Depth Of Maximum Dose Research Articles

Commissioning and validation of the electron Monte Carlo dose calculation at extended source to surface distance from a medical linear accelerator

Introduction: Radiotherapy is one of the major modality for cancer management playing curative, adjuvant, and palliative and sometimes has an alternative role to chemotherapy. Radiotherapy is practiced in two ways viz. External beam therapy and Brachytherapy. Electron beam therapy is widely used in the management of cancers. An electron beam is characterized by a finite range of penetration with a rapid dose fall off towards a slowly decaying x-ray background as the electrons traverse through tissues. The electron monte carlo (eMC) dose calculation algorithm for eclipse treatment system has been introduced by Varian Medical systems. The algorithm is commissioned and validated by comparing percentage depth dose (PDD) and gamma index. Methods: Percentage depth dose curves were generated for all the energies for 4x4 cm2 and 10x10 cm2 field sizes. The depth of maximum dose (R100), therapeutic depth (R85), depth of 50% isodose (R50) and the relative surface (Ds) were compared with the measured and calculated PDD curves. Results: The eMC calculated fluence and measured fluence were compared for all the energies and cones at nominal source to surface distance and extended distances. For 4x4 cm2 field size the maximum shift in R100 was 5 mm, R85 was 1.9 mm, R50 was 0.9 mm and the variation in the relative surface (DS) was about 25Gamma analysis shows excellent agreements with greater than 98% of the pixels passing the gamma requirements. Conclusion: We have successfully commissioned and validated the electron monte carlo dose calculation at extended source to surface distance.

Alternate calibration method of radiochromic EBT3 film for quality assurance verification of clinical radiotherapy treatments

EBT3 film is utilized as a dosimetry quality assurance tool for the verification of clinical radiotherapy treatments. In this work, we suggest a percentage-depth-dose (PDD) calibration method that can calibrate several EBT3 film pieces together at different dose levels because photon beams provide different dose levels at different depths along the axis of the beam. We investigated the feasibility of the film PDD calibration method based on PDD data and compared the results those from the traditional film calibration method. Photon beams at 6 MV were delivered to EBT3 film pieces for both calibration methods. For the PDD-based calibration, the film pieces were placed on solid phantoms at the depth of maximum dose (dmax) and at depths of 3, 5, 8, 12, 17, and 22 cm, and a photon beam was delivered twice, at 100 cGy and 400 cGy, to extend the calibration dose range under the same conditions. Fourteen film pieces, to maintain their consistency, were irradiated at doses ranging from approximately 30 to 400 cGy for both film calibrations. The film pieces were located at the center position on the scan bed of an Epson 1680 flatbed scanner in the parallel direction. Intensity-modulated radiation therapy (IMRT) plans were created, and their dose distributions were delivered to the film. The dose distributions for the traditional method and those for the PDD-based calibration method were evaluated using a Gamma analysis. The PDD dose values using a CC13 ion chamber and those obtained by using a FC65-G Farmer chamber and measured at the depth of interest produced very similar results. With the objective test criterion of a 1% dosage agreement at 1 mm, the passing rates for the four cases of the three IMRT plans were essentially identical. The traditional and the PDD-based calibrations provided similar plan verification results. We also describe another alternative for calibrating EBT3 films, i.e., a PDD-based calibration method that provides an easy and time-saving approach to film calibration.

SU‐F‐T‐524: Investigation of the Dosimertric Benefits of Interchangeable Source Size of a Novel Rotating Gamma System

Purpose:Tremendous technological developments were made for conformal therapy techniques with linear accelerators, while less attention was paid to cobalt‐60 units. The aim of the current study is to explore the dosimetric benefits of a novel rotating gamma ray system enhanced with interchangeable source sizes and multi‐leaf collimator (MLC).Material and Methods:CybeRT is a novel rotating gamma ray machine with a ring gantry that ensures an iso‐center accuracy of less than 0.3 mm. The new machine has a 70cm source axial distance allowing for improved penumbra compared to conventional machines. MCBEAM was used to simulate Cobalt‐60 beams from the CybeRT head, while the MCPLAN code was used for modeling the MLC and for phantom/patient dose calculation. The CybeRT collimation will incorporate a system allowing for interchanging source sizes. In this work we have created phase space files for 1cm and 2cm source sizes. Evaluation of the system was done by comparing CybeRT beams with the 6MV beams in a water phantom and in patient geometry. Treatment plans were compared based on isodose distributions and dose volume histograms.Results:Profiles for the 1cm source were comparable to that from 6MV in the order of 6mm for 10×10 cm2 field size at the depth of maximum dose. This could ascribe to Cobalt‐60 beams producing lowerenergy secondary electrons. Although, the 2cm source have a larger penumbra however it could be still used for large targets with proportionally increased dose rate. For large lung targets, the difference between cobalt and 6MV plans is clinically insignificant. Our preliminary results showed that interchanging source sizes will allow cobalt beams for volumetric arc therapy of both small lesions and large tumors.Conclusion:The CybeRT system will be a cost effective machine capable of performing advanced radiation therapy treatments of both small tumors and large target volumes.

TU‐AB‐BRA‐12: Quality Assurance of An Integrated Magnetic Resonance Image Guided Adaptive Radiotherapy Machine Using Cherenkov Imaging

Purpose:To investigate the viability of using Cherenkov imaging as a fast and robust method for quality assurance tests in the presence of a magnetic field, where other instruments can be limited.Methods:Water tank measurements were acquired from a clinically utilized adaptive magnetic resonance image guided radiation therapy (MR‐IGRT) machine with three multileaf‐collimator equipped 60Co sources. Cherenkov imaging used an intensified charge coupled device (ICCD) camera placed 3.5m from the treatment isocenter, looking down the bore of the 0.35T MRI into a water tank. Images were post‐processed to make quantitative comparison between Cherenkov light intensity with both film and treatment planning system predictions, in terms of percent depth dose curves as well as lateral beam profile measurements. A TG‐119 commissioning test plan (C4: C‐Shape) was imaged in real‐time at 6.33 frames per second to investigate the temporal and spatial resolution of the Cherenkov imaging technique.Results:A .33mm/pixel Cherenkov image resolution was achieved across 1024×1024 pixels in this setup. Analysis of the Cherenkov image of a 10.5×10.5cm treatment beam in the water tank successfully measured the beam width at the depth of maximum dose within 1.2% of the film measurement at the same point. The percent depth dose curve for the same beam was on average within 2% of ionization chamber measurements for corresponding depths between 3–100mm. Cherenkov video of the TG‐119 test plan provided qualitative agreement with the treatment planning system dose predictions, and a novel temporal verification of the treatment.Conclusions:Cherenkov imaging was successfully used to make QA measurements of percent depth dose curves and cross beam profiles of MRI‐IGRT radiotherapy machines after only several seconds of beam‐on time and data capture; both curves were extracted from the same data set. Video‐rate imaging of a dynamic treatment plan provided new information regarding temporal dose deposition.This study has been funded by NIH grants R21EB17559 and R01CA109558, as well as Norris Cotton Cancer Center Pilot funding.

TH-CD-201-06: Experimental Characterization of Acoustic Signals Generated in Water Following Clinical Photon and Electron Beam Irradiation

Purpose: Previous work has demonstrated the detectability of acoustic waves induced following the irradiation of high density metals with radiotherapy linac photon beams. This work demonstrates the ability to experimentally detect such acoustic signals following both photon and electron irradiation in a more radiotherapy relevant material. The relationship between induced acoustic signal properties in water and the deposited dose distribution is explored, and the feasibility of exploiting such signals for radiotherapy dosimetry is demonstrated. Methods: Acoustic waves were experimentally induced in a water tank via the thermoacoustic effect following a single pulse of photon or electron irradiation produced by a clinical linac. An immersion ultrasound transducer was used to detect these acoustic waves in water and signals were read out on an oscilloscope. Results: Peaks and troughs in the detected acoustic signals were found to correspond to the location of gradients in the deposited dose distribution following both photon and electron irradiation. Signal amplitude was linearly related to the dose per pulse deposited by photon or electron beams at the depth of detection. Flattening filter free beams induced large acoustic signals, and signal amplitude decreased with depth after the depth of maximum dose. Varying the field size resulted in a temporal shift of the acoustic signal peaks and a change in the detected signal frequency. Conclusion: Acoustic waves can be detected in a water tank following irradiation by linac photon and electron beams with basic electronics, and have characteristics related to the deposited dose distribution. The physical location of dose gradients and the amount of dose deposited can be inferred from the location and magnitude of acoustic signal peaks. Thus, the detection of induced acoustic waves could be applied to photon and electron water tank and in vivo dosimetry. This work was supported in part by CIHR grants MOP-114910 and MOP-136774. S.H. acknowledges support by the NSERC CREATE Medical Physics Research Training Network grant 432290.

SU‐F‐T‐460: Dosimetric Matching Between Trilogy Tx and TrueBeam STx

Purpose:To compare the commissioned beam data for one flattening filter photon mode (6 MV) and two flattening filter‐free (FFF) photon modes (6 and 10 MV‐FFF) between Trilogy Tx and TrueBeam STx and evaluate the possibility of dosimetric matchingMethods:Dosimetric characteristics of the new Trilogy Tx including percent depth doses (PDDs), profiles, and output factors were measured for commissioning. Linear diode array detector and ion chambers were used to measure dosimetric data. The depth of dose maximum (dmax) and PDD at 10 cm (PDD10) were evaluated: 3×3 cm2, 10×10 cm2, and 40×40 cm2. The beam profiles were compared and then penumbras were evaluated. As a further test of the dosimetric matching, the same VMAT plans were delivered, measured with film, and compared with TPS calculation.Results:All the measured PDDs matched well across the two units. PDD10 showed less than 0.5% variation and dmax were within 1.5 mm at the field sizes evaluated. Within the central 80% of transverse axis, profile data were almost identical. TrueBeam data resulted in a slightly greater penumbra width (up to 1.9 mm). The greatest differences of output factors were found at 40 × 40 cm2: 2.40%, 2.03%, and 2.22% for 6 MV, 6 MV‐FFF, and 10 MV‐FFF, respectively. For smaller field sizes, less than 1% differences were observed. The film measurements demonstrated over 97.3% pixels passing‐gamma analysis (2%/2mm). The results showed excellent agreement between measurements of two machines.Conclusion:The differences between Trilogy Tx and TrueBeam STx found could possibly affect small field and also very large field sizes in dosimetric matching considerations. These differences encountered are mostly related with the changes in the head design of the TrueBeam. Although it cannot guarantee full interchangeability of two machines, dosimetric matching by field size of 25 × 25 cm2 might be clinically acceptable.

SU‐F‐T‐663: Cerenkov Radiation Enhanced Radiotherapy with Titanium Dioxide Nanoparticle: A Monte Carlo Study

Purpose:A recent publication has shown that by delivering titanium dioxide nanoparticles (titania) as a photosensitizer into tumors, Cerenkov radiation (CR) produced by radionuclides could be used for substantially boosting damage to cancer cells. The present work compares CR production by various clinically relevant radiation sources including internal radionuclides and external beam radiotherapy (EBRT), and provides preliminarily computational results of CR absorption by titania.Methods:1) Geant4.10.1 was used to simulate ionizing radiation‐induced CR production in a 1cm diameter spherical volume using external radiotherapy sources: Varian Clinac IX 6MV and Eldorado 60Co, both with 10*10 cm2 field size. In each case the volume was placed at the maximum dose depth (1.5cm for 6MV source and 0.5cm for 60Co). In addition, 18F, 192Ir and 60Co were simulated using Geant4 radioactive decay models as internal sources. Dose deposition and CR production spectra in 200nm‐400nm range were calculated as it is the excitation range of titania. 2) Using 6MV external source, the absorption by titania was calculated via the track length of CR in the spherical volume. The nanoparticle concentration was varied from 0.25 to 5µg/g.Results:Among different radioactive sources, results showed that 18F induced the highest amount of CR per disintegration, but 60Co had the highest yield per unit dose. When compared with external sources, 6MV source was shown to be the most efficient for the the same delivered dose. Simulations indicated increased absorption for increasing concentrations, with up to 68% absorption of generated CR for 5µg/g titania concentration.Conclusion:The results demonstrate that 6MV beam is favored with a higher CR yield, compared to radionuclides, and that the use of higher concentrations of titania may increase photosensitization. From the findings, we propose that if sufficiently potent concentrations of titania are delivered to tumors this could substantially boost EBRT.

SU‐F‐T‐428: An Optimization‐Based Commissioning Tool for Finite Size Pencil Beam Dose Calculations

Purpose:Finite size pencil beam (FSPB) algorithms are commonly used to pre‐calculate the beamlet dose distribution for IMRT treatment planning. FSPB commissioning, which usually requires fine tuning of the FSPB kernel parameters, is crucial to the dose calculation accuracy and hence the plan quality. Yet due to the large number of beamlets, FSPB commissioning could be very tedious. This abstract reports an optimization‐based FSPB commissioning tool we have developed in MatLab to facilitate the commissioning.Methods:A FSPB dose kernel generally contains two types of parameters: the profile parameters determining the dose kernel shape, and a 2D scaling factors accounting for the longitudinal and off‐axis corrections. The former were fitted using the penumbra of a reference broad beam's dose profile with Levenberg‐Marquardt algorithm. Since the dose distribution of a broad beam is simply a linear superposition of the dose kernel of each beamlet calculated with the fitted profile parameters and scaled using the scaling factors, these factors could be determined by solving an optimization problem which minimizes the discrepancies between the calculated dose of broad beams and the reference dose.Results:We have commissioned a FSPB algorithm for three linac photon beams (6MV, 15MV and 6MVFFF). Dose of four field sizes (6*6cm2, 10*10cm2, 15*15cm2 and 20*20cm2) were calculated and compared with the reference dose exported from Eclipse TPS system. For depth dose curves, the differences are less than 1% of maximum dose after maximum dose depth for most cases. For lateral dose profiles, the differences are less than 2% of central dose at inner‐beam regions. The differences of the output factors are within 1% for all the three beams.Conclusion:We have developed an optimization‐based commissioning tool for FSPB algorithms to facilitate the commissioning, providing sufficient accuracy of beamlet dose calculation for IMRT optimization.

Determining the Effective Source-Surface Distance for Therapeutic Electron Beams

Background: The source point of the irradiated electron beam must be considered to estimate the output factor and dose distribution during electron therapy. Objectives: The aim of this study was to determine the effective source-surface distance (SSDeff) of an electron linear accelerator (Linac), and its dependence on energy and depth. Materials and Methods: A Varian Linac 2100CD with electron energies of 4, 6, 9, 12, and 15 MeV, electron applicator size of 20 × 20 cm2, nominal SSDs of 97 to 113 cm, and air gaps of 2 to 18 cm were studied. Using a Farmer (0.13 cc) ionizing chamber, the percentage depth doses were measured in the water phantom (50 cm3) and then the SSDeff was calculated by applying the inverse square law. Results: For the 100% PDD, the SSDeff values were calculated as 79, 91, 92, 93, and 92 cm for 4, 6, 9, 12, and 15 MeV, respectively. At a depth with a certain PDD, increasing energy also increases SSDeff, and a similar increase is observed at a distinctive energy by increasing the PDD. Conclusions: Using the maximum dose depth from PDD curves and the inverse square law, the required SSDeff to calculate the dose distribution of the electron beam can be calculated.

Absorbed dose measurements in the build-up region of flattened versus unflattened megavoltage photon beams

This study evaluated absorbed dose measurements in the build-up region of conventional (FF) versus flattening filter-free (FFF) photon beams. The absorbed dose in the build-up region of static 6 and 10MV FF and FFF beams was measured using radiochromic film and extrapolation chamber dosimetry for single beams with a variety of field sizes, shapes and positions relative to the central axis. Removing the flattening filter generally resulted in slightly higher relative build-up doses. No considerable impact on the depth of maximum dose was found.

Electron beam energy QA - a note on measurement tolerances.

Monthly QA is recommended to verify the constancy of high‐energy electron beams generated for clinical use by linear accelerators. The tolerances are defined as 2%/2 mm in beam penetration according to AAPM task group report 142. The practical implementation is typically achieved by measuring the ratio of readings at two different depths, preferably near the depth of maximum dose and at the depth corresponding to half the dose maximum. Based on beam commissioning data, we show that the relationship between the ranges of energy ratios for different electron energies is highly nonlinear. We provide a formalism that translates measurement deviations in the reference ratios into change in beam penetration for electron energies for six Elekta (6‐18 MeV) and eight Varian (6‐22 MeV) electron beams. Experimental checks were conducted for each Elekta energy to compare calculated values with measurements, and it was shown that they are in agreement. For example, for a 6 MeV beam a deviation in the measured ionization ratio of ±15% might still be acceptable (i.e., be within ±2 mm), whereas for an 18 MeV beam the corresponding tolerance might be ±6%. These values strongly depend on the initial ratio chosen. In summary, the relationship between differences of the ionization ratio and the corresponding beam energy are derived. The findings can be translated into acceptable tolerance values for monthly QA of electron beam energies.PACS number(s): 87.55, 87.56

Analytical Consideration of Surface Dose and Kerma for Megavoltage Photon Beams in Clinical Radiation Therapy.

In radiation therapy, estimation of surface doses is clinically important. This study aimed to obtain an analytical relationship to determine the skin surface dose, kerma and the depth of maximum dose, with energies of 6 and 18 megavoltage (MV). To obtain the dose on the surface of skin, using the relationship between dose and kerma and solving differential equations governing the two quantities, a general relationship of dose changes relative to the depth was obtained. By dosimetry all the standard square fields of 5x5cm to 40x40cm, an equation similar to response to differential equations of the dose and kerma were fitted on the measurements for any field size and energy. Applying two conditions: a) equality of the area under dose distribution and kerma changes in versus depth in 6 and 18 MV, b) equality of the kerma and dose at x=dmax and using these results, coefficients of the obtained analytical relationship were determined. By putting the depth of zero in the relation, amount of PDD and kerma on the surface of the skin, could be obtained. Using the MATLAB software, an exponential binomial function with R-Square >0.9953 was determined for any field size and depth in two energy modes 6 and 18MV, the surface PDD and kerma was obtained and both of them increase due to the increase of the field, but they reduce due to increased energy and from the obtained relation, depth of maximum dose can be determined. Using this analytical formula, one can find the skin surface dose, kerma and thickness of the buildup region.

Cherenkov imaging method for rapid optimization of clinical treatment geometry in total skin electron beam therapy.

A method was developed utilizing Cherenkov imaging for rapid and thorough determination of the two gantry angles that produce the most uniform treatment plane during dual-field total skin electron beam therapy (TSET). Cherenkov imaging was implemented to gather 2D measurements of relative surface dose from 6 MeV electron beams on a white polyethylene sheet. An intensified charge-coupled device camera time-gated to the Linac was used for Cherenkov emission imaging at sixty-two different gantry angles (1° increments, from 239.5° to 300.5°). Following a modified Stanford TSET technique, which uses two fields per patient position for full body coverage, composite images were created as the sum of two beam images on the sheet; each angle pair was evaluated for minimum variation across the patient region of interest. Cherenkov versus dose correlation was verified with ionization chamber measurements. The process was repeated at source to surface distance (SSD) = 441, 370.5, and 300 cm to determine optimal angle spread for varying room geometries. In addition, three patients receiving TSET using a modified Stanford six-dual field technique with 6 MeV electron beams at SSD = 441 cm were imaged during treatment. As in previous studies, Cherenkov intensity was shown to directly correlate with dose for homogenous flat phantoms (R(2) = 0.93), making Cherenkov imaging an appropriate candidate to assess and optimize TSET setup geometry. This method provided dense 2D images allowing 1891 possible treatment geometries to be comprehensively analyzed from one data set of 62 single images. Gantry angles historically used for TSET at their institution were 255.5° and 284.5° at SSD = 441 cm; however, the angles optimized for maximum homogeneity were found to be 252.5° and 287.5° (+6° increase in angle spread). Ionization chamber measurements confirmed improvement in dose homogeneity across the treatment field from a range of 24.4% at the initial angles, to only 9.8% with the angles optimized. A linear relationship between angle spread and SSD was observed, ranging from 35° at 441 cm, to 39° at 300 cm, with no significant variation in percent-depth dose at midline (R(2) = 0.998). For patient studies, factors influencing in vivo correlation between Cherenkov intensity and measured surface dose are still being investigated. Cherenkov intensity correlates to relative dose measured at depth of maximum dose in a uniform, flat phantom. Imaging of phantoms can thus be used to analyze and optimize TSET treatment geometry more extensively and rapidly than thermoluminescent dosimeters or ionization chambers. This work suggests that there could be an expanded role for Cherenkov imaging as a tool to efficiently improve treatment protocols and as a potential verification tool for routine monitoring of unique patient treatments.

Analysis of small field percent depth dose and profiles: Comparison of measurements with various detectors and effects of detector orientation with different jaw settings.

The advent of modern technologies in radiotherapy poses an increased challenge in the determination of dosimetric parameters of small fields that exhibit a high degree of uncertainty. Percent depth dose and beam profiles were acquired using different detectors in two different orientations. The parameters such as relative surface dose (DS), depth of dose maximum (Dmax), percentage dose at 10 cm (D10), penumbral width, flatness, and symmetry were evaluated with different detectors. The dosimetric data were acquired for fields defined by jaws alone, multileaf collimator (MLC) alone, and by MLC while the jaws were positioned at 0, 0.25, 0.5, and 1.0 cm away from MLC leaf-end using a Varian linear accelerator with 6 MV photon beam. The accuracy in the measurement of dosimetric parameters with various detectors for three different field definitions was evaluated. The relative DS(38.1%) with photon field diode in parallel orientation was higher than electron field diode (EFD) (27.9%) values for 1 cm ×1 cm field. An overestimation of 5.7% and 8.6% in D10 depth were observed for 1 cm ×1 cm field with RK ion chamber in parallel and perpendicular orientation, respectively, for the fields defined by MLC while jaw positioned at the edge of the field when compared to EFD values in parallel orientation. For this field definition, the in-plane penumbral widths obtained with ion chamber in parallel and perpendicular orientation were 3.9 mm, 5.6 mm for 1 cm ×1 cm field, respectively. Among all detectors used in the study, the unshielded diodes were found to be an appropriate choice of detector for the measurement of beam parameters in small fields.

On beam quality and flatness of radiotherapy megavoltage photon beams.

Ratio of percentage depth dose (PDD) at two depths, PDD at a depth of 10 cm (PDD10), and beam flatness are monitored regularly for radiotherapy beams for quality assurance. The purpose of this study is to understand the effects of changes in one of these parameters on the other. Is it possible to monitor only the beam flatness and not PDD? The investigation has two components. Naturally occurring i.e., unintended changes in PDD ratio and in-plane flatness for 6 and 10 MV photon beams for one particular Siemens Artiste Linac are monitored for a period of about 4 years. Secondly, deliberate changes in the beam parameters are induced by changing the bending magnet current (BMI). Relationships between various beam parameters for unintended changes as well as deliberate changes are characterized. Long term unintentional changes of PDD ratio are found to have no systematic trend. The flatness in the inplane direction for 6 and 10 MV beams show slow increase of 0.43 and 0.75% respectively in about 4 years while the changes in the PDD ratio show no such trend. Over 10% changes in BMI are required to induce changes in the beam quality indices at 2% level. PDD ratio for the 10 MV beam is found to be less sensitive, while the depth of maximum dose, d(max), is more sensitive to the changes in BMI compared to the 6 MV beam. Tolerances are more stringent for PDD10 than PDD ratio for the 10 MV beam. PDD ratio, PDD10, and flatness must be monitored independently. Furthermore, off axis ratio alone cannot be used to monitor flatness. The effect of beam quality change in the absolute dose is clinically insignificant.

2D EPID dose calibration for pretreatment quality control of conformal and IMRT fields: A simple and fast convolution approach

PurposeThis work presents an original algorithm that converts the signal of an electronic portal imaging device (EPID) into absorbed dose in water at the depth of maximum. MethodsThe model includes a first image pre-processing step that accounts for the non-uniformity of the detector response but also for the perturbation of the signal due to backscatter radiation. Secondly, the image is converted into absorbed dose to water through a linear conversion function associated with a dose redistribution kernel. These two computation parameters were modelled by correlating the on-axis EPID signal with absorbed dose measurements obtained on square fields by using an ionization chamber placed in water at the depth of maximum dose. The accuracy of the algorithm was assessed by comparing the dose determined from the EPID signal with the dose derived by the treatment planning system (TPS) using the ϒ-index. These comparisons were performed on 8 conformal radiotherapy treatment fields (3DCRT) and 18 modulated fields (IMRT). ResultsFor a dose difference and a distance-to-agreement set to 3% of the maximum dose and 2 mm respectively, the mean percentage of points with a ϒ-value less than or equal to 1 was 99.8% ± 0.1% for 3DCRT fields and 96.8% ± 2.7% for IMRT fields. Moreover, the mean gamma values were always less than 0.5 whatever the treatment technique. ConclusionThese results confirm that our algorithm is an accurate and suitable tool for clinical use in a context of IMRT quality assurance programmes.

Dosimetric comparison of water phantoms, ion chambers, and data acquisition modes for LINAC characterization

PurposeIn this study a dosimetric comparison utilizing continuous data acquisition and discrete data acquisition is examined using IBA Blue Phantom (IBA Dosimetry, Schwarzenbruck, Germany) and PTW (PTW, Freiberg, Germany) MP3-M water tanks. The tanks were compared according to several factors including set up time, ease of use, and data acquisition times. A tertiary objective is to study the response of several ionization chambers in the two tanks examined. MethodsMeasurements made using a Varian 23EX LINAC (Varian Medical Systems, Palo Alto, CA) include PDDs and beam profiles for various field sizes with IBA CC13, PTW Semiflex 31010, PTW Pinpoint N31016, and PTW 31013 ion chambers for photons (6, 18 MV) and electrons (6, 9, 12, 15, and 18 MeV). Radial and transverse profile scans were done at depths of maximum dose, 5 cm, 10 cm, and 20 cm using the same set of tanks and detectors for the photon beams. Radial and transverse profile scans were done at depth of maximum dose for the electron beams on the same tanks and chambers. Data processing and analysis was performed using PTW's MEPHYSTO Navigator software and IBA's OmniPro Accept version 6.6 for the respective water tank systems. ResultsPDD values agree to within 1% and dmax to within 1 mm for the PTW MP3-M tank using PTW 31010 and Blue Phantom using IBA CC13 chamber, respectively and larger discrepancy with the PTW PinPoint N31016 chamber at 6 MV. With respect to setup time the PTW MP3-M and IBA Blue phantom tank took about 20 and 40 min, respectively. Scan times were longer by 5–15 min per field size in the PTW MP3-M tank for the square field sizes from 1 cm to 40 cm as compared to the IBA Blue phantom. However, data processing times were higher by 7 min per field size with the IBA system. ConclusionsTank measurements showed little deviation with the higher energy photons as compared to the lower energy photons with regards to the PDD measurements. Chamber construction as well as tank set up may be causing the slight deviation in data. It is important to identify the exact source of the potential errors to ensure that proper tank usage is performed when making such measurements to ensure that patient safety is in compliance. Beam profiles done with different chambers and tanks showed little to no deviation from one to another. With regards to continuous versus discrete data measurements the main difference was in the data processing technique used. Discrete data obtained required less data processing as compared to the continuous data acquired.

Angular dependence of dose sensitivity of nanoDot optically stimulated luminescent dosimeters in different radiation geometries.

A type of in vivo dosimeter, an optically stimulated luminescent dosimeter, OSLD, may have dose sensitivity that depends on the angle of incidence of radiation. This work measures how angular dependence of a nanoDot changes with the geometry of the phantom in which irradiation occurs and with the intrinsic structure of the nanoDot. The OSLDs used in this work were nanoDot dosimeters (Landauer, Inc., Glenwood, IL), which were read with a MicroStar reader (Landauer, Inc., Glenwood, IL). Dose to the OSLDs was delivered by 6 MV x-rays. NanoDots with various intrinsic sensitivities were irradiated in numerous phantoms that had geometric shapes of cylinders, rectangles, and a cube. No angular dependence was seen in cylindrical phantoms, cubic phantoms, or rectangular phantoms with a thickness to width ratio of 0.3 or 1.5. An angular dependence of 1% was observed in rectangular phantoms with a thickness to width of 0.433-0.633. A group of nanoDots had sensitive layers with mass density of 2.42-2.58 g/cm(3) and relative sensitivity of 0.92-1.09 and no difference in their angular dependence. Within experimental uncertainty, nanoDot measurements agree with a parallel-plate ion chamber at a depth of maximum dose. When irradiated in cylindrical, rectangular, and cubic phantoms, nanoDots show a maximum angular dependence of 1% or less at an incidence angle of 90°. For a sample of 78 new nanoDots, the range of their relative intrinsic sensitivity is 0.92-1.09. For a sample of ten nanoDots, on average, the mass in the sensitive layer is 73.1% Al2O3:C and 26.9% polyester. The mass density of the sensitive layer of a nanoDot disc is between 2.42 and 2.58 g/cm(3). The angular dependence is not related to Al2O3:C loading of the nanoDot disc. The nanoDot at the depth of maximum dose has no more angular dependence than a parallel-plate ion chamber.

Spectroscopic separation of Čerenkov radiation in high-resolution radiation fiber dosimeters.

We have investigated Čerenkov radiation generated in phosphor-based optical fiber dosimeters irradiated with clinical electron beams. We fabricated two high-spatial resolution fiber-optic probes, with 200 and 400 μm core diameters, composed of terbium-based phosphor tips. A generalizable spectroscopic method was used to separate Čerenkov radiation from the transmitted signal by the fiber based on the assumption that the recorded signal is a linear superposition of two basis spectra: characteristic luminescence of the phosphor medium and Čerenkov radiation. We performed Monte Carlo simulations of the Čerenkov radiation generated in the fiber and found a strong dependence of the recorded Čerenkov radiation on the numerical aperture of the fiber at shallow phantom depths; however, beyond the depth of maximum dose that dependency is minimal. The simulation results agree with the experimental results for Čerenkov radiation generated in fibers. The spectroscopic technique used in this work can be used for development of high-spatial resolution fiber micro dosimeters and for optical characterization of various scintillating materials, such as phosphor nanoparticles, in ionizing radiation fields of high energy.

Continuous versus step-by-step scanning mode of a novel 3D scanner for CyberKnife measurements

The purpose of the study is to investigate the continuous versus step-by-step scanning mode of a commercial circular 3D scanner for commissioning measurements of a robotic stereotactic radiosurgery system. The 3D scanner was used for profile measurements in step-by-step and continuous modes with the intent of comparing the two scanning modes for consistency. The profile measurements of in-plane, cross-plane, 15 degree, and 105 degree were performed for both fixed cones and Iris collimators at depth of maximum dose and at 10cm depth. For CyberKnife field size, penumbra, flatness and symmetry analysis, it was observed that the measurements with continuous mode, which can be up to 6 times faster than step-by-step mode, are comparable and produce scans nearly identical to step-by-step mode. When compared with centered step-by-step mode data, a fully processed continuous mode data gives rise to maximum of 0.50% and 0.60% symmetry and flatness difference respectfully for all the fixed cones and Iris collimators studied.