Fetal radiation dose in three common CT examinations during pregnancy – Monte Carlo study

PurposeTo determine fetal doses in different stages of pregnancy in three common computed tomography (CT) examinations: pulmonary CT angiography, abdomino-pelvic and trauma scan with Monte Carlo (MC) simulations.MethodsAn adult female anthropomorphic phantom was scanned with a 64-slice CT using pulmonary angiography, abdomino-pelvic and trauma CT scan protocols. Three different sized gelatin boluses placed on the phantom’s abdomen simulated different stages of pregnancy. Intrauterine dose was used as a surrogate to a dose absorbed to the fetus. MC simulations were performed to estimate uterine doses. The simulation dose levels were calibrated with volumetric CT dose index (CTDI_vol) measurements and MC simulations in a cylindrical CTDI body phantom and compared with ten point doses measured with metal-oxide-semiconductor field-effect-transistor dosimeters. Intrauterine volumes and uterine walls were segmented and the respective dose volume histograms were calculated.ResultsThe mean intrauterine doses in different stages of pregnancy varied from 0.04 to 1.04 mGy, from 4.8 to 5.8 mGy, and from 9.8 to 12.6 mGy in the CT scans for pulmonary angiography, abdomino-pelvic and trauma CT scans, respectively. MC simulations showed good correlation with the MOSFET measurement at the measured locations.ConclusionsThe three studied examinations provided highly varying fetal doses increasing from sub-mGy level in pulmonary CT angiography to notably higher levels in abdomino-pelvic and trauma scans where the fetus is in the primary exposure range. Volumetric dose distribution offered by MC simulations in an appropriate anthropomorphic phantom provides a comprehensive dose assessment when applied in adjunct to point-dose measurements.     

Effect of diagnostic cone-beam computed tomography protocols on image quality,patient dose,and lesion detection

ObjectiveTo evaluate the effect of cone-beam computed tomography (CBCT) image acquisition protocols on image quality, lesion detection, delineation, and patient dose.Methods100-patients and a CTDI phantom combined with an electron density phantom were examined using four different CBCT-image acquisition protocols during image-guided transarterial chemoembolization (TACE). Protocol-1 (time: 6 s, tube rotation: 360°), protocol-2 (5 s, 300°), protocol-3 (4 s, 240°) and protocol-4 (3 s, 180°) were used. The protocols were first investigated using a phantom. The protocols that were found to be clinically appropriate in terms of image quality and radiation dose were then assessed on patients. A higher radiation dose and/or a poor image quality were inappropriate for the patient imaging. Patient dose (patient-entrance dose and dose-area product), image quality (Hounsfield Unit, noise, signal-to-noise ratio and contrast-to-noise ratio), and lesion delineation (tumor-liver contrast) were assessed and compared using appropriate statistical tests. Lesion detectability, sensitivity, and predictive values were estimated for CBCT-image data using pre-treatment patient magnetic resonance imaging.ResultsThe estimated patient dose showed no statistical significance (p > 0.05) between protocols-2 and -3; the assessed image quality between these protocols manifested insignificant difference (p > 0.05). Two other phantom protocols were not considered for patient imaging due to significantly higher dose (protocols-1) and poor image quality (protocol-4). Lesion delineation and detection were insignificant (p > 0.05) between protocols-2 and -3. Lesion sensitivities generated were 81–89% (protocol-2) and 81–85% (protocol-3) for different lesion types.ConclusionData acquisition using protocols-2 and -3 provided good image quality, lesion detection and delineation with acceptable patient dose during CBCT-imaging mainly due to similar frame numbers acquired.     

Évaluation des paramètres et méthode de contrôle de qualité de la correction d’atténuation en tomographie par émission de positons

ObjectiveThe aim of this study is to evaluate the performance of the Computed Tomography based Attenuation Correction (CTAC) for Positron Emission Tomography (PET) data. Attenuation maps containing linear attenuation coefficients at 511 keV (LAC_511 keV) are calculated by trilinear conversion of Hounsfield Units (HU) obtained from CT slices after matrix size-reduction and gaussian filtering. Our work focusses on this trilinear conversion.Materials and methodsCT slices of an electron density phantom, composed of 17 cylindrical inserts made of different tissue-equivalent materials, were acquired using a Discovery ST4^® PET-CT. Data were processed with a customized version of CT quality control software, giving automatically the experimental conversion function: LAC_511 keV = f(HU). Furthermore, data from patient datasets were assessed using both smoothed CT slices and attenuation maps.ResultsLAC_511 keV extracted from phantom data are in good correlation with the expected theoretical values, except for the standard 10 mm diameter dense bone insert, where the obtained CTAC values are underestimated. Assuming a sample size issue, similar acquisitions were performed with a special 30 mm-diameter dense bone insert, confirming the underestimation as a consequence of the sample size. This effect, caused partly by a too smooth Gaussian filter of the CT images, could be limited by reducing the strength of the filter. Measurements from patients’ data showed the same underestimation of CAL_511 keV for high-density tissues.ConclusionWe assessed an underestimation of the CTAC obtained-values related to the sample size of the insert. A quality control was developed to this effect.     

Megavoltage 2D topographic imaging: An attractive alternative to megavoltage CT for the localization of breast cancer patients treated with TomoDirect

PurposeTo show the usefulness of topographic 2D megavoltage images (MV2D) for the localization of breast cancer patients treated with TomoDirect (TD), a radiotherapy treatment technique with fixed-angle beams performed on a TomoTherapy system.MethodsA method was developed to quickly localize breast cancer patients treated with TD by registering the MV2D images produced before a TD treatment with reference images reconstructed from a kilovoltage CT simulation scanner and by using the projection of the beam-eye-view TD treatment field. Dose and image quality measurements were performed to determine the optimal parameters for acquiring MV2D images. A TD treatment was simulated on a chest phantom equipped with a breast attachment. MVCT and MV2D images were performed for 7 different shifted positions of the phantom and registered by 10 different operators with the simulation kilovoltage CT images.ResultsCompared to MVCT, MV2D imaging reduces the dose by a factor of up to 45 and the acquisition time by a factor of up to 49. Comparing the registration shift values obtained for the phantom images obtained with MVCT in the coarse mode to those obtained with MV2D, the mean difference is 1.0 ± 1.1 mm, −1.1 mm ± 1.1, and −0.1 ± 2.2 mm, respectively, in the lateral, longitudinal, and vertical directions.ConclusionsWith dual advantages (very fast imaging and a potentially reduced dose to the heart and contralateral organs), MV2D topographic images may be an attractive alternative to MVCT for the localization of breast cancer patients treated with TomoDirect.     

Comparison of CT artifacts and image recognition of various fiducial markers including two types of thinner fiducial markers for CyberKnife treatment

AimThe aim of the study was to evaluate computed tomography (CT) artifacts and image recognition of the CyberKnife system. Regarding fiducial markers, VISICOIL of 0.5 mm × 5.0 mm and 0.75 mm × 5.0 mm, ball-shaped Gold Anchor (GA) of 0.28 mm × 10 mm and 0.28 mm × 20 mm, were compared with the standard cylinder marker of 0.9 mm × 3.0 mm (ACCULOC).BackgroundRecently, various kinds of commercial fiducial markers have been available in CyberKnife treatment.Materials and methodsThe CT images of a water equivalent gel with each fiducial marker were acquired for the evaluation of CT artifacts. The evaluation was performed using the standard deviation of Hounsfield Unit (HU) value for a rectangle region near the fiducial marker. Then, to evaluate the image recognition, each fiducial marker was located to overlap in the target locating system (TLS) for the two sites; the vertebral bone and the pubic bone.ResultsFor CT artifacts, the standard deviations of the VISICOIL of 0.5 mm × 5.0 mm was the smallest. The image recognition of four fiducial markers had a value close to the standard cylinder marker and was feasible for common use, but was slightly poorer when using GA of 0.28 mm × 10 mm in the dynamic conditions.ConclusionOur results indicated that VISICOIL 0.5 × 5.0 mm and the GAs can be used nearly always for CyberKnife treatment in spite of their much thinner needles than those of cylinder types.     

Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9 MeV electron beams

In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0–15 mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9 MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5 cm depth occurred at 4, 9, and 15 mm with 4, 6, and 9 MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others.     

Radiochromic films for dental CT dosimetry: A feasibility study

Dental CT dose evaluations are commonly performed using thermoluminescent dosimeters (TLD) inside anthropomorphic phantoms. Radiochromic films with good sensitivity in the X-ray diagnostic field have recently been developed and are commercially available as GAFCHROMIC XR-QA. There are potential advantages in the use of radiochromic films such as a more comprehensive dosimetry thanks to the adjustable size of the film samples. The purpose of this study was to investigate the feasibility of using radiochromic films for dental CT dose evaluations.Film samples were cut with a width of 5 mm and a length of 25 mm (strips), the same size as the Alderson Rando anthropomorphic phantom holes used in this study. Dental CT dose measurements were performed using simultaneously both TLD and radiochromic strips in the same phantom sites. Two equipment types were considered for dental CT examinations: a 16 slice CT and a cone beam CT. Organ equivalent doses were then obtained averaging the measurements from the sites of the same organ and effective doses were calculated using ICRP 103 weighting factors. The entire procedure was repeated four times for each CT in order to compare also the repeatability of the two dosimeter types.A linear correlation was found between the absorbed dose evaluated with radiochromic films and with TLD, with slopes of 0.930 and 0.944 (correlation r > 0.99). The maximum difference between the two dosimeter’s measurements was 25%, whereas the average difference was 7%. The measurement repeatability was comparable for the two dosimeters at cumulative doses above 15 mGy (estimated uncertainty at 1 sigma level of about 5%), whereas below this threshold radiochromic films show a greater dispersion of data, of about 10% at 1 sigma level. We obtained, using respectively Gafchromic and TLD measurements, effective dose values of 107 μSv and 117 μSv (i.e. difference of 8.6%) for the cone beam CT and of 523 μSv and 562 μSv (i.e. difference of 7%) for the multislice CT.This study demonstrates the feasibility of radiochromic films for dental CT dosimetry, pointing out a good agreement with the results obtained using TLD, with potential advantages and the chance of a more extensive dose investigation.     

Does gantry rotation time influence accuracy of volume computed tomography dose index (CTDIvol) in modern CT?

PurposeThe purpose of this study was to develop a gantry overrun corrected CTDI_vol (cCTDI_vol) dosimetry and evaluate the differences between the displayed CTDI_vol (dCTDI_vol), measured CTDI_vol (mCTDI_vol), and the cCTDI_vol.Methods and materialsThe each 8 rotation times between 275 and 1000 ms of two CT scanners were investigated. Rotation time (T_rot) and the beam-on time (T_beam) in axial scanning were measured accurately to determine the gantry overrun time (T_over) as T_beam − T_rot. Subsequently, mCTDI_vol was measured by using a 100 mm ionization chamber and CTDI phantoms. Furthermore, we introduced a gantry overrun correction factor (C_o = T_rot/T_beam) to obtain cCTDI_vol. Upon completion of the data acquisition, the dCTDI_vol and mCTDI_vol were compared with the cCTDI_vol.ResultsThe discrepancies of T_rot were 0.2 ± 0.2 ms as compared to the preset rotation times, and T_over was machine-specific and almost constant (22.4 ± 0.5 ms or 45.1 ± 0.3 ms) irrespective of the preset rotation time. Both dCTDI_vol and mCTDI_vol were increasingly overestimated compared to cCTDI_vol as the faster the preset rotation time was selected (1.7–23.5%).ConclusionThe rotation time influences the accuracy of CTDI_vol in modern CT, and should be taken into consideration when assessing the radiation output in modern CT.     

Incorrect dosimetric leaf separation in IMRT and VMAT treatment planning: Clinical impact and correlation with pretreatment quality assurance

PurposeDynamic treatment planning algorithms use a dosimetric leaf separation (DLS) parameter to model the multi-leaf collimator (MLC) characteristics. Here, we quantify the dosimetric impact of an incorrect DLS parameter and investigate whether common pretreatment quality assurance (QA) methods can detect this effect.Methods16 treatment plans with intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) technique for multiple treatment sites were calculated with a correct and incorrect setting of the DLS, corresponding to a MLC gap difference of 0.5 mm. Pretreatment verification QA was performed with a bi-planar diode array phantom and the electronic portal imaging device (EPID). Measurements were compared to the correct and incorrect planned doses using gamma evaluation with both global (G) and local (L) normalization. Correlation, specificity and sensitivity between the dose volume histogram (DVH) points for the planning target volume (PTV) and the gamma passing rates were calculated.ResultsThe change in PTV and organs at risk DVH parameters were 0.4–4.1%. Good correlation (>0.83) between the PTV_mean dose deviation and measured gamma passing rates was observed. Optimal gamma settings with 3%L/3 mm (per beam and composite plan) and 3%G/2 mm (composite plan) for the diode array phantom and 2%G/2 mm (composite plan) for the EPID system were found. Global normalization and per beam ROC analysis of the diode array phantom showed an area under the curve <0.6.ConclusionsA DLS error can worsen pretreatment QA using gamma analysis with reasonable credibility for the composite plan. A low detectability was demonstrated for a 3%G/3 mm per beam gamma setting.     

Objective assessment of low-contrast computed tomography images with iterative reconstruction

ObjectiveThis study aims to assess low-contrast image quality using a low-contrast object specific contrast-to-noise ratio (CNR_LO) analysis for iterative reconstruction (IR) computed tomography (CT) images.MethodsA phantom composed of low-contrast rods placed in a uniform material was used in this study. Images were reconstructed using filtered back projection (FBP) and IR (Adaptive Iterative Dose Reduction 3D). Scans were performed at six dose levels: 1.0, 1.8, 3.1, 4.6, 7.1 and 13.3 mGy. Objective image quality was assessed by comparing CNR_LO with CNR using a human observer test.ResultsCompared with FBP, IR yielded increased CNR at the same dose levels. The results of CNR_LO and observer tests showed similarities or only marginal differences between FBP and IR at the same dose levels. The coefficient of determination for CNR_LO was significantly better (R² = 0.86) than that of CNR (R² = 0.47).ConclusionFor IR, CNR_LO could potentially serve as an objective index reflective of a human observer assessment. The results of CNR_LO test indicated that the IR algorithm was not superior to FBP in terms of low-contrast detectability at the same radiation doses.     

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.     

Dosimetric impact of 4DCT artifact in carbon-ion scanning beam treatment: Worst case analysis in lung and liver treatments

IntroductionWe evaluated the impact of 4DCT artifacts on carbon-ion pencil beam scanning dose distributions in lung and liver treatment.Methods & materials4DCT was performed in 20 liver and lung patients using area-detector CT (original 4DCT). 4DCT acquisition by multi-detector row CT was simulated using original 4DCT by selecting other phases randomly (plus/minus 20% phases). Since tumor position can move over the respiratory range in original 4DCT, mid-exhalation was set as reference phase. Total prescribed dose of 60 Gy (RBE) was delivered to the clinical target volume (CTV). Reference dose distribution was calculated with the original CT, and actual dose distributions were calculated with treatment planning parameters optimized using the simulated CT (simulated dose). Dose distribution was calculated by substituting these parameters into the original CT.ResultsFor liver cases, CTV-D95 and CTV-Dmin values for the reference dose were 97.6 ± 0.5% and 89.8 ± 0.6% of prescribed dose, respectively. Values for the simulated dose were significantly degraded, to 88.6 ± 14.0% and 46.3 ± 26.7%, respectively. Dose assessment results for lung cases were 84.8 ± 12.8% and 58.0 ± 24.5% for the simulated dose, showing significant degradation over the reference dose of 95.1 ± 1.5% and 87.0 ± 2.2%, respectively.Conclusions4DCT image quality should be closely checked to minimize degradation of dose conformation due to 4DCT artifacts. Medical staff should pay particular attention to checking the quality of 4DCT images as a function of respiratory phase, because it is difficult to recognize 4DCT artifact on a single phase in some cases     

Heart position variability during voluntary moderate deep inspiration breath-hold radiotherapy for breast cancer determined by repeat CBCT scans

Voluntary moderate deep inspiration breath hold (vmDIBH) in left-sided breast cancer radiotherapy reduces cardiac dose. The aim of this study was to investigate heart position variability in vmDIBH using CBCT and to compare this variability with differences in heart position between vmDIBH and free breathing (FB).For 50 patients initial heart position with respect to the field edge (HP-FE) was measured on a vmDIBH planning CT scan. Breath-hold was monitored using an in-house developed vertical plastic stick. On pre-treatment CBCT scans, heart position variability with respect to the field edge (Δ_HP-FE) was measured, reflecting heart position variability when using an offline correction protocol. After registering the CBCT scan to the planning CT, heart position variability with respect to the chest wall (Δ_HP-CW) was measured, reflecting heart position variability when using an online correction protocol. As a control group, vmDIBH and FB computed tomography (CT) scans were acquired for 30 patients and registering both scans on the chest wall.For 34 out of 50 patients, the average HP-FE and HP-CW increased over the treatment course in comparison to the planning CT. Averaged over all patients and all treatment fractions, the Δ_HP-FE and the Δ_HP-CW was 0.8 ± 4.2 mm (range −9.4–+10.6 mm) and 1.0 ± 4.4 mm (range −8.3–+10.4 mm) respectively. The average gain in heart to chest wall distance was 11.8 ± 4.6 mm when using vmDIBH instead of FB. In conclusion, substantial variability in heart position using vmDIBH was observed during the treatment course.     

Monte Carlo study of the influence of energy spectra,mesh size,high Z element on dose and PVDR based on 1-D and 3-D heterogeneous mouse head phantom for Microbeam Radiation Therapy

PurposeTo evaluate the influence of energy spectra, mesh sizes, high Z element on dose and PVDR in Microbeam Radiation Therapy (MRT) based on 1-D analogy-mouse-head-model (1-D MHM) and 3-D voxel-mouse-head-phantom (3-D VMHP) by Monte Carlo simulation.MethodsA Microbeam-Array-Source-Model was implemented into EGSnrc/DOSXYZnrc. The microbeam size is assumed to be 25 μm, 50 μm or 75 μm in thickness and fixed 1 mm in height with 200 μm c-t-c. The influence of the energy spectra of ID17@ESRF and BMIT@CLS were investigated. The mesh size was optimized. PVDR in 1-D MHM and 3-D VMHP was compared with the homogeneous water phantom. The arc influence of 3-D VMHP filled with water (3-D VMHWP) was compared with the rectangle phantom.ResultsPVDR of the lower BMIT@CLS spectrum is 2.4 times that of ID17@ESRF for lower valley dose. The optimized mesh is 5 µm for 25 µm, and 10 µm for 50 µm and 75 µm microbeams with 200 µm c-t-c. A 500 μm skull layer could make PVDR difference up to 62.5% for 1-D MHM. However this influence is limited (<5%) for the farther homogeneous media (e.g. 600 µm). The peak dose uniformity of 3-D VMHP at the same depth could be up to 8% for 1.85 mm × 1 mm irradiation field, whereas that of 3-D VMHWP is <1%. The high Z element makes the dose uniformity enhance in target. The surface arc could affect the superficial PVDR (from 44% to 21% in 0.2 mm depth), whereas this influence is limited for the more depth (<1%).ConclusionAn accurate MRT dose calculation algorithm should include the influence of 3-D heterogeneous media.     

Full Monte Carlo and measurement-based overall performance assessment of improved clinical implementation of eMC algorithm with emphasis on lower energy range

New version 13.6.23 of the electron Monte Carlo (eMC) algorithm in Varian Eclipse™ treatment planning system has a model for 4 MeV electron beam and some general improvements for dose calculation. This study provides the first overall accuracy assessment of this algorithm against full Monte Carlo (MC) simulations for electron beams from 4 MeV to 16 MeV with most emphasis on the lower energy range. Beams in a homogeneous water phantom and clinical treatment plans were investigated including measurements in the water phantom. Two different material sets were used with full MC: (1) the one applied in the eMC algorithm and (2) the one included in the Eclipse™ for other algorithms. The results of clinical treatment plans were also compared to those of the older eMC version 11.0.31. In the water phantom the dose differences against the full MC were mostly less than 3% with distance-to-agreement (DTA) values within 2 mm. Larger discrepancies were obtained in build-up regions, at depths near the maximum electron ranges and with small apertures. For the clinical treatment plans the overall dose differences were mostly within 3% or 2 mm with the first material set. Larger differences were observed for a large 4 MeV beam entering curved patient surface with extended SSD and also in regions of large dose gradients. Still the DTA values were within 3 mm. The discrepancies between the eMC and the full MC were generally larger for the second material set. The version 11.0.31 performed always inferiorly, when compared to the 13.6.23.     

TEMP/TDM et scintigraphie osseuse en cancérologie : impact d’une acquisition scannographique basse dose chez les patients avec foyer isolé suspect ou de nature indéterminée

AimTo evaluate the usefulness of a low dose SPECT/CT and the added value of an additional “diagnostic” centred CT-scan in cancer patients with a solitary focus observed on planar whole-body bone scintigraphy (PWBS) and classified as indeterminate or suspicious.Material and methodsSixty consecutive patients underwent a low dose SPECT/CT acquisition (120 kV, 30 mAs, 3 mm slice thickness) followed by a “diagnostic” CT-scan (120 kV, 100 mAs, 1.25 mm slice thickness) centred on the focus. The first observer considered prospectively WBS, low-dose SPECT/CT and finally the centred SPECT/CT. A blinded review was performed by a second observer.ResultsPWBS depicted solitary indeterminate or suspicious foci in 38 and 22 patients, respectively. SPECT/CT acquisitions clarified 73% (44/60) of the foci. Additional diagnostic CT-scan altered low-dose SPECT/CT results in nine patients. Additional foci (not found by PWBS) located outside the scanning area of the centred diagnostic CT-scan were found in 20 patients. Inter observer agreement for PWBS, low-dose SPECT/CT and diagnostic SPECT/CT was equal to 0.542, 0.68 and 0.694, respectively. ROC analysis showed no difference between low-dose SPECT/CT and diagnostic SPECT/CT for observer 1 and observer 2.ConclusionThis study shows that a conventional low-dose SPECT/CT in patients presenting with a solitary focus on PWBS is sufficient to improve both accuracy and inter observer variability of bone scanning. A CT volume session should not be limited to the area of the solitary focus since additional foci located outside the centred CT-scan frequently occurred.     

Virtual patient 3D dose reconstruction using in air EPID measurements and a back-projection algorithm for IMRT and VMAT treatments

PurposeAt our institute, a transit back-projection algorithm is used clinically to reconstruct in vivo patient and in phantom 3D dose distributions using EPID measurements behind a patient or a polystyrene slab phantom, respectively. In this study, an extension to this algorithm is presented whereby in air EPID measurements are used in combination with CT data to reconstruct ‘virtual’ 3D dose distributions. By combining virtual and in vivo patient verification data for the same treatment, patient-related errors can be separated from machine, planning and model errors.Methods and materialsThe virtual back-projection algorithm is described and verified against the transit algorithm with measurements made behind a slab phantom, against dose measurements made with an ionization chamber and with the OCTAVIUS 4D system, as well as against TPS patient data. Virtual and in vivo patient dose verification results are also compared.ResultsVirtual dose reconstructions agree within 1% with ionization chamber measurements. The average γ-pass rate values (3% global dose/3 mm) in the 3D dose comparison with the OCTAVIUS 4D system and the TPS patient data are 98.5 ± 1.9%(1SD) and 97.1 ± 2.9%(1SD), respectively. For virtual patient dose reconstructions, the differences with the TPS in median dose to the PTV remain within 4%.ConclusionsVirtual patient dose reconstruction makes pre-treatment verification based on deviations of DVH parameters feasible and eliminates the need for phantom positioning and re-planning. Virtual patient dose reconstructions have additional value in the inspection of in vivo deviations, particularly in situations where CBCT data is not available (or not conclusive).     

Visualization of air and metal inhomogeneities in phantoms irradiated by carbon ion beams using prompt secondary ions

PurposeNon-invasive methods for monitoring of the therapeutic ion beam extension in the patient are desired in order to handle deteriorations of the dose distribution related to changes of the patient geometry. In carbon ion radiotherapy, secondary light ions represent one of potential sources of information about the dose distribution in the irradiated target. The capability to detect range-changing inhomogeneities inside of an otherwise homogeneous phantom, based on single track measurements, is addressed in this paper.MethodsAir and stainless steel inhomogeneities, with PMMA equivalent thickness of 10 mm and 4.8 mm respectively, were inserted into a PMMA-phantom at different positions in depth. Irradiations of the phantom with therapeutic carbon ion pencil beams were performed at the Heidelberg Ion Beam Therapy Center. Tracks of single secondary ions escaping the phantom under irradiation were detected with a pixelized semiconductor detector Timepix. The statistical relevance of the found differences between the track distributions with and without inhomogeneities was evaluated.ResultsMeasured shifts of the distal edge and changes in the fragmentation probability make the presence of inhomogeneities inserted into the traversed medium detectable for both, 10 mm air cavities and 1 mm thick stainless steel. Moreover, the method was shown to be sensitive also on their position in the observed body, even when localized behind the Bragg-peak.ConclusionsThe presented results demonstrate experimentally, that the method using distributions of single secondary ion tracks is sensitive to the changes of homogeneity of the traversed material for the studied geometries of the target.