Slab Phantom Research Articles

Internal breast dosimetry in mammography: Experimental methods and Monte Carlo validation with a monoenergetic x-ray beam.

To investigate the performance, such as energy dependence and sensitivity, of thermoluminescent dosimeters (TLD), metal oxide semiconductor field-effect transistor dosimeters (MOSFET), and GafChromic™ films, and to validate the estimates of local dose deposition of a Monte Carlo (MC) simulation for breast dosimetry applications. Experimental measurements were performed using a monoenergetic beam at the ELETTRA synchrotron radiation light source (Trieste, Italy). The three types of dosimeters were irradiated in a plane transversal to the beam axis and calibrated in terms of air kerma. The sensitivity of MOSFET dosimeters and GafChromic™ films was evaluated in the range of 18-28 keV. Three different calibration curves for the GafChromic™ films were tested (logarithmic, rational, and exponential functions) to evaluate the best-fit curve in the dose range of 1-20 mGy. Internal phantom dose measurements were performed at 20 keV for four different depths (range 0-3 cm, with 1 cm steps) using a homogeneous 50% glandular breast phantom. A GEANT4 MC simulation was modified to match the experimental setup. Thirty sensitive volumes, on the axial-phantom plane were included at each depth in the simulation to characterize the internal dose variation and compare it to the experimental TLD and MOSFET measurements. Experimental 2D dose maps were obtained with the GafChromic™ films and compared to the simulated 2D dose distributions estimated with the MC simulations. The sensitivity of the MOSFET dosimeters and GafChromic™ films increased with x-ray energy, by up to 37% and 48%, respectively. Dose-response curves for the GafChromic™ film result in an uncertainty lower than 5% above 6 mGy, when a logarithmic relationship is used in the dose range of 1-10 mGy. All experimental values fall within the experimental uncertainty and a good agreement (within 5%) is found against the MC simulation. The dose decreased with increasing phantom depth, with the reduction being ~80% after 3 cm. The uncertainty of the empirical measurements makes the experimental values compatible with a flat behavior across the phantom slab for all the investigated depths, while the MC points to a dose profile with a maximum toward the center of the phantom. The calibration procedures and the experimental methodologies proposed lead to good accuracy for internal breast dose estimation. In addition, these procedures can be successfully applied to validate MC codes for breast dosimetry at the local dose level. The agreement among the experimental and MC results not only shows the correctness of the empirical procedures used but also of the simulation parameters.

Ethylene-vinyl acetate foam as a new lung substitute in radiotherapy.

The purpose of this study was to evaluate ethylene-vinyl acetate (EVA) foam as a new lung substitute in radiotherapy and to study its physical and dosimetric characteristics. We calculated the ideal vinyl acetate (VA) content of EVA foam sheets to mimic the physical and dosimetric characteristics of the ICRU lung tissue. We also computed the water-to-medium mass collision stopping power ratios, mass attenuation coefficients, CT numbers, effective atomic numbers and electron densities for: ICRU lung tissue, the RANDO commercial phantom, scaled WATER and EVA foam sheets with varying VA contents in a range between the minimum and maximum values supplied by the manufacturer. For all these substitutes, we simulated percent depth-dose curves with EGSnrc Monte Carlo (MC PDDs) in a water-lung substitute-water slab phantom expressed as dose-to-medium and dose-to-water for 3×3- and 10×10-cm2 field sizes. PDD for the 10×10-cm2 field size was also calculated with the MultiGrid Superposition algorithm (MGS PDD) for a relative electron density to water ratio of 0.26. The latter was compared with the MC PDDs in dose-to-water for scaled WATER and EVA foam sheets with the VA content that was most similar to the calculated ideal content that is physically achievable in practice. We calculated an ideal VA content of 55%; however, the maximum physically achievable content with current manufacturing techniques is 40%. The physical characteristics of the EVA foam sheets with a VA content of 40% (EVA40) are very close to those of the ICRU lung reference. The physical densities of the EVA40 foam sheets ranged from 0.030 to 0.965g/cm3 , almost covering the entire physical density range of the inflated/deflated lung (0.260-1.050g/cm3 ). Its mass attenuation coefficient at the effective energy of a 6-MV photon beam agrees within 0.8% of the ICRU reference value, and its CT number agrees within 6HU. The effective atomic number for EVA40 varies by less than 0.42 of the ICRU value, and its effective electron density is within 0.9%. PDDs expressed in dose-to-medium and dose-to-water agree with the ICRU curve within 2% in all regions. PDDs calculated with both MC and MGS were within 1.5%. The EVA40 is an excellent cork-like lung substitute for radiotherapy applications. From a sole material used in footwear, it is possible to obtain a lung substitute that mimics the physical and dosimetric characteristics of ICRU lung tissue even better than the RANDO commercial phantom.

Fabrication and Characterization of Optical Tissue Phantoms Containing Macrostructure.

The rapid development of new optical imaging techniques is dependent on the availability of low-cost, customizable, and easily reproducible standards. By replicating the imaging environment, costly animal experiments to validate a technique may be circumvented. Predicting and optimizing the performance of in vivo and ex vivo imaging techniques requires testing on samples that are optically similar to tissues of interest. Tissue-mimicking optical phantoms provide a standard for evaluation, characterization, or calibration of an optical system. Homogenous polymer optical tissue phantoms are widely used to mimic the optical properties of a specific tissue type within a narrow spectral range. Layered tissues, such as the epidermis and dermis, can be mimicked by simply stacking these homogenous slab phantoms. However, many in vivo imaging techniques are applied to more spatially complex tissue where three dimensional structures, such as blood vessels, airways, or tissue defects, can affect the performance of the imaging system. This protocol describes the fabrication of a tissue-mimicking phantom that incorporates three-dimensional structural complexity using material with optical properties of tissue. Look-up tables provide India ink and titanium dioxide recipes for optical absorption and scattering targets. Methods to characterize and tune the material optical properties are described. The phantom fabrication detailed in this article has an internal branching mock airway void; however, the technique can be broadly applied to other tissue or organ structures.

Evaluation of targeted image-guided radiation therapy treatment planning system by use of american association of physicists in medicine task group-119 test cases

This study aimed to evaluate the overall accuracy of the beam commissioning criteria of targeted image‑guided radiation therapy (TiGRT) treatment planning system (TPS) based on the American Association of Physicists in Medicine (AAPM) Task Group Report 119 (TG‑119). The work was performed using 6 MV energy LINAC with a variable dose rate of 200 MU/min which equipped with the high‑quality external TiGRT dynamic multileaf collimator model H. The AAPM TG‑119 intensity‑modulated radiation therapy (IMRT) commissioning tests are composed of two preliminary tests and four clinical test cases. The clinical tests consisted of mock prostate, mock head and neck, C‑shaped target, and multitarget. EDR2 flm was used for evaluating the IMRT plans and point dose measured by a Pinpoint chamber positioned in slab phantom. The flm analysis was done with the Sun Nuclear Corporation patient software. The dose prescription for each fraction was 200 cGy in mock prostate, mock head and neck, C‑shaped target, and multitarget. Dose distributions were analyzed using gamma criteria of 3% and 2% dose difference (DD) and 3 and 2 mm distance to agreement. In all test cases, the gamma criteria for 2%/2 and 3%/3 were found to be 94% and 98%, respectively. Results showed that the average gamma criteria result was in the range of 99.1% to 93% (3%/3, 2%/2) overall test cases. Findings were favorable and in some tests were comparable with the other studies. The dose point values were within the mean values of the range reported by TG‑119. Overall, the TiGRT TPS is needed to apply IMRT technique in radiation therapy centers.

Evaluation of targeted image-guided radiation therapy treatment planning system by use of american association of physicists in medicine task group-119 test cases

This study aimed to evaluate the overall accuracy of the beam commissioning criteria of targeted image-guided radiation therapy (TiGRT) treatment planning system (TPS) based on the American Association of Physicists in Medicine (AAPM) Task Group Report 119 (TG-119). The work was performed using 6 MV energy LINAC with a variable dose rate of 200 MU/min which equipped with the high-quality external TiGRT dynamic multileaf collimator model H. The AAPM TG-119 intensity-modulated radiation therapy (IMRT) commissioning tests are composed of two preliminary tests and four clinical test cases. The clinical tests consisted of mock prostate, mock head and neck, C-shaped target, and multitarget. EDR2 film was used for evaluating the IMRT plans and point dose measured by a Pinpoint chamber positioned in slab phantom. The film analysis was done with the Sun Nuclear Corporation patient software. The dose prescription for each fraction was 200 cGy in mock prostate, mock head and neck, C-shaped target, and multitarget. Dose distributions were analyzed using gamma criteria of 3% and 2% dose difference (DD) and 3 and 2 mm distance to agreement. In all test cases, the gamma criteria for 2%/2 and 3%/3 were found to be 94% and 98%, respectively. Results showed that the average gamma criteria result was in the range of 99.1% to 93% (3%/3, 2%/2) overall test cases. Findings were favorable and in some tests were comparable with the other studies. The dose point values were within the mean values of the range reported by TG-119. Overall, the TiGRT TPS is needed to apply IMRT technique in radiation therapy centers.

Design of a slab phantom for breast dosimetry applications.

Design of phantoms for use in radiotherapy should consider the complex geometry of breast tissue and inhomogeneity. The aim of this study is design of a slab phantom for breast dosimetry applications. In this study, an anatomical slab phantom was designed with cork lung inhomogeneity and plexi colored heart part, also describes the different size of breast and chest wall phantom that have been designed and constructed for dosimetry. Three size different phantoms have been manufactured that installed in one trunk, as "small," "medium," and "large," two breast size fixed and one size was movable on a chest wall phantom. Two different dosimeters selected to dosimetry in this phantom, film was chosen for this dosimetry since it provides good spatial resolution and suitable for two-dimensional dosimetry also measure dose distribution used a point dosimetry with thermoluminescent dosimeter (TLD). The results were shown near date due to either software or phantom calculation. Application include assessment dose in the junction region between the tangential fields and the supraclavicular fossa field, as well as assess dose in inhomogeneities, the phantom were formed from a variety of tissue substitute materials.

Evaluation on dose distribution of longitudinal profile on helical tomotherapy Gha-Jung Kim

In this study is aimed to evaluate the accuracy of longitudinal dose distribution during helical tomotherapy slice treatment using ion chamber and EBT film, and measure the flatness in the longitudinal profile for each radiation field width. As the experiment condition, the radiation field widths were set to 10 mm, 25 mm, and 50 mm, respectively, to acquire and evaluate a longitudinal beam profile. A slab phantom and A1SL ion chamber was used to evaluate the accuracy of longitudinal dose distribution in helical tomotherapy. Also, solid phantom and EBT film was used to measurement of dose flatness in the longitudinal axis (Y axis). The difference between reference and measured data values of FWHM at the radiation field width of 10 mm was 0.546 ± 0.186% on average, and the maximum γ-index was 0.522 ± 0.056. The difference between reference and measured data values of FWHM at the radiation field width of 25 mm was 0.133 ± 0.113% on average, and the maximum γ-index was 0.417 ± 0.102. The difference between reference and measured data values of FWHM at the radiation field width of 50 mm was 0.120 ± 0.072% on average, and the maximum γ-index was 0.467 ± 0.077. The dose flatness within a range of 250 mm longitudinal axis (Y axis) for each radiation field width was 1.71 ± 0.46% on average, which was a value within 3%. The results exhibited that the dose flatness became more even as the radiation field width became wider. The above study results will contribute to clarifying the accuracy of radiation dose delivered to real patients, or the DQA, which is an important QA item in the IMRT using tomotherapy.

Imaging and Dosimetric Study on Direct Flat-Panel Detector-Based Digital Mammography System.

Introduction:Image quality of digital mammography system is generally defined by three primary physical parameters, namely, contrast, resolution, and noise. Quantification of these metrics can be done by measuring objective image quality parameters defined as contrast-to-noise ratio (CNR), modulation transfer function (MTF), and noise power spectra (NPS).Materials and Methods:In the present study, various imaging metrics such as CNR, contrast detail resolution, MTF, and NPS were evaluated for a direct flat-panel detector-based digital mammography system following the European Guidelines. Furthermore, system performance relating to both image quality and doses were evaluated using figure of merit (FOM) in terms of CNR2/mean glandular dose (MGD) under automatic exposure control (AEC) and clinically used OPDOSE operating mode.Results and Conclusion:Under AEC mode, FOM values for the 4.5 cm thick BARC polymethyl methacrylate (PMMA) phantom were found to be 15.02, 15.88, and 19.82 at Mo/Mo, Mo/Rh, and W/Rh target/filter (T/F), respectively. Under OPDOSE mode, FOM values were found to 65.32, 11.80, and 1.14 for the BARC PMMA phantom thickness of 2, 4.5, and 8 cm, respectively. Under OPDOSE mode, the calculated MGD values for three Computerized Imaging Reference Systems slab phantoms having total thickness of 7 cm were observed to be 3.03, 2.32, and 1.75 mGy with glandular/adipose tissue compositions of 70/30, 50/50, and 30/70, respectively, whereas for the 2–8-cm thick BARC PMMA phantom, the calculated MGDs were found to be in the range of 0.57–3.32 mGy. All the calculated MGDs values were found to be lower than the acceptable level of dose limits provided in European Guidelines.

Underdosing of the maxillary sinus for small fields used in newer radiotherapy techniques: Comparison of thermoluminescent dosimeter and Monte Carlo data.

To evaluate the underdosing of the maxillary sinus at its distal end produced by air cavity in the path of the 6 MV photon beam. A cubic solid water slab phantom of dimensions 18 cm × 18 cm × 18 cm with 4 cm × 4 cm × 4 cm air cavity 3 cm away from its anterior surface was used in this study. The percentage depth dose (PDD) for 6 MV X-rays along the central axis of the cubical air cavity was measured using thermoluminescent dosimeter-100 chips. The EGSnrc/DOSXYZnrc Monte Carlo (MC) code was used to estimate the PDD values in both homogeneous and inhomogeneous conditions. The dose data were generated for 1 cm × 1 cm, 2 cm × 2 cm, 3 cm × 3 cm, and 5 cm × 5 cm field sizes. Average percentage dose reductions at 1 mm beyond the distal surface of the maxillary sinus for the field sizes 1 × 1, 2 × 2, and 3 × 3 cm2 are 42.4%, 39.5%, and 29.4%, respectively. However, for 5 cm × 5 cm field size, there is a dose enhancement (i.e., overdosing) at 1 mm from the distal surface of the maxillary sinus and the average percentage dose enhancement is 5.9%. Also, beyond 1 cm from the air-water interface, there is dose enhancement for all the field sizes. This study showed that the significant dose reduction occurs near the air-water interface for the treatment techniques using small photon fields such as intensity-modulated radiotherapy or other newer techniques. MC-based treatment planning calculation predicts realistic dose distribution while using small photon fields in the treatment of maxillary sinus.

Evaluation of Superficial Dosimetry in Postmastectomy Radiotherapy using High-energy Electromagnetic Radiation Treatment

The present study evaluated surface and superficial doses delivered by high-energy electromagnetic radiation treatment in patients who received postmastectomy radiotherapy. Computed tomography was performed using an RW3 slab phantom, and hypothetical target volumes were delineated. 6MV electromagnetic radiation beams were generated with five treatment plans: 2-field 3-dimentional conformal radiation therapy, 4-field intensitymodulated radiation therapy, 7-field intensity-modulated radiation therapy, TomoHelical 3-dimentional conformal radiation therapy, and TomoHelical intensity-modulated radiation therapy. Film dosimetry was performed with Gafchromic EBT3 film for dose measurement of high-energy electromagnetic radiation. The dose profile at the surface and superficial regions (1-6 mm depth) of the phantom obtained for each treatment technique. Compared to other techniques, Tomo 3-dimentional conformal radiation therapy had the highest surface dose (47-71 %). The superficial doses of TomoHelical 3-dimentional conformal radiation therapy and TomoHelical intensity-modulated radiation therapy were > 75 %, 80 %, and 90 % of the prescribed dose at 1, 2, and 5 mm depths, respectively. For postmastectomy radiotherapy, TomoHelical 3-dimentional conformal radiation therapy and TomoHelical intensity-modulated radiation therapy had higher surface and superficial doses than linear accelerator-based treatment techniques, with a sufficient dose of ≥ 75 % being delivered to the skin region at depths of 1 mm.

Independent Monte-Carlo dose calculation for MLC based CyberKnife radiotherapy

This work aims to develop, implement and validate a Monte Carlo (MC)-based independent dose calculation (IDC) framework to perform patient-specific quality assurance (QA) for multi-leaf collimator (MLC)-based CyberKnife® (Accuray Inc., Sunnyvale, CA) treatment plans. The IDC framework uses an XML-format treatment plan as exported from the treatment planning system (TPS) and DICOM format patient CT data, an MC beam model using phase spaces, CyberKnife MLC beam modifier transport using the EGS++ class library, a beam sampling and coordinate transformation engine and dose scoring using DOSXYZnrc.The framework is validated against dose profiles and depth dose curves of single beams with varying field sizes in a water tank in units of cGy/Monitor Unit and against a 2D dose distribution of a full prostate treatment plan measured with Gafchromic EBT3 (Ashland Advanced Materials, Bridgewater, NJ) film in a homogeneous water-equivalent slab phantom. The film measurement is compared to IDC results by gamma analysis using 2% (global)/2 mm criteria. Further, the dose distribution of the clinical treatment plan in the patient CT is compared to TPS calculation by gamma analysis using the same criteria.Dose profiles from IDC calculation in a homogeneous water phantom agree within 2.3% of the global max dose or 1 mm distance to agreement to measurements for all except the smallest field size. Comparing the film measurement to calculated dose, 99.9% of all voxels pass gamma analysis, comparing dose calculated by the IDC framework to TPS calculated dose for the clinical prostate plan shows 99.0% passing rate. IDC calculated dose is found to be up to 5.6% lower than dose calculated by the TPS in this case near metal fiducial markers.An MC-based modular IDC framework was successfully developed, implemented and validated against measurements and is now available to perform patient-specific QA by IDC.

Experimental assessment of the Advanced Collapsed-cone Engine for scalp brachytherapy treatments

PurposeTo experimentally assess the performance of the Advanced Collapsed-cone Engine (ACE) for 192Ir high-dose-rate brachytherapy treatment planning of nonmelanoma skin cancers of the scalp. Methods and MaterialsA layered slab phantom was designed to model the head (skin, skull, and brain) and surface treatment mold using tissue equivalent materials. Six variations of the phantom were created by varying skin thickness, skull thickness, and size of air gap between the mold and skin. Treatment planning was initially performed using the Task Group 43 (TG-43) formalism with CT images of each phantom variation. Doses were recalculated using standard and high accuracy modes of ACE. The plans were delivered to Gafchromic EBT3 film placed between different layers of the phantom. ResultsDoses calculated by TG-43 and ACE and those measured by film agreed with each other at most locations within the phantoms. For a given phantom variation, average TG-43– and ACE-calculated doses were similar, with a maximum difference of (3 ± 12)% (k = 2). Compared to the film measurements, TG-43 and ACE overestimated the film-measured dose by (13 ± 12)% (k = 2) for one phantom variation below the skull layer. ConclusionsTG-43– and ACE-calculated and film-measured doses were found to agree above the skull layer of the phantom, which is where the tumor would be located in a clinical case. ACE appears to underestimate the attenuation through bone relative to that measured by film; however, the dose to bone is below tolerance levels for this treatment.

Abstract ID: 111 Monte Carlo modeling of Orthovoltage treatment fields

Computational power and anthropomorphous phantoms advances in the last years have extended greatly the number of potential applications of Monte Carlo methods in the medical field. Aiming at an orthovoltage treatment plan definition, the results of Monte Carlo simulations performed with the MCNP6 code on two different typologies of computational models have been compared: Unstructured Mesh (UM) model and Voxel model. The capability to simulate the particle transport directly on the unstructured mesh model geometry has been recently introduced in the release 6 of MCNP [1] , which has been chosen for this study. These kind of models overcome the insurgence of voxel effects [2] and provide a more accurate description of volumes and complex surfaces. The simulations were performed on anthropomorphous models based on anonymous CT scans. Firstly, the UM model was obtained from the CT scans thanks to the SCAN-IP mesh tools (Simpleware), and secondly the CT was voxelized with different voxel sizes in order to compare and gauge the voxel effects. The radiation source simulates the Orthovoltage X-Ray tube in use at IRCCS-ASMN in Reggio Emilia, tested in different configurations and tube voltages. The computational source has been validated with experimental measures on the real one applied on heterogeneous slab phantoms. The validation showed good agreement of the simulated setups with the experiments, with deviations varying accordingly to the source configuration, but always below 5–6%. The comparison between the two models showed how, with decreasing voxel sizes, the voxel models results partially converge to the UM models values. Furthermore, it was highlighted that the major deviations between the models resulted where the dose gradient is higher, along the field borders and when tissue heterogeneity is more and more considered. The study shows how the UM model can be implemented for evaluating the dose deposition within anthropomorphous phantoms and, in perspective, be used in treatment planning.

Improvement of Dose Calculation with Scattered Photon-Corrected kV Cone-Beam CT Image Using the Kernel Deconvolution Method

Intensity modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) has to be accomplished with a dose distribution in terms of changes in the tumor size and position for minimizing adverse effects on neighboring organs. If cone-beam computed tomography (CBCT) would facilitate dose calculation attached to the IMRT or SBRT apparatus, we could use the CBCT-based adaptive radiation therapy (ART). However, a CBCT image cannot achieve quantitative electron density values owing to scattered photons; thus, we cannot use it directly for dose calculation. In this study, we calculated the distribution of scattered photons in CBCT images using the Monte Carlo (MC) method and corrected the scattered photon effect with scatter kernels in the process of image reconstruction with the maximum likelihood expectation maximization (MLEM) method. Moreover, we investigated the accuracy of dose calculation by this segmented region method in comparison with typical methods by replacing CBCT values with ideal density values. We calculated the distribution of scattered photons with slab phantoms using the MC method and acquired scattered kernels of three materials (bone, water, and lungs) having different thicknesses until 20 cm every 0.1 cm. These kernels were used in the MLEM method for correcting scattered photons in lung and pelvis CBCT images. Next, we segmented CBCT and multislice CT (MSCT) images into three regions (lungs, bones, and soft tissues) and compared segmentation accuracy between MSCT and scatter-corrected CBCT images. Moreover, we evaluated the effectiveness of dose calculation by CBCT images. In the CBCT to density conversion method, we converted the CBCT value using individual three tables in each organ, and in the density overriding method, we observed calculated doses on accurate organ-segmented images using γ and dose–volume histogram (DVH) analyses. Segmentation results showed that the threshold value of bone and soft tissue separation was similar to the ideal threshold value used in MSCT images; the segmentation error ratio improved (w/o: 63.0%, w/: 4.7%). Similarly, segmentation error ratios for lungs and soft tissues improved by 8.6% compared with those observed on the original CBCT image. In the CBCT to density conversion method, differences in target doses of MSCT and CBCT with scatter corrections were small in the DVH analysis. In the density overriding method, γ analysis results showed that the calculated dose distributions with scatter-corrected images were similar to those seen in MSCT; the pass rate was significantly higher with an accurate segmentation effect (w/o: 70.1%, w/: 95.9%). We evaluated the effectiveness of scattered photon-corrected CBCT image using the kernel deconvolution method for organ segmentation and the effect of this contribution on dose calculation. Improvement of the segmentation accuracy with scattered photon correction can be observed using CBCT dose calculation for ART.

Comparative Evaluation of Treatment Planning Systems Used in Lung SBRT

The effectiveness of lung SBRT depends on the choice of treatment planning system (TPS) used for dose calculations. Whereas Monte-Carlo (MC) based TPS calculate the most accurate dose, conventional planning systems cannot adequately account for dose variation caused by heterogeneous lung tissue. We aim to evaluate the performance of several commercial TPS used in lung SBRT. We hypothesize that the use of MC-based TPS will result in more accurate and consistent delivered doses in lung SBRT. SBRT plans for 30 early-stage lung cancer patients (31 targets) were retrospectively evaluated in this multi-institutional study. Patient data with structure sets were anonymized and sent to four participating institutions to plan on five different TPS: one pencil-beam (PB), two convolution-superposition (CS1 & CS2), and two Monte Carlo (MC1 & MC2) systems. A test case with homogenous slab phantom was also included to allow benchmarking of each TPS. Patients were grouped according to PTV size: small (<25cc), medium (25 – 60cc), large (>60cc); and location: island, adjacent to chest-wall/mediastinum, and mixed. All patients were initially planned with MC1TPS to deliver 50Gy PTV dose in 5 fractions using 7-12 non-coplanar, conformal, 6 MV fields. Treatment plans were optimized for > 95% PTV dose coverage as per the Radiation Therapy Oncology Group guidelines. Using the same beam parameters, calculation geometry, and MUs, all patient plans were re-computed on other TPS. Dose-volume-histograms (DVH) for PTV and organs-at-risk (OARs) including lung, esophagus, trachea, heart, and spinal cord were evaluated. Two-tailed student t-test was used for statistical analysis. Average PTV was 53.5cc (range: 8.6 – 149.4cc). The largest dose difference was found between MC1 and PB for small island targets. Compared to MC1, average values of PTV Dmean for all patients were higher by 12.5%, 3%, 0.5% and 2% respectively in PB, CS1, CS2 and MC2 TPS (p < 0.01). Larger dose differences were seen for PTV Dmin: 21.8%, 4.6%, 10.6% and 12.8% (p < 0.01), indicating greater sensitivity of dose calculation algorithms in the target periphery due to density changes. Whereas the prescription dose conformality (CI) was similar in CS and MC TPS, dose spillage (D2cm) was 6.2%, 4.8%, 1.5% and 0.9% higher respectively for PB, CS1, CS2 and MC2 (p< 0.01). Compared to MC1, average lung V20 showed an 11.4% increase in PB, but was 2.4%, 2.4% and 4.4% lower in CS1, CS2 and MC2 TPS respectively (p < 0.01). Other OARs showed similar trend with MC1 having closest agreement with MC2, followed by CS2, CS1 and PB. Conventional TPS for lung SBRT result in significant dose variation depending on the calculation algorithm. MC based TPS provide the greatest accuracy and consistency in delivered doses, thereby allowing easier plan comparison among different institutions. In light of observed dose differences, current RTOG dose guidelines may require further evaluation or revision.

Tissue-like phantoms for quantitative birefringence imaging.

Birefringence imaging, including polarization sensitive optical coherence tomography (PS-OCT), can provide valuable insight into the microscopic structure and organization of many biological tissues. In this paper, we report on a method to fabricate tissue-like birefringence phantoms for such imaging modalities. We utilize the photo-elastic effect, wherein birefringence is induced by stretching a polymer sample after heating it above its glass-transition temperature. The cooled samples stably exhibit homogeneous birefringence, and were assembled into phantoms containing multiple well-defined regions of distinct birefringence. We present planar slab phantoms for microscopy applications and cylindrical phantoms for catheter-based imaging and demonstrate quantitative analysis of the birefringence within individual regions of interest. Birefringence phantoms enable testing, validating, calibrating, and improving PS-OCT acquisition systems and reconstruction strategies.

Responses of LiF Thermoluminescence Dosimeters to Diagnostic 60Co Teletherapy Beams

Thermoluminescence Dosimeter (TLD) is used as an ionizing radiation detector in the field of clinical radiotherapy, diagnostic radiology, personal radiation monitoring as well as in environmental radiation dosimetry. The sensitivity of all the TLDs is not the same even though the manufacturer produced them in the same batch. The response of TLD at various doses is also not always linear. In the present study fifty-seven rod shaped LiF TLDs had been irradiated by doses of 50cGy, 100cGy, 150cGy and 200cGy using 60Co teletherapy unit of Dhaka Medical College (Alcyon II, CGR, McV, France). The TLDs were placed in the grooves of a slab phantom and then covered by a 0.5cm thick plexiglass sheet. The phantom was then irradiated in the reference condition (10x10cm2 field size, 80cm SSD). After irradiation, readout of TLDs was done using a Harshaw TLD reader (Model 3500). Measurements were performed three times for each dose value and their average was taken. For convenience all fifty-seven TLDs were divided into nineteen groups. Grouping was done according to the response of the TLDs for a fixed dose value. In a particular group of TLDs, variation of the response of the dosimeters is the smallest one. The dose (cGy) – response (nC) linearity curve of TLDs for each of the nineteen groups showed to be very close to unity.Bangladesh Journal of Medical Physics Vol.8 No.1 2015 14-21

A back-projection algorithm in the presence of an extra attenuating medium: towards EPID dosimetry for the MR-Linac

In external beam radiotherapy, electronic portal imaging devices (EPIDs) are frequently used for pre-treatment and for in vivo dose verification. Currently, various MR-guided radiotherapy systems are being developed and clinically implemented. Independent dosimetric verification is highly desirable. For this purpose we adapted our EPID-based dose verification system for use with the MR-Linac combination developed by Elekta in cooperation with UMC Utrecht and Philips. In this study we extended our back-projection method to cope with the presence of an extra attenuating medium between the patient and the EPID. Experiments were performed at a conventional linac, using an aluminum mock-up of the MRI scanner housing between the phantom and the EPID. For a 10 cm square field, the attenuation by the mock-up was 72%, while 16% of the remaining EPID signal resulted from scattered radiation. 58 IMRT fields were delivered to a 20 cm slab phantom with and without the mock-up. EPID reconstructed dose distributions were compared to planned dose distributions using the -evaluation method (global, 3%, 3 mm). In our adapted back-projection algorithm the averaged was , while in the conventional it was . Dose profiles of several square fields reconstructed with our adapted algorithm showed excellent agreement when compared to TPS.

Measurement of neutron dose in the compensator IMRT treatment

A radiation treatment delivery technique, intensity modulated radiation therapy (IMRT), has found widespread use in the treatment of cancers. One of IMRT implementing methods is IMRT compensator based, which the modulation are done by high Z materials. When photons with energies higher than 8MV interact with high Z material in path, Photoneutrons are produced. In this study, the effect of compensator on photoneutron production was investigated. The Monte Carlo code MCNPX was used to calculate the neutron dose equivalent as a function of the depth in phantom with and without compensator. Measurements were made using CR-39 track-etched detectors. CR-39 detectors, were cut in dimensions of 2.5×2.5 cm2 by laser, placed in different depths of slab phantom and then irradiated by 18MV photons. Same procedure was performed with the compensator present and absent. The measured data were compared with MCNP calculations. In both experimental and simulation results, neutron dose equivalent when compensator used, was less than non-compensator field. The calculated neutron dose equivalent was maximum at surface and decreased exponentially by increasing depth, but in experimental data, the neutron dose equivalent reached a maximum at approximately 3cm depth in the phantom and beyond which decreased with depth.CR-39 calibration was carried out in air, by considering that neutron energy spectrum changes toward thermal neutrons by depth in phantom increasing, it is suggested that for measuring equivalent neutron dose at phantom depth, should have proper neutron calibration in terms of energy spectrum.

A Simple Method for 2-D In Vivo Dosimetry by Portal Imaging.

Purpose:To improve patient safety and treatment quality, verification of dose delivery in radiotherapy is desirable. We present a simple, easy-to-implement, open-source method for in vivo planar dosimetry of conformal radiotherapy by electronic portal imaging device (EPID).Methods:Correlation ratios, which relate dose in the mid-depth of slab phantoms to transit EPID signal, were determined for multiple phantom thicknesses and field sizes. Off-axis dose is corrected for by means of model-based convolution. We tested efficacy of dose reconstruction through measurements with off-reference values of attenuator thickness, field size, and monitor units. We quantified the dose calculation error in the presence of thickness changes to simulate anatomical or setup variations. An example of dose calculation on patient data is provided.Results:With varying phantom thickness, field size, and monitor units, dose reconstruction was almost always within 3% of planned dose. In the presence of thickness changes from planning CT, the dose discrepancy is exaggerated by up to approximately 1.5% for 1 cm changes upstream of the isocenter plane and 4% for 1 cm changes downstream.Conclusion:Our novel electronic portal imaging device in vivo dosimetry allows clinically accurate 2-dimensional reconstruction of dose inside a phantom/patient at isocenter depth. Due to its simplicity, commissioning can be performed in a few hours per energy and may be modified to the user’s needs. It may provide useful dose delivery information to detect harmful errors, guide adaptive radiotherapy, and assure quality of treatment.