uncertainties. A study of the conversion of silicon microdosimetric spectra to the diamond equivalent for microdosimeters with different geometry of the sensitive volumes is performed, including the use of different stopping power databases. METHOD: Diamond and silicon microdosimeters were irradiated under the same conditions, aligned at the same depth in a carbon-ion beam at the MedAustron ion therapy center. In order to estimate the microdosimetric quantities, the readout electronic linearity was investigated with three different methods, that is, the first being a single linear regression, the second consisting of a double linear regression with a channel transition and last a multiple linear regression by splitting the data into odd and even groups. The uncertainty related to each of these methods was estimated as well. The edge calibration was performed using the intercept with the horizontal axis of the tangent through the inflection point of the Fermi function approximation multi-channel analyzer spectrum. It was assumed that this point corresponds to the maximum energy difference of particle traversing the sensitive volume (SV) for which the residual range difference in the continuous slowing down approximation is equal to the thickness of the SV of the microdosimeter. Four material conversion methods were explored, the edge method, the density method, the maximum-deposition energy method and the bin-by-bin transformation method. The uncertainties of the microdosimetric quantities resulting from the linearization, the edge calibration and the detectors thickness were also estimated. RESULTS: It was found that the double linear regression had the lowest uncertainty for both microdosimeters. The propagated standard (k = 1) uncertainties on the frequency-mean lineal energy y ¯ F ${\bar{y}}_{\rm{F}}$ and the dose-mean lineal energy y ¯ D ${\bar{y}}_{\rm{D}}$ values from the marker point, in the spectra, in the plateau were 0.1% and 0.2%, respectively, for the diamond microdosimeter, whilst for the silicon microdosimeter data converted to diamond, the uncertainty was estimated to be 0.1%. In the range corresponding to the 90% of the amplitude of the Bragg Peak at the distal part of the Bragg curve (R90 ) the uncertainty was found to be 0.1%. The uncertainty propagation from the stopping power tables was estimated to be between 5% and 7% depending on the method. The uncertainty on the y ¯ F ${\bar{y}}_{\rm{F}}$ and y ¯ D ${\bar{y}}_{\rm{D}}$ coming from the thickness of the detectors varied between 0.3% and 0.5%. CONCLUSION: This article demonstrate that the linearity of the readout electronics affects the microdosimetric spectra with a difference in y ¯ F ${\bar{y}}_{\rm{F}}$ values between the different linearization methods of up to 17.5%. The combined uncertainty was dominated by the uncertainty of stopping power on the edge.

Application of a novel diamond detector for commissioning of FLASH radiotherapy electron beams.

PURPOSE: A diamond detector prototype was recently proposed by Marinelli et al. (Medical Physics 2022, https://doi.org/10.1002/mp.15473) for applications in ultrahigh-dose-per-pulse (UH-DPP) and ultrahigh-dose-rate (UH-DR) beams, as used in FLASH radiotherapy (FLASH-RT). In the present study, such so-called flashDiamond (fD) was investigated from the dosimetric point of view, under pulsed electron beam irradiation. It was then used for the commissioning of an ElectronFlash linac (SIT S.p.A., Italy) both in conventional and UH-DPP modalities. METHODS: Detector calibration was performed in reference conditions, under 60 Co and electron beam irradiation. Its response linearity was investigated in UH-DPP conditions. For this purpose, the DPP was varied in the 1.2-11.9 Gy range, by changing either the beam applicator or the pulse duration from 1 to 4 µs. Dosimetric validation of the fD detector prototype was then performed in conventional modality, by measuring percentage depth dose (PDD) curves, beam profiles, and output factors (OFs). All such measurements were carried out in a motorized water phantom. The obtained results were compared with the ones from commercially available dosimeters, namely, a microDiamond, an Advanced Markus ionization chamber, a silicon diode detector, and EBT-XD GAFchromic films. Finally, the fD detector was used to fully characterize the 7 and 9 MeV UH-DPP electron beams delivered by the ElectronFlash linac. In particular, PDDs, beam profiles, and OFs were measured, for both energies and all the applicators, and compared with the ones from EBT-XD films irradiated in the same experimental conditions. RESULTS: The fD calibration coefficient resulted to be independent from the investigated beam qualities. The detector response was found to be linear in the whole investigated DPP range. A very good agreement was observed among PDDs, beam profiles, and OFs measured by the fD prototype and reference detectors, both in conventional and UH-DPP irradiation modalities. CONCLUSIONS: The fD detector prototype was validated from the dosimetric point of view against several commercial dosimeters in conventional beams. It was proved to be suitable in UH-DPP and UH-DR conditions, for which no other commercial real-time active detector is available to date. It was shown to be a very useful tool to perform fast and reproducible beam characterizations in standard clinical motorized water phantom setups. All of the previously mentioned demonstrate the suitability of the proposed detector for the commissioning of UH-DR linac beams for preclinical FLASH-RT applications.

Design, realization, and characterization of a novel diamond detector prototype for FLASH radiotherapy dosimetry.

PURPOSE: FLASH radiotherapy (RT) is an emerging technique in which beams with ultra-high dose rates (UH-DR) and dose per pulse (UH-DPP) are used. Commercially available active real-time dosimeters have been shown to be unsuitable in such conditions, due to severe response nonlinearities. In the present study, a novel diamond-based Schottky diode detector was specifically designed and realized to match the stringent requirements of FLASH-RT. METHODS: A systematic investigation of the main features affecting the diamond response in UH-DPP conditions was carried out. Several diamond Schottky diode detector prototypes with different layouts were produced at Rome Tor Vergata University in cooperation with PTW-Freiburg. Such devices were tested under electron UH-DPP beams. The linearity of the prototypes was investigated up to DPPs of about 26 Gy/pulse and dose rates of approximately 1 kGy/s. In addition, percentage depth dose (PDD) measurements were performed in different irradiation conditions. Radiochromic films were used for reference dosimetry. RESULTS: The response linearity of the diamond prototypes was shown to be strongly affected by the size of their active volume as well as by their series resistance. By properly tuning the design layout, the detector response was found to be linear up to at least 20 Gy/pulse, well into the UH-DPP range conditions. PDD measurements were performed by three different linac applicators, characterized by DPP values at the point of maximum dose of 3.5, 17.2, and 20.6 Gy/pulse, respectively. The very good superimposition of three curves confirmed the diamond response linearity. It is worth mentioning that UH-DPP irradiation conditions may lead to instantaneous detector currents as high as several mA, thus possibly exceeding the electrometer specifications. This issue was properly addressed in the case of the PTW UNIDOS electrometers. CONCLUSIONS: The results of the present study clearly demonstrate the feasibility of a diamond detector for FLASH-RT applications.

S-band hybrid amplifiers based on hydrogenated diamond FETs.

The first realizations of S-band hybrid amplifiers based on hydrogenated-diamond (H-diamond) FETs are reported. As test vehicles of the adopted H-diamond technology at microwave frequencies, two designs are proposed: one, oriented to low-noise amplification, the other, oriented to high-power operation. The two amplifying stages are so devised as to be cascaded into a two-stage amplifier. The activities performed, from the technological steps to characterization, modelling, design and realization are illustrated. Measured performance demonstrates, for the low-noise stage, a noise figure between 7 and 8 dB in the 2-2.5 GHz bandwidth, associated with a transducer gain between 5 and 8 dB. The OIP3 at 2 GHz is 21 dBm. As to the power-oriented stage, its transducer gain is 5-6 dB in the 2-2.5 GHz bandwidth. The 1-dB output compression point at 2 GHz is 20 dBm whereas the OIP3 is 33 dBm. Cascading the measured S-parameters of the two stages yields a transducer gain of 15 ± 1.2 dB in the 2-3 GHz bandwidth.

Microdosimetric characterization of clinical carbon-ion beams using synthetic diamond detectors and spectral conversion methods.

PURPOSE: To investigate for the first time the potentialities of obtaining microdosimetric measurements in scanned clinical carbon-ion beams using synthetic single crystal diamond detector and to verify the spectral conversion methods. METHODS: Microdosimetric measurements were performed at different depths in a water phantom at the therapeutic scanned carbon-ion beam of the National Center of Oncological Hadrontherapy (CNAO) in Pavia, using waterproof encapsulated diamond microdosimeter developed at "Tor Vergata" University. A monoenergetic carbon-ion beam of 195 MeV/µ scanned over a square field of 2 × 2 cm2 was used. Experimental microdosimetric spectra were compared with those obtained with a propane-filled Tissue Equivalent Proportional Counters (TEPCs) microdosimeter in the same facility at the same conditions. To this purpose, the spectra in diamond were converted to the spectra that would have been collected with a propane-filled cylindrical sensitive volume by means of a novel analytic methodology, recently developed at MedAustron. RESULTS: The microdosimetric spectra acquired by the diamond microdosimeter show different shapes in the 10 keV µm-1  ÷ 103  keV µm-1 lineal-energy range at different water depths. In spite of the high counting rate, no spectral distortion, due to pile-up events and polarization effects, were observed. The experimental spectra have a low detection threshold of about 6 keV µm-1 due to the electronic noise in the irradiation room. The comparison between the spectra converted to propane from diamond detector and the spectra collected directly with propane-filled TEPC shows a good agreement in the whole lineal-energy range. Furthermore this comparison confirms that diamond detector response is LET independent. The frequency- and dose-mean lineal energy values were also assessed for all spectra. The frequency-mean values obtained with diamond microdosimeter at different depths scales rather well with the absorbed dose values. CONCLUSIONS: Microdosimetric characterization of a synthetic single crystal diamond detector in high-energy scanned carbon-ion beams was performed. The results of the present study showed that this detector is suitable for microdosimetry of clinical carbon ion beams. In addition, the good agreement between the converted diamond spectra and those obtained with TEPC provides the first experimental validation of the spectra conversion methodologies as valuable tools for the comparison of spectra collected with different detectors.

MICRODOSIMETRY OF CLINICAL ION BEAMS: CONVERTING SPECTRA FROM DIAMOND SLAB TO WATER OF DIFFERENT SHAPES.

Microdosemeters are frequently used today to specify the radiation quality in the framework of ion-beam therapy. The heterogeneity of the detector shapes and the materials limits the possibility of comparing directly spectra and mean lineal energies. A method was recently studied to convert the spectra obtained with unidirectional ion beams in slab detectors to those obtained with detectors of different in shape and material. The method is based on the observation that the lineal-energy spectra of slab detector, in a restricted energy interval, approximate the Linear Energy Transfer distributions at corresponding material and particle type and energies. In this study, the experimental spectra collected with a slab diamond detector are converted to the spectra that would be obtained using water detectors of spherical and cylindrical shapes.

Effective-dose estimation in interventional radiological procedures.

Interventional radiology is based on minimally invasive procedures that allow diagnosis and percutaneous treatment of diseases in almost all organ systems. Such procedures have many benefits, but they also contribute significantly to collective radiation dose. In this regard, effective dose (E) is a convenient quantity to estimate patients' stochastic radiation risk. However, E cannot be accurately evaluated immediately. In the present study, we aimed to estimate the E value in 15 selected interventional procedures. The estimation was based on dose area product (DAP) measurements and used case-specific conversion coefficients. The E values ranged from 3.3 to 69.9 mSv, depending on the kind of procedure. This wide range was mainly due to the broad variation in DAP values, which in turn depend on the details of how the procedures are performed. This suggests that to ensure valid comparative studies and universal reference levels, all interventional procedures should be well classified.

Is the PTW 60019 microDiamond a suitable candidate for small field reference dosimetry?

A systematic study of the PTW microDiamond (MD) output factors (OF) is reported, aimed at clarifying its response in small fields and investigating its suitability for small field reference dosimetry. Ten MDs were calibrated under 60Co irradiation. OF measurements were performed in 6 MV photon beams by a CyberKnife M6, a Varian DHX and an Elekta Synergy linacs. Two PTW silicon diodes E (Si-D) were used for comparison. The results obtained by the MDs were evaluated in terms of absorbed dose to water determination in reference conditions and OF measurements, and compared to the results reported in the recent literature. To this purpose, the Monte Carlo (MC) beam-quality correction factor, [Formula: see text], was calculated for the MD, and the small field output correction factors, [Formula: see text], were calculated for both the MD and the Si-D by two different research groups. An empirical function was also derived, providing output correction factors within 0.5% from the MC values calculated for all of the three linacs. A high reproducibility of the dosimetric properties was observed among the ten MDs. The experimental [Formula: see text] values are in agreement within 1% with the MC calculated ones. Output correction factors within +0.7% and -1.4% were obtained down to field sizes as narrow as 5 mm. The resulting MD and Si-D field factors are in agreement within 0.2% in the case of CyberKnife measurements and 1.6% in the other cases. This latter higher spread of the data was demonstrated to be due to a lower reproducibility of small beam sizes defined by jaws or multi leaf collimators. The results of the present study demonstrate the reproducibility of the MD response and provide a validation of the MC modelling of this device. In principle, accurate reference dosimetry is thus feasible by using the microDiamond dosimeter for field sizes down to 5 mm.

Response of synthetic diamond detectors in proton, carbon, and oxygen ion beams.

PURPOSE: In this work, the LET-dependence of the response of synthetic diamond detectors is investigated in different particle beams. METHOD: Measurements were performed in three nonmodulated particle beams (proton, carbon, and oxygen). The response of five synthetic diamond detectors was compared to the response of a Markus or an Advanced Markus ionization chamber. The synthetic diamond detectors were used with their axis parallel to the beam axis and without any bias voltage. A high bias voltage was applied to the ionization chambers, to minimize ion recombination, for which no correction is applied (+300 V and +400 V were applied to the Markus and Advanced Markus ionization chambers respectively). RESULTS: The ratio between the normalized response of the synthetic diamond detectors and the normalized response of the ionization chamber shows an under-response of the synthetic diamond detectors in carbon and oxygen ion beams. No under-response of the synthetic diamond detectors is observed in protons. For each beam, combining results obtained for the five synthetic diamond detectors and considering the uncertainties, a linear fit of the ratio between the normalized response of the synthetic diamond detectors and the normalized response of the ionization chamber is determined. The response of the synthetic diamond detectors can be described as a function of LET as (-6.22E-4 ± 3.17E-3) • LET + (0.99 ± 0.01) in proton beam, (-2.51E-4 ± 1.18E-4) • LET + (1.01 ± 0.01) in carbon ion beam and (-2.77E-4 ± 0.56E-4) • LET + (1.03 ± 0.01) in oxygen ion beam. Combining results obtained in carbon and oxygen ion beams, a LET dependence of about 0.026% (±0.013%) per keV/µm is estimated. CONCLUSIONS: Due to the high LET value, a LET dependence of the response of the synthetic diamond detector was observed in the case of carbon and oxygen beams. The effect was found to be negligible in proton beams, due to the low LET value. The under-response of the synthetic diamond detector may result from the recombination of electron/hole in the thin synthetic diamond layer, due to the high LET-values. More investigations are required to confirm this assumption.

In phantom assessment of superficial doses under TomoTherapy irradiation.

PURPOSE: Aim of this work is the assessment of build-up and superficial doses of different clinical Head&Neck plans delivered with Helical TomoTherapy (HT) (Accuray, Sunnyvale, CA). Depth dose profiles and superficial dose points were measured in order to evaluate the Treatment Planning System (TPS) capability of an accurate dose modeling in regions of disequilibrium. Geometries and scattering conditions were investigated, similar to the ones generally encountered in clinical treatments. METHODS: Measurements were performed with two dosimeters: Gafchromic® EBT3 films (Ashland Inc., Wayne, NJ) and a synthetic single crystal diamond detector (PTW-Frieburg microDiamond, MD). A modified version of the Alderson RANDO phantom was employed to house the detectors. A comparison with TPS data was carried out in terms of dose difference (DD) and distance-to-agreement (DTA). RESULTS: DD between calculated data and MD measurements are within 4% even in points with high spatial dose variation. For depth profiles, EBT3 data show a DDmax of 3.3% and DTAmax of 2.2mm, in low and high gradient regions, respectively, and compare well with MD data. EBT3 superficial points always results in measured doses lower than TPS evaluated ones, with a maximum DTA value of 1.5mm. CONCLUSIONS: Doses measured with the two devices are in good agreement and compare well with calculated data. The deviations found in the present work are within the reference tolerance level, suggesting that the HT TPS is capable of a precise dose estimation both in superficial regions and in correspondence with interfaces between air and PMMA.

CyberKnife beam output factor measurements: A multi-site and multi-detector study.

PURPOSE: New promising detectors are available for measuring small field size output factors (OFs). This study focused on a multicenter evaluation of two new generation detectors for OF measurements on CyberKnife systems. METHODS: PTW-60019 microDiamond and W1 plastic scintillation detector (PSD) were used to measure OFs on eight CyberKnife units of various generations for 5-60mm fixed cones. MicroDiamond and PSD OF were compared to routinely used silicon diodes data corrected applying published Monte Carlo (MC) factors. PSD data were corrected for Cerenkov Light Ratio (CLR). The uncertainties related to CLR determination were estimated. RESULTS: Considering OF values averaged over all centers, the differences between MC corrected diode and the other two detectors were within 1.5%. MicroDiamond exhibited an over-response of 1.3% at 7.5mm and a trend inversion at 5mm with a difference of 0.2%. This behavior was consistent among the different units. OFs measured by PSD slightly under-responded compared to MC corrected diode for the smaller cones and the differences were within 1%. The observed CLR variability was 2.5% and the related variation in OF values was 1.9%. CONCLUSION: This study indicates that CyberKnife microDiamond OF require corrections below 2%. The results are enhanced by the consistency observed among different units. Scintillator shows a good agreement to MC corrected diode but CLR determination remains critical requiring further investigations. The results emphasized the value of a multi-center validation over a single center approach.

Multicenter evaluation of a synthetic single-crystal diamond detector for CyberKnife small field size output factors.

PURPOSE: The aim of the present work was to evaluate small field size output factors (OFs) using the latest diamond detector commercially available, PTW-60019 microDiamond, over different CyberKnife systems. OFs were measured also by silicon detectors routinely used by each center, considered as reference. METHODS: Five Italian CyberKnife centers performed OFs measurements for field sizes ranging from 5 to 60mm, defined by fixed circular collimators (5 centers) and by Iris(™) variable aperture collimator (4 centers). Setup conditions were: 80cm source to detector distance, and 1.5cm depth in water. To speed up measurements two diamond detectors were used and their equivalence was evaluated. MonteCarlo (MC) correction factors for silicon detectors were used for comparing the OF measurements. RESULTS: Considering OFs values averaged over all centers, diamond data resulted lower than uncorrected silicon diode ones. The agreement between diamond and MC corrected silicon values was within 0.6% for all fixed circular collimators. Relative differences between microDiamond and MC corrected silicon diodes data for Iris(™) collimator were lower than 1.0% for all apertures in the totality of centers. The two microDiamond detectors showed similar characteristics, in agreement with the technical specifications. CONCLUSIONS: Excellent agreement between microDiamond and MC corrected silicon diode detectors OFs was obtained for both collimation systems fixed cones and Iris(™), demonstrating the microDiamond could be a suitable detector for CyberKnife commissioning and routine checks. These results obtained in five centers suggest that for CyberKnife systems microDiamond can be used without corrections even at the smallest field size.

The role of a microDiamond detector in the dosimetry of proton pencil beams.

In this work, the performance of a microDiamond detector in a scanned proton beam is studied and its potential role in the dosimetric characterization of proton pencil beams is assessed. The linearity of the detector response with the absorbed dose and the dependence on the dose-rate were tested. The depth-dose curve and the lateral dose profiles of a proton pencil beam were measured and compared to reference data. The feasibility of calibrating the beam monitor chamber with a microDiamond detector was also studied. It was found the detector reading is linear with the absorbed dose to water (down to few cGy) and the detector response is independent of both the dose-rate (up to few Gy/s) and the proton beam energy (within the whole clinically-relevant energy range). The detector showed a good performance in depth-dose curve and lateral dose profile measurements; and it might even be used to calibrate the beam monitor chambers-provided it is cross-calibrated against a reference ionization chamber. In conclusion, the microDiamond detector was proved capable of performing an accurate dosimetric characterization of proton pencil beams.

Evaluation of a synthetic single-crystal diamond detector for relative dosimetry on the Leksell Gamma Knife Perfexion radiosurgery system.

PURPOSE: To evaluate the new commercial PTW-60019 synthetic single-crystal microDiamond detector (PTW, Freiburg, Germany) for relative dosimetry measurements on a clinical Leksell Gamma Knife Perfexion radiosurgery system. METHODS: Detector output ratios (DORs) for 4 and 8 mm beams were measured using a microDiamond (PTW-60019), a stereotactic unshielded diode [IBA stereotactic field detector (SFD)], a shielded diode (IBA photon field detector), and GafChromic EBT3 films. Both parallel and transversal acquisition directions were considered for PTW-60019 measurements. Measured DORs were compared to the new output factor reference values for Gamma Knife Perfexion (0.814 and 0.900 for 4 and 8 mm, respectively). Profiles in the three directions were also measured for the 4 mm beam to evaluate full width at half maximum (FWHM) and penumbra and to compare them with the corresponding Leksell GammaPlan profiles. RESULTS: FWHM and penumbra for PTW-60019 differed from the calculated values by less than 0.2 and 0.3 mm, for the parallel and transversal acquisitions, respectively. GafChromic films showed FWHM and penumbra within 0.1 mm. The output ratio obtained with the PTW-60019 for the 4 mm field was 1.6% greater in transverse direction compared to the nominal value. Comparable differences up to 0.8% and 1.0% for, respectively, GafChromic films and SFD were found. CONCLUSIONS: The microDiamond PTW-60019 is a suitable detector for commissioning and routine use of Gamma Knife with good agreement of both DORs and profiles in the three directions.

Patient skin dose measurements using a cable free system MOSFETs based in fluoroscopically guided percutaneous vertebroplasty, percutaneous disc decompression, radiofrequency medial branch neurolysis, and endovascular critical limb ischemia.

The purpose of this work has been to dosimetrically investigate four fluoroscopically guided interventions: the percutaneous vertebroplasty (PVP), the percutaneous disc decompression (PDD), the radiofrequency medial branch neurolysis (RF) (hereafter named spine procedures), and the endovascular treatment for the critical limb ischemia (CLI). The X-ray equipment used was a Philips Integris Allura Xper FD20 imaging system provided with a dose-area product (DAP) meter. The parameters investigated were: maximum skin dose (MSD), air kerma (Ka,r), DAP, and fluoroscopy time (FT). In order to measure the maximum skin dose, we employed a system based on MOSFET detectors. Before using the system on patients, a calibration factor Fc and correction factors for energy (CkV) and field size (CFD) dependence were determined. Ka,r, DAP, and FT were extrapolated from the X-ray equipment. The analysis was carried out on 40 patients, 10 for each procedure. The average fluoroscopy time and DAP values were compared with the reference levels (RLs) proposed in literature. Finally, the correlations between MSD, FT, Ka,r, and DAP values, as well as between DAP and FT values, were studied in terms of Pearson's product-moment coefficients for spine procedures only. An Fc value of 0.20 and a very low dependence of CFD on field size were found. A third-order polynomial function was chosen for CkV. The mean values of MSD ranged from 2.3 to 10.8cGy for CLI and PVP, respectively. For these procedures, the DAP and FT values were within the proposed RL values. The statistical analysis showed little correlation between the investigated parameters. The interventional procedures investigated were found to be both safe with regard to deterministic effects and optimized for stochastic ones. In the spine procedures, the observed correlations indicated that the estimation of MSD from Ka,r or DAP was not accurate and a direct measure of MSD is therefore recommended.

Comparison between small radiation therapy electron beams collimated by Cerrobend and tubular applicators.

The purpose of this study was to compare the dosimetric properties of small field electron beams shaped by circular Cerrobend blocks and stainless steel tubular applicators. Percentage depth dose curves, beam profiles, and output factors of small-size circular fields from 2 to 5 cm diameter, obtained either by tubular applicators and Cerrobend blocks, were measured for 6, 10, and 15 MeV electron beam energies. All measurements were performed using a PTW microDiamond 60019 premarket prototype. An overall similar behavior between the two collimating systems can be observed in terms of PDD and beam profiles. However, Cerrobend collimators produce a higher bremsstrahlung background under irradiation with high-energy electrons. In such irradiation condition, larger output factors are observed for tubular applicators. Similar dosimetric properties are observed using circular Cerrobend blocks and stainless steel tubular applicators at lower beam energies. However, Cerrobend collimators allow the delivery of specific beam shapes, conformed to the target area. On the other hand, in high-energy irradiation conditions, tubular applicators produce a lower bremsstrahlung contribution, leading to lower doses outside the target volume. In addition, the higher output factors observed at high energies for tubular applicators lead to reduced treatment times.