Depths In Water Phantom Research Articles

Improvement of neutron sensitivity for lithium formate EPR dosemeters: a Monte Carlo analysis.

This work presents the computational analysis of the sensitivity improvements that could be achieved in lithium formate monohydrate (LFM) electron paramagnetic resonance (EPR) dosemeters exposed to neutron beams. Monte Carlo (MC) simulations were performed on LFM pellets exposed to neutron beams with different energy spectra at various depths inside a water phantom. Various computations were carried out by considering different enrichments of 6Li inside the LFM matrix as well as addition of different amounts of gadolinium oxide inside the pellet blend. The energy released per unit mass was calculated with the aim of predicting the increase in dose achievable by the addition of sensitizers inside the pellets. As expected, a larger amount of 6Li induces an increase of energy released because of the charged secondary particles (i.e. 3H ions and α-particles) produced after neutron capture. For small depths in water phantom and low-energy neutron spectra the dose increase due to 6Li enrichment is high (more than three orders of magnitude with respect to the case of with 7Li). In case of epithermal neutron beams the energy released in 6Li-enriched LFM compound is smaller but larger than in the case of fast neutron beams. On the other hand, the computational analysis evidenced that gadolinium is less effective than 6Li in improving neutron sensitivity of the LFM pellets. Discussion based on the features of MC transport code is provided. This result suggests that 6Li enrichment of LFM dosemeters would be more effective for neutron sensitivity improvement and these EPR dosemeters could be tested for dosimetric applications in Neutron Capture Therapy.

Depth dose measurement in water phantom for two X-ray energies (6MeV and 10MeV) in comparison with actual planning

The purpose of this study is to measure doses delivered at different depths in water phantom at vertical position in comparison with the actual planning in order to verify the dose delivered to the tumor in addition to the measurement of the effect penumbra dose to assess the dose leaking to the healthy soft tissue.
 Percentage depth dose (PDD) values was measured at field sizes (5×5,10×10,15×15, and 20×20) cm2, and the depth dose was measured between (0-16) cm deep at 4cm intervals, for both energies 6 MeV and 10 MeV X-ray beam. Other readings were taken at different distances 1cm and 2cm outside of the actual beam in orthogonal directions at depth of 4 cm. These measurements were designed to measure the penumbra dose produced outside the central beam.
 Results show that the high similarity between water phantom and actual tissue for this reason water is taken as phantom for Quality Assurance (QA) and calculation the depth dose. The similar results may appear strange as the actual planning depth dose is taken in the chest wall where there is bone and soft tissue. The increase in the field size, increases the percentage of surface dose, this could be caused by an increase in the amount of scattering in the larger fields. There is almost no difference in depth dose between homogenous and non homogenous planning also similar to the water phantom. Because of higher photon energy 6MeV and 10MeV the bone has no influence in absorption from the soft tissue. A slight change in the depth dose with increase in the field size may be caused by the scattered radiation.

Effect of scattering and differential attenuation on beam profile in the presence of high-density intensity modifying compensator.

The aim of this study is to investigate the effect of scattering and differential attenuation on dose profile of 6 MV photon beam in the presence of cadmium (Cd)-free compensator which has been used in compensator-based intensity-modulated radiotherapy. Totally, 10 slabs of Cd-free compensator having thicknesses ranging from 2.4 to 61.4 mm have been prepared. Dose profiles have been taken using computer-controlled radiation field analyzer for five field sizes from 30 mm × 30 mm to 200 mm × 200 mm and at three depths in water phantom. Off-axis dose variation (ODV) has been measured with off-axis percentage depth dose scan and with ion chamber by measuring point dose at two diagonal points with respect to dose at central axis point in a plane and at three depths. A decrease in beam flatness has been observed with increase in compensator thickness and depth in phantom. ODV has been found to increase with compensator thickness. Selective beam hardening has been observed due to differential attenuation from compensator. Point dose measurements show approximately 20% and 23% underdose region at 70 and 106 mm off-axis diagonal point, respectively, as compared to dose at central axis point for a field size of 200 mm × 200 mm at a depth of 15 mm, with 30.2-mm slab thickness. Significant increase in scattered penumbra has been observed with field size and thickness of compensator due to increase in scattered photon. The presence of compensator changes photon beam mean energy along the cross-section resulted in decreased beam flatness and increased scattering. This may lead to overestimation of dose along off-axis within radiation field if change in flatness is not taken into account and more exposure to healthy tissues in penumbral region due to large-angle scattering.

Dosimetry of indigenously developed 177Lu patch source for surface brachytherapy—Experimental and Monte Carlo methods

This paper describes the evaluation of dosimetry characteristics of an in-house developed 177Lu skin patch source for treatment of non-melanoma skin cancer. A 177Lu skin patch source based on Nafion-115 membrane backbone containing 3.46 ± 0.01 mCi of activity was used. Activity measurement of the patch source was based on gamma ray spectrometry using a HPGe detector. The efficiencies of the HPGe detector were fitted using an orthogonal polynomial function. The absorbed dose rate to water at 5 μm depth in water was determined using an extrapolation chamber, EBT3 Gafchromic film and compared with Monte Carlo methods. The correction factors such as Bragg-Gray stopping power ratio of water-to-air and chamber wall material being different from water, needed to be applied on measurements for establishing the dose rate at 5 μm depth, were calculated using the Monte Carlo method. Absorbed dose rate at 5 μm depth in water (surface dose rate) measured using an extrapolation chamber and EBT3 Gafchromic film were 9.9 ± 0.7 and 8.2 ± 0.1 Gy h−1 mCi−1 respectively for the source activity of 3.46 ± 0.01 mCi. The surface dose rate calculated using the Monte Carlo method was 8.7 ± 0.2 Gy h−1 mCi−1, which agrees reasonably well with measurement. The measured dose rate per mCi offers scope for ascertaining treatment time required to deliver the dose for propitious therapeutic outcome. Additionally, on-axis depth dose and lateral dose profiles at 5 μm and 1 mm depth in water phantom were also calculated using the Monte Carlo method.

Dosimetry Investigation and Evaluation for Removing Flattening Filter Configuration of Linac: Monte Carlo Study

The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible and then to investigate the removing of flattening filter impact on dosimetry for a high radiotherapy efficiency. Monte Carlo codes used in this work were BEAMnrc code to simulate photons beam and DOSXYZnrc code to examinate absorbed dose in water phantom. PDDs and beam profiles were calculated for 6 × 6 cm2 and 10 × 10 cm2 field sizes. Good agreement was found between calculated PDD and beam profile compared to measurements. Gamma index acceptance rate was more than 98% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc, DOSXYZnrc and BEAMDP codes package. Varian Clinac 2100 with removing flattening filer could increase the dose by a gain was approximately 80% for 6 × 6 cm2 field size and it was approximately 110% for 10 × 10 cm2 at the build-up dose region but for all depth in water phantom, the dose of without FF configuration of linac was increased by more than 40% of dose of with FF configuration of linac.

Monte Carlo simulation of energy absorbed in phenolic ESR dosimeters added with gadolinium exposed to thermal, epithermal and fast neutrons

Abstract In this work analyses of the energy released per unit mass in phenolic compound exposed to neutron beams were performed with the aim of predicting the increase in dose achievable by addition of gadolinium (Gd) inside the pellets. In particular, Monte Carlo (MC) simulations were carried out for IRGANOX® 1076 phenolic compound irradiated with neutron beams with different energy spectra at various depths inside a water phantom. The addition of gadolinium increases sensitivity of phenolic ESR (electron spin resonance) dosimeters to neutrons thanks to the high gadolinium cross section for neutron capture and to the large number of secondary particles (mainly Auger and internal conversion electrons) which are able to release energy inside the sensitive material layers. For small depths in water phantom and low energy neutron spectra the increase in dose due to gadolinium is large (more than a factor 50). The enhancement is smaller in case of epithermal neutron beam, whereas the increase in dose for fast neutrons is less than 50%. In order to have a comparison with other ESR dosimeters the energy released per unit mass in phenolic compound was compared with that calculated in alanine pellets. For thermal neutron beams the energy released in phenolic compound with gadolinium is comparable to that released in alanine for small depths in phantom, whereas it is larger than in alanine for large depths. In case of epithermal and fast neutron beams the energy released in phenolic compound is larger than in alanine samples because the elastic scattering with hydrogen nuclei is more probable for high neutron energies and this phenolic compound is characterized by an higher number of 1H nuclei than alanine. All results here found suggest that these phenolic pellets could be fruitfully used for dosimetric applications in Neutron Capture Therapy.

Dosimetric characterization of small fields using a plastic scintillator detector: A large multicenter study

PurposeIn modern radiation therapy accurate small fields dosimetry is a challenge and its standardization is fundamental to harmonize delivered dose in different institutions. This study presents a multicenter characterization of MLC-defined small field for Elekta and Varian linear accelerators. Measurements were performed using the Exradin W1 plastic scintillator detector. Materials and methodsThe project enrolled 24 Italian centers. Each center performed Tissue Phantom Ratio (TPR), in-plane and cross-plane dose profiles of 0.8×0.8cm2 field, and Output Factor (OF) measurements for square field sizes ranging from 0.8 to 10cm. Set-up conditions were 10cm depth in water phantom at SSD 90cm. Measurements were performed using two twin Exradin W1 plastic scintillator detectors (PSD) correcting for the Cerenkov effect as proposed by the manufacturer. ResultsData analysis from 12 Varian and 12 Elekta centers was performed. Measurements of 7 centers were not included due to cable problems. TPR measurements showed standard deviations (SD)<1%; SD<0.4mm for the profile penumbra was obtained, while FWHM measurements showed SD<0.5mm. OF measurements showed SD<1.5% for field size greater than 2×2cm2. Median OFs values were in agreement with the recent bibliography. ConclusionsHigh degree of consistency was registered for all the considered parameters. This work confirmed the importance of multicenter dosimetric intercomparison. W1 PSD could be considered as a good candidate for small field measurements.

Comparison of dosimeter response of TLD-100 and ionization chamber for high energy photon beams at KIRAN Karachi in Pakistan

This study was conducted by measuring point dose at different depths of water phantom by using two types of dosimeters (PTW N30013 ionization chamber and TLD-100 chips) at Karachi institute of radiotherapy and nuclear medicine (KIRAN), Pakistan. Two different TLD chips were used (circular pallets and square chips). The main aim of this study was to compare the responses of two different types of dosimeter irradiated with 6MV X-rays using same parameters. Both types of dosimeter were irradiated with different dose value ranging from 25cGy to 500cGy. The deviation between different shapes of TLD and ionization chamber remained within 5% limit. Maximum deviations of circular pallets reading from that of ion chamber are 3.56% at 1.5cm depth and 4.91% at 5cm depth. For square chips maximum deviation happened to be 4.38% at 1.5cm depth and 4.23% at 5cm depth. This measurement shows that TLDs are reliable tool for dosimetry regardless of their shape or manufacturer and they can be used as re-validation tool for ion chamber dosimetry.

Dosimetric characterization of linac small beams using a plastic scintillator detector: A multicenter study

Purpose In modern radiation therapy accurate dosimetry within small photon fields is a challenge. This study presents a multicenter characterization of MLC-defined small field for the two major linear accelerator manufacturers. Materials and methods The project enrolled 31 Italian centers, 15 equipped with Elekta Linacs and 16 equipped with Varian Linacs. Each center performed TPR, in-plane and cross-plane dose profile of 0.8 × 0.8 cm2 field and OFs measurements for field sizes ranging from 0.6 × 0.6 to 10 × 10 cm2. Set-up conditions were 10 cm depth in water phantom at SSD 90 cm. Measurements were performed using two Exradin W1 plastic scintillator detectors correcting for the Cerenkov effect as proposed by the manufacturer. Results Analysis of the measurements performed by 12 Varian and 13 Elekta centers was performed. TPR measurements showed standard deviations within 0.6%; penumbra values of dose profiles showed standard deviations within 0.5 mm. FWHM measurements showed a greater variability. OF measurements showed standard deviations within 1.5% for field size greater than 2 × 2 cm2; for field size less than 2 × 2 cm2 measurements’ variability increases with decreasing field size. OF values show no dependence from the effective field size. Conclusions Relatively high degree of consistency regarding TPR and penumbra values was registered. FWHM and OF instead show greater variability. Our work confirmed the importance of multicenter dosimetric studies and W1 PSD as a candidate for small field clinical radiation dosimetry in advanced radiation therapy techniques.

SU‐E‐T‐46: A Monte Carlo Investigation of Radiation Interactions with Gold Nanoparticles in Water for 6 MV, 85 KeV and 40 KeV Photon Beams

Purpose:To determine the effect of gold‐nanoparticles (AuNPs) on energy deposition in water for different irradiation conditions.Methods:TOPAS version B12 Monte Carlo code was used to simulate energy deposition in water from monoenergetic 40 keV and 85 keV photon beams and a 6 MV Varian Clinac photon beam (IAEA phase space file, 10x10 cm2, SSD 100 cm). For the 40 and 85 keV beams, monoenergetic 2x2 mm2 parallel beams were used to irradiate a 30x30x10 µm 3 water mini‐phantom located at 1.5 cm depth in a 30x30x50 cm3 water phantom. 5000 AuNPs of 50 nm diameter were randomly distributed inside the mini‐phantom. Energy deposition was scored in the mini‐phantom with the AuNPs’ material set to gold and then water. For the 6 MV beam, we created another phase space (PHSP) file on the surface of a 2 mm diameter sphere located at 1.5 cm depth in the water phantom. The PHSP file consisted of all particles entering the sphere including backscattered particles. Simulations were then performed using the new PHSP as the source with the mini‐phantom centered in a 2 mm diameter water sphere in vacuum. The g4em‐livermore reference list was used with “EMRangeMin/EMRangeMax = 100 eV/7 MeV” and “SetProductionCutLowerEdge = 990 eV” to create the new PHSP, and “SetProductionCutLowerEdge = 100 eV” for the mini‐phantom simulations. All other parameters were set as defaults (“finalRange = 100 µm”).Results:The addition of AuNPs resulted in an increase in the mini‐phantom energy deposition of (7.5 ± 8.7)%, (1.6 ± 8.2)%, and (−0.6 ± 1.1)% for 40 keV, 85 keV and 6 MV beams respectively.Conclusion:Enhanced energy deposition was seen at low photon energies, but decreased with increasing energy. No enhancement was observed for the 6 MV beam. Future work is required to decrease the statistical uncertainties in the simulations.This research is partially supported from institutional funds from the Center for Radiation Oncology Research, The University of Texas MD Anderson Cancer Center.

Treatment planning validation for symmetric and asymmetric motorized wedged fields

Purpose: Wedged beam are often used in clinical radiotherapy to compensate missing tissues and dose gradients. The Elekta Precise linear accelerator supports an internal motorized wedge, which is a single large, physical wedge on a motorized carriage. In this study, the dosimetric performance of Elekta precise three dimensional treatment planning system (3DTPS) is evaluated by comparing the calculated and measured doses. Methods: The calculations were performed by the 3DTPS for symmetric as well as asymmetric fields in a source to skin distance (SSD) setup at the depth of maximum dose (d max ) as well as at 5, 10, and 20 cm depths in water phantom using 60° motorized wedges for field sizes of 4 × 4, 10 × 10, and 20 × 20 cm 2 for 6 and 15 MV photon beams. Measurements were produced by Elekta Precise linear accelerator using 0.125 cc volume ionization chamber. Results: Good agreement between the measured and calculated isodose lines were found, with the maximum difference not exceed 5%. The difference between the calculated and measured data increases as the field size decreases, and the deviation in symmetric setting was less than that of asymmetric setting. The increase in wedge angle led to increase in the difference between calculated and measured data. Conclusion: The results from this study showed that the accuracy of Elekta Precise 3DTPS used with the motorized wedges for symmetric and asymmetric fields is adequate for the clinical applications under the studied experimental conditions.

Skin dose measurements using radiochromic films, TLDS and ionisation chamber and comparison with Monte Carlo simulation

Estimation of the surface dose is very important for patients undergoing radiation therapy. The purpose of this study is to investigate the dose at the surface of a water phantom at a depth of 0.007 cm as recommended by the International Commission on Radiological Protection and International Commission on Radiation Units and Measurement with radiochromic films (RFs), thermoluminescent dosemeters and an ionisation chamber in a 6-MV photon beam. The results were compared with the theoretical calculation using Monte Carlo (MC) simulation software (MCNP5, BEAMnrc and DOSXYZnrc). The RF was calibrated by placing the films at a depth of maximum dose (d(max)) in a solid water phantom and exposing it to doses from 0 to 500 cGy. The films were scanned using a transmission high-resolution HP scanner. The optical density of the film was obtained from the red component of the RGB images using ImageJ software. The per cent surface dose (PSD) and percentage depth dose (PDD) curve were obtained by placing film pieces at the surface and at different depths in the solid water phantom. TLDs were placed at a depth of 10 cm in a solid water phantom for calibration. Then the TLDs were placed at different depths in the water phantom and were exposed to obtain the PDD. The obtained PSD and PDD values were compared with those obtained using a cylindrical ionisation chamber. The PSD was also determined using Monte Carlo simulation of a LINAC 6-MV photon beam. The extrapolation method was used to determine the PSD for all measurements. The PSD was 15.0±3.6% for RF. The TLD measurement of the PSD was 16.0±5.0%. The (0.6 cm(3)) cylindrical ionisation chamber measurement of the PSD was 50.0±3.0%. The theoretical calculation using MCNP5 and DOSXYZnrc yielded a PSD of 15.0±2.0% and 15.7±2.2%. In this study, good agreement between PSD measurements was observed using RF and TLDs with the Monte Carlo calculation. However, the cylindrical chamber measurement yielded an overestimate of the PSD. This is probably due to the ionisation chamber calibration factor that is only valid in charged particle equilibrium condition, which is not achieved at the surface in the build-up region.

SU-GG-T-57: Collimated Beams for HDR Brachytherapy Dosimetry

Purpose: To construct and test a small 192Ir collimator for HDR brachytherapydosimetry purposes. Material and methods: The device is made up of a cerrobend cube to produce a collimated beam when a catheter from an HDR 192Ir source is inserted in the center of the cube. The cube is mounted in a calibration bench which allows the positioning of a 15 × 15 × 15 cm3 water and/or solid water phantoms in the central axis of the radiation beam at a fixed distance. The field size at the fixed calibration distance of 10 cm is 5 cm diameter. The radiation beam profile at this distance was measured with a gafchromic film, ionization chamber and calculated with the PENELOPE Monte Carlo code. Such code was also used to determine the spectra of radiation in air and at 0.5‐cm depth of both phantoms. Simulations were performed with 109 histories. Results: Calculated data have shown that both profiles (x and the z‐axis which is along the source) at a distance 10 cm away from the source is flat within 0.7%. This beam flatness was confirmed with the measurements done with the ionization chamber and with the gaphchromic film as well. Within the statistical uncertainty of the calculations (0.4%), there was no observable change in the mean energy of the input photon spectra and at 0.5‐cm depth in water phantom. Conclusion: The miniature irradiatior can be used for QA of HDR brachytherapy equipments. If the system is provided with a standard calibrated ionization chamber and appropriate phantoms, it can be used to calibrate HDR sources, TLDs films and to determine the Fricke chemical yield.

광섬유 방사선량계를 이용한 Co-60 방사선원으로부터 조사되는 상대선량측정

In this study, we have fabricated a scintillating fiber-optic dosimeter for a radiotherapy dosimetry. And <TEX>${\gamma}$</TEX>-rays generated by a Co-60 are measured using a scintillating fiber-optic dosimeter and percent depth dose curves are obtained according to the different depths of solid water phantoms. Also, Cerenkov radiations generated by primary or secondary electrons are measured at different depths of water phantom using a background optical fiber.

Monte Carlo evaluations of the absorbed dose and quality dependence of in radiotherapy photon beams

The purpose of this work was to evaluate the absorbed dose to AL2O3 dosimeter at various depths of water phantom in radiotherapy photon beams by Monte Carlo simulation and evaluate the beam quality dependence. The simulations were done using EGSnrc. The cylindrical Al2O3 dosimeter (Phi4 mm x 1 mm) was placed at the central axis of the water phantom (Phi16 cm x 16 cm) at depths between 0.5 and 8 cm. The incident beams included monoenergetic photon beams ranging from 1 to 18 MeV, 60Co gamma beams, Varian 6 MV beams using phase space files based on a full simulation of the linac, and Varian beams between 4 and 24 MV using Mohan's spectra. The absorbed dose to the dosimeter and the water at the corresponding position in the absence of the dosimeter, as well as absorbed dose ratio factor fmd, was calculated. The results show that fmd depends obviously on the photon energy at the shallow depths. However, as the depth increases, the change in fmd becomes small, beyond the buildup region, the maximum discrepancy of fmd to the average value is not more than 1%. These simulation results confirm the use of Al2O3 dosimeter in radiotherapy photon beams and clearly indicate that more attention should be paid when using such a dosimeter in the buildup region of high-energy radiotherapy photon beams.

Thermal and temporal response of ionization chambers in radiation dosimetry.

The temporal and thermal response of various commonly used ion chambers (NEL, Exradin, PTW) with different wall and electrode materials is studied in a water phantom. Measurements were taken in heating water bath having an automatic temperature control mechanism. All chambers were submersed at 3 cm depth in water phantom and connected to separate electrometers for simultaneous readings for a given dose from a 6 MV beam. The temporal response was studied at approximately +/- 10 degrees C from room temperature, i.e., 10 degrees C and 30 degrees C. Temporal results show that all chambers reach quick equilibrium response within < 2 minutes of submersion in water. The steady-state thermal response was dependent upon the water temperature and wall and electrode compositions. The temperature and pressure corrected response of the NEL chamber was least affected by the changes in water temperature, where as the Exradin chamber has positive and PTW has negative slope with water temperature. The response varied within +/-1.5% between 10 degrees C-50 degrees C temperature and is mainly dependent on volume changes rather than the humidity. A correction factor based on thermal coefficients is derived for each chamber. It is concluded that ion chamber correction factors can be divided into first order; temperature and pressure connection, second order; volume correction for thermal expansion, and third order; humidity correction. To eliminate dosimetric error, the temporal and thermal response should be known for a chamber or water phantom temperature should be maintained close to room temperature.