Depth In Phantom Research Articles

Evaluation of dose calculation accuracy of treatment planning systems in the presence of tissue heterogeneities

Background: Accuracy of five dose calculation algorithms within three treatment planning systems (TPS): BrainLAB iPlan 4.2 (BL: pencil beam and Monte Carlo), Philips Pinnacle (PL: Collapsed Cone Calculation, CCC), and Varian Eclipse (VR: AAA and Acuros XB) is investigated in this multi-institutional study. Methods: A Monte-Carlo based TPS (BL) was first validated against benchmark measurements in heterogeneous-slab phantoms consisting of tissue-equivalent plastic, lung-equivalent cork, and bone density materials. Ion chamber/EDR film measurements of depth-dose and dose-profiles for 6MV enface photons in a range of field-sizes (12 mm × 12 mm – 60 mm × 60 mm) were performed. A common CT dataset for each phantom was sent to four participating institutions. Measured versus calculated dose differences for all dose algorithms considered in this study were quantified using two dose-profile indices: D diff within the central 80% of photon field, and D spill for dose outside the field-edge (50–10% dose). Results: BrainLAB TPS BL : MC and measured doses agreed well ( D diff <3%) for all field-sizes and phantom depths investigated. The agreement improved with increasing field size (2.6% for 12 mm × 12 mm vs. 1.1% for 60 mm × 60 mm at 60 mm depth in lung phantom). In contrast, for lung phantom, pencil beam (PB) calculations significantly over-predicted the measured dose (34% and 6.7% respectively). In general, PB vs. measured dose differences increased with decreasing field-size, decreasing phantom density and increasing depth within heterogeneity. Pinnacle TPS PL : pinnacle showed the best agreement with measured data in the presence of tissue heterogeneities and modeled dose changes at the tissue-lung interface better than AAA and PB. Varian TPS VR : AAA over-predicted measured results and was unable to replicate dose variation near heterogeneities. In contrast, Acuros XB showed good agreement with measurements for heterogeneous media as well as superior agreement in the interface region. Conclusions: Based on the results of this multi-institutional study, appropriate corrections may be applied to lung stereotactic body radiotherapy (SBRT) plans for patients enrolled in RTOG protocols.

Analytical investigation of magnetic field effects on Proton lateral deflection and penetrating depth in the water phantom: A relativistic approach.

BackgroundIntegrated proton therapy - MRI systems are capable of delivering high doses to the target tissues near sensitive organs and achieve better therapeutic results; however, the applied magnetic field for imaging, influences the protons path, changes the penetration depth and deflects the particles, laterally, leading to dose distribution variations.ObjectiveTo determine the effects of a magnetic field on the range and the lateral deflection of protons, analytically.MethodsAn analytical survey based on protons energy and range power law relation, without using small angle assumption was done. The penetration depth and lateral deflection of protons with therapeutic energy ranges 60–250 MeV in the presence of uniform magnetic fields of 0–10T intensities, were calculated analytically. Calculations were done for relativistic conditions with Mathematica software version 7.0, and MATLAB 7.0 was applied to plot curves and curve fittings.ResultsIn the presence of a magnetic field, the depth of Bragg peak was decreased and it was shifted laterally. A second order polynomial model with power equation for its coefficients and a power model with quadratic polynomial coefficients predicted the maximum lateral deflection (ymax) and maximum penetration depth (zmax) variations with energy and magnetic field intensity, respectively.ConclusionThe applied correction for deflection angle will give more reliable results in initial energy of 250 MeV and 3T magnetic field intensity. For lower energies and magnetic field intensities the differences are negligible, clinically.

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 simulations of a kilovoltage external beam radiotherapy system on phantoms and breast patients.

To determine the most suitable lesion size and depth for radiotherapy treatments with a prototype kilovoltage x-ray arc therapy (KVAT) system through Monte Carlo simulations of the dose delivered to lesion, dose homogeneity, and lesion-to-skin ratio. Monte Carlo simulations were used to calculate dose distributions generated by a novel low-energy kilovoltage x-ray system to a variety of clinically relevant lesion sizes and depths in phantoms and for hypothetical partial breast irradiations of patients in supine and prone positions. The treatments by 200 kV KVAT system were modeled for four sizes of tumor (1-4 cm diameter) at three depths (superficial, middle, and deep) in two sizes of cylindrical water phantoms (16.2-cm and 32.2-cm diameter). In addition, treatments of 3-cm and 4-cm diameter lesions were modeled for two breast patients in prone and supine positions. Dose distributions were calculated using the EGSnrc/DOSXYZnrc code package. Phantom study metrics included lesion-to-skin ratio, dose delivered to isocenter (cGy/min), dose homogeneity, dose profiles, and cumulative dose volume histograms. Lesion-to-skin ratio, lesion-to-rib ratio, dose profiles, and cumulative dose volume histograms were used to evaluate simulated breast patient treatments. Supine breast irradiations were compared to 6-MV VMAT plans. The criterion applied to evaluate the dose distributions was derived from NSABP-B39/RTOG 0413 for accelerated partial breast irradiation. Skin dose was limited to a maximum of 250 cGy for a prescribed lesion dose of 385 cGy per fraction (with the whole treatment being delivered in 10 fractions). This produced the minimum lesion-to-skin dose ratio of 1.5 that served as the main guideline, along with other metrics, for evaluation of future clinical viability of treatments. Phantom dose distributions in the centrally located lesions treated with 360-degree KVAT were found to be superior to dose distributions in off-center lesions with the exception of isocenter dose, which was highest for lesions located closer to the phantom surface. Dose metrics were more favorable for smaller lesions, suggesting that KVAT might be most suitable for treatment of lesions of 1-2 cm in diameter down to depths of 8.1 cm along with 3 cm lesions at depths from 3 cm to 8.1 cm. In addition, treatments of 4-cm lesions were found to be acceptable down to the depths of 4.1 cm (in the 16.2-cm phantom) and 8.1 cm (in the 32.2-cm phantom). At depths from 8.1-cm to 16.1-cm, treatments of 1-cm to 4-cm lesions are possible at the cost of decreased dose rate. KVAT breast treatments in the supine patient position demonstrated that increasing the arc angle and decreasing lesion size improved lesion-to-skin ratio and lesion-to-rib ratio. Supine breast data indicate that 3-cm lesions are treatable at a minimum depth of 3 cm. The 6-MV VMAT plan resulted in lower doses to the ipsilateral lung and the body, but a higher heart dose compared to the KVAT plans. Dose distributions for the prone breast phantoms were superior to the supine cases due to the increased treatment angle of 360-degrees. Although nonoptimized KVAT dose distributions presented here were of inferior quality to VMAT plans, this work has demonstrated the feasibility of delivering low-energy kilovoltage x-rays to lesions up to 4 cm in diameter to depths of 8.1 cm while sparing surrounding tissue.

Characterization of proton pencil beam scanning and passive beam using a high spatial resolution solid-state microdosimeter.

This work aims to characterize a proton pencil beam scanning (PBS) and passive double scattering (DS) systems as well as to measure parameters relevant to the relative biological effectiveness (RBE) of the beam using a silicon on insulator (SOI) microdosimeter with well-defined 3D sensitive volumes (SV). The dose equivalent downstream and laterally outside of a clinical PBS treatment field was assessed and compared to that of a DS beam. A novel silicon microdosimeter with well-defined 3D SVs was used in this study. It was connected to low noise electronics, allowing for detection of lineal energies as low as 0.15keV/μm. The microdosimeter was placed at various depths in a water phantom along the central axis of the proton beam, and at the distal part of the spread-out Bragg peak (SOBP) in 0.5mm increments. The RBE values of the pristine Bragg peak (BP) and SOBP were derived using the measured microdosimetric lineal energy spectra as inputs to the modified microdosimetric kinetic model (MKM). Geant4 simulations were performed in order to verify the calculated depth-dose distribution from the treatment planning system (TPS) and to compare the simulated dose-mean lineal energy to the experimental results. For a 131MeV PBS spot (124.6mm R90 range in water), the measured dose-mean lineal energy yD¯ increased from 2keV/μm at the entrance to 8keV/μm in the BP, with a maximum value of 10keV/μm at the distal edge. The derived RBE distribution for the PBS beam slowly increased from 0.97±0.14 at the entrance to 1.04±0.09 proximal to the BP, then to 1.1±0.08 in the BP, and steeply rose to 1.57±0.19 at the distal part of the BP. The RBE distribution for the DS SOBP beam was approximately 0.96±0.16 to 1.01±0.16 at shallow depths, and 1.01±0.16 to 1.28±0.17 within the SOBP. The RBE significantly increased from 1.29±0.17 to 1.43±0.18 at the distal edge of the SOBP. The SOI microdosimeter with its well-defined 3D SV has applicability in characterizing proton radiation fields and can measure relevant physical parameters to model the RBE with submillimeter spatial resolution. It has been shown that for a physical dose of 1.82Gy at the BP, the derived RBE based on the MKM model increased from 1.14 to 1.6 in the BP and its distal part. Good agreement was observed between the experimental and simulation results, confirming the potential application of SOI microdosimeter with 3D SV for quality assurance in proton therapy.

Abstract ID: 68 Heterogeneous multiscale simulations of radiation therapy with gold nanoparticles

Purpose An outstanding challenge for gold nanoparticle (GNP) dose-enhanced radiation therapy (GNPT) is accurate computation of energy deposition in microscopic volumes of interest within macroscopic tumour and normal tissues. This work presents calculations for GNPT within the Heterogeneous MultiScale (HetMS) model, a general Monte Carlo framework. Methods The HetMS approach involves combining distinct models of varying level of detail on different length scales within a single simulation. Using EGSnrc, GNPT HetMS models considering varying extra/intracellular GNP distributions are implemented. Groups of > 3000 cells containing discretely-modelled GNPs are embedded at different positions throughout a cylindrical phantom (2 cm diameter, 3 cm length) consisting of a gold-tissue mixture. Dose enhancement factors (DEFs; ratios of doses with and without gold present) are calculated for cell nucleus and cytoplasm compartments for various photon source energies, cell/nucleus sizes, and gold concentrations. HetMS simulations are validated via comparisons with simulations of > 1.5E10 discretely-modelled cells and GNPs throughout the cylindrical phantom. Results DEFs are highly sensitive to depth in phantom, GNP distribution and concentration, cell/nucleus size, and source energy. Considering an isolated cell (not at depth in phantom), nuclear DEFs change with cell/nucleus and GNP intracellular distribution, ranging from 1.6 (GNPs in an endosome, 5 mg of gold/g of tissue, 50 keV photons) to 18.7 (GNPs just outside the cell nucleus, 20 mg/g, 20 keV). Nuclear DEFs are consistently higher for GNPs clustered about the nuclear membrane compared to GNPs within endosomes in the cell cytoplasm. Nuclear DEFs decrease considerably with depth in the phantom, e.g., changes of up to 64% are observed over the first centimeter. HetMS simulation results are in agreement (within statistical uncertainties) with those of simulations of discrete cells and GNPs throughout the phantom, but are considerably more efficient with HetMS simulations > 1000 times faster. Conclusions This work demonstrates considerable variation in DEFs, due to effects such as decreasing fluence because of gold in the macroscopic phantom and local enhancement because of nanoparticle distribution within cells. Results underline the importance of quantifying DEFs across tumour volumes while considering intracellular GNP distribution. The HetMS approach enables accurate and relatively efficient simulations of GNPT.

Comparison of Measurements in Small Fields from an MR-LINAC

To investigate beam profiles for small square fields from a 1.5 T MRI – 7 MV linear accelerator system (MR-Linac) measured with Gafchromic film, Fricke gel films, and a microDiamond detector. EBT-3 radiochromic films were irradiated to 4 Gy at isocenter distance at 5 cm depth inside a water phantom. 2x2, 3x3, 5x5, and 10x10 cm2 fields were delivered by an MR-Linac while the magnetic field (B-field) was turned off. Fricke-type gels were manufactured in-house, poured into 5-mm thick sheets and stored at 4⁰C for 24 hours prior to irradiation. The gel sheets were placed at isocenter at 5 cm depth in a solid water phantom and irradiated to 4Gy for each field size. All gels and films were scanned with an Epson 10000XL flatbed scanner pre- and post-irradiation. In-plane and cross-plane beam profiles were measured for all four field sizes with a PTW60019 microDiamond detector at 5 cm depth inside a water phantom under the same conditions. In-plane and cross-plane profiles measured with the planar detectors were analyzed and compared to profiles obtained with the microDiamond detector. All measurements were repeated with the B-field turned on. With the B-field off, in-plane and cross-plane profiles measured with film agreed to approximately 1% with microDiamond measurements. Compared to the microdiamond data, the 10x10 cm2 cross-plane profile from film measurements indicated slightly more than 1% higher signal more than 3 cm away from the central axis. The 80/20 penumbra regions overlapped for both detectors. The shape of the cross-plane profiles measured with gel dosimetry conformed with both film and microDiamond. The gel measurements exhibited more signal variations inside the radiation field and reported lower doses in the penumbra. With the B-field on, the effect of the magnetic field was clearly seen. In-plane profiles appeared symmetric while cross-plane profiles were noticably asymmetric. Film and microDiamond measurements agreed within 1-2% for all four fields. The shape of the profiles measured with gels agreed with both film and microDiamond. In regions of non-uniform thickness the gels demonstrated variations in signal. Both microDiamond and EBT-3 film offer a valuable means to measure small fields with steep dose gradients from the MR-Linac. Maintaining good quality control during the fabrication process of gel sheets is crucial to obtain uniform sheet thickness. Further investigation of gel sheets will be pursued as an alternative to film measurements.

Comparison and Assessment of Profile and Output Factor Measurement for a Frameless Robotic Radiosurgery System Using Different Detectors

To compare and assess different detectors for the profile and output factor (OF) measurement for a Cyberknife system, and provide reference and suggestions for selecting and using the correct detectors. Profiles were acquired using the detectors of PTW-60017, PTW-60018, PTW-60019 and IBA-SFD, at different depths for different collimator sizes, with the detectors stem oriented both perpendicular and parallel to the central beam axis. The differences of profiles among different detectors and orientation of detectors were analyzed. Output factors were measured using PTW ionization chamber 30013, 31010, diode detector PTW 60017, PTW60018 and diamond detector 60019 for different fixed aperture collimators from 5 mm to 60 mm at SAD 800 mm and depth of water 50 mm. EBT3 film measured OF at 5 cm depth in a water phantom with a SAD of 800mm was used as reference for comparison. All full width at half maximum (FWHM) of field measured by four detectors in parallel orientation were larger than actual field size. The deviation increased with the size of collimator and measurement depth, with the maximal deviation of 0.6mm. The deviation of FWHM among four detectors increased with the increase of measurement depth, with the maximum deviation of 0.2mm. The penumbra was the smallest for IBA-SFD, and largest for PTW-60019. The maximal deviation of penumbra was 0.3mm. The IBA-SFD agreed with other detectors but tended to over-respond in the out-of-field region when the collimator size was larger than 30mm. Both FWHM and penumbra in perpendicular orientation were smaller than in parallel orientation for PTW-60017, PTW-60018 and PTW-60019, especially for the 5mm collimator. However, the trend was opposite for IBA-SFD. With the increase of collimator aperture, the difference between the right and left penumbra acquired by four detectors increased, with more obvious stem effects observed. A good agreement among the output factors obtained from these five detectors was found for the collimator size more than 30mm, with the deviations less than 1%. When the collimator size was less than 30mm, the deviations of the OF became large and obviously increased with the decrease of the collimator size. The measured OFs of PTW60019 were basically consistent with EBT3 film, with the deviation less than 2%. The OFs were higher for diodes detectors, and much smaller for Ionization chambers than EBT3 film. Similar profiles were acquired by PTW-60017, PTW-60018, PTW-60019 and IBA-SFD, but the detectors’ characteristics and effects of detector orientations should be considered. PTW 30013, 31010, PTW 60017, PTW60018 and 60019 could be directly used for the OFs measurement, when field was greater than 30mm. But if the field was less than 30mm, the measurement results obtained by these detectors need to be corrected. Sensitive volume, the effective measure length and material density of detector affected the measurement of OFs, especially for small filed.

Prompt gamma spectroscopy for range control with CeBr3

Abstract The ultimate goal of radiotherapy using external beams is to maximize the dose delivered to the tumor while minimizing the radiation given to surrounding healthy critical organs. Prompt Gamma Spectroscopy (PGS) has been proposed for range control of particle beams along with the determination of the elemental composition of irradiated tissues. We aim at developing a PGS system for the German Cancer Research Center – DKFZ that takes advantage of the superior selectivity of Helium and Carbon beams accelerated at the Heidelberg Ion-Beam Therapy Center. Preliminary tests with protons accelerated with an IBA C230 cyclotron located at the Universitäts Protonen Therapie Dresden were performed at OncoRay – National Center for Radiation Re-search in Oncology. We present results obtained with a PGS system composed of CeBr3 detectors (Ø 2’’ × 2’’) and (Ø 1.5’’ × 3’’) coupled to a Hamamatsu R13089 photomulti-plier tube and plugged to a Target U100 Spectrometer. Such system provides accurate time-of-flight measurements to increase the signal-to-noise ratio relative to neutron-induced background. First measurements resulted from the irradiation of PMMA and water phantoms, and graphite and aluminum bricks. Several PG energy lines ranging from 0.511 MeV up to 8 MeV were identified and compared with reference re-sults. Two further experiments consisted in irradiating PMMA phantoms in a slit- and semi-collimated configuration with mono-energetic proton beams of 165 MeV and 224 MeV, respectively. Results acquired by means of trans-versal PGS at different phantom depths, ranging from 6 cm before the Bragg peak (BP) to 3.5 cm beyond the BP in 5 mm steps with a 1 cm slit collimation (tungsten) showed a slight decrease of PG yields after the BP. Similar measurements with a semi-opened collimation configuration demonstrated a steeper decrease of PG yields after the BP.

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.

Carbon and oxygen minibeam radiation therapy: An experimental dosimetric evaluation.

To perform dosimetric characterization of a minibeam collimator in both carbon and oxygen ion beams to guide optimal setup geometry and irradiation for future radiobiological studies. Carbon and oxygen minibeams were generated using a prototype tungsten multislit collimator presenting line apertures 700μm wide, which are spaced 3500μm centre-to-centre distance apart. Several radiation beam spots generated the desired field size of 15×15mm2 and production of a 50mm long spread out Bragg peak (SOBP) centered at 80mm depth in water. Dose evaluations were performed with two different detectors: a PTW microDiamond® single crystal diamond detector and radiochromic films (EBT3). Peak-to-valley dose ratio (PVDR) values, output factors (OF), penumbras, and full width at half maximum (FWHM) were measured. Measured lateral dose profiles exhibited spatial fractionation of dose at depth in a water phantom in the expected form of peaks and valleys for both carbon and oxygen radiation fields. The diamond detector and radiochromic film provided measurements of PVDR in good agreement. PVDR values at shallow depth were about 60 and decreased to about 10 at 80mm depth in water. OF in the center of the SOBP was about 0.4; this value is larger than the corresponding one in proton minibeam radiation therapy measured using a comparable collimator due to a reduced lateral scattering for carbon and oxygen minibeams. Carbon and oxygen minibeams may be produced by a mechanical collimator. PVDR values and output factors measured in this first study of these minibeam radiation types indicate there is potential for their therapeutic use. Optimization of minibeam collimator design and the number and size of focal spots for irradiation are advocated to improve PDVR values and dose distributions for each specific applied use.

Measurement of neutron dose in the compensator IMRT treatment

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

Dose calculation for spot scanning proton therapy with the application of a range shifter

Purpose. To build a correction factor dataset to improve accuracy of dose calculations for spot-scanning proton therapy with a range shifter employed. Material and method. Our synchrotron-based spot scanning proton therapy system has an energy range of 72.5–221.8 MeV. The Bragg peak chamber was used to perform the integral depth dose measurements in a water phantom with different air-gap sizes between the range shifter and the water phantom. Three typical energies of 151.0, 181.1 and 219.3 MeV and three air-gap sizes of 10, 20 and 30 cm were selected to perform the measurements. The measured data were used as the standard metrics for evaluating the accuracy of Monte Carlo simulations. The Monte Carlo platform TOPAS was used to simulate the transport of protons in the nozzle with a range shifter and the water phantom. The Monte Carlo generated dose data were used to build the dataset of dose correction factors for air gap effect. A treatment field from a multi-field treatment plan of a brain cancer case was selected to demonstrate the clinical significance of applying the dose correction factors. Result. The beam parameters in Monte Carlo simulations were finely tuned to make the simulated dose distributions match the measurements with the mean difference in the magnitude of 1.0%. The Monte Carlo generated correction factors for different beam energies and different depths in the water phantom show that for each specific beam energy the dose correction factor increases with the increase of air gap and decreases with the increase of depth in water, and larger correction factors are needed for higher beam energies. The gamma pass rate for the selected brain treatment field was improved from 80.4% to 97.8% after applying the dose correction factors. Conclusion. Applying the dose correction factor dataset for different air gaps between the range shifter and the target is a practical, low-cost solution to improve the accuracy of dose calculations for SSPT treatment plans.

Study of 6 MV Photon Beam Dose Profile Peaks, Investigation and Evaluation of the Backscattering Effects on the Beam Dose Profiles

The dose profile peaks are mainly produced by the flattening filter of the head of the medical linear accelerator due to the backscattering effects on the beam dose profiles; in this study we have investigated the backscattering effects on the beam profile peaks; the purpose was to evaluate the backscattering effects at the beam edge due to the flattening filter shape by calculating the increasing relative dose on the beam profile peaks as functions of depth along the central axis and filed size. The accent is on the photon beam dose profiles at different depths in a water phantom along central beam axis for a set of irradiation field sizes. The measures of dose profiles of the 6 MV photon beam in a water phantom, source to surface distance is 100 cm, for the set of the irradiation field sizes of 3×3 cm², 6×6 cm², 10×10 cm², 15×15 cm², and 20×20 cm² at different depths on the beam central axis, the linear accelerator is VARIAN Clinac 2100. First, we have plotted the dose profiles as function of the normalized depth to show the beam profile peaks; then we have calculated the increasing relative dose on the beam profile peaks as a function of the field size and depth along central beam axis from the measurements data. Then we have normalized the increasing relative dose on the beam profile peaks to the increasing relative dose on the beam profile peak of the standard filed size of 10×10 cm². We have plotted the results for interpretation and evaluation the backscattering effects on the beam dose profiles; the pervious study have shown that the flattening filter produced a flat profile at a depth of 10 cm and beam profile peaks; and in our study we have quantified the increasing relative dose on the beam profile peaks; then, we have shown that the variation of the normalized increasing relative dose presented the peaks at depth of 10 cm along the central beam axis, e.g. We have found that the increasing relative dose at depth of 10 cm along central beam axis for the field size of 25×25 cm² is almost 20 times of the standard field size at the same depth, but the increasing relative dose is very close for both filed sizes at a depth of 20 cm along central beam axis. Throughout this work we have use a method to evaluate the backscattering effects on the dose profiles; we have found the backscattering effects are very high at a depth 10 cm for the large irradiation field size.

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.

Photoneutron depth dose equivalent distributions in high-energy X-ray medical accelerators by a novel position-sensitive dosimeter

PurposeThe purpose of this study was to; (1) investigate employing a novel position-sensitive mega-size polycarbonate (MSPC) dosimeter for photoneutron (PN) depth, profile and dose equivalent distributions studies in a multilayer polyethylene phantom in a Siemens ONCOR accelerator, and (2) develop depth dose equivalent distribution matrix data at different depths and positions of the phantom for patient PN dose equivalent determination and in particular for PN secondary cancer risk estimation. MethodsPosition-sensitive MSPC dosimeters were successfully exposed at 9 different depths of the phantom in a 10×10cm2 X-ray field. The dosimeters were processed in mega-size electrochemical chambers at optimum conditions. Each MSPC dosimeter was placed at a known phantom depth for PN depth dose equivalents and profiles on transverse, longitudinal and diagonal axes and isodose equivalent distribution studies in and out of the X-ray beam. ResultsPN dose equivalent distributions at any depth showed the highest value at the beam central axis and decreases as the distance increases. PN dose equivalent at any position studied in the axes has a maximum value on the phantom surface which decreases as depth increases due to flux reduction by multi-elastic scattering interactions. ConclusionsExtensive PN dose equivalent matrix data at different depths and positions in the phantom were determined. The position-sensitive MSPC dosimeters proved to be highly efficient for PN depth, profile and isodose equivalent distribution studies. The extensive data obtained highly assists for determining PN dose equivalent of a patient undergoing high-energy X-ray therapy and for PN secondary cancer risk estimation.

Mailed dosimetric audit of therapeutic proton beams using thermoluminescence MTS-N (LiF:Mg,Ti) powder – First results

Institute of Oncology in Warsaw provides mailed dosimetric service of radiotherapy units in Poland according to the procedure implemented by IAEA. The service applies the dosimetric thermoluminescence (TLD) system with detectors in form of capsules filled with MTS-N (LiF: Mg, Ti) powder. In these studies dosimetric properties of the TLD system were determined to adapt it for mailed dosimetric audits of proton radiotherapy units. Dose response of TLD capsules filled with MTS-N powder was studied for doses between 1.75 Gy and 2.25Gy in the middle of 5 × 10 × 10 cm3 Spread Out Bragg Peak (SOBP). The SOBP was located at the different position (depth) in the water phantom in order to determine the energy dependence of the system. All exposures were performed at the isocentric gantry with Pencil Beam Scanning field at the Bronowice Cyclotron Center, Institute of Nuclear Physics (IFJ PAN). It was found that for TLD system with MTS-N powder the correction for the supralinear dose response should be applied for doses exceeding 2 Gy. No significant influence of response on the position (depth) of SOBP was observed. The preliminary results show that the standard mailed TLD dosimetry system based on MTS-N (LiF:Mg,Ti) can be adapted for auditing proton radiotherapy units.

Proton therapy dosimetry using the scintillation of the silica fibers.

We investigate the feasibility of proton therapy dose measurement by using scintillation of a bare silica glass fiber. The emission spectra of the optical fiber at various depths in tissue-mimicking phantoms, irradiated with proton beams of energies 100-225MeV show two distinct peaks at 460 and 650nm whose nature is connected with the silica point defects. Our experimental results and Monte Carlo simulation showed that the Čerenkov radiation cannot be responsible for such a phenomenon. We showed that the intensity of the peak at 650nm correlates with the proton dose with a minimal effect of ionization quenching, while the intensity peak at 460nm under-reports the radiation dose.

RBE study using solid state microdosimetry in heavy ion therapy

The response of two types of 10 μm thick silicon-on-insulator (SOI) detectors in a 12C ion therapy beam with extremely high spatial resolution are presented in this work. The two detectors used are the “bridge” microdosimeter with isolated 3D sensitive volumes (SVs) and the ultra-thin 3D (U3DTHIN) detector with n+ and p+ 3D columnar structures. Both detectors were investigated at various depths in a water phantom along the central axis of the spread-out Brag peak (SOBP) of a 290 MeV/u 12C ion beam at the Heavy Ion Medical Accelerator in Chiba, Japan. Based on the Microdosimetric Kinetic (MK) model and the microdosimetric quantities measured with the two detectors, the relative biological effectiveness (RBE) values were derived and compared to results obtained with the tissue-equivalent proportional counter (TEPC). Derived RBE10 values obtained with the U3DTHIN detector were considerably higher than those obtained with the bridge microdosimeter and the TEPC along the SOBP. Due to the high spatial resolution of the microdosimeters, more detailed measurements were obtained at the end of the SOBP compared to the TEPC. The maximum derived RBE10 found using the U3DTHIN detector and bridge microdosimeter were approximately 2.66 and 2.58, respectively which are higher than the RBE10 value of 2.35 obtained with the TEPC due to the lack of high spatial resolution in the TEPC. The discrepancy in the results obtained using the two detectors is due to the difference in geometry of the SVs in the two detectors. This work presents an application of different types of SOI micodosimeters in a 12C ion therapy beam and has demonstrated that the microdosimeter with micron sized 3D SVs is more desirable for accurate lineal energy measurement and RBE determination. Silicon microdosimetry has demonstrated a simple, fast and accurate method for routine Quality Assurance in charged particle therapy.

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.