A surface energy spectral study on the bone heterogeneity and beam obliquity using the flattened and unflattened photon beams

Aim

Using flattened and unflattened photon beams, this study investigated the spectral variations of surface photon energy and energy fluence in the bone heterogeneity and beam obliquity.

Background

Surface dose enhancement is a dosimetric concern when using unflattened photon beam in radiotherapy. It is because the unflattened photon beam contains more low-energy photons which are removed by the flattening filter of the flattened photon beam.

Materials and methods

We used a water and bone heterogeneity phantom to study the distributions of energy, energy fluence and mean energy of the 6 MV flattened and unflattened photon beams (field size = 10 cm × 10 cm) produced by a Varian TrueBEAM linear accelerator. These elements were calculated at the phantom surfaces using Monte Carlo simulations. The photon energy and energy fluence calculations were repeated with the beam angle turned from 0° to 15°, 30° and 45° in the water and bone phantom.

Results

Spectral results at the phantom surfaces showed that the unflattened photon beams contained more photons concentrated mainly in the low-energy range (0–2 MeV) than the flattened beams associated with a flattening filter. With a bone layer of 1 cm under the phantom surface and within the build-up region of the 6 MV photon beam, it is found that both the flattened and unflattened beams had slightly less photons in the energy range <0.4 MeV compared to the water phantom. This shows that the presence of the bone decreased the low-energy photon backscatters to the phantom surface. When both the flattened and unflattened photon beams were rotated from 0° to 45°, the number of photon and mean photon energy increased. This indicates that both photon beams became more hardened or penetrate when the beam angle increased. In the presence of bone, the mean energies of both photon beams increased. This is due to the absorption of low-energy photons by the bone, resulting in more beam hardening.

Conclusions

This study explores the spectral relationships of surface photon energy and energy fluence with bone heterogeneity and beam obliquity for the flattened and unflattened photon beams. The photon spectral information is important in studies on the patient''s surface dose enhancement using unflattened photon beams in radiotherapy.     

A novel high-resolution 2D silicon array detector for small field dosimetry with FFF photon beams

PurposeFlattening filter free (FFF) beams are increasingly being considered for stereotactic radiotherapy (SRT). For the first time, the performance of a monolithic silicon array detector under 6 and 10 MV FFF beams was evaluated. The dosimeter, named “Octa” and designed by the Centre for Medical Radiation Physics (CMRP), was tested also under flattened beams for comparison.MethodsOutput factors (OFs), percentage depth-dose (PDD), dose profiles (DPs) and dose per pulse (DPP) dependence were investigated. Results were benchmarked against commercially available detectors for small field dosimetry.ResultsThe dosimeter was shown to be a ‘correction-free’ silicon array detector for OFs and PDD measurements for all the beam qualities investigated. Measured OFs were accurate within 3% and PDD values within 2% compared against the benchmarks. Cross-plane, in-plane and diagonal DPs were measured simultaneously with high spatial resolution (0.3 mm) and real time read-out. A DPP dependence (24% at 0.021 mGy/pulse relative to 0.278 mGy/pulse) was found and could be easily corrected for in the case of machine specific quality assurance applications.ConclusionsResults were consistent with those for monolithic silicon array detectors designed by the CMRP and previously characterized under flattened beams only, supporting the robustness of this technology for relative dosimetry for a wide range of beam qualities and dose per pulses. In contrast to its predecessors, the design of the Octa offers an exhaustive high-resolution 2D dose map characterization, making it a unique real-time radiation detector for small field dosimetry for field sizes up to 3 cm side.     

On the use of EBT3 film for relative dosimetry of kilovoltage X ray beams

EBT3 films were evaluated for relative dosimetry in water, in the energy range of therapeutic kV X ray beams. A film batch was calibrated in air for all nine beam qualities of a clinical unit (XStrahl 200). Monte Carlo (MC) simulations using MCNP v.6 facilitated the calculation of the film absorbed dose (f), and beam quality (k_bq) energy dependences in air. Results were found in agreement with corresponding data in the literature. Film samples from the same batch were irradiated in water along the central beam axis for each beam quality. Experimental percentage depth dose (PDD) results obtained using calibration data in air showed quality and depth dependent differences from corresponding MC simulations. These differences increased beyond film dosimetry uncertainty (<3.3%), reaching up to 8% at increased depth. The observed differences reduced only slightly when spectral variation as a function of measurement point was accounted for, using photon effective energy. PDD measurements and corresponding MC results facilitated the determination of f and k_bq in water. Results showed that the origin of the observed differences between experimental and MC PDD results is the difference between film response in air and water, as a result of radiation field perturbation from the film oriented along the central beam axis. This implies a directional dependence of film response which necessitates that the angular distribution of photons impinging on the film is the same in the calibration and measurement geometries.     

Dosimetric characterization of a small-scale (Zn,Cd)S:Ag inorganic scintillating detector to be used in radiotherapy

PurposeIn modern radiotherapy techniques, to ensure an accurate beam modeling process, dosimeters with high accuracy and spatial resolution are required. Therefore, this work aims to propose a simple, robust, and a small-scale fiber-integrated X-ray inorganic detector and investigate the dosimetric characteristics used in radiotherapy.MethodsThe detector is based on red-emitting silver-activated zinc-cadmium sulfide (Zn,Cd)S:Ag nanoclusters and the proposed system has been tested under 6 MV photons with standard dose rate used in the patient treatment protocol. The article presents the performances of the detector in terms of dose linearity, repeatability, reproducibility, percentage depth dose distribution, and field output factor. A comparative study is shown using a microdiamond dosimeter and considering data from recent literature.ResultsWe accurately measured a small field beam profile of 0.5 × 0.5 cm² at a spatial resolution of 100 µm using a LINAC system. The dose linearity at 400 MU/min has shown less than 0.53% and 1.10% deviations from perfect linearity for the regular and smallest field. Percentage depth dose measurement agrees with microdiamond measurements within 1.30% and 2.94%, respectively for regular to small field beams. Besides, the stem effect analysis shows a negligible contribution in the measurements for fields smaller than 3x3 cm². This study highlights the drastic decrease of the convolution effect using a point-like detector, especially in small dimension beam characterization. Field output factor has shown a good agreement while comparing it with the microdiamond dosimeter.ConclusionAll the results presented here anticipated that the developed detector can accurately measure delivered dose to the region of interest, claim accurate depth dose distribution hence it can be a suitable candidate for beam characterization and quality assurance of LINAC system.     

Segmented photon beams technique for irradiation of postmastectomy patients

AimTo present the segmented photon beams technique (SPBT) for irradiation of postmastectomy patients.BackgroundIn majority of techniques for irradiation of posmastectomy patients, a few adjacent photon or electron beams were usually implemented in order to encompass different parts of the target. In the presented SPBT technique, the radiotherapy plan consists of 6 isocentric photon beams and the area CTV includes both the chest wall and the supraclavicular area. This makes it possible to provide a uniform dose to the CTV with no hot and cold points and enables the determination of doses for the entire volume of critical organs.Methods and materialThe treatment forward-IMRT plan comprises six isocentric 4 and 15 MV photon beams. Modulation of the dose distribution for each field was obtained by applying three segments on average. The total dose of 45 Gy was administered in 20 fractions. Dose distributions in target volume and organs at risk were evaluated for 70 randomly chosen patients.ResultsOn average, 94.8% of the CTV volume received doses within 95–107% of the prescribed dose. The average volume of the heart receiving a dose of 30 Gy and lager was 2% for patients with left breast cancer. The average dose to the lung on the irradiation side was always lower than 15.5 Gy and the average V20 Gy was below 35.5%.ConclusionsThe SPBT complies with requirements for high dose homogeneity within the target volume and satisfactory level of sparing of organs at risk.     

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

PurposeThe 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.MethodsFive Italian CyberKnife centers performed OFs measurements for field sizes ranging from 5 to 60 mm, defined by fixed circular collimators (5 centers) and by Iris^™ variable aperture collimator (4 centers). Setup conditions were: 80 cm source to detector distance, and 1.5 cm 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.ResultsConsidering 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.ConclusionsExcellent 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.     

Determination of the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams

PurposeTo determine the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams.MethodsAn AGMS-DM+ semiconductor detector was calibrated in terms of air kerma measured with an ionization chamber. Air kerma was measured for 20 x-ray beams with tube voltages of 50–140 kVp and a half-value layer (HVL) of 2.2–9.7 mm Al for given quality index (QI) values of 0.4, 0.5, and 0.6, and converted to the surface dose. Finally, the air kerma and HVL measured by the AGMS-DM+ detector were expressed as a ratio of the surface dose for 10 × 10 and 20 × 20 cm² fields. The ratio of both was represented as a function of HVL for the given QI values and verified by comparing it with that calculated using the Monte Carlo method.ResultsThe air kerma calibration factor, CF, for the AGMS-DM+ detector ranged from 0.986 to 1.016 (0.9% in k = 1). The CF values were almost independent of the x-ray fluence spectra for the given QI values. The ratio of the surface dose to the air kerma determined by the PTW 30,013 chamber and the AGMS-DM+ detector was less than 1.8% for the values calculated using the Monte Carlo method, and showed a good correlation with the HVL for the given QI values.ConclusionIt is possible to determine the surface dose of a water phantom from the air kerma and HVL measured by a semiconductor detector for given QI values.     

A pilot study of a postal dosimetry system using the Fricke dosimeter for research irradiators

Cobalt-60 irradiators and soft X-ray machines are frequently used for research purposes, but the dosimetry is not always performed using the recommended protocols. This may lead to confusing and untrustworthy results within the conducted research. Postal dosimetry systems have already been approved by the IAEA, with thermoluminescence dosimeters (TLD) and optically stimulated luminescence (OSL) as the most commonly used dosimeter systems in these cases. The present study tests the Fricke dosimeter properties as a potential system to be used in postal dosimetry for a project using research irradiators. The Fricke solution was prepared according to the literature, and the linearity and fading tests were performed accordingly. All calculated doses were measured using a NE2571 Farmer ionization chamber as a reference. Doses ranging from 25 to 300 Gy were delivered by a research irradiator, with 150 kV and 22 mA to the Fricke solutions inside polyethylene (PE) bags (4 × 4 × 0.2 cm³). The results compared with the ionization chamber showed a linear response to the range of doses used. Fading tests showed no significant difference for the absorbed doses over 9 days, with a maximum difference of 1.5% found between days 0 and 3. The Fricke dosimeter presented good linearity, for low and high doses, and low uncertainties for the fading even for 9 days after irradiation. These preliminary results are motivating, and as the next step, we intend to design a postal dosimetry system using the PE bags of Fricke solution.     

Dosimetric impact of failing to apply correction factors to ion recombination in percentage depth dose measurements and the volume-averaging effect in flattening filter-free beams

PurposeTo examine whether it is essential to apply correction factors for ion recombination (k_S) to percentage depth dose (PDD) measurements and to the volume-averaging effect (k_vol) to ensure accurate absolute dose calibration for flattening filter-free (FFF) beams for the most commonly used ionization chambers.MethodsWe surveyed medical physicists worldwide (n = 159) to identify the five most common ionization chamber combinations used for absolute and relative reference dosimetry of FFF beams. We then assessed the overall absolute dose calibration error for FFF beams of the Artiste Siemens and TrueBeam Varian linear accelerators resulting from failing to apply correction factors k_S in the PDD(10) and the volume-averaging effect (k_vol) to such chamber combinations.ResultsAll the chamber combinations examined—the Farmer PTW 30013 ionization chamber used for absolute dosimetry, and the PTW 31010, PTW 30013, IBA CC04, IBA CC13, and PTW 31021 ionization chambers used for PDD curves measurements—showed non-negligible errors (≥0.5%). The largest error (1.6%) was found for the combination of the Farmer PTW 30013 chamber with the IBA CC13 chamber, which was the most widely used chamber combination in our survey.ConclusionsBased on our findings, we strongly recommend assessing the impact of failing to apply correction factors k_S in the PDD(10) and k_vol prior to using any chamber type for FFF beam reference dosimetry purposes.     

Energy dependence of radiochromic dosimetry films for use in radiotherapy verification

Aim

The purpose of the study was to examine the energy dependence of Gafchromic EBT radiochromic dosimetry films, in order to assess their potential use in intensity-modulated radiotherapy (IMRT) verifications.

Materials and methods

The film samples were irradiated with doses from 0.1 to 12 Gy using photon beams from the energy range 1.25 MeV to 25 MV and the film response was measured using a flat-bed scanner. The samples were scanned and the film responses for different beam energies were compared.

Results

A high uncertainty in readout of the film response was observed for samples irradiated with doses lower than 1 Gy. The relative difference exceeds 20% for doses lower than 1 Gy while for doses over 1 Gy the measured film response differs by less than 5% for the whole examined energy range. The achieved uncertainty of the experimental procedure does not reveal any energy dependence of Gafchromic EBT film response in the investigated energy range.

Conclusions

Gafchromic EBT film does not show any energy dependence in the conditions typical for IMRT but the doses measured for pre-treatment plan verifications should exceed 1 Gy.     

Implementation and evaluation of a transit dosimetry system for treatment verification

PurposeTo evaluate a formalism for transit dosimetry using a phantom study and prospectively evaluate the protocol on a patient population undergoing 3D conformal radiotherapy.MethodsAmorphous silicon EPIDs were calibrated for dose and used to acquire images of delivered fields. The measured EPID dose map was back-projected using the planning CT images to calculate dose at pre-specified points within the patient using commercially available software, EPIgray (DOSIsoft, France). This software compared computed back-projected dose with treatment planning system dose. A series of tests were performed on solid water phantoms (linearity, field size effects, off-axis effects). 37 patients were enrolled in the prospective study.ResultsThe EPID dose response was stable and linear with dose. For all tested field sizes the agreement was good between EPID-derived and treatment planning system dose in the central axis, with performance stability up to a measured depth of 18 cm (agreement within −0.5% at 10 cm depth on the central axis and within −1.4% at 2 cm off-axis). 126 transit images were analysed of 37 3D-conformal patients. Patient results demonstrated the potential of EPIgray with 91% of all delivered fields achieved the initial set tolerance level of Δ_D of 0 ± 5-cGy or %Δ_D of 0 ± 5%.ConclusionsThe in vivo dose verification method was simple to implement, with very few commissioning measurements needed. The system required no extra dose to the patient, and importantly was able to detect patient position errors that impacted on dose delivery in two of cases.     

Characterization of a low-cost PIN photodiode for dosimetry in diagnostic radiology

A commercial silicon PIN-photodiode was tested and characterized as ionizing radiation detector for radiological applications. A current-to-voltage amplification stage was designed and realized in order to acquire the photodiode signal in current mode. The system was tested with clinical beams routinely used for radiography and mammography. A Monte Carlo simulation of the detector was performed with the MCNPX code in order to model and fully understand, in particular, the impact of the sensor casing on the low energy response of the device. A reproducible output linearity was found over the dose range 0.03–4.5 mGy of great clinical relevance. The system sensitivity was found to be stable at 0.2 V s Gy⁻¹ for effective X-ray energies between 17 and 40 keV. The batch-to-batch reproducibility of the diodes was also experimentally investigated for two different batches of 14 diodes each. An inter-comparison with dosimeters routinely used in medical physics (i.e. Barracuda MPD RTI) showed a linear correlation between PIN-photodiode readout and absorbed dose measured with Barracuda, in the range of doses received by mammography and radiology patients.     

KCl:Dy phosphor for thermoluminescence dosimetry of ionizing radiation

The thermoluminescence (TL) characterizations of γ‐irradiated KCl:Dy phosphor for radiation dosimetry are reported. All phosphors were synthesized via a wet chemical route. Minimum fading of TL intensity is recorded in the prepared material. TL in samples containing different concentrations of Dy impurity was studied at different γ‐irradiation doses. Peak TL intensities varied sublinearly with γ‐ray dose in all samples, but were linear between 0.08 to 0.75 kGy for the KCl:Dy (0.1 mol%) sample. This material may be useful for dosimetry within this range of γ‐ray dose. TL peak height was found to be dependant on the concentration (0.05–0.5 mol%) of added Dy in the host. Copyright © 2012 John Wiley & Sons, Ltd.     

A new approach in the design of electronic portal imaging devices for portal dosimetry in radiotherapy

A CCD-based EPID using new crystal-assembly X-ray (CAX) converters is investigated for radiotherapy dosimetry. The proposed EPID design consists in replacing the common phosphor X-ray converters of current CCD-based EPIDs with high-stopping-power CAX converters. A Test Imaging Device (TID), consisting of a 30-mm-thick CAX converter made of Bismuth Germanate (BGO), coupled to a highly sensitive CCD camera, was used to evaluate the accessible imaging and dosimetric performance of the proposed design. The system response to dose and its dependence on photon beam energy were investigated. The effects of ghosting, dose rate, field size and phantom thickness were evaluated as well. The same measurements were also performed with our clinically used aSi-EPID so that comparisons of performance could be directly inferred. The TID displayed no detectable ghosting or sensitivity to dose rate. Its response to MU exposure was found to be linear within about ±1%. The level of glare induced in the TID and the aSi-EPID were equivalent. The TID resolution was higher than that of the aSi-EPID on the axis, but was found to decrease with off-axis distance. Finally, the image quality, assessed on the basis of signal-to-noise ratio in low dose radiographs of the larynx of a patient, was higher for the TID. The imaging performance accessible with the TID proved to be satisfying and its dosimetric capability was found to be superior to that of the current aSi-EPID.     

EPID-based in vivo dosimetry for stereotactic body radiotherapy of non-small cell lung tumors: Initial clinical experience

PurposeEPID-based in vivo dosimetry (IVD) has been implemented for stereotactic body radiotherapy treatments of non-small cell lung cancer to check both isocenter dose and the treatment reproducibility comparing EPID portal images.Methods15 patients with lung tumors of small dimensions and treated with volumetric modulated arc therapy were enrolled for this initial experience. IVD tests supplied ratios R between in vivo reconstructed and planned isocenter doses. Moreover a γ-like analysis between daily EPID portal images and a reference one, in terms of percentage of points with γ-value smaller than 1, P_γ<1, and mean γ-values, γ_mean, using a local 3%–3 mm criteria, was adopted to check the treatment reproducibility. Tolerance levels of 5% for R ratio, P_γ<1 higher than 90% and γ_mean lower than 0.67 were adopted.ResultsA total of 160 EPID images, two images for each therapy session, were acquired during the treatment of the 15 patients. The overall mean of the R ratios was equal to 1.005 ± 0.014 (1 SD), with 96.9% of tests within ± 5%. The 2 D image γ-like analysis showed an overall γ_mean of 0.39 ± 0.12 with 96.1% of tests within the tolerance level, and an average P_γ<1 value equal to 96.4 ± 3.6% with 95.4% of tests with P_γ<1 > 90%. Paradigmatic discrepancies were observed in three patients: a set-up error and a patient morphological change were identified thanks to CBCT image analysis whereas the third discrepancy was not fully justified.ConclusionsThis procedure can provide improved patient safety as well as a first step to integrate IVD and CBCT dose recalculation.     

Determination of the correction factors used in Fricke dosimetry for HDR 192Ir sources employing the Monte Carlo method

PurposeFricke dosimetry has shown great potential in the direct measurement of the absolute absorbed dose for ¹⁹²Ir sources used in HDR brachytherapy. This work describes the determination of the correction factors necessary to convert the absorbed dose in the Fricke solution to the absorbed dose to water. Methods: The experimental setup for Fricke irradiation using a ¹⁹²Ir source was simulated. The holder geometry used for the Fricke solution irradiation was modelled for MC simulation, using the PENELOPE. Results: The values of the factors determined for validation purposes demonstrated differences of less than 0.2% when compared to the published values. Four factors were calculated to correct: the differences in the density of the solution (1.0004 ± 0.0004); the perturbations caused by the holder (0.9989 ± 0.0004); the source anisotropy and the water attenuation effects (1.0327 ± 0.0012); and the distance from the center of the detection volume to the source (7.1932 ± 0.0065). Conclusion: Calculated corrections in this work show that the largest correction comes from the inverse squared reduction of the dose due to the point of measurement shift from the reference position of 1 cm. This situation also causes the correction due to volume averaging and attenuation in water to be significant. Future versions of the holder will aim to reduce these effects by having a position of measurement closer to the reference point thus requiring smaller corrections.