Depth Of Maximum Dose Research Articles

Generation and verification of QFix kVue Calypso-compatible couch top model for a dedicated stereotactic linear accelerator with FFF beams.

This study details the generation, verification, and implementation of a treatment planning system (TPS) couch top model for patient support system used in conjunction with a dedicated stereotactic linear accelerator. Couch top model was created within the TPS using CT simulation images of the kVue Calpyso‐compatible couchtop (with rails). Verification measurements were compared to TPS dose prediction for different energies (6 MV FFF and 10 MV FFF) and rail configurations (rails in and rails out) using: 1) central axis point‐dose measurements with pinpoint chamber in water‐equivalent phantom at 42 gantry angles for various field sizes (2×2 cm2,4×4 cm2,10×10 cm2); and 2) Gafchromic EBT3 film parallel to beam in acrylic slab to assess changes in surface and percent depth doses in PA geometry. To assess sensitivity of delivered dose to variations in patient lateral position, measurements at central axis using the pinpoint chamber geometry were taken at lateral couch displacements of 2, 5, and 10 mm for 6 MV FFF. The maximum percent difference for point‐dose measurements was 3.24% (6 MV FFF) and 2.30% (10 MV FFF). The average percent difference for point‐dose measurements was less than 1.10% for all beam energies and rail geometries. The maximum percent difference between calculated and measured dose can be as large as 13.0% if no couch model is used for dose calculation. The presence of the couch structures also impacts surface dose and PDD, which was evaluated with Gafchromic film measurements. The upstream shift in the depth of dose maximum (dmax) was found to be 10.5 mm for 6 MV FFF and 5.5 mm for 10 MV FFF for ‘Rails In’ configuration. Transmission of the treatment beam through the couch results in an increase in surface dose (absolute percentage) of approximately 50% for both photon energies (6 MV FFF and 10 MV FFF). The largest sensitivity to lateral shifts occurred at the lateral boundary of the rail structures. The mean magnitude (standard deviation) of the deviation between shifted and centered measurements over all field sizes tested was 0.61% (0.61%) for 2 mm shifts, 0.46% (0.67%) for 5 mm shifts, and 0.86% (1.46%) for 10 mm shifts.PACS numbers: 87.56.‐v, 87.56.Da, 87.56.Fc

A comparison between direct TMR measurements and TMRs calculated from PDDs using BJR Supplement 25 data for flattened and unflattened photon beams.

This study assessed the validity of the conversion from percentage depth dose (PDD) to tissue maximum ratio (TMR) using BJR Supplement 25 data for flattened and flattening filter free (FFF) beams. PDD and TMR scans for a variety of field sizes were measured in water using a Sun Nuclear Corporation 3D SCANNER™ on a Varian TrueBeam linear accelerator in 6 MV, 10 MV and 6 MV FFF beams. The BJR Supplement 25 data was used to convert the measured PDDs to TMRs and these were compared with the directly measured TMR data. The TMR plots calculated from PDD were within 1% for the 10 MV and 6 MV flattened beams, for field sizes 3 cm × 3 cm to 40 cm × 40 cm inclusive, at depths measured beyond the depth of maximum dose. The disagreement between the measured and calculated TMR plots for the 6 MV FFF beam increased with depth and field size to a maximum of 1.7% for a 40 cm × 40 cm field. The results found in this study indicate that the BJR Supplement 25 data should not be used for field sizes larger than 20 cm × 20 cm at depths greater than 15 cm for the 6 MV FFF beam. It is advised that PDD to TMR conversion for FFF beams should be done with phantom scatter ratios appropriate to FFF beams, or the TMR should be directly measured if required.

SU‐C‐BRB‐03: Novel Technique to Implement GRID Therapy in a Commercial Treatment Planning System

Purpose:Due to the difficulty of creating an accurate multi‐aperture block in treatment planning systems (TPS), spatially fractionated radiation therapy (GRID Therapy) is limited to clinical setups using simple monitor unit calculations. In this work, we present a novel approach to accurately model a grid block in a TPS and evaluate its dosimetric accuracy.Methods:An acrylic GRID block was fashioned such that the hole size and spacing were the same dimensions of the brass GRID block (dotDecimal) at isocenter. Holes were non‐divergent and drilled 1 cm deep. CT scans (RT Lightspeed, GE Healthcare) of the block were acquired using axial slices with a thickness of 0.625 mm. These were imported into Eclipse (version 11, Varian Medical Systems) and holes were auto‐contoured into structures. The GRID block was created in Eclipse by forming apertures around the hole structures using a simple three step process. Treatment plans were created in a water‐equivalent phantom using 6 and 10 MV beams, delivered with a linear accelerator (Clinac iX, Varian Medical Systems) and measured using a water tank, film and a diode array.Results:Comparisons of percent depth dose curves and profiles at depths of maximum dose, 5cm, and 10cm for field sizes ranging from 5 cm2 to 25 cm2 using water and film were within 3% of their respective positions in Eclipse. GRID plans utilizing various jaw sizes and MLC blocking measured with film and a diode array showed pass rates with an average of 97% using Gamma analysis at 3%/ 3 mm and no lower than 93%.Conclusion:We have developed a simple yet accurate method of incorporating a GRID block into a commercially available TPS and demonstrated that the TPS accurately reflects the dose delivered. This will allow for real‐time planning using patient scans and encourage new methods for GRID therapy.Support provided by dotDecimal, Sanford, FL

SU‐E‐T‐706: Small Field Dosimetry: Output Factors for Fixed Cones, Iris Collimator and MLC Fields for the CyberKnife M6™ System with Newly Released InCise™ Multileaf Collimator

Purpose:Accuray has recently released a new collimator, the InCise™ Multileaf Collimator (MLC), for clinical use with the CyberKnife M6™ System. This work reports the results of measurements of output factors (OF) for fields shaped by the InCise™ Multileaf collimator, Iris collimator and fixed cones.Methods:The MLC consists of 41 pairs of 2.5 mm wide leaves projecting a minimum and maximum field size of 7.6 mm × 7.5 mm and 110 mm × 97.5 mm at 800 mm SAD. OF measurements were made using 6 different detectors: PTW stereotactic and electron diodes, PTW microDiamond detector, Sun Nuclear Edge diode, IBA SFD diode and PFD 3G photon diode. Measurements were made for 14 MLC field sizes and 12 cone/Iris field sizes (5mm to 60 mm diameter field sizes). All measurements were made in a Wellhofer water tank at the depth of maximum dose. Correction factors from Francescon et al (PMB 59, 2014, N11‐N17; 17, 2012 3741–3748) were used to correct the diode responses for the fixed cone and Iris measurements.Results:For the MLC fields no correction factors are currently available. For the cones and Iris collimator, the average of corrected OF for all diodes ranged from 0.651 ± 0.014 (5mm cone) to 0.994 ± 0.001 (50 mm cone); for the Iris collimator the corresponding values are 0.510 ±0.004 and 0.997 ±0.000 respectively. The OF for the MLC field sizes ranged from 0.806 ±0.009 for the 7.6 mm × 7.5 mm field to 1.023± 0.003 for the 110 mm × 97.5 mm field size.Conclusion:The present results are the first set of output factor data reported for the InCise™ MLC, Iris collimator and the fixed cones for the CyberKnife M6™ system. Good agreement is obtained among all the corrected data for the cones and Iris data.

SU‐E‐T‐454: Impact of Air Gap On PDDs of 6 MV Photon Beam for Various Field Sizes in Inhomogeneous Phantoms

Purpose:To investigate how the shape of air gap and its size will impact the percent depth doses (PDDs) of a 6MV photon beam for various field sizes.Methods:Two in‐house phantoms were manufactured containing rectangular (phantom A) and circular (phantom B) air gaps. Both phantoms A and B were composed of same top layer (solid‐water; 30×30cm2,5cm thickness) and bottom layer (solid‐water; 30×30cm 2,8cm thickness), but middle layer was varied to observe air gap effects and scatter contribution to the measurement point. In phantom A, a rectangular shaped air gap (30×30cm2,7cm thickness) was created by placing Styrofoam blocks between top and bottom layers of the phantom. In phantom B, middle layer was replaced by “inhomogenous block”, composed of acrylic plate (30×30cm2, 4cm thickness) followed by PVC(30×30cm2,3cm thickness). Additionally, circular air gap was created by drilling a hole (diameter=2.8cm, length=7cm) at the center of “inhomogenous block”. In both phantoms, measurement readings were obtained at 13cm depth (i.e., 1cm after air gap) and depth of maximum dose(6MV energy; 100 MUs; field sizes ranged from 3×3cm2 to 10×10cm2). The PDDs at 13cm depth were compared in both phantoms.Results:The measurements in both phantoms A and B showed an almost linear increase in PDDs with increasing field size, especially for smaller field sizes (from 3×3 to 7×7cm2). For each field size, the PDD in phantom A was smaller compared to the one in phantom B. The difference in PDDs between two phantoms decreased with an increase in field size as the PDD difference decreased from 9.0% to 6.4%.Conclusion:The shape and size of air gap affect the PDD measurements in secondary build‐up region as 6 MV primary beam traverses through the center of air gap. The scatter contribution due to increase in field size was more noticeable for field sizes ≤7×7cm2.

SU‐E‐T‐193: Commissioning of An IntraOp Mobetron

Purpose:This study will describe the commissioning process of an Intra‐Op Mobetron following the guidance of TG72 and includes a detailed method for measuring gap factors.Methods:Beam data was collected for 6, 9, and 12MeV electrons on the Mobetron using a PTW water tank and a Markus chamber. Commissioning data was collected for 3 sets of cones (0°, 15°, and 30° with respect to the water surface) that ranged in size from 3cm to 10cm diameter in half centimeter increments. Profiles were collected at six depths for all integer cones. PDI's were taken for all cones in a path perpendicular to the water surface and then converted to dose using MEPHYSTO's internal algorithm. The depth of maximum dose was determined from the PDD's and used for measurement of output factors. Output factors were normalized to the 10cm flat cone. Gap factors were measured for all flat cones by raising the cone away from the water surface in defined gaps. These gap factors were measured at dmax for gaps of 1cm to 5cm in 1cm increments Gap factors and effective SSD values were calculated.Results:Output factors measured for all cones were within 3% of the representative beam data for flat cones. There is no published data on beveled cones, but the angled cone output factors agreed with the flat cones within 5%. Gap factors follow a linear pattern with the output decreasing 3–7% per centimeter gap depending on the size of the cone and energy. Effective SSD values calculated and fit a 3rd order polynomial curve with R2>0.98 for all energies.Conclusion:Commissioning of an Intra‐Op Mobetron was completed with general agreement to published data. New data was published for beveled cones and a method for determining gap factors was established.

SU‐E‐T‐203: Comparison of a Commercial MRI‐Linear Accelerator Based Monte Carlo Dose Calculation Algorithm and Geant4

Purpose:An MRI‐linear accelerator is currently being developed by the vendor Elekta™. The treatment planning system that will be used to model dose for this unit uses a Monte Carlo dose calculation algorithm, GPUMCD, that allows for the application of a magnetic field. We tested this radiation transport code against an independent Monte‐Carlo toolkit Geant4 (v.4.10.01) both with and without the magnetic field applied.Methods:The setup comprised a 6 MeV mono‐energetic photon beam emerging from a point source impinging on a homogeneous water phantom at 100 cm SSD. The comparisons were drawn from the percentage depth doses (PDD) for three different field sizes (1.5 × 1.5 cm2, 5 × 5 cm2, 10 × 10 cm2) and dose profiles at various depths. A 1.5 T magnetic field was applied perpendicular to the direction of the beam. The transport thresholds were kept the same for both codes.Results:All of the normalized PDDs and profiles agreed within ± 1 %. In the presence of the magnetic field, PDDs rise more quickly reducing the depth of maximum dose. Near the beam exit point in the phantom a hot spot is created due to the electron return effect. This effect is more pronounced for the larger field sizes. Profiles selected parallel to the external field show no effect, however, the ones selected perpendicular to the direction of the applied magnetic field are shifted towards the direction of the Lorentz force applied by the magnetic field on the secondary electrons. It is observed that these profiles are not symmetric which indicates a lateral build up of the dose.Conclusion:There is a good general agreement between the PDDs/profiles calculated by both algorithms thus far. We are proceeding towards clinically relevant comparisons in a heterogeneous phantom for polyenergetic beams.Funding for this work has been provided by Elekta.

SU-E-T-276: Dose Calculation Accuracy with a Standard Beam Model for Extended SSD Treatments

Purpose: While most photon treatments are delivered near 100cm SSD or less, a subset of patients may benefit from treatment at SSDs greater than 100cm. A proposed rotating chair for upright treatments would enable isocentric treatments at extended SSDs. The purpose of this study was to assess the accuracy of the Pinnacle3 treatment planning system dose calculation for standard beam geometries delivered at extended SSDs with a beam model commissioned at 100cm SSD. Methods: Dose to a water phantom at 100, 110, and 120cm SSD was calculated with the Pinnacle 3 CC convolve algorithm for 6x beams for 5×5, 10×10, 20×20, and 30×30cm2 field sizes (defined at the water surface for each SSD). PDDs and profiles (depths of 1.5, 12.5, and 22cm) were compared to measurements in water with an ionization chamber. Point-by-point agreement was analyzed, as well as agreement in field size defined by the 50% isodose. Results: The deviations of the calculated PDDs from measurement, analyzed from depth of maximum dose to 23cm, were all within 1.3% for all beam geometries. In particular, the calculated PDDs at 10cm depth were all within 0.7% of measurement. For profiles, the deviations within the central 80% of the field were within 2.2% for all geometries. The field sizes all agreed within 2mm. Conclusion: The agreement of the PDDs and profiles calculated by Pinnacle3 for extended SSD geometries were within the acceptability criteria defined by Van Dyk (±2% for PDDs and ±3% for profiles). The accuracy of the calculation of more complex beam geometries at extended SSDs will be investigated to further assess the feasibility of using a standard beam model commissioned at 100cm SSD in Pinnacle3 for extended SSD treatments.

SU‐E‐T‐182: Clinical Implementation of TG71‐Based Electron MU Calculation and Comparison with a Commercial Secondary Calculation

Purpose:The TG‐71 report was published in 2014 to present standardized methodologies for MU calculations and determination of dosimetric quantities. This work explores the clinical implementation of a TG71‐based electron MU calculation algorithm and compares it with a recently released commercial secondary calculation program–Mobius3D (Mobius Medical System, LP).Methods:TG‐71 electron dosimetry data were acquired, and MU calculations were performed based on the recently published TG‐71 report. The formalism in the report for extended SSD using air‐gap corrections was used. The dosimetric quantities, such PDD, output factor, and f‐air factors were incorporated into an organized databook that facilitates data access and subsequent computation. The Mobius3D program utilizes a pencil beam redefinition algorithm. To verify the accuracy of calculations, five customized rectangular cutouts of different sizes–6×12, 4×12, 6×8, 4×8, 3×6 cm2–were made. Calculations were compared to each other and to point dose measurements for electron beams of energy 6, 9, 12, 16, 20 MeV. Each calculation / measurement point was at the depth of maximum dose for each cutout in a 10×10 cm2 or 15×15cm2 applicator with SSDs 100cm and 110cm. Validation measurements were made with a CC04 ion chamber in a solid water phantom for electron beams of energy 9 and 16 MeV.Results:Differences between the TG‐71 and the commercial system relative to measurements were within 3% for most combinations of electron energy, cutout size, and SSD. A 5.6% difference between the two calculation methods was found only for the 6MeV electron beam with 3×6 cm2cutout in the 10×102cm applicator at 110cm SSD. Both the TG‐71 and the commercial calculations show good consistency with chamber measurements: for 5 cutouts, <1% difference for 100cm SSD, and 0.5–2.7% for 110cm SSD.Conclusions:Based on comparisons with measurements, a TG71‐based computation method and a Mobius3D program produce reasonably accurate MU calculations for electron‐beam therapy.

SU‐E‐T‐676: Reproducibility and Consistency of Two SunNuclear 3D Scanning Tanks

Purpose:To determine if two Sun Nuclear 3D Scanning (SNC 3DS) tanks collect reproducible and consistent data and test the precision of the SNC Dosimetry auto‐setup.Methods:Percent depth doses (PDDs) and profiles were collected on two SNC 3DS tanks with a Varian TrueBeam linear accelerator. SNC Dosimetry auto‐setup application was used with CC13 ionization chambers. After auto‐setup, collimator light field was checked manually against the position of the chamber. Comparing measured data for repeated measurements with tank 1 allowed evaluation of SNC‐3DS auto‐setup and tank reproducibility. Comparing measured data between tanks 1 and 2 allowed evaluation of consistency between tanks.Results:Preliminary results showed reproducibility of depth of maximum dose (Dmax) of 0.38mm for a 10cmx10cm field and 0.67mm for 30cmx30cm on a single tank. PDD values at 5cm, 10cm, and 20cm depths were reproducible within 0.26%. Consistency of Dmax between tanks was 0.17mm for a 10cmx10cm field and 0.44mm for 30cmx30cm. PDD values at 5cm, 10cm, and 20cmwere consistent within 0.06%. Profiles showed reproducibility in field width within 0.4mm for a 10cmx10cm field and 0.7mm for a 30cmx30cm field. Profiles showed consistency in field width within 0.2mm for 10cmx10cm and 30cmx30cm field sizes. Penumbra width was reproducible and consistent to under 0.5mm except for 30cmx30cm field size at 30cm depth where the reproducibility was 2.2mm and the consistency was 2.6mm.Conclusion:In conclusion, the SNC 3DS tank shows good reproducibility in measured data. Since the tank to tank variation in measured data is within the uncertainty of repeated single tank measurements the tanks also perform consistently.

SU‐E‐T‐220: Computational Accuracy of Adaptive Convolution (AC) and Collapsed Cone Convolution (CCC) Algorithms in the Presence of Air Gaps

Purpose:To compare the percentage depth dose (PDD) computational accuracy of Adaptive Convolution (AC) and Collapsed Cone Convolution (CCC) algorithms in the presence of air gaps.Methods:A 30×30×30 cm3 solid water phantom with two 5cm air gaps was scanned with a CT simulator unit and exported into the Phillips Pinnacle™ treatment planning system. PDDs were computed using the AC and CCC algorithms. Photon energy of 6 MV was used with field sizes of 3×3 cm2, 5×5 cm2, 10×10 cm2, 15×15 cm2, and 20×20 cm2. Ionization chamber readings were taken at different depths in water for all the field sizes. The percentage differences in the PDDs were computed with normalization to the depth of maximum dose (dmax). The calculated PDDs were then compared with measured PDDs.Results:In the first buildup region, both algorithms overpredicted the dose for all field sizes and under‐predicted for all other subsequent buildup regions. After dmax in the three water media, AC under‐predicted the dose for field sizes 3×3 and 5×5 cm2 and overpredicted for larger field sizes, whereas CCC under‐predicted for all field sizes. Upon traversing the first air gap, AC showed maximum differences of –3.9%, −1.4%, 2.0%, 2.5%, 2.9% and CCC had maximum differences of −3.9%, −3.0%,–3.1%, −2.7%, −1.8% for field sizes 3×3, 5×5, 10×10, 15×15, and 20×20 cm2 respectively.Conclusion:The effect of air gaps causes a significant difference in the PDDs computed by both the AC and CCC algorithms in secondary build‐up regions. AC computed larger values for the PDDs except at smaller field sizes. For CCC, the size of the errors in prediction of the PDDs has an inverse relationship with respect to field size. These effects should be considered in treatment planning where significant air gaps are encountered.

A geometrical model for the Monte Carlo simulation of the TrueBeam linac

Monte Carlo simulation of linear accelerators (linacs) depends on the accurate geometrical description of the linac head. The geometry of the Varian TrueBeam linac is not available to researchers. Instead, the company distributes phase-space files of the flattening-filter-free (FFF) beams tallied at a plane located just upstream of the jaws. Yet, Monte Carlo simulations based on third-party tallied phase spaces are subject to limitations. In this work, an experimentally based geometry developed for the simulation of the FFF beams of the Varian TrueBeam linac is presented. The Monte Carlo geometrical model of the TrueBeam linac uses information provided by Varian that reveals large similarities between the TrueBeam machine and the Clinac 2100 downstream of the jaws. Thus, the upper part of the TrueBeam linac was modeled by introducing modifications to the Varian Clinac 2100 linac geometry. The most important of these modifications is the replacement of the standard flattening filters by ad hoc thin filters. These filters were modeled by comparing dose measurements and simulations. The experimental dose profiles for the 6 MV and 10 MV FFF beams were obtained from the Varian Golden Data Set and from in-house measurements performed with a diode detector for radiation fields ranging from 3 × 3 to 40 × 40 cm2 at depths of maximum dose of 5 and 10 cm. Indicators of agreement between the experimental data and the simulation results obtained with the proposed geometrical model were the dose differences, the root-mean-square error and the gamma index. The same comparisons were performed for dose profiles obtained from Monte Carlo simulations using the phase-space files distributed by Varian for the TrueBeam linac as the sources of particles. Results of comparisons show a good agreement of the dose for the ansatz geometry similar to that obtained for the simulations with the TrueBeam phase-space files for all fields and depths considered, except for the 40 × 40 cm2 field where the ansatz geometry was able to reproduce the measured dose more accurately. Our approach overcomes some of the limitations of using the Varian phase-space files. It makes it possible to: (i) adapt the initial beam parameters to match measured dose profiles; (ii) reduce the statistical uncertainty to arbitrarily low values; and (iii) assess systematic uncertainties (type B) by using different Monte Carlo codes. One limitation of using phase-space files that is retained in our model is the impossibility of performing accurate absolute dosimetry simulations because the geometrical description of the TrueBeam ionization chamber remains unknown.

Comparing the dosimetric characteristics of the electron beam from dedicated intraoperative and conventional radiotherapy accelerators.

The specific design of the mobile dedicated intraoperative radiotherapy (IORT) accelerators and different electron beam collimation system can change the dosimetric characteristics of electron beam with respect to the conventional accelerators. The aim of this study is to measure and compare the dosimetric characteristics of electron beam produced by intraoperative and conventional radiotherapy accelerators. To this end, percentage depth dose along clinical axis (PDD), transverse dose profile (TDP), and output factor of LIAC IORT and Varian 2100C/D conventional radiotherapy accelerators were measured and compared. TDPs were recorded at depth of maximum dose. The results of this work showed that depths of maximum dose, R90,R50, and RP for LIAC beam are lower than those of Varian beam. Furthermore, for all energies, surface doses related to the LIAC beam are substantially higher than those of Varian beam. The symmetry and flatness of LIAC beam profiles are more desirable compared to the Varian ones. Contrary to Varian accelerator, output factor of LIAC beam substantially increases with a decrease in the size of the applicator. Dosimetric characteristics of beveled IORT applicators along clinical axis were different from those of the flat ones. From these results, it can be concluded that dosimetric characteristics of intraoperative electron beam are substantially different from those of conventional clinical electron beam. The dosimetric characteristics of the LIAC electron beam make it a useful tool for intraoperative radiotherapy purposes.PACS number: 87.56.‐v, 87.56.bd

Moving gantry method for electron beam dose profile measurement at extended source-to-surface distances.

A novel method has been put forward for very large electron beam profile measurement. With this method, absorbed dose profiles can be measured at any depth in a solid phantom for total skin electron therapy. Electron beam dose profiles were collected with two different methods. Profile measurements were performed at 0.2 and 1.2 cm depths with a parallel plate and a thimble chamber, respectively. 108 cm×108 cm and 45 cm×45 cm projected size electron beams were scanned by vertically moving phantom and detector at 300 cm source‐to‐surface distance with 90° and 270° gantry angles. The profiles collected this way were used as reference. Afterwards, the phantom was fixed on the central axis and the gantry was rotated with certain angular steps. After applying correction for the different source‐to‐detector distances and incidence of angle, the profiles measured in the two different setups were compared. Correction formalism has been developed. The agreement between the cross profiles taken at the depth of maximum dose with the ‘classical’ scanning and with the new moving gantry method was better than 0.5 % in the measuring range from zero to 71.9 cm. Inverse square and attenuation corrections had to be applied. The profiles measured with the parallel plate chamber agree better than 1%, except for the penumbra region, where the maximum difference is 1.5%. With the moving gantry method, very large electron field profiles can be measured at any depth in a solid phantom with high accuracy and reproducibility and with much less time per step. No special instrumentation is needed. The method can be used for commissioning of very large electron beams for computer‐assisted treatment planning, for designing beam modifiers to improve dose uniformity, and for verification of computed dose profiles.PACS numbers: 87.53.Bn, 87.53.Jw, 87.56.jf

Comparison of Bolus Materials vs Highly Absorbent Polypropylene and Rayon Cloth

Bolus materials are tissue equivalent substances used to modify the depth of maximum dose. Currently at the Stronach Regional Cancer Centre several types of bolus materials are used, such as superflab, thermoplastic and dental wax. Bolus materials are intended to lie flush to the skin; however, this is not always possible due to the anatomy of the area to be treated and the type of bolus material used. The bolus material to be evaluated is a highly absorbent polypropylene and rayon cloth. This study is to determine the effective use of a highly absorbent polypropylene and rayon cloth as a tissue equivalent material. The study will investigate and compare the densities, cost and practical use of a highly absorbent polypropylene and rayon cloth and currently used bolus materials. It was hypothesized that an absorbent polypropylene and rayon cloth will absorb and retain water, as well as conform and lie flush to a patient's contour. Bolus materials will be CT scanned to determine their respective densities. Materials requiring water will be scanned dry and wet (with specific volumes of water poured on them ie. 100, 200, 500 ml). Each bolus material will be scanned on a thorax phantom with an irregular contour to assess stand-off of the bolus from the phantom's external contour. A tangent breast plan will be generated according to our clinical practice. This treatment plan will include two opposing 6 MV open beams, followed by a number of inverse planned segments to increase dose homogeneity within the breast. Dose distributions of these treatment fields will be calculated on each CT scan with various bolus materials. At the time of CT simulation, five measurement points will be identified. The treatment plan will be delivered to the phantom with each type of bolus material and saturation level if applicable. Skin dose measurements will be performed using the Best Medical mobile MOSFET system at the five measurement locations determined at CT. Comparison will be made between the skin dose measurements and the skin doses indicated in the planning system at five different locations for each bolus material. Pending.

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.

Dosimetric effects on small-field beam-modeling for stereotactic body radiation therapy

The treatment planning of stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) requires high accuracy of dosimetric data for small radiation fields. The dosimetric effects on the beam-modeling process of a treatment planning system (TPS) were investigated using different measured small-field data sets. We performed small-field dosimetry with three detectors: a CC13 ion chamber, a CC01 ion chamber, and an edge detector. Percentage depth doses (PDDs) and dose profiles for field sizes given by 3 × 3 cm2, 2 × 2 cm2, and 1 × 1 cm2 were obtained for 6 MV and 15 MV photon beams. Each measured data set was used as data input for a TPS, in which a beam-modeling process was implemented using the collapsed cone convolution (CCC) algorithm for dose calculation. The measured data were used to generate six beam-models based on each combination of detector type and photon energy, which were then used to calculate the corresponding PDDs and dose profiles for various depths and field sizes. Root mean square differences (RMSDs) between the calculated and the measured doses were evaluated for the PDDs and the dose profiles. The RMSDs of PDDs beyond the maximum dose depth were within an accuracy of 0.2–0.6%, being clinically acceptable. The RMSDs of the dose profiles corresponding to the CC13, the CC01, and the edge detector were 2.80%, 1.49%, and 1.46% for a beam energy of 6 MV and 2.34%, 1.15%, and 1.44% for a beam energy of 15 MV, respectively. The calculated results for the CC13 ion chamber showed the most discrepancy compared to the measured data, due to the relatively large sensitive volume of this detector. However, the calculated dose profiles for the detectors were not significantly different from another. The physical algorithm used in the beam-modeling process did not seem to be sensitive to blurred data measured with detectors with large sensitive volumes. Each beam-model was used to clinically evaluate lung and lymphatic node SBRT plans, yielding almost equal dose distributions for the treatment targets, while the mean doses related to the organs at risk (OARs) deviated by approximately 0.7–1.2%. The use of the measured data sets from different detectors for the beam-modeling process still provided acceptable dose distributions with accuracies within 2%.

Analytical Study of Flatness and Symmetry of Electron Beam with 2D Array Detectors

Aims: The main aim of our study is to analyse Electron Beam Profiles like Flatness, Symmetry with 2D Array Detectors. The 2D Array can be used as an alternative device to measure the Electron beam profiles. Introduction: Flatness and symmetry of a radiation beam: The flatness of the beam is defined by the following formula: Flatness (%) = Dmax/Dmin × 100% (1) Where Dmax and Dmin are the maximum and minimum doses respectively within the area. Radiation field symmetry is defined as the maximum ratio of doses at two symmetric points relative to the central axis of the field. Symmetry (%) = [D(x, y)]/[D (-x,-y)] × 100% (2) Methods and Material: The beam symmetry is easily defined and is not very dependent on the depth of measurement. However, the flatness of the beam depends on the size and shape of the measurement phantom. However, 3% is sufficient measurement accuracy for a quick check. We measured the electron profiles of energies (4 MeV, 6 MeV, 8 MeV, 10 MeV, 12 MeV, 15 MeV and 18 MeV) from Elekta Synergy Linear accelerator for different electron applicators (6 × 6, 10 × 10, 14 × 14 and 20 × 20) at their depth of maximum dose (dm) respectively and Source-Surface-Distance (SSD) of 100 cm using PTW 729 2D array detectors. Results: We analyzed the variations in Flatness and Symmetry of electron energies obtained from 729 2D Array Detector and compared with the standard values obtained from the Radiation Field Analyzer (RFA) during Commissioning. Conclusions: We found that there is no significant variation in Flatness and symmetry obtained from the 729 2D Array detector as compared to the standard Flatness and symmetry obtained from the RFA .Thus we conclude that 729 2 D Array detector can be used for the routine measurement of electron beam profiles.

Monte-Carlo-derived insights into dose–kerma–collision kerma inter-relationships for 50 keV–25 MeV photon beams in water, aluminum and copper

The relationships between D, K and Kcol are of fundamental importance in radiation dosimetry. These relationships are critically influenced by secondary electron transport, which makes Monte-Carlo (MC) simulation indispensable; we have used MC codes DOSRZnrc and FLURZnrc. Computations of the ratios D/K and D/Kcol in three materials (water, aluminum and copper) for large field sizes with energies from 50 keV to 25 MeV (including 6–15 MV) are presented. Beyond the depth of maximum dose D/K is almost always less than or equal to unity and D/Kcol greater than unity, and these ratios are virtually constant with increasing depth. The difference between K and Kcol increases with energy and with the atomic number of the irradiated materials. D/K in ‘sub-equilibrium’ small megavoltage photon fields decreases rapidly with decreasing field size. A simple analytical expression for , the distance ‘upstream’ from a given voxel to the mean origin of the secondary electrons depositing their energy in this voxel, is proposed: , where is the mean initial secondary electron energy. These agree well with ‘exact’ MC-derived values for photon energies from 5–25 MeV for water and aluminum. An analytical expression for D/K is also presented and evaluated for 50 keV–25 MeV photons in the three materials, showing close agreement with the MC-derived values.

Derivation of equations to define inflection point and its analysis in flattening filter free photon beams based on the principle of polynomial function

Purpose: The objective of this work is to (1) present a mechanism for calculating inflection points on profiles at various depths and field sizes, and (2) study the doses at the inflection points for various field sizes at depth of maximum dose (D max ) for flattening filter free (FFF) photon beam profiles. Methods: Graphical representation was done on percentage of dose versus inflection points. Also, using the polynomial function, the author formulated equations for calculating spot-on inflection point on the profiles for both the 6MV and 10 MV energies for different field sizes at various depths. Results: In a 10 MV FFF radiation beam, the dose at inflection point of the profile decreases as the field size increases. However, in 6MV FFF radiation beam, the dose at the inflection point initially increases with an increase in the field size up to 10 ×10 cm 2 and decreases after 10 ×10 cm 2 . The polynomial function was fitted for both the 6 MV and 10 MV FFF beams for all field sizes and depths. Conclusion: Polynomial function is one of the easiest ways of identifying the inflection point in FFF beam for various field sizes and depths. Graphical representation of dose versus inflection point for both FFF energies was derived.