Thimble Chamber Research Articles

Investigations on the beam quality correction factor for ionization chambers in high-energy brachytherapy dosimetry

Objective. To enhance the investigations on MC calculated beam quality correction factors of thimble ionization chambers from high-energy brachytherapy sources and to develop reliable reference conditions in source and detector setups in water. Approach. The response of five different ionization chambers from PTW-Freiburg and Standard Imaging was investigated for irradiation by a high dose rate Ir-192 Flexisource in water. For a setup in a Beamscan water phantom, Monte Carlo simulations were performed to calculate correction factors for the chamber readings. After exact positioning of source and detector the absorbed dose rate at the TG-43 reference point at one centimeter nominal distance from the source was measured using these factors and compared to the specification of the calibration certificate. The Monte Carlo calculations were performed using the restricted cema formalism to gain further insight into the chamber response. Calculations were performed for the sensitive volume of the chambers, determined by the methods currently used in investigations of dosimetry in magnetic fields. Main results. Measured dose rates and values from the calibration certificate agreed within the combined uncertainty (k = 2) for all chambers except for one case in which the full air cavity was simulated. The chambers showed a distinct directional dependence. With the restricted cema formalism calculations it was possible to examine volume averaging and energy dependence of the perturbation factors contributing to the beam quality correction factor also differential in energy. Significance. This work determined beam quality correction factors to measure the absorbed dose rate from a brachytherapy source in terms of absorbed dose to water for a variety of ionization chambers. For the accurate dosimetry of brachytherapy sources with ionization chambers it is advisable to use correction factors based on the sensitive volume of the chambers and to take account for the directional dependence of chamber response.

Spectral analysis of Monte Carlo calculated fluence correction and cema conversion factors for high-energy photon beams at different depths

Purpose: The aim of this study is to investigate the depth-dependent detector response of detailed thimble air-filled ionization chambers by calculating spectral charged particle fluence correction factors at different depths in water. Those spectral correction factors will help to understand, how the detector response varies at different depths and what kind of influences disparate effects have on the spectral detector response.Methods: The cema-approach can be used to obtain spectral charged particle fluence-based correction factors for various measurement conditions by substituting the commonly well-known dose conversion factor with a conversion factor based on the dosimetric quantity cema (“converted-energy per unit mass”). The resulting spectral fluence correction factors were calculated with the EGSnrc software toolkit and analyzed for two air-filled cylindrical ionization chambers (PTW type 31021 Semiflex 3D, SNC125c™) at different depths in a water phantom irradiated with a 6 MV linear accelerator x-ray spectrum. The ionization chamber models have been stepwise decomposed to investigate the perturbation caused by internal and external effects on the fluence distribution within the detector.Results: Monte Carlo calculated fluence-based perturbation correction factors revealed that for all investigated detectors, considerable fluence disturbances occur, especially in the build-up region of depth-dose curves. Our results have shown that even slight variations in depth can have major consequences on the differential charged particle fluence within the ionization chamber, mainly due to internal cavity-specific effects. Furthermore, the results showed that in the case of relative dose measurements, the depth-depending detector response can significantly differ from unity in a range of 1.4%–2.8% depending on the ionization chamber design.Conclusion: The complexity of different effects on the fluence disturbance could be broken down with regard to their influence on the spectral fluence distribution in the sensitive volume of the investigated detectors. It could be demonstrated, that the displacement of water is a depth-depending effect, which can not be compensated or corrected ideally for each investigated water depth by the shift of the effective point of measurement. Generally, the spectral analysis of those energy-dependent correction factors serves to a deeper understanding of the detector response under various conditions.

Small static radiosurgery field dosimetry with small volume ionization chambers

PurposeTo evaluate the response of the four smallest active volume thimble type ionization chambers commercially available (IBA-dosimetry RAZOR Nano Chamber, Standard Imaging Exradin A16, IBA-dosimetry CC01 and PTW T31022) when measuring SRS cone collimated Flattening Filter Free (FFF) fields. MethodsWe employed Monte Carlo simulation for calculating correction factors as defined in IAEA TRS-483. Monte Carlo simulation beam model and ion chamber geometry definitions were supported by an extensive set of measurements. Type A and B uncertainty components were evaluated. ResultsCommissioning of Monte Carlo 6 MV and 10 MV FFF beam models yielded relative differences between measured and simulated dose distributions lower than 1.5%. Monte Carlo simulated output factors for 5 mm SRS field agree with experimental values within 1% local relative difference for all chambers. Smallest active volume ion chamber (IBA-dosimetry RAZOR Nano Chamber) exhibits smallest correction, being compatible with unity. Correction factor combined uncertainties range between 0.7% and 0.9%. Smallest uncertainties were recorded for smallest and largest active volume ion chambers, although the latter exhibited largest correction factor. Highest contribution to combined uncertainty was type B component associated with beam model initial electron spatial Full Width Half Maximum (FWHM) uncertainty. ConclusionsAmong the investigated chambers, the IBA RAZOR Nano Chamber was found to be an excellent choice for narrow beam output factor measurement since it requires minimum correction (in line with IAEA TRS-483 recommendations). This is caused by its tiny size and tissue equivalence materials which produce minimum volume averaging and fluence perturbation.

Dosimetric accuracy of a cross-calibration coefficient for plane-parallel ionization chamber obtained in low-energy electron beams using various cylindrical dosimeters

Abstract Introduction: The accuracy of the cross-calibration procedure depends on ionization chamber type, both used as reference one and under consideration. Also, the beam energy and phantom medium could influence the precision of cross calibration coefficient, resulting in a systematic error in dose estimation and thus could influence the linac beam output checking. This will result in a systematic mismatch between dose calculated in treatment planning system and delivered to the patient. Material and methods: The usage of FC65-G, CC13 and CC01 thimble reference chambers as well as 6, 9, and 15 MeV electron beams has been analyzed. A plane-parallel PPC05 chamber was calibrated since scarce literature data are available for this dosimeter type. The influence of measurement medium and an effective point of measurement (EPOM) on obtained results are also presented. Results: Dose reconstruction precision of ~0.1% for PPC05 chamber could be obtained when cross-calibration is based on a thimble CC13 chamber. Nd,w,Qcross obtained in beam ≥ 9MeV gives 0.1 – 0.5% precision of dose reconstruction. Without beam quality correction, 15 MeV Nd,w,Qcross is 10% lower than Co-60 Nd,w,0. Various EPOM shifts resulted in up to 0.6% discrepancies in Nd,w,Qcross values. Conclusions: Ionization chamber with small active volume and tissue-equivalent materials supplies more accurate cross-calibration coefficients in the range of 6 – 15 MeV electron beams. In the case of 6 and 9 MeV beams, the exact position of an effective point of measurement is of minor importance. In-water cross-calibration coefficient can be used in a solid medium without loss of dose accuracy.

Feasibility of operating a millimeter-scale graphite calorimeter for absolute dosimetry of small-field photon beams in the clinic.

To characterize and build a cylindrically layered graphite calorimeter the size of a thimble ionization chamber for absolute dosimetry of small fields. This detector has been designed in a familiar probe format to facilitate integration into the clinical workflow. The feasibility of operating this absorbed dose calorimeter in quasi-adiabatic mode is assessed for high-energy accelerator-based photon beams. This detector, herein referred to as Aerrow MK7, is a miniaturized version of a previously validated aerogel-insulated graphite calorimeter known as Aerrow. The new model was designed and developed using numerical methods. Medium conversion factors from graphite to water, small-field output correction factors, and layer perturbation factors for this dosimeter were calculated using the EGSnrc Monte Carlo code system. A range of commercially available aerogel densities were studied for the insulating layers, and an optimal density was selected by minimizing the small-field output correction factors. Heat exchange within the detector was simulated using a five-body compartmental heat transfer model. In quasi-adiabatic mode, the sensitive volume (a 3mm diameter cylindrical graphite core) experiences a temperature rise during irradiation on the order of 1.3mK·Gy-1 . The absorbed dose is obtained by calculating the product of this temperature rise with the specific heat capacity of the graphite. The detector was irradiated with 6MV ( =63.5%) and 10MV ( =71.1%) flattening filter-free (FFF) photon beams for two field sizes, characterized by dimensions of 2.16 and 11.0cm. The dose readings were compared against a calibrated Exradin A1SL ionization chamber. All dose values are reported at in water. The field output correction factors for this dosimeter design were computed for field sizes ranging from =0.54 to 11.0cm. For all aerogel densities studied, these correction factors did not exceed 1.5%. The relative dose difference between the two dosimeters ranged between 0.3% and 0.7% for all beams and field sizes. The smallest field size experimentally investigated, =2.16cm, which was irradiated with the 10MV FFF beam, produced readings of 84.4cGy (±1.3%) in the calorimeter and 84.5cGy (±1.3%) in the ionization chamber. The median relative difference in absorbed dose values between a calibrated A1SL ionization chamber and the proposed novel graphite calorimeter was 0.6%. This preliminary experimental validation demonstrates that Aerrow MK7 is capable of accurate and reproducible absorbed dose measurements in quasi-adiabatic mode.

Evaluation of two-dimensional dosimeter based on thallium (I) bromide for measurement of surface dose in radiotherapy

In radiotherapy, side effects can occur on the skin because each patient's skin has a different level of sensitivity to radiation. Therefore, in clinical practice, point dose is measured to verify skin doses, although there is a problem, in that it is difficult to analyse the dose distribution for the body surface area. In this study, we constructed a film dosimeter based on polycrystalline thallium (I) bromide, evaluated its performance, and analysed the electrical properties according to the number of bending cycles to evaluate the mechanical flexibility. Further, an array dosimeter was constructed, and the dose distribution was analysed to evaluate the clinical effectiveness. As a result of the performance evaluation, reproducibility, linearity, and dose-rate dependence all satisfied the evaluation criteria. Percent depth dose showed similar results, and a difference of 0.027 when the slope of the depth in the range of 20–10 cm was compared with that of the thimble chamber. With regard to mechanical flexibility, as the number of bending cycles increased, the sensitivity tended to decrease. The clinical efficacy evaluation showed that the difference in dose distribution between the boundary field and non-field was approximately ≥ 40%, which decreased exponentially as the distance from the boundary field increased. The gap dose distribution showed an average difference of approximately ≥ 47% compared with the beam profile central axis. This suggests the possibility of application as a radiotherapy digital dosimeter, as well as the possibility of analysing the dose distribution on the body surface.

Evaluation of monoxide film-based dosimeters for surface dose detection in electron therapy.

Generally, electron therapy is applied to tumors on or close to the skin surface. However, this causes a variety of skin-related side effects. To alleviate the risk of these side effects, clinical treatment uses skin dosimeters to verify the therapeutic dose. However, dosimeters suffer from poor accuracy, because their attachment sites are approximated with the help of naked eyes. Therefore, a dosimeter based on a flexible material that can adjust to the contours of the human body is required. In this study, the reproducibility, linearity, dose-rate dependence, and percentage depth ionization (PDI) of PbO and HgO film-based dosimeters are evaluated to explore their potential as large-scale flexible dosimeters. The results demonstrate that both dosimeters deliver impressive reproducibility (within 1.5%) and linearity (≥ 0.9990). The relative standard deviations of the dose-rate dependence of the PbO and HgO dosimeters were 0.94% and 1.16% at 6 MeV, respectively, and 1.08% and 1.25% at 9 MeV, respectively, with the PbO dosimeter outperforming the 1.1% of existing diodes. The PDI analysis of the PbO and HgO dosimeters returned values of 0.014 cm (–0.074 cm) and 0.051 cm (–0.016 cm), respectively at 6 MeV (9 MeV) compared to the thimble chamber and R50. Therefore, the maximum error of each dosimeter is within the allowable range of 0.1 cm. In short, the analysis reveals that the PbO dosimeter delivers a superior performance relative to its HgO counterpart and has strong potential for use as a surface dosimeter. Thus, flexible monoxide materials have the necessary qualities to be used for dosimeters that meet the requisite quality assurance standards and can satisfy a variety of radiation-related applications as flexible functional materials.

Efficacy of several compensating materials on the high-energy electron beam irradiation

This study aimed to use different dose verification methods Treatment Planning System (TPS), thimble ionization chamber and two-dimensional matrix ionization chamber (MatriXX) to explore the compensating effects of several materials on high-energy electron beam irradiation. In this study, we positioned plexiglass panels, wet medical gauze, colloidal compensatory material and Vaseline compensator, all of the same thickness, flat on the surface of a MatriXX. The above four materials were placed on the surface of a thimble ionization chamber and irradiated with a linear accelerator. Under the same irradiation parameters, the compensation dose of of the tested materials was worked out by a dosimeter, which was connected at the thimble ionization chamber. Under vertical irradiation from the linear accelerator, the accelerator used the same irradiation conditions for the four materials, which were flat on the surface of the MatriXX, then determined the distribution of compensation doses by the soft system matching with the MatriXX. The results showed that the compensation dose of moist medical gauze was lower than the other three materials, the compensation dose of Plexiglass plate, colloidal compensatory material and vaseline compensation were approximate. The dose compensation effect of the vaseline compensation material made by our research group was close to the plexiglass plate and colloidal compensatory material used in radiotherapy in real life. Its physical and chemical properties are stable, easy to manufacture and store, and economical and practical.

Dosimetric impact and modeling of prone breast positioning device and couch structures

In prone breast radiation, as the medial tangential beam usually passes through the immobilization board and couch, it is necessary to quantify the attenuation effect and the potential skin dose enhancement from these external structures. The prone breast board studied consists of an insert on which the contralateral breast rests and a board base indexed to the couch. Two different Varian couch systems were also studied. Transmission factors (TF) of the board were measured using a Farmer chamber at 4 cm depth. Couch TFs were measured using a thimble chamber centered in a cylindrical phantom. A custom support model was created in the treatment planning system (TPS). TFs were then computed in the TPS for comparison. Selected clinical plans were recomputed in the TPS incorporating external structures for target coverage evaluation. The correction for the attenuation effect in the TPS was also demonstrated. Skin dose effects were evaluated using a Markus parallel plate chamber with a 1 mm buildup cap. Measured insert TFs ranged 0.976 to 0.983 for 6 MV and 0.990 to 0.999 for 23 MV. Board base TFs ranged 0.979 to 0.985 for 6 MV and 0.989 to 0.998 for 23 MV. TPS values agreed within 0.9% and 0.5% for the insert and board base, respectively. Assigned Hounsfield units (HUs) providing the best agreement were 200, −100, and −900 for the insert, the board “base shell” and “base inside,” respectively. Varian Exact Couch and Exact IGRT Couch TFs varied with respect to couch angle, with minimum values of 0.837 and 0.956, respectively, for 6 MV. The clinical treatment volume (CTV) and whole breast receiving 95% of the prescription dose (CTV-V95 and WB-V95) of selected patients demonstrated reduced coverage due to attenuation of external structures. Close proximity to the base increased skin dose by up to 25% to 30%. Contacting the insert increased skin dose by 65% to 93% for 6 MV and 117% to 157% for 23 MV, respectively. Results have shown reduced coverage by attenuating external structures. Proper modeling of immobilization devices and couch structures in the TPS should be implemented for accurate dose calculation. Increased surface doses were observed due to direct contact to the insert or close proximity to the base. Further study is required to quantify such a skin dose enhancement effect and its correlation to clinically apparent skin effects and toxicity.

Dosimetry evaluation of the GammaPod stereotactic radiosurgery device based on established AAPM and IAEA protocols.

The GammaPod is a novel dedicated prone breast stereotactic radiosurgery (SRS) device recently developed at the University of Maryland Medical Center. This device utilizes multiple rotating Co-60 sources to create highly conformal dose distributions for breast treatments, including boosts, partial breast irradiation, or presurgery SRS. However, due to its small field sizes and nonstandard geometry, existing calibration protocols cannot be directly applied. In this study, we adapt and implement the American Association of Physicists in Medicine Task Group 21 (TG-21) and International Atomic Energy Agency (IAEA) Technical Report Series 483 (TRS 483) protocols for reference dose measurements for the GammaPod. This represents the first published dosimetric investigation GammaPod and is meant to serve as a reference to future users commissioning and calibrating these devices. Reference dose measurements were performed following the TG-21/IAEA TRS 483 protocols using an ADCL-calibrated Exradin A1SL thimble chamber in a polymethyl methacrylate (PMMA) breast-mimicking phantom. Monte Carlo calculations and measurements were also performed in water to determine chamber-specific quality conversion factor converting reference field size (fref ) to machine-specific field sizes (fmsr ) (25-mm) as well as , the conversion factor from the (fmsr ) to the clinical field size (fclin ) (15mm). Verification was performed using the thermoluminescent dosimeter remote monitoring service from the Imaging and Radiation Oncology Core (IROC) in Houston, TX. The (fref ) to (fmsr ) chamber-specific factor was 0.992 while the (fmsr ) to (fclin ) chamber-specific factor was 1.014. The radiation absorbed dose to water measured in the PMMA phantom based on the TG-21/IAEA TRS 483 formalism agreed with IROC values to within 1% and 2% for the 25- and 15-mm collimators, respectively. We successfully implemented the TG-21 and TRS 483 reference dosimetry protocols for the GammaPod. These results show agreement between measurements performed with different reference dosimetry protocols and independent thermoluminescent measurements.

Extraction of phytochemicals and study of its antimicrobial and antioxidant activity of Helicteres isora L.

BackgroundHerbal medicines are used for the treatment of many infectious diseases throughout the history of mankind. The increasing antibiotic resistance exhibited by microorganisms has led to the phytochemical screening of medicinal plants for antimicrobial activity. Many infections can be treated by phytochemicals possessing potent antibacterial efficiency. The pharmacological activities of any plant is because of the presence of primary metabolites, secondary metabolites and its secretary products, comprising the phenolic compounds, alkaloids, tannins, saponins, carbohydrates, glycosides, flavanoids, steroids, etc. Helicteres isora L. is a medicinal plant used in traditional medicine. The root and bark are expectorant, demulcent, astringent to the bowels, antigalactagogue, lessen gripping; a cure for scabies when applied topically.MethodSoxhlet extraction or hot continuous extraction: In this method, finely ground sample was placed in thimble chamber of the Soxhlet apparatus. Extraction solvents was heated in the bottom flask, vaporizes into the sample thimble, condenses in the condenser and drip back. When the liquid content reaches the siphon arm, the liquid contents emptied into the bottom flask again and the process is continued. Gram positive bacteria Staphylococcus aureus NCIM 2079, Gram negative bacteria Escherichia coli NCIM 2109, Fungi (Yeast) Candida albicans NCIM 3471, Fungi (Mould) Aspergillus niger NCIM 545 are used as reference Strain for antimicrobial activity. Antioxidant activity was tested by DPPH method.ResultChloroform Extracts of leaves, Stem and fruits of Helicteres isora L. exhibited potent antimicrobial activity against Escherichia coli, Staphylococcus aureus, Candida albicans and Aspergillus, niger. The stem extract showed highest antimicrobial activity against Escherichia coli with zone of inhibition of 19.24 mm and leaves lowest activity against Candida albicans with zone of inhibition of 11.26 mm. Antioxidant activity was tested by DPPH method.ConclusionThe use of herbal crude drugs, in tracts and their remedies have significantly increased throughout the world. Efforts must be made to ensure safe, effective and affordable treatments for wide range of diseases by traditional methods which use locally available medicinal plants. The scientific and authentic researches on these aspects are to be done in order to exploit traditional knowledge of medicinal plants.

Evaluation of polycrystalline thallium (I) bromide dosimeter for radiotherapy quality assurance

In radiotherapy, dose assessment is performed as part of quality assurance (QA) to verify the dose accuracy of the treatment administered to patients. Various dosimeters are used depending on the part of the body and the desired purpose, and tools such as the thermoluminescent dosimeter (TLD) and semiconductor dosimeter are used as in vivo dosimeters. However, TLDs have demonstrated measurement errors of approximately 11.8%, and the diodes used in semiconductor dosimeters fluctuate in performance owing to radiation damage. Consequently, various photoconductor materials are being researched to replace diodes. In particular, Thallium (I) bromide (TlBr) is being used as a substitute material for semiconductors because of its low cost and high performance. Therefore, this study determined the optimised performance of TlBr, which is a potential photoconductor material to replace other materials commercially, using the particle-in-binder deposition method. Upon evaluating the reproducibility and linearity, all sensors demonstrated remarkable performance at 6 MV and 15 MV as the results demonstrated a relative standard deviation (RSD) less than 1.5% and R2 value above 0.9998, which were the evaluation criteria. A 3:1 material ratio was chosen, as the performance demonstrated in this case was superior to that of other material ratios. The results of the monitor unit rate dependence and percent depth dose (PDD) were compared with those of silicon diode and thimble chambers, which are widely used for dose measurement. According to the monitor unit rate dependence results, the RSD was 0.66% at 6 MV and 0.27% at 15 MV with the standard set at 100 MU, indicating that the diode was outperformed by less than 1%. Similar results were obtained for the PDD with the ion chamber, and the surface dose required for in vivo dose measurement indicated an error within 8.67%. Therefore, this study provides basic data for polycrystalline TlBr dosimeters, which can replace the existing semiconductor dosimeters for QA.

Detector response in the buildup region of small MV fields.

The model used to calculate dose distributions in a radiotherapy treatment plan relies on the data entered during beam commissioning. The quality of these data heavily depends on the detector choice made, especially in small fields and in the buildup region. Therefore, it is necessary to identify suitable detectors for measurements in the buildup region of small fields. To aid the understanding of a detector's limitations, several factors that influence the detector signal are to be analyzed, for example, the volume effect due to the detector size, the response to electron contamination, the signal dependence on the polarity used, and the effective point of measurement chosen. We tested the suitability of different small field detectors for measurements of depth dose curves with a special focus on the surface-near area of dose buildup for fields sized between 10×10 and 0.6×0.6cm2 . Depth dose curves were measured with 14 different detectors including plane-parallel chambers, thimble chambers of different types and sizes, shielded and unshielded diodes as well as a diamond detector. Those curves were compared with depth dose curves acquired on Gafchromic film. Additionally, the magnitude of geometric volume corrections was estimated from film profiles in different depths. Furthermore, a lead foil was inserted into the beam to reduce contaminating electrons and to study the resulting changes of the detector response. The role of the effective point of measurement was investigated by quantifying the changes occurring when shifting depth dose curves. Last, measurements for the small ionization chambers taken at opposing biasing voltages were compared to study polarity effects. Depth-dependent correction factors for relative depth dose curves with different detectors were derived. Film, the Farmer chamber FC23, a 0.13cm3 scanning chamber CC13 and a plane-parallel chamber PPC05 agree very well in fields sized 4×4 and 10×10cm2 . For most detectors and in smaller fields, depth dose curves differ from the film. In general, shielded diodes require larger corrections than unshielded diodes. Neither the geometric volume effect nor the electron contamination can account for the detector differences. The biggest uncertainty arises from the positioning of a detector with respect to the water surface and from the choice of the detector's effective point of measurement. Depth dose curves acquired with small ionization chambers differ by over 15% in the buildup region depending on sign of the biasing voltage used. A scanning chamber or a PPC40 chamber is suitable for fields larger than 4×4cm2 . Below that field size, the microDiamond or small ionization chambers perform best requiring the smallest corrections at depth as well as in the buildup region. Diode response changes considerably between the different types of detectors. The position of the effective point of measurement has a huge effect on the resulting curves, therefore detector specific rather than general shifts of half the inner radius of cylindrical ionization chambers for the effective point of measurement should be used. For small ionization chambers, averaging between both polarities is necessary for data obtained near the surface.

Development of CT Pelvimetry Using Deep Learning Based Reconstruction

X-ray pelvimetry is typically performed for the diagnosis of the cephalopelvic disproportion (CPD). The purpose of this study was to assess the utility of new computed tomography (CT) reconstruction "deep learning based reconstruction (DLR) " in ultra-low dose CT pelvimetry. CT pelvimetry was performed 320-row CT. All CT images were reconstructed with and without DLR and transferred for workstation to processing martius and guthmann view. Radiologist and obstetrician-gynecologist subjectively ranked overall image quality of each CT image from the best to the worst. Exposure dose of the CT pelvimetry used a following calculated value, displayed CT dose index (CTDI) vol multiplied by measured value using the thimble chamber and pelvic phantom, and of the X-ray pelvimetry used Japan-Diagnositic Refernce Levels 2015 as a reference, were compared. 3D images obtained from CT pelvimetry with DLR showed accurate biparietal diameter and obstetric conjugate as compared to without DLR. Radiation dose of CT pelvimetry is 0.39 mGy, of X-ray pelvimetry is 1.18 mGy, respectively. Conculusion: Although the visualizing high contrast object, such as bone morphology, is likely to reduce exposure dose in CT examination generally, DLR enable to further dose reduction to keep image quality. 3D image processing from CT pelvimetry solves the problem of expansion rate in X-P pelvimetry and provide accurate measurements. Furthermore, CT pelvimetry can undergo more comfortable position for Pregnant Woman in Labor.

Investigation of LINAC Structural Effects on Photoneutron Specified Parameters Using FLUKA code

Introduction: The utilization of high-energy photons in the medical linear accelerator can lead to photoneutron production. This study aimed to evaluate the effect of the physical components of the head, including flattening filter (FF) andmultileaf collimator (MLC), as well as the dependence of therapeutic field size on the photoneutron spectrum, dose, and flux. Material and Methods: The present study reported the simulation of the fundamental linac head components of the Varian Clinac 2100 performing in X-ray mode with 18 MV energy by the FLUKA code. The percentage depth dose and lateral dose profile were measured using a PTW thimble chamber to ensure the simulation reliability. Results: Photoneutron spectrum analysis indicated that neutrons with highest relative biological effectiveness were delivered to the phantom surface, and opening the field from 0×0 to 40×40 shifted the spectrum by 24.545% to the higher energies. The target and the vicinity parts played the most prominent roles in neutron contamination. The relationship between the field size and the photoneutron dose was non-linear, and it reached a peak of 20×20 . Although using small fields formed by the MLC contribute to a lower dose compared to those shaped by the jaws, MLC-equipped machines result in 21.98% higher dose. Moreover, the flattening filter removal unexpectedly increased the isocenter photoneutron dose by 11.63%. This undesirable dose can be up to 2.54 mSv/Gy for the reference field at the isocenter while the out-of-field dose is about 0.5 mSv/Gy for most of the field dimensions. Conclusion: As a result, it is critical to consider this unwanted absorbed dose, which is seriously influenced by the implemented therapeutic conditions.

Pretreatment Dose Verification in Volumetric Modulated Arc Therapy Using Liquid Ionization Chamber.

Purpose:The purpose of the present study was to evaluate the practicability of liquid ionization chamber (LIC) for pretreatment dose verification of the advanced radiotherapy techniques such as volumetric modulated arc therapy (VMAT).Materials and Methods:The dosimetric characteristics of LIC such as repeatability, sensitivity, monitor unit linearity, dose rate dependence, angular dependence, voltage-current response, and output factors were investigated in 6 MV therapeutic X-ray beams. The LIC was cross-calibrated against 0.125-cc air-filled thimble ionization chamber. A dedicated dosimetry insert made up of Perspex to incorporate the LIC at proper location in the intensity-modulated radiation therapy thorax phantom was locally fabricated. The collection efficiency and ion recombination correction factor was determined using the two-dose rate method. Pretreatment dose verification measurement of VMAT treatment plans were carried out using the liquid ionization chamber as well as small volume (0.125 cc) air-filled thimble ionization chamber. The measured dose values by the two dosimeters and TPS calculated dose at a given point were compared.Results:The relative percentage differences between the TPS calculated and measured doses were within ± 1.57% for LIC and ± 2.21% for 0.125 cc ionization chamber, respectively.Conclusions:The measured dose values by the two dosimeters and TPS calculated dose at a given point were found comparable suggesting that the LIC could be a good choice of dosimeter for pretreatment dose verification in VMAT.

P123] Reference dosimetry in flattening filter-free beams: Experience with IPSM-1990/IPEM Topical Report 1–2016 and IAEA TRS-398

Introduction Flattening Filter Free beams have become widespread in current radiotherapy practice; therefore, an assessment of different dosimetric approaches is necessary. The British Institute of Physics and Engineering in Medicine (IPEM) Topical Report 1 (2016) recommendations include a FFF spectral correction. IAEA TRS-398 has not yet included this type of correction. Materials and methods Three Varian Truebeam linacs were used, with 6FFF and 10FFF beams. A PTW Semiflex thimble chamber and an IBA CCU electrometer were used for PDD measurements for IPSM 1990, a PTW Roos and an IBA CCU electrometer were used for IAEA TRS-398 PDD measurements. For absolute dose measurements, a NE 2571 Farmer chamber and a PTW Unidos E electrometer were used. Absolute dose was measured following both protocols recommendations: 5 cm (IPSM 1990) and 10 cm (TRS-398) deep. Polarity and recombination factors were also measured at each recommended depth. Our secondary standard chamber and electrometer were calibrated at the National Physical Laboratory (UK), and all our measurements are subsequently traceable to NPL. Ion chamber intercomparisons, using the replacement method recommended for FFF beams by IPEM Topical Report 1, were performed with our secondary standard (NE 2561) and NE 2571 at each of the recommended depth and for both beam qualities. Results NPL N D , w calibration factors were used for each beam quality for IPSM 1990; whereas for TRS-398, the recommended option of using N D , w for Co-60 along with its tabulated k q factors was utilized. TRS-398 and IPSM 1990/IPEM Topical Report 1 absolute dose measurements are within 0.5%, falling well within uncertainty estimates. Conclusions A detailed and thorough absolute dose study was carried out according to the two above mentioned codes of practice: correction factors, ion chamber intercomparisons and absolute dose measurements at recommended depths. All experimental results are accompanied by a rigorous uncertainty analysis, according to ISO GUM. Our results for 6 FFF and 10 FFF show that, within uncertainty, absorbed dose to water according to both codes of practice agree.

OA154] A comparison of dosimetry codes practice for FFF beams: IAEA TRS-398 and IPSM 1990

Purpose Flattening Filter Free beams have become widespread in current radiotherapy practice; therefore, an assessment of different dosimetric approaches is necessary. The British Institute of Physics and Engineering in Medicine (IPEM) recommendations, Topical Report 1 (2016), include a FFF spectral correction. IAEA TRS-398 has not yet included this type of correction. A comparison of IAEA TRS-398 and the British IPSM 1990 codes of practice (including recent IPEM recommendations) was carried out. Methods Three Varian Truebeam linacs were used, with 6FFF and 10FFF beams. A PTW Semiflex thimble chamber and an IBA CCU electrometer were used for PDD measurements for IPSM 1990, a PTW Roos and an IBA CCU electrometer were used for IAEA TRS-398 PDD measurements. For absolute dose measurements, a NE 2571 Farmer chamber and a PTW Unidos E electrometer were used. Absolute dose was measured following both protocols recommendations: 5cm (IPSM 1990) and 10 cm (TRS-398) deep. Polarity and recombination factors were also measured at each recommended depth. Our secondary standard chamber and electrometer were calibrated at the National Physical Laboratory (UK), and all our measurements are subsequently traceable to NPL. Ion chamber intercomparisons, using the replacement method recommended for FFF beams by IPEM Topical Report 1, were performed with our secondary standard (NE 2561) and NE 2571 at each of the recommended depth and for both beam qualities. Results NPL N D , w calibration factors were used for each beam quality for IPSM 1990; whereas for TRS-398, the recommended option of using N D , w for Co-60 along with its tabulated k q factors was utilized. TRS-398 and IPSM 1990/IPEM Topical Report 1 absolute dose measurements are within 0.5%, falling well within uncertainty estimates. Conclusions A detailed and thorough absolute dose study was carried out according to the two above mentioned codes of practice: correction factors, ion chamber intercomparisons and absolute dose measurements at recommended depths. All experimental results are accompanied by a rigorous uncertainty analysis, according to ISO GUM. Our results for 6 FFF and 10 FFF show that, within uncertainty, absorbed dose to water according to both codes of practice agree.

Study of different dose calculation algorithms for the phantom of metallic implants

Objective To compare the dose difference between the 12-bit and 16-bit CT images containing metallic implants calculated by different algorithms. Methods The titanium alloy rod was inserted into the phantom and subject to CT scan and then the 12-bit and 16-bit CT images were reconstructed. The CT images were online transmitted to the Monaco planning system and a 0 ° of single field was designed. The dose distribution was calculated by PB (Pencil Beam), CC (Collapsed Cone) and MC (Monte Carlo) algorithms, respectively. The CT-ED curve was expanded and the dose was recalculated. The depth dose curve through the center of the metallic implants along with the direction of the field was obtained by using the Matlab 8.3 statistical software. The dose distribution curves between 12-bit and 16-bit CT images calculated by different algorithms and the dose difference of varying distances between the incident and the exit surfaces of metallic implants were statistically compared. The dose was measured by thimble chamber. Results The 16-bit CT images accurately read the CT values of the metallic implants. After the CT-ED curve was expanded, the dose on the incident surface of metallic implant was reduced by 5.43% and that on the exit surface was increased by 25.56% calculated by PB algorithm compared with MC algorithm. The dose on the posterior exit surface was higher than that of MC algorithm. The dose on the incident surface of metallic surface was decreased by 4.5%, whereas that on the exit surface was reduced by 4.31% using CC algorithm. The dose on the posterior exit surface was more significantly reduced. The calculated values by MC algorithm were the most close to the measured values. Conclusions Application of 16-bit CT image, CT-ED curve expansion of the treatment planning system combined with MC algorithm can enhance the accuracy of dose calculation for the patients containing metallic implants during radiotherapy. Key words: Monte Carlo; Radiotherapy dose; Metallic implant; CT value

Status of ionization chambers calibration for radiation therapy in Brazil

CNEN makes a constant effort to keep updated with international standards and national needs to strengthen the radiological protection status of the country. The guidelines related to radiation treatment facilities have been revised in the last five years in order to take in consideration the most relevant aspects of the growing technology as well as to mitigate the accidents or incidents observed in practice. Hence, clinical dosimeters have gained special importance as significant items in Brazilian regulation. In the present work we discuss the importance of inspections from the point of view of equipment dosimetry and instruments quality control. The dosimeter sets based on thimble and well ionization chambers need periodic calibration, and this calibration becomes a fundamental task in order to guarantee the dose prescribed-delivered to patients.Thus Brazilian guidelines enforce the need of at least two sets of clinical dosimeters with thimble chambers calibrated and one set of electrometer with well ionization chamber for hdr equipment. We call attention to the fact that inspections are a very valuable tool in order to enforce the application of guidelines around the country both by enlightening the weaker aspects of facilities concerning radiological protection and by stating in loco that reasons which lead the regulatory body to enforce such guidelines items.