Depth Dose Data Research Articles

SU‐E‐T‐159: Development of An Independent, Monte Carlo, Dose Calculation, Quality Assurance Tool for Clinical Trials

Purpose: To commission a multiple‐source Monte Carlo model of Elekta linear accelerator beams of nominal energies 6MV and 10MV. Methods: A three source, Monte Carlo model of Elekta 6 and 10MV therapeutic x‐ray beams was developed in a two‐step process. Energy spectra of each of three sources, a primary source corresponding to photons created in the target, an extra‐focal source corresponding to photons originating from scattered events in the linac head, and an electron contamination source, were determined. The two photon sources were determined by an optimization process that fit the relative fluence of 0.25 MeV energy bins to the product of Fatigue‐Life and Fermi functions to match calculated percent depth dose (PDD) data with that measured in water for a 10×10cm2 field. Off‐axis effects were modeled by fitting the off‐axis fluence to a piece‐wise linear function through optimization of relative fluence to match calculated dose profiles with measured dose profiles for a 40×40cm2 field. A 3rd degree polynomial was used to describe the off‐axis half‐value layer as a function of off‐axis angle. The model was then commissioned by comparing calculated PDDs and dose profiles for field sizes ranging from 3×3cm2 to 30×30cm2 to those obtained from measurements. Results: Agreement between calculated and measured data was evaluated using 2%/2mm global gamma criterion for field sizes of 3×3, 5×5, 10×10, 15×15, 20×20, and 30×30cm2. Along the central axis of the beam 99.5% and 99.6% of all data passed the criterion for 6 and 10MV models, respectively. Dose profiles at depths of dmax, 5, 10, 20, and 25cm agreed with measured data for 95.4% and 99.2% of data tested for 6 and 10MV models, respectively. Conclusion: A Monte Carlo multiple‐source model for Elekta 6 and 10MV therapeutic x‐ray beams has been developed as a quality assurance tool for clinical trials. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.

SU‐E‐T‐498: Implementation of Clinical Monte Carlo Dose Calculation for CyberKnife On a Web‐Based Treatment Planning System WebTPS

Purpose: The scope of this study is to implement an accurate Cyberknife model on a web‐based tool (WebTPS), which uses the EGSnrc Monte Carlo dose calculation engine. WebTPS will be mostly used as a reference to evaluate clinical treatment plans in highly heterogeneous phantoms. Methods: The WebTPS dose calculation module is linked to the user code DOSxyznrc. WebTPS automatically converts CyberKnife clinical plans to DOSxyznrc input files. Phantoms are created using a tissue segmentation method from HU‐ED calibrated curves and materials are assigned based on CT data and contours performed by radiation oncologists. Parallel computation is run on a high‐performance cluster (Compute Canada) to achieve reasonable simulation time. The CyberKnife model is built on the BEAMnrc system using manufacturer's specifications. Simulated and experimental data are compared to estimate the optimal electron beam parameters. The beam energy estimation is based on percent depth dose (PDD) data comparison, while the spot size is validated using output factor (OF) and off‐axis ratio (OAR) data. An egs_chamber model of a PTW60012 diode is used to simulate OF experimental measurements for different collimator sizes. Results: A preliminary linac model optimization yields a 0.5% agreement between experimental and simulation PDD data; a 0.5% or 1 mm agreement for OAR data and a 2% agreement for OF data. Full treatment plan simulations are achieved with the CyberKnife model using patient heterogeneous phantoms. Uncertainties under 1% are achieved for less than 2 hours of CPU time. Conclusion: This work aims to develop a suitable model for reference plan dose calculation. WebTPS will be used in several clinical and research applications where the CyberKnife embedded ray‐tracing algorithm show significant limitations. Further improvements are yet to be achieved to match experimental data to a level of 1%.

SU-E-T-541: An Optimum Normalization Technique for Electron Monte Carlo Treatment Planning

Purpose: To evaluate the optimum normalization technique using the treatment planning system (TPS) to calculate dose with Electron Monte Carlo (eMC) algorithms. Methods: Eclipse TPS was used to generate 6 and 9 MeV plans for field apertures ranging from 3 cm to 10 cm, calculation grid sizes from 1 mm to 5mm, source to surface distance (SSD) from 100 to 110, and beam incident angles from 0 to 30 degrees. Each plan was normalized to the (1) TPS global maximum and (2) the physical dmax depth on the central axis given by measured percent depth dose (PDD) data. To validate the eMC relative dose distributions, PDD and beam profiles (at dmax) were obtained in a water phantom. An irregular surface phantom and a small domed phantom were constructed for dose measurements. Results: In all cases, the TPS' PDD agreed with the measured PDD within 2.5 % beyond a depth of 0.5cm. Absolute dose measurements on the irregular surface and domed phantoms also agreed with an average difference of 2.5%. Normalizing to the physical dmax on the central axis resulted in MUs up to 10% higher than normalizing to TPS global maximum. This was due to the eMC dmax shifting as much as 6mm shallower in depth, or shifting away from the central axis vary based on field apertures and beam angle incidences. MU calculations based on tabular data from beam commissioning differed from measured by 11.1% on average when oblique angles and cutouts were used. The tabulated data are based on measurements performed in a flat phantom with no beam obliquity. Conclusion: The planned MUs using the global maximum are more closely matched with the measured values as compared with MUs generated from tabulated data. Thus, the TPS generated MUs should more accurately reflect dose delivered for patient treatments.

SU-E-T-133: The Accuracy of Dose Calculations for the Brainlab Imaging Couch Top with the Eclipse Treatment Planning System

Purpose: To evaluate the accuracy of the Brainlab imaging couch top model within the Eclipse treatment planning system for dose calculations. Methods: A 25cm3 polystyrene phantom was irradiated at 6MV with an ion chamber (IC) approximately 12cm deep. A second setup was constructed for film collection of depth dose data to the surface of the phantom. Irradiations were performed with and without the couch in the beam path and compared to the calculated dose using Eclipse version 10 and AAA (Varian Medical Systems, Palo Alto, CA). Dose and water equivalent depth (WED) were calculated using the standard Brainlab couch model (−1000HU interior, −300HU surface) and a modified model (−890HU interior, +700HU surface) taken from the iPlan treatment planning system. A comparison was performed using triple channel dosimetry in FilmQA Pro (Ashland, Wayne, NJ) with a gamma metric using 2%/2mm as a tolerance. Results: The mean HU values on CBCT were −925HU in the interior and −425HU on the surface of the couch. WED through the couch at the couch-phantom surface was 0.47cm without the adjusted HU values and 1.4cm with the modified values. This agreed better with the Brainlab published WED of 1.2cm. IC agreement to the treatment plan was 1.58% for the AP field and −1.78% for the PA field with the default HU and 1.16% with the adjusted HU values. Film depth dose measurements showed a decrease in gamma from 99.8% to 94.4% points passing with the default values. Dose at the couch - phantom interface was approximately 20% different on film for the default values and 1% for the adjusted HU values. Conclusion: The default couch HU values were found to be similar to what was observed on CT. However, for our configuration they proved to be insufficient in accurately calculating dose, especially dose to the phantom surface.

SU-E-T-520: Deriving Energy Spectrum From Depth Dose Measurements for X-Rays of CT Scanner

Purpose: The goal of this study is to derive x-ray energy spectrum from the measured depth dose data for any CT scanner without a priori knowledge of x-ray tube configurations. The obtained energy spectrum, together with other beam characteristics, can be used as an effective way to estimate the patient-specific organ doses from CT scans clinically. Methods: EGSnrc Monte Carlo code was used to generate a series of percent depth dose (PDD) curves in water for mono-energetic x-rays with energy ranging from 11 keV to 150 keV. Specifically, a fan beam was simulated with a fan angle of 55 degrees and 2 cm thickness along superior-inferior direction. Based on BEAMnrc Monte Carlo characterization of x-rays from CT scanners, an empirical function with three variables was proposed to represent the energy spectrum of those beams. An iterative multivariate search algorithm has been developed to derive the energy spectrum based on the correlation coefficient between the measured PDD and the fitted PDD. The developed algorithm has been tested with a BEAMnrc simulated x-ray spectrum serving as the ‘original’ spectrum and the subsequent dose simulation serving as the ‘measured’ PDD. Results: Except for the characteristic energy peaks, the derived energy spectrum reproduced the ‘original’ spectrum very well with a smooth energy dependency, peaking around 39 keV. The maximum error between the measured PDD and the fitted PDD was less than 1%, indicating the validity of our proposed method for spectrum derivation. Conclusion: According to this study, it is possible to derive the energy spectrum from the measured PDD data for the x-rays of a CT scanner, without a priori knowledge of x-ray tube configurations. Future work will seek to demonstrate its clinical efficacy by establishing a patient-specific dosimetry system for any CT scanners, particularly for organ dose estimation on real patients.

SU-E-T-92: Comparison of the Depth Dose in the Build-Up Region and Surface Dose for 6MV Flattened and 7MV Unflattened Photon Beams with Different Detectors

Purpose: Comparison of the depth dose in the build-up region and Surface dose for 6MV flattened and 7MV unflattened photon beams with different detectorsMethods: The percentage depth dose in the build-up region and the surface dose for the 6MV-FB and 7MV-UFB photon beams from a Siemens Artiste medical linear accelerator were measured for square field sizes of 5×5, 10×10 and 15×15 cm 2 using Scanditronix NACP ion chamber, CC 13, and Stereotactic field Detectors (SFD) along the central axis of the beam at 100 cm source to surface distance with IBA blue phantom 2 and solid phantom. Results: The consistency between the measured percentage depth dose data from all four detector types were observed for depths beyond the depth of the maximum dose, but were all clearly different from each other data in the build-up region this is observed for both 6MV-FB and 7MV-UFB beams. The measured surface dose (16.2 Percent) applied with improved Velkley correction factors used for 10X10 cm2 of 6MV-FB using NACP parallel plate chamber values were all in good agreement with previously published data. The measured percentage surface doses obtained using the NACP parallel plate chamber, CC 13 and SFD without any correction factor values are 0.3885,0.4330 and 0.4751, 0.4485,0.4922 and 0.5323,0.3652,0.4076 and 0.4580 respectively for field sizes of 5X5,10X10 and 15×15 cm2 for the 6MV-FB photon beams and 0.3800,0.4117 and 0.4398, 0.4364, 0.4655 and 0.4965, 0.3611,0.4100 and 0.4349 for the 7MV-UFB photon beams. Conclusion: The measured surface dose clearly increases with increasing field size, regardless of the detector used in the measurement for both 6MV-FB and 7MV-UFB photon beams. Compare to NACP parallel plate chamber CC 13 chamber is showing over response and SFD is showing lesser surface dose.

Monte Carlo simulation of electron modes of a Siemens Primus linac (8, 12 and 14 MeV)

AbstractBackgroundElectron mode is used for treatment of superficial tumours in linac-based radiotherapy.PurposeThe aim of present study is simulation of 8, 12 and 14 MeV electrons from a Siemens Primus linac using MCNPX Monte Carlo (MC) code and verification of the results based on comparison of the results with the measured data.Materials and methodsElectron mode for 8, 12 and 14 MeV electron energies of a Siemens Primus linac was simulated using MCNPX MC code. Percent depth dose (PDD) data for 10 × 10, 15 × 15 and 25 × 25 cm2 applicators obtained from MC simulations were compared with the corresponding measured data.ResultsGamma index values were less than unity in most of points for all the above-mentioned energies and applicators. However, for 25 × 25 cm2 applicator in 8 MeV energy, 10 × 10 cm2 applicator and 15 × 15 cm2 applicator in 14 MeV energy, there were four data points with gamma indices higher than unity. However among these data points, there are a number of cases with relatively large value of gamma index, these cases are positioned on the bremsstrahlung tail of the PDD curve which is not normally used in treatment planning.ConclusionThere was good agreement between the results of MC simulations developed in this study and the measured values. The obtained simulation programmes can be used in dosimetry of electron mode of Siemens Primus linac in the cases in which it is not easily feasible to perform experimental in-phantom measurements.

Evaluation of a novel triple-channel radiochromic film analysis procedure using EBT2

A novel approach to read out radiochromic film was introduced recently by the manufacturer of GafChromic film. In this study, the performance of this triple-channel film dosimetry method was compared against the conventional single-red-channel film dosimetry procedure, with and without inclusion of a pre-irradiation (pre-IR) film scan, using EBT2 film and kilo- and megavoltage photon beams up to 10 Gy. When considering regions of interest averaged doses, the triple-channel method and both single-channel methods produced equivalent results. Absolute dose discrepancies between the triple-channel method, both single-channel methods and the treatment planning system calculated dose values, were no larger than 5 cGy for dose levels up to 2.2 Gy. Signal to noise in triple-channel dose images was found to be similar to signal to noise in single-channel dose images. The accuracy of resulting dose images from the triple- and single-channel methods with inclusion of pre-IR film scan was found to be similar. Results of a comparison of EBT2 data from a kilovoltage depth dose experiment to corresponding Monte Carlo depth dose data produced dose discrepancies of 9.5 ± 12 cGy and 7.6 ± 6 cGy for the single-channel method with inclusion of a pre-IR film scan and the triple-channel method, respectively. EBT2 showed to be energy sensitive at low kilovoltage energies with response differences of 11.9% and 15.6% in the red channel at 2 Gy between 50–225 kVp and 80–225 kVp photon spectra, respectively. We observed that the triple-channel method resulted in non-uniformity corrections of ±1% and consistency values of 0–3 cGy for the batches and dose levels studied. Results of this study indicate that the triple-channel radiochromic film read-out method performs at least as well as the single-channel method with inclusion of a pre-IR film scan, reduces film non-uniformity and saves time with elimination of a pre-IR film scan.

SU-E-T-537: Photon Beam Modeling and Verification of Collapsed Cone Convolution Algorithm for Dose Calculation in a Radiation Treatment Planning System.

The aim of this study is to evaluate the accuracy the collapsed cone convolution (CCC) algorithm for dose calculation in a radiation treatment planning system (TPS). We modeled various photon beams for various setup conditions in a radiation treatment planning system (CorePLANTM, Seoul C&J, Korea). The beam models were generated at various set-up conditions such as open beam or wedged beam, 6 MV or 15 MV beam and field sizes from 4×4 cm2 to 40 × 40 cm2 . Each beam model was optimized by spectrum modeling from measured percent depth dose (PDD) data, dose profile modeling from a measured profile at a specific depth (10 cm) data. Dose calculation was performed using conventional CCC algorithm. All measured data were acquired from a Clinac 21EX (Varian Medical System, Palo Alto, CA, USA) linear accelerator with the setting of SSD = 100 cm. All calculated PDD and dose profiles at various depths from generated beam models were compared to the measured data. Calculated dose data from each beam model showed good agreements within 2% of difference to the measured PDD and within 3% dose profiles at various depths. Some regions such as penumbra region at 20 × 20 cm2 field size and horn region at wedge field showed dose discrepancies over 3%. The results of PDD at all situations showed well agreement with measured data under the 10×10 cm2 field size. For wedged cases, however, under the 5 cm depths, some inconsistency at penumbra region were appeared. In this study, we verified the accuracy of CCC algorithm in the TPS. Calculated results by our implemented algorithm was well satisfied with measured dose at small field size (〈20 7 times; 20 cm2 ). Our next study will perform to compensate theses inconsistencies.

SU-E-T-223: High-Energy Photon Standard Dosimetry Data: A Quality Assurance Tool.

Describe the Radiological Physics Center's (RPC) extensive standard dosimetry data set determined from on-site audits measurements. Measurements were made during on-site audits to institutions participating in NCI funded cooperative clinical trials for 44 years using a 0.6cc cylindrical ionization chamber placed within the RPC's water tank. Measurements were made on Varian, Siemens, and Elekta/Philips accelerators for 11 different energies from 68 models of accelerators. We have measured percent depth dose, output factors, and off-axis factors for 123 different accelerator model/energy combinations for which we have 5 or more sets of measurements. The RPC analyzed these data and determined the 'standard data' for each model/energy combination. The RPC defines 'standard data' as the mean value of 5 or more sets of dosimetry data or agreement with published depth dose data (within 2%). The analysis of these standard data indicates that for modern accelerator models, the dosimetry data for a particular model/energy are within ï,±2%. The RPC has always found accelerators of the same make/model/energy combination have the same dosimetric properties in terms of depth dose, field size dependence and off-axis factors. Because of this consistency, the RPC can assign standard data for percent depth dose, average output factors and off-axis factors for a given combination of energy and accelerator make and model. The RPC standard data can be used as a redundant quality assurance tool to assist Medical Physicists to have confidence in their clinical data to within 2%. The next step is for the RPC to provide a way for institutions to submit data to the RPC to determine if their data agrees with the standard data as a redundant check. This work was supported by PHS grants CA10953 awarded by NCI, DHHS.

SU‐E‐T‐274: Monte Carlo Simulations of Output Factors for a Small Animal Irradiator

Measurement of dosimetric parameters of small photon beams, with field sizes as small 1 mm in diameter, is particularly challenging. This work utilizes Monte Carlo techniques to calculate percent depth dose (PDD) and output factors for small photon fields from a kV x-ray based small animal irradiator. Absolute dose calibration of a commercial small animal stereotactic irradiator (XRAD225, Precision X-ray) was performed in accordance with the recommendations of AAPM TG-61 protocol. Both in-air and in-water calibrations were performed at a 30.4 cm source-to-surface distance (SSD) for a reference collimator 50 mm in diameter. The BEAM/EGS was used to model 225 kV photon beams used for most therapeutic applications. The Monte Carlo model was provided good agreement with measured beam characteristics, e.g. PDD and off-axis ratios. Subsequently, output factors for various square and circular applicators were measured using an ionization chamber and radiochromic film, and compared with MC simulations. Directional Bremsstrahlung splitting (DBS) was utilized for variance reduction to improve efficiency of the output factor simulations. The statistical uncertainty on the MC- calculated results is between 0.5% and 1% for most points. The absolute dose measured for reference collimator at 30.4 cm SSD in water and in air is 4.1 and 4.12 Gy/min. The agreement between simulated and measured output factors was excellent, ranging from 1% to 2.84%. The MC- simulated and measured depth dose data, normalized at the surface, show excellent agreement, with a maximum deviation is approximately 2.5 %. Monte Carlo simulation provides an indispensible tool for validating measurements of the smallest field sizes used in preclinical small animal irradiation.

Dosimetric impacts of microgravity: an analysis of 5th, 50th and 95th percentile male and female astronauts

Computational phantoms serve an important role in organ dosimetry and risk assessment performed at the National Aeronautics and Space Administration (NASA). A previous study investigated the impact on organ dose equivalents and effective doses from the use of the University of Florida hybrid adult male (UFHADM) and adult female (UFHADF) phantoms at differing height and weight percentiles versus those given by the two existing NASA phantoms, the computerized anatomical man (CAM) and female (CAF) (Bahadori et al 2011 Phys. Med. Biol. 56 1671–94). In the present study, the UFHADM and UFHADF phantoms of different body sizes were further altered to incorporate the effects of microgravity. Body self-shielding distributions are generated using the voxel-based ray tracer (VoBRaT), and the results are combined with depth dose data from the NASA codes BRYNTRN and HZETRN to yield organ dose equivalents and their rates for a variety of space radiation environments. It is found that while organ dose equivalents are indeed altered by the physiological effects of microgravity, the magnitude of the change in overall risk (indicated by the effective dose) is minimal for the spectra and simplified shielding configurations considered. The results also indicate, however, that UFHADM and UFHADF could be useful in designing dose reduction strategies through optimized positioning of an astronaut during encounters with solar particle events.

EBT2 film as a depth-dose measurement tool for radiotherapy beams over a wide range of energies and modalities

One of the fundamental parameters used for dose calculation is percentage depth-dose, generally measured employing ionization chambers. There are situations where use of ion chambers for measuring depth-doses is difficult or problematic. In such cases, radiochromic film might be an alternative. The EBT-2 model GAFCHROMIC™ film was investigated as a potential tool for depth-dose measurement in radiotherapy beams over a broad range of energies and modalities. Pieces of the EBT-2 model GAFCHROMIC™ EBT2 film were exposed to x-ray, electron, and proton beams used in radiotherapy. The beams employed for this study included kilovoltage x-rays (75 kVp), (60)Co gamma-rays, megavoltage x-rays (18 MV), electrons (7 and 20 MeV), and pristine Bragg-peak proton beams (126 and 152 MeV). At each beam quality, film response was measured over the dose range of 0.4-8.0 Gy, which corresponds to optical densities ranging from 0.05 to 0.4 measured with a flat-bed document scanner. To assess precision in depth-dose measurements with the EBT-2 model GAFCHROMIC™ film, uncertainty in measured optical density was investigated with respect to variation in film-to-film and scanner-bed uniformity. For most beams, percentage depth-doses measured with the EBT-2 model GAFCHROMIC™ film show an excellent agreement with those measured with ion chambers. Some discrepancies are observed in case of (i) kilovoltage x-rays at larger depths due to beam-hardening, and (ii) proton beams around Bragg-peak due to quenching effects. For these beams, an empirical polynomial correction produces better agreement with ion-chamber data. The EBT-2 model GAFCHROMIC™ film is an excellent secondary dosimeter for measurement of percentage depth-doses for a broad range of beam qualities and modalities used in radiotherapy. It offers an easy and efficient way to measure beam depth-dose data with a high spatial resolution.

Bremsstrahlung Spectrum Reconstruction from Gradient Depth Dose Curves Obtained in a Water Phantom

Megavoltage sources are commonly used in radiotherapy treatments, and the determination of the spectral distribution of a photon beam is extremely important for exact dosimetry and for the calculation of therapeutic dose distributions. Since direct measurements of the spectrum are very difficult, we present a technique to accurately calculate the bremsstrahlung spectra based on a numerical reconstruction upon central-axis depth dose data measured in a water tank using inverse methods.The basic idea of this technique is that the measured depth dose curve can be expressed as a weighted superposition of monoenergetic depth dose curves. While traditional approaches directly use the measured depth dose data, we show the improvement of using the gradient of these data for reconstruction. The inverse problem in terms of gradients is shown to be markedly less ill-conditioned than the usual inverse problem. In each case, a Tikhonov regularization is introduced to minimize the effects of noise due to measurement and computation. We illustrate this theory to calculate a 6-MeV photon beam from an Elekta Precise radiotherapy unit utilizing the gradient of depth dose measurements in a water tank.

SU‐E‐T‐687: Experimental Assessment of the Accuracy of a Semi‐Empirical Model and a Monte Carlo System for Proton Dose Calculations for Highly Inhomogeneous Media

Purpose: To experimentally assess the accuracy of proton dose distributions, computed with a semi‐empirical algorithm and a Monte Carlo (MC) system, for inhomogeneous media. Methods: A complex range compensator for a lung patient, being treated with passively‐scattered proton therapy (PSPT) was used to represent the inhomogeneity. Planar dose distributions for a PSPT beam of exactly the same characteristics as the one used to treat the patient, incident normally on a water equivalent phantom, were measured at a series of depths using a calibrated chamber array detector. Semi‐empirical algorithm and MC (MCNPX) calculations were performed with exactly the same geometry. Absolute depth doses and lateral profiles at multiple depths were compared. Results: Agreement between MC and measured depth dose data was found to be within 2% or 2 mm except at the distal edge where the dose gradient was the highest. The corresponding differences between measurements and the semi‐empirical model were up to 5% in the regions of low gradients and distance to agreement in the distal edge region exceeded 1 cm. Comparison of measured vs. MC vs. se‐empirical profiles showed that, in some regions measured data agreed better with MC, whereas in other regions the agreement was better with semi‐empirical model. Generally, the agreement within the beam boundary was superior for measured vs. MC. The gamma analysis (points within 3% and 3 mm) indicated that the percentage of points passing was comparable for both modes of calculations. Conclusions: The differences between measured and semi‐empirical model dose distributions might be explainable based on the approximations in the latter. However, differences between measurements and MC require further examination of numerous sources of uncertainties, e.g., in modeling of beams and compensators, and the measurements (e.g., the depth of sensitive region of the chamber).NCI P01CA021239

Monte Carlo electron source model validation for an Elekta Precise linac

Electron radiation therapy is used frequently for the treatment of skin cancers and superficial tumors especially in the absence of kilovoltage treatment units. Head-and-neck treatment sites require accurate dose distribution calculation to minimize dose to critical structures, e.g., the eye, optic chiasm, nerves, and parotid gland. Monte Carlo simulations can be regarded as the dose calculation method of choice because it can simulate electron transport through any tissue and geometry. In order to use this technique, an accurate electron beam model should be used. In this study, a two point-source electron beam model developed for an Elekta Precise linear accelerator was validated. Monte Carlo data were benchmarked against measured water tank data for a set of regular and circular fields and at 95, 100, and 110 cm source-to-skin-distance. EDR2 Film dose distribution data were also obtained for a paranasal sinus treatment case using a Rando phantom and compared with corresponding dose distribution data obtained from Monte Carlo simulations and a CMS XiO treatment planning system. A partially shielded electron field was also evaluated using a solid water phantom and EDR2 film measurements against Monte Carlo simulations using the developed source model. The major findings were that it could accurately replicate percentage depth dose and beam profile data for water measurements at source-to-skin-distances ranging between 95 and 110 cm over beam energies ranging from 4 to 15 MeV. This represents a stand-off between 0 and 15 cm. Most percentage depth dose and beam profile data (better than 95%) agreed within 2%/2 mm and nearly 100% of the data compared within 3%/3 mm. Calculated penumbra data were within 2 mm for the 20 x 20 cm2 field compared to water tank data at 95 cm source-to-skin-distance over the above energy range. Film data for the Rando phantom case showed gamma index map data that is similar in comparison with the treatment planning system and the Monte Carlo source model. The gamma index showed good agreement (2%/2 mm) between the Monte Carlo source model and the film data. Percentage depth dose and beam profile data were in most cases within a tolerance of 2%/2 mm. The biggest discrepancies were in most cases recorded in the first 6 mm of the water phantom. Circular fields showed local dose agreement within 3%/3mm. Good agreement was found between calculated dose distributions for a paranasal sinus case between Monte Carlo, film measurements and a CMS XiO treatment planning system. The electron beam model can be easily implemented in the BEAMnrc or DOSXYZnrc Monte Carlo codes enabling quick calculation of electron dose distributions in complex geometries.

Dependence of wedge transmission factor on co-60 teletherapy treatment depths and techniques

Measuring the wedge factor (WF) for radiation field of 10 x 10 cm2 at a specified depth and Source to Surface Distance (SSD), and applying the value to all treatment depths and technique could introduce errors > ± 5 % of threshold stipulated for patient radiation dose delivery. Therefore, some Treatment Planning Systems (TPSs) provide for inputs of separate Percentage Depth Dose (PDD) and Tissue Phantom Ratio (TPR) data for wedged fields to account for WF dependence of treatment depths and techniques. Hence, relatively more measurements than usual are taken per wedge filter and photon energy to establish a TPS and obtain dosimetric data for estima-ting treatment time for wedged fields, which required sophisticated equipment and procedures. While many On-cology Centres rely on International PDD and Tissue Maximum Ratio (TMR) data, benchmark data for wedged beams are not readily available. To provide radiotherapy dosimetry of high accuracy and expediency, two emp-irical equations were developed for a GWGP 80 Cobalt-60 teletherapy machine at the Oncology Department, Korle Bu Teaching Hospital (Ghana). The equations were validated via linear interpolations by measuring WFs at various treatment depths using Source Axial distance (SAD) and SSD treatment techniques. The approach required only measurements of WF for a 10 x 10 cm2 field at depth of 5 cm employing SSD treatment technique per wedge filter. Using the empirical equations, WFs were determined to within ± 0.50 % of the measured valu-es over the entire treatment depth range of 1.5 to 15.5 cm for SAD and SSD treatment techniques respectively; and WFs could be obtained for any treatment depth and technique.

Radio-physical properties of micelle leucodye 3D integrating gel dosimeters

Recently, novel radiochromic leucodye micelle hydrogel dosimeters were introduced in the literature. In these studies, gel measured electron depth dose profiles were compared with ion chamber depth dose data, from which it was concluded that leucocrystal violet-type dosimeters were independent of dose rate. Similar conclusions were drawn for leucomalachite green-type dosimeters, only after pre-irradiating the samples to a homogeneous radiation dose. However, in our extensive study of the radio-physical properties of leucocrystal violet- and leucomalachite green-type dosimeters, a significant dose rate dependence was found. For a dose rate variation between 50 and 400 , a maximum difference of 75% was found in optical dose sensitivity for the leucomalachite green-type dosimeter. Furthermore, the measured optical dose sensitivity of the leucomalachite green-type dosimeter was four times lower than the value previously reported in the literature. For the leucocrystal violet-type dosimeter, a maximum difference in optical dose sensitivity of 55% was found between 50 and 400 . A modified composition of the leucomalachite green-type dosimeter is proposed. This dosimeter is composed of gelatin, sodium dodecyl sulfate, chloroform, trichloroacetic acid and leucomalachite green. The optical dose sensitivity amounted to (dose rate 400 ). No energy dependence for photon energies between 6 and 18 MV was found. No temperature dependence during readout was found notwithstanding a temperature dependence during irradiation of 1.90 cGy °C−1 increase on a total dose of 100 cGy. The novel gel dosimeter formulation exhibits an improved spatial stability (= 0.088 )) and good water/soft tissue equivalence. Nevertheless, the novel formulation was also found to have a significant, albeit reduced, dose rate dependence, as a maximum difference of 33% was found in optical dose sensitivity when the dose rate varied between 50 and 400 . By pre-irradiating the novel leucomalachite green-type dosimeter to 500 cGy, the apparent difference in dose response between 200 and 400 was eliminated, similar to earlier findings. However, a dose response difference of 38% between 50 and 200 was still measured. On the basis of these experimental results it is concluded that the leucodye micelle gel dosimeter is not yet optimal for dose verifications of high precision radiation therapy treatments. This study, however, indicates that the dose rate dependence has a potential for improvement. Future research is necessary to further minimize the dose rate dependence through extensive chemical analysis and optimization of the gel formulation. Some insights into the physicochemical mechanisms were obtained and are discussed in this paper.

Estimation of inhomogenity correction factors for a Co-60 beam using Monte Carlo simulation

The aim was to obtain inhomogenity correction factors (ICFs) for lung tissue inhomogenity for a Co-60 teletherapy beam using Monte Carlo simulation and to compare them with factors obtained from a commercially available treatment planning system. The Monte Carlo simulation code of EGSnrc is used for the depth dose calculations. Two clinical like situations were simulated-dose calculation point beyond the lung tissue volume and dose calculation point within the lung tissue volume. The variation of ICF with lung thicknesses and positions was studied. ICF values were obtained for the similar situations from a commercially available treatment planning system, Theraplan Plus. Percentage depth dose data obtained from Monte Carlo simulation is well matching with the published measurement data. ICFs for lung tissue inhomogenity calculated using the Monte Carlo code are in good agreement with Theraplan Plus TPS values for small inhomogenity thicknesses. These results can be used for the verification of TPS calculation or manual treatment time calculation.

The accuracy of dose calculations by anisotropic analytical algorithms for stereotactic radiotherapy in nasopharyngeal carcinoma

Nasopharyngeal tumors are commonly treated with intensity-modulated radiotherapy techniques. For photon dose calculations, problems related to loss of lateral electronic equilibrium exist when small fields are used. The anisotropic analytical algorithm (AAA) implemented in Varian Eclipse was developed to replace the pencil beam convolution (PBC) algorithm for more accurate dose prediction in an inhomogeneous medium. The purpose of this study was to investigate the accuracy of the AAA for predicting interface doses for intensity-modulated stereotactic radiotherapy boost of nasopharyngeal tumors. The central axis depth dose data and dose profiles of phantoms with rectangular air cavities for small fields were measured using a 6 MV beam. In addition, the air–tissue interface doses from six different intensity-modulated stereotactic radiotherapy plans were measured in an anthropomorphic phantom. The nasopharyngeal region of the phantom was especially modified to simulate the air cavities of a typical patient. The measured data were compared to the data calculated by both the AAA and the PBC algorithm. When using single small fields in rectangular air cavity phantoms, both AAA and PBC overestimated the central axis dose at and beyond the first few millimeters of the air–water interface. Although the AAA performs better than the PBC algorithm, its calculated interface dose could still be more than three times that of the measured dose when a 2 × 2 cm2 field was used. Testing of the algorithms using the anthropomorphic phantom showed that the maximum overestimation by the PBC algorithm was 20.7%, while that by the AAA was 8.3%. When multiple fields were used in a patient geometry, the dose prediction errors of the AAA would be substantially reduced compared with those from a single field. However, overestimation of more than 3% could still be found at some points at the air–tissue interface.