Source-to-surface Distance Research Articles

Comparison of Calculated Percentage Depth Doses at Extended Source-to-Surface Distance for 6 MV And 15 MV Photon Beam of a Linear Accelerator


 
 
 
 Background: Research findings from percentage depth dose (PDD) are crucial in evaluating patient doses received in radiation therapy.
 Objective: To compare calculated percentage depth doses at an extended source-to-surface distance (SSD) for 6 MV and 15 MV photon beams of a linear accelerator.
 Methodology: Measured PDD values of the 100 cm source to surface distance (SSD) and calculated values at extended SSDs for 6 MV and 15 MV photon beams of an Elekta NHA SLi 1998 linear accelerator were analyzed. The PDD data was collected by placing ionization chamber inside water phantom for depths ranging from z = 0 - 30 cm in a water phantom and using a square field sizes of 10 x 10 cm2. Photon energies of 6 MV and 15 MV were used for the measurement, with both gantry and collimator angles fixed at zero degree. PDD was calculated at extended SSD of 110 cm, 120 cm, 130 cm, and 140 cm from the measured PDD values of 100 cm SSD for both 6 MV and 15 MV photon energies using Mayneord factor.
 Results: The depth dose maximum (Dmax) for field size 10 x 10 cm2 for 6 MV and 15 MV photon energies were 1.62 cm and 2.65 cm, respectively and the PDD at 10 cm (D10) were 67.9% and 75.9%, respectively. The mean deviation of the calculated PDD at extended SSDs was found to be between 0.2% and 1%.
 Conclusion: The calculated PDD values at extended SSDs are considered suitable for clinical use at all clinically relevant depths and field sizes.
 
 
 
 

Three-dimensional printers applied for the production of beam blocks in total body irradiation treatment.

PurposeTotal body irradiation (TBI) in extended source surface distance (SSD) is a common treatment technique before hematopoietic stem cell transplant. The lungs are organs at risk, which often are treated with a lower dose than the whole body.MethodsThis can be achieved by the application of blocks. Three‐dimensional (3D) printers are a modern tool to be used in the production process of these blocks.ResultsWe demonstrate the applicability of a specific printer and printing material, describe the process, and evaluate the accuracy of the product.ConclusionThe blocks and apertures were found to be applicable in clinical routine.

A novel technique for peripheral dose measurements in external beam radiation therapy

Abstract Introduction: In radiotherapy, the dose delivered outside the field is known as peripheral dose (PD). In this study, we have attempted to develop a dataset using the PD values measured with a two-dimensional array, IMatrixx. Methods: The IMatrixx was used to measure the PD up to a distance of 45 cm from the field edge, in a Varian Clinac 2100-C machine. Solid water slabs and water phantom were used to get the required geometry for the PD measurements. The measurements were done for different field sizes, collimator angles, source to surface distance (SSD) and depths. The influence of gantry angles and photon energies on the PD was studied. The surface dose measurements were carried out using thermoluminescent detectors (TLD). Results: The dataset shows that the PD increased significantly with field size and depth and its increase was insignificant for collimator rotation and SSD. The influence of gantry angle was less pronounced at dmax than at the surface. The TLD measurements at the surface of patients were in agreement with the IMatrixx measurements. Conclusions: The IMatrixx can be used for the generation of PD values and it is less time-consuming, accurate, and commonly available in all radiotherapy departments.

Dose calculations for preclinical radiobiology experiments conducted with single-field cabinet irradiators.

To provide percentage depth dose (PDD) data along the central axis for dosimetry calculations in small-animal radiation biology experiments performed in cabinet irradiators. The PDDs are provided as a function of source-to-surface distance (SSD), field size, and animal size. The X-ray tube designs for four biological cabinet irradiators, the RS2000, RT250, MultiRad350, and XRAD320, were simulated using the BEAMnrc Monte Carlo code to generate 160, 200, 250, and 320kVp photon beams, respectively. The 320kVp beam was simulated with two filtrations: a soft F1 aluminium filter and a hard F2 thoraeus filter made of aluminium, tin, and copper. Beams were collimated into circular fields with diameters of 0.5-10cm at SSDs of 10-60cm. Monte Carlo dose calculations in 1-5-cm diameter homogeneous (soft tissue) small-animal phantoms as well as in heterogeneous phantoms with 3-mm diameter cylindrical lung and bone inserts (rib and cortical bone) were performed using DOSXYZnrc. The calculated depth doses in three test-cases were estimated by applying SSD, field size, and animal size correction factors to a reference case (40-cm SSD, 1-cm field, and 5-cm animal size), and these results were compared with the specifically simulated (i.e., expected) doses to assess the accuracy of this method. Dosimetry for two test-case scenarios of 160 and 250 kVp beams (representative of end-user beam qualities) was also performed, whereby the simulated PDDs at two different depths were compared with the results based on the interpolation from reference data. The depth doses for three test-cases calculated at 200, 320kVp F1, and 320kVp F2 with half value layers (HVLs) ranging from ∼0.6to 3.6mm Cu, agreed well with the expected doses, yielding dose differences of 1.2%, 0.1%, and 1.0%, respectively. The two end-user test-cases for 160 and 250 kVp beams with respective HVLs of ∼0.8 and 1.8mm Cu yielded dose differences of 1.4% and 3.2% between the simulated and the interpolated PDDs. The dose increase at the bone-tissue proximal interface ranged from 1.2 to 2.5 times the dose in soft tissue for rib and 1.3 to 3.7 times for cortical bone. The dose drop-off at 1-cm depth beyond the bone ranged from 1.3% to 6.0% for rib and 3.2% to 11.7% for cortical bone. No drastic dose perturbations occurred in the presence of lung, with lung-tissue interface dose of>99% of soft tissue dose and<3% dose increase at 1-cm depth beyond lung. The developed dose estimation method can be used to translate the measured dose at a point to dose at any depth in small-animal phantoms, making it feasible for preclinical calculation of dose distributions in animals irradiated with cabinet-style irradiators. The dosimetric impact of bone must be accurately quantified as dramatic dose perturbations at and beyond the bone interfaces can occur due to the relative importance of the photoelectric effect at kilovoltage energies. These results will help improve dosimetric accuracy in preclinical experiments.

Experimental validation of a linac head Geant4 model under a grid computing environment

Background and purpose . This work aims to present the strategy to simulate a clinical linear accelerator based on the geometry provided by the manufacturer and summarize the corresponding experimental validation. Simulations were performed with the Geant4 Monte Carlo code under a grid computing environment. The objective of this contribution is reproducing therapeutic dose distributions in a water phantom with an accuracy less than 2%. Materials and methods. A Geant4 Monte Carlo model of an Elekta Synergy linear accelerator has been established, the simulations were launched in a large grid computing platform. Dose distributions are calculated for a 6 MV photon beam with treatment fields ranging from 5 × 5 cm2 to 20 × 20 cm2 at a source—surface distance of 100 cm. Results. A high degree of agreement is achieved between the simulation results and the measured data, with dose differences of about 1.03% and 1.96% for the percentage depth dose curves and lateral dose profiles, respectively. This agreement is evaluated by the gamma index comparisons. Over 98% of the points for all simulations meet the restrictive acceptability criteria of 2%/2 mm. Conclusion. We have demonstrated the possibility to establish an accurate linac head Monte Carlo model for dose distribution simulations and quality assurance. Percentage depth dose curves and beam quality indices are in perfect agreement with the measured data with an accuracy of better than 2%.

Evaluation of the virucidal efficacy of Klaran UVC LEDs against surface-dried norovirus.

Human norovirus (HuNoV) is a highly contagious pathogenic virus that is transmitted through contaminated food, water, high-touch surfaces and aerosols. Globally, there are an estimated 685 million infections annually due to norovirus, including 200 million affecting children under the age of 5. HuNoV causes approximately 50, 000 child deaths per year and costs an estimated USD $60 billion annually in healthcare. This study sought to determine the inactivation profile of ultraviolet subtype C (UVC) against norovirus using a UVC light-emitting diode (LED) array, KL265-50V-SM-WD. The array emitted radiation at 269 nm peak wavelength and a measured fluence of 1.25 mW cm−2 at a 7 cm source–surface distance. Since the HuNoV is not cultivable, the study utilized feline calicivirus (FCV) ATCC VR-782, a recommended surrogate as challenge organism. The test followed modified ASTM E2197. Assessment of virus inactivation was performed using a plaque assay method. With irradiance at a UVC dose of 22.5 mJ cm−2, the study obtained 99.9 % virus reduction (3 log reduction). The results demonstrate that the UVC LED array can provide effective inactivation of HuNoV.

Design and Evaluation of Structural Shielding of a Typical Radiotherapy Facility Using EGSnrc Monte Carlo Code.

Purpose:This study aimed to evaluate the shielding integrity of a typical radiotherapy facility using the Monte Carlo (MC) method.Materials and Methods:EGSnrc MC code was used to design a radiotherapy bunker with appropriate materials and thicknesses. A concrete density of 2.36 g/cm3 was used as a shielding material for primary and secondary barriers. The lead slab was used in the entrance door. The complex geometries of the bunker were modeled by using the egs++ application code embedded in the software. Phase-space generated from a linac machine built with BEAMnrc codes was used as a source of 18 MV X-ray beam set at 100 cm source–surface distance with a field size of 40 cm × 40 cm. Energy deposited in each geometrical region was evaluated and analyzed.Results:Energy deposited at the entrance door, supervised and controlled areas were found to be approximately 0%. No significant difference in the energy deposition on the geometries was observed when the gantry angles were set at either 90° or 270° (P = 1).Conclusion:The findings in this study revealed that the EGSnrc MC code can be used as a veritable tool in the design and evaluation of structural shielding efficiency in a radiotherapy facility.

Measurement of Neutron Dose Equivalent within and Outside of a LINAC Treatment Vault Using a Neutron Survey Meter

This work concerns neutron doses associated with the use of a Siemens Primus M5497 electron accelerator, which is operated in the photon mode at 15 MV. The conditions offer a situation within which a fraction of the bremsstrahlung emission energies exceed the photoneutron threshold. For different field sizes, an investigation has been made of neutron dose equivalent values at various measurement locations, including: (i) At the treatment table, at a source-surface distance of 100 cm; (ii) at the level of the floor directly adjacent to the treatment table; and (iii) in the control room and patient waiting area. The evaluated neutron dose equivalent was found to range from 0.0001 to 8.6 mSv/h, notably with the greatest value at the level of the floor directly adjacent to the treatment couch (8.6 mSv/h) exceeding the greatest value on the treatment table (5.5 mSv/h). Low values ranging from unobservable to between 0.0001 to 0.0002 mSv/h neutron dose were recorded around the control room and patient waiting area. For measurements on the floor, the study showed the dose equivalent to be greatest with the jaws closed. These data, most particularly concerning neutron distribution within the treatment room, are of great importance in making steps towards improving patient safety via the provision of protective measures.

Comparing the dosimeter-specific corrections for influence quantities of some parallel-plate ionization chambers in conventional electron beam dosimetry

The performance characteristics of some widely employed parallel-plate ionization chambers in dosimetry of conventional high energy electron beams were evaluated and compared in the present study following the recommendations of the IAEA TRS-398 reference dosimetry protocol.Three different types of PTW-made parallel-plate ionization chambers including Roos (TM34001), Markus (TM23343), and Advanced Markus (TM34045) were employed, and correction factors for polarity (kpol), recombination (ks), and quality conversion factor (kQ,Q0) were determined at different nominal electron energies of 4, 6, 9, 12, 16, and 20 MeV produced by a Varian Trilogy clinical Linac. All measurements were performed inside a MP3-M automatic water phantom in the reference condition of 100 cm SSD (source to surface distance), reference measurement depth (zref), and 10 × 10 cm2 field size at the phantom surface.The maximum and minimum range of kpol deviations from unity were respectively found for Markus and Roos ionization chambers. The maximum ks values also belonged to the Markus ionization chamber, while the minimum ks values were observed for the Advanced Markus chamber. The measured ks values through recommendations of the TRS-398 dosimetry protocol were in good accordance with those obtained by Jaffe-plot analysis for all considered ionization chambers. The type of employed ionization chamber can minimally affect the measured electron beam quality index (R50), while it can have a more considerable impact on kQ,Q0 value, especially in the case of the Markus chamber.From the results, it can be concluded that the Roos and Advanced Markus ionization chambers have a superior performance in the case of electron beam dosimetry, although all considered ionization chambers fulfilled the criteria requested by relevant reference dosimetry protocols.

In vivo dosimetry for testicular and scalp shielding in total skin electron therapy using a radiophotoluminescence glass dosimeter

Mycosis fungoides (MF) is a common, low-grade non-Hodgkin’s lymphoma of skin-homing T lymphocytes that can be treated via skin-directed radiotherapy. Our institution has implemented total skin electron therapy (TSET) with a 4.3 m source-to-surface distance (SSD) and 6 MeV electron beams with a beam spoiler. A 35-year-old male undergoing TSET desired to avoid radiotherapy-induced hair loss and temporary infertility; therefore, leakage dose to scalp and testicles was reduced with a special radiation shield composed of stacked lead sheets. The shields for the scalp and scrotal were of 3 mm and 6 mm, respectively. To assess leakage doses, a radiophotoluminescence glass dosimeter (RPLD) was placed at every fraction. The difference dose between the measured and prescribed dose at the calibration point was 2%. The top of the head and scrotal surface exhibited 18 cGy and 10 cGy, respectively. Thus, the dose to the scrotal surface was not beyond the testicular tolerance dose of 20 cGy. Results of semen analysis two months postradiotherapy were normal. There was no hair loss during or after radiation therapy. Therefore, the RPLD is a useful in vivo dosimeter that provides technical information on radiation shielding to allow for completion of TSET without hair loss or temporary infertility.

Determination of geometric information and radiation field overlaps on the skin in percutaneous coronary interventions with computer-aided design-based X-ray beam modeling.

PurposeThis study aimed to develop a method for the determination of the source‐to‐surface distance (SSD), the X‐ray beam area in a plane perpendicular to the beam axis at the entrance skin surface (Ap ), and the X‐ray beam area on the actual skin surface (As ) during percutaneous coronary intervention (PCI).Materials and MethodsMale and female anthropomorphic phantoms were scanned on a computed tomography scanner, and the data were transferred to a commercially available computer‐aided design (CAD) software. A cardiovascular angiography system with a 200 × 200 mm flat‐panel detector with a field‐of‐view of 175 × 175 mm was modeled with the CAD software. Both phantoms were independently placed on 40 mm thick pads, and the examination tabletop at the patient entrance reference point. Upon panning, the heart center was aligned to the central beam axis. The SSD, Ap , and As were determined with the measurement tool and Boolean intersection operations at 10 gantry angulations.ResultsThe means and standard deviations of the SSD, Ap , and As for the male and female phantoms were 573 ± 15 and 580 ± 15 mm, 8799 ± 1009 and 9661 ± 1152 mm2, 10495 ± 602 and 11913 ± 600 mm2, respectively. The number of As overlaps for the male and female phantoms were 15/45 and 21/45 view combinations, respectively.ConclusionsCAD‐based X‐ray beam modeling is useful for the determination of the SSD, Ap , and As . Furthermore, the knowledge of the As distribution helps to reduce the As overlap in PCI.

Feasibility of Clinically Practical Ultra-High Dose Rate (FLASH) Radiation Delivery by a Reversible Configuration of a Standard Clinical-Use Linear Accelerator

Multiple preclinical in-vivo studies of ultra-high dose rate (FLASH) radiation therapy (RT) have shown reduced normal tissue toxicity with equivalent tumor control when compared to conventional dose rate RT, with benefits seen at dose rates ≥40 Gray (Gy) per second. Leveraging these benefits for clinical translation could profoundly impact RT. Extending an institutionally established reversible configuration of a linear accelerator (LINAC) for FLASH, we explored the feasibility of LINAC-based FLASH RT for potential clinical application.A standard, clinical-use treatment delivery system was used. A single decommissioned beam's program circuit board was replaced with a dedicated experimental board, holding control parameters for RF power and gun current. Dose rates at an electron beam energy of ∼16 MeV were maximized at a gun current setting of 11.8V grid voltage. Notably, this setup kept the scattering foil in the beam, maintaining the advantages of the clinical utility of a standard scattered high-energy electron beam, and its inherent achievable field sizes and depth doses. For measurements, gantry and collimator rotations were set to 0° with open primary jaws. Dose rates were measured at central axis using radiographic film, with a 3 cm buildup of solid water and 20 cm downstream. FLASH treatments were delivered in 90 pulses, with 2 independent measurements at 4 source-to-surface distance (SSD) positions, including at the machine head (59 cm SSD). Percentage depth dose (PDD) was measured with a film strip placed between two vertical 5 cm thick solid water phantoms, obtained at 70, 80, and 100 cm SSD using FLASH, and 100 cm SSD with conventional dose rate 16 MeV electrons.FLASH dose rates > 100 Gy/s were obtained using a standard, scattered electron beam at clinically relevant SSD's and depths with a clinical-use LINAC. Measured average dose rates at 100, 80, 70, and 59 cm SSD were 36.82, 59.52, 82.01, and 112.83 Gy/s, respectively. Average doses per pulse measured were 0.21, 0.33, 0.46, and 0.63 Gy/pulse, respectively. FLASH PDD's emulated 16-18 MeV energies, aligning closely with a conventional 16 MeV PDD. At 70, 80, and 100 cm SSD FLASH, and 100 cm SSD conventional 16 MeV, 90% fall-off measured at 5.2, 5.3, 5.5, and 5.2 cm depth, 80% at 5.9, 6.0, 6.1, and 5.7 cm and 50% at 7.0, 7.0, 7.1, and 6.8 cm depth, respectively.Using a readily reversible configuration of a standard LINAC, FLASH dose rates > 100 Gy/s was achievable at clinically applicable SSDs and depths. Balancing clinical feasibility with FLASH, further studies characterizing the FLASH beam at 70 to 90 cm SSD are underway, including development of a cone-less electron field shaping system and solutions for energy modulation. Advantages of our setup, using a standard scattering foil with high energy electrons, bring us a step closer to clinical practicality of FLASH RT delivery using a standard LINAC.

Validation of a Novel Cone-Less Set-up for Electron FLASH Radiation Delivery on a Clinical-Use Linear Accelerator

<h3>Purpose/Objective(s)</h3> Ultra-high dose rate (FLASH) radiotherapy (RT) at ≥40 Gray per second has been shown preclinically to achieve improved normal tissue sparing while maintaining tumor control similar to that of conventional dose-rate RT. Many published FLASH-RT delivery models are inaccessible for widespread use and/or not applicable for clinical translation. We previously described a model for reversible configuration of a clinical linear accelerator (LINAC) that can deliver ∼16-18 MeV electron FLASH-RT at source-to-surface distances (SSDs) < 95 cm. However, it precludes the use of a standard electron cone. This proof-of-concept study will characterize the beam profiles from a novel cone-less set-up, to verify its feasibility as a widely accessible method of electron FLASH-RT delivery for the intention of clinical treatment. <h3>Materials/Methods</h3> This novel set-up employs a flat 25 × 25 cm applicator with custom cut-outs that lies within the accessory mount at 66.6 cm SSD of a standard treatment delivery system. We tested a 10 × 10 cm circular field size at a target distance of 80 cm SSD (selected to maintain FLASH dose rates). Radiographic film and an IC Profiler were used to evaluate penumbra width, beam flatness, symmetry, and percentage depth dose (PDD). FLASH-RT was delivered in 20 pulses with 3 independent measurements each using film and the profiler. This was compared to a conventional 16 MeV beam using a standard cone with a 10 × 10 cm field size at 100 cm SSD. <h3>Results</h3> Our novel cone-less FLASH set-up achieved a beam profile that was similar to the conventional electron set-up. Respectively, for the cone-less and conventional set-up, measured at a standardized depth of half R85, penumbra width was 1.13 cm and 1.15 cm, and uniformity index was 0.72 and 0.74 (defined as the ratio of the areas inside the 90% and 50% isodose lines); measured at a standardized depth of R95, flatness (defined as the maximum dose deviation from the central axis at a specific depth over an area confined within lines 2 cm inside the geometric edge of fields) was 1.5% and 4.1%, beam diameter containing dose > 80% of the central axis was 10.6 cm and 9.6 cm, and symmetry (defined as the maximum dose difference between any two symmetric points about the central axis in the flat portion of the beam, as a percentage of central dose) was 2.24% and 2.20%. PDD values for the cone-less set-up at Dmax, R95, R85, and R50 were, respectively, 3.1, 4.1, 5.3, and 6.8 cm. <h3>Conclusion</h3> We have developed a novel cone-less set-up that allows for the delivery of electron FLASH-RT utilizing a clinical LINAC at practical SSDs and verified that its beam profiles are clinically feasible. Further work will be done to characterize additional SSDs and field sizes, along with the addition of a custom skin collimator to further shape the beam. This will pave the way for practical treatment using electron FLASH-RT to a host of oncologic indications, including cutaneous malignancies and partial breast irradiation in the future.

Dosimetric comparison of AP/PA and bilateral geometries for total body irradiation treatment.

Total body irradiation (TBI) is an external radiotherapy technique. Its aim is to deliver a therapeutic dose uniformly within ± 10% of the absorbed dose to the prescription point. In the present study, the TBI technique was implemented in anterior/posterior (AP/PA), and bilateral geometry with photons from a 6 [Formula: see text] and 18 [Formula: see text] accelerator. The TBI technique was implemented on an Alderson Rando phantom at 312 [Formula: see text] source surface distance. During bilateral fraction, rice bags were applied as tissue compensators. To reduce the lung's absorbed dose to the acceptance level, in AP/PA geometry lung blocks made of Cerrobend were used. The required monitor unit (MU) for each fraction was calculated regarding depending on the prescribed dose and beam output. Gafchromic EBT3 films were used for dosimetry between the phantom layers in eight selected points. It is demonstrated that dose uniformity for AP/PA geometry with 6 [Formula: see text] and 18 [Formula: see text] photons was within ± 10%. In contrast, for the bilateral geometry the dose uniformity was not acceptable for both studied energies; However, the results for 18 [Formula: see text] were better than those for 6 [Formula: see text]. Dose accuracy for all measurements was within ± 5 of the prescribed dose. The absorbed dose to the lungs was successfully reduced using the lung blocks. By combining different therapeutic geometries and energies over six fractions, the results of uniformity and accuracy of dose delivery could be improved. It is concluded that the introduced TBI method achieved good dose accuracy and acceptable dose uniformity. Lungs absorbed dose was lower than 10 [Formula: see text] using the lungs blocks. Based on these results, the TBI technique can now be implemented in radiotherapy at Tehran's Imam Hospital. The approach developed in the present study can be used and adapted to match with the conditions at other hospitals.

Investigation of backscatter factor in medical radiography using anthropomorphic phantom by optically stimulated luminescence dosimeter

At the diagnostic reference level (DRL) related to medical radiation, DRL quantity for general radiography is the entrance surface dose (ESD). Calculation of the ESD in medical radiography requires the backscatter factor (BSF), but derivation of the BSF requires assessment of an irradiated simulation of a human body. The present study used optically stimulated luminescence (OSL) dosimeters and an anthropomorphic phantom as the irradiated body, and the BSF was calculated for different half value layer (HVL)s and field sizes. The need for different BSFs for different regions was also investigated by derivationing of the BSFs for different regions. The pelvis of a RANDO phantom was irradiated under the conditions of the HVL of 2.0, 3.1, and 4.6 mmAl; tube current of 200 mA; irradiation time of 0.1 s; source surface distance of 100 cm; and field sizes of 10 × 10 cm2, 20 cm2, 30 cm2, and 40 cm2. Measurement in air was performed under the same conditions. Several threads were stretched through the air with tissue paper placed on them and the nanoDot dosimeters placed on the paper. Four dosimeters were placed, and measurement was performed 5 times under each set of conditions. The compared radiographed regions were the skull, chest, and pelvis. The BSF increased with increasing HVL size and with increasing field size. The larger the HVL, the larger the difference between field sizes of 10 × 10 cm2 and 40 × 40 cm2 and the larger the increase in BSF relative to the increase in field size. The BSF differed by region, from large to small in the order chest, pelvis, and skull. The results thus showed that the BSF differs by the radiographed region. Thus, it is desirable to determine the BSF in each radiographed region by investigation with an anthropomorphic phantom.

SURFACE DOSE ESTIMATION BY A KAP METER FOR KILOVOLTAGE X-RAY BEAMS.

This study aims to estimate the entrance surface dose (ESD) of a water phantom for kilovoltage x-ray beams using an air kerma area product meter (KAP meter) equipped in an x-ray unit. The KAP meter was calibrated in terms of the ESD determined by a plane-parallel ionization chamber based on a 60Co absorbed dose-to-water calibration coefficient, ${N}_{D,w}^{{}^{60}\mathrm{C}\mathrm{o}}$. The ESD measured using the KAP meter was verified by comparing it with that estimated by the air kerma calibration coefficient, NK, for x-ray beam qualities. The ratio of ESDs based on ${N}_{D,w}^{{}^{60}\mathrm{C}\mathrm{o}}$ and NK was 1.003 on average and independent of the beam quality. The ESD by the KAP meter was an agreement within ±1.5% with that measured using the plane-parallel chamber for 10×10-30×30cm2 fields with a source-surface distance of 75-150cm. It was possible to estimate the ESD directly in a water phantom for x-ray beams without correction factors compared to the existing air kerma calibration, using a KAP meter calibrated based on ${N}_{D,w}^{{}^{60}\mathrm{C}\mathrm{o}}$.

Synthesis and evaluation of thermoluminescence properties of ZrO2:Mg for radiotherapy dosimetry.

This study aimed to investigate the thermoluminescent properties of ZrO2:Mg irradiated with a 6MV X-ray beam and its potential application in radiotherapy dosimetry. ZrO2 powder was synthesized using the sol-gel method and Mg was used as a dopant. Irradiations were performed with ZrO2:Mg chips located at the center of a 10 × 10cm2 radiation field at a source surface distance of 100cm, below a stack of solid water slabs, at the depth of maximum absorbed dose. The investigated characteristics of the material included linearity with radiation dose, reproducibility, accuracy, sensitivity and fading. Regarding the intrinsic difference of the samples, the glow curves of the investigated ZrO2:Mg chips exposed to 1Gy of 6 MV X-rays exhibited three or four peaks. The ZrO2:Mg samples showed a 47% fading at 24h after irradiation, and the reproducibility of the thermoluminescence reading of ZrO2:Mg for equal irradiation conditions was ± 21%. The thermoluminescence response of the investigated ZrO2:Mg samples to various absorbed doses from 0.5 to 2.5Gy showed a gentle increase of the thermoluminescence intensity with increasing absorbed dose. The obtained results show that ZrO2:Mg is not an appropriate candidate for X-ray photons in radiotherapy, due to low thermoluminescence peak temperature, low reproducibility, low sensitivity to various absorbed doses and significant fading.

Radioluminescence of cylindrical and flat Ge-doped silica optical fibers for real-time dosimetry applications

Investigation has been made of the radioluminescence dose response of Ge-doped silica flat and cylindrical fibers subjected to 6 and 10 MV photon beams. The fibers have been custom fabricated, obtaining Ge dopant concentrations of 6 and 10 mol%, subsequently cut into 20 mm lengths. Each sample has been exposed under a set of similar conditions, with use made of a fixed field size and source to surface distance (SSD). Investigation of dosimetric performance has involved radioluminescence linearity, dose-rate dependence, energy dependence, and reproducibility. Mass for mass, the 6 mol% Ge-doped samples provided the greater radioluminescence yield, with both flat and cylindrical fibers responding linearly to the absorbed dose. Further found has been that the cylindrical fibers provided a yield some 38% greater than that of the flat fibers. At 6 MV, the cylindrical fibers were also found to exhibit repeatability variation of <1%, superior to that of the flat fibers, offering strong potential for use in real-time dosimetry applications.

Dose divergence between monte carlo simulation and experimental measurements in the build-up region for a photon beam

The aim of this work was to study the divergence between simulation and experimental measurements in the build-up region using two types of medical accelerators Elekta Synergy MLCi2 and Varian clinac 2100C and also for two types of ionization chambers. The Monte Carlo simulation was performed using the G4Linac-MT code. By demonstrating the variation of the particles in the build-up region, this work allows us to give an exact conclusion on the divergence between the Monte Carlo simulation and the experimental measurements.Using multi-threading code G4Linac-MT and the performance of the HPC-Marwan computing grid, we carried out simulations of large events with a saving in computing time. After optimizing the initial beam parameters, we looked for the ideal voxel size which corresponds to a better agreement between simulation and measurements to avoid simulation errors. The divergence between simulation and measurements in the build-up region is studied according to the type of ionization chamber, the irradiation fields, the source-to-surface distance (SSD) and the photon energy.The results obtained indicate that the dose divergence in the build-up region increases with the appearance of the contamination particles. The secondary particles detected in the build-up region are electrons and positrons for the 6 MV photon beam and other particles are added for the high energy beam 18 MV, including protons and neutrons. The dose accuracy of the comparison between the simulation and the measurements of the Varian 2100C and Elekta Synergy MLCi2 accelerators increases if the voxel size is small. A good agreement between the simulation and the experimental measurements in the build-up region is that which shows almost stable contamination of the particles up to the depth of electronic equilibrium.

Characteristics of very high-energy electron beams for the irradiation of deep-seated targets.

Driven by advances in accelerator technology and the potential of exploiting the FLASH effect for the treatment of deep-seated targets (>5cm), there is an active interest in the construction of devices to deliver very high-energy electron (VHEE) beams for radiation therapy. The application of novel VHEE devices, however, requires an assessment of the tradeoffs between the different beam parameter choices including beam energies, beam divergences, and maximal field sizes. This study systematically examines the dosimetric beam properties of VHEE beams, determining their clinical usefulness while marking their limits of applications for different beam configurations. We performed Monte Carlo simulations of the dose distributions of electron beams for different energies (25-250MeV), source-to-surface distances (SSD) (50cm, 100cm, parallel), and field sizes (2cm2 ×2cm2 to 15cm2 ×15cm2 ) in water using a research version of the RayStation treatment planning system (RaySearch Labs 9A IONPG). The beam was simulated using a monoenergetic point source and perfect collimation. Central axis percentage depth dose (PDD) and transverse dose profiles at multiple depths were evaluated and compared to those of MV photon beams. Profile characteristics including therapeutic range (TR) at 90%, proximal fall-off (PFO) at 90%, lateral penumbra (LP) at 90%-10%, and field width (FW) at 90% were obtained. Very high-energy electrons beams with SSD 100cm and parallel beams (infinite SSD) exhibit a linear to near-linear increase of TR as a function of energy in the simulated energy range and reach values well beyond the typical depths of lesions encountered in clinics (<20cm). Their TR show a marked field size dependence only for field sizes <10cm2 ×10cm2 . For VHEE beams with SSD 50cm, TR are largely reduced (4-8cm). For beam energies >150MeV with large SSD (>100cm), for many configurations, there is no substantial difference in PDD when adding an opposed beam. This may potentially reduce the number of VHEE beams needed for treatment by a factor of two compared to a treatment using lower energies and lower SSD. In order to cover deep-seated targets homogeneously, VHEE devices with a parallel beam must provide a maximum field size up to several centimeters larger than the tumor size. For the investigated diverging beams, there is not such a significant field width reduction with depth for larger fields as it is compensated by divergence. Penumbrae of VHEE beams are smaller than those of clinical MV photon beams for lower depths (<5cm) but increase quickly for larger depths. There is only a relatively small dependence of penumbra on the SSD of the beam. The findings presented in this study assess the performance of VHEE beams and offer a first estimate of treatment indications and tradeoffs for a given design of a VHEE device. SSD >100cm results in clinically more favorable PDD. Beam energies of 100MeV and above are needed to cover common tumors (5-15cm in-depth) conformally. Higher energies provide an additional benefit specifically for small and deep-seated lesions due to their reduced lateral penumbrae.