Out-of-field scattering from the Integral Quality Monitor® in megavolt photon beams.

PURPOSE: To investigate out-of-field scattered doses from the Integral Quality Monitor (IQM) transmission detector in megavoltage photon beams. MATERIALS AND METHODS: We measured out-of-field point doses for 20 × 20 cm2 6-15 MV photon beams using 10 × 2 cm2 Gafchromic EBT3 film strips placed across the surfaces of 5-cm thick water-equivalent RW3 slabs. The films were placed at 10 cm intervals from the central axis (CAX) of each beam, up to 1.0 m away on opposite sides of the CAX. The measurements were conducted at 80 cm and 100 cm source-to-surface distances (SSD) without the IQM, and were repeated with the IQM in the paths of the beams. Measurements were also performed at 90 cm SSD for 20 × 20 and 30 × 30 cm2 15 MV fields. Surface dose profiles were then constructed from the measurements for each beam setup with and without the IQM to examine the differences in scattered dose off-axis. The dose profile for each beam setup was normalised to dose maximum measured on the CAX. RESULTS: Overall, surface dose profiles acquired with the IQM in the paths of the beams were higher than the corresponding profiles without the IQM. The out-of-field dose increased with increase in photon energy, field size, and shorter SSDs, and decreased with off-axis distance. At 80 cm SSD for the 20 × 20 cm2 field, the IQM-induced surface dose ranged from -0.6% ÷ 1.9%, -0.3% ÷ 3.0%, and 0.3% ÷ 6.8% for 6, 10, and 15 MV beams, respectively. CONCLUSION: The higher surface dose profiles measured with the IQM attached to the linac compared to the profiles without the IQM indicates that the device is acting as an additional source of scattered radiation.

Characterization of Nylon-12 as a water-equivalent solid phantom material for dosimetric measurements in therapeutic photon and electron beams.

The tissue- or water-equivalence of dosimetry phantoms used as substitutes for water is essential for absorbed dose measurements in radiotherapy. At our institution, a heterogeneous pelvic phantom that consists of stacked Nylon-12 layers has recently been manufactured for Gafchromic film dosimetry. However, data on the use of Nylon as tissue-mimicking media for dosimetric applications are scarce. This study characterizes the water-equivalence of Nylon-12 for dosimetric measurements in therapeutic photon and electron beams. Employing an Elekta Synergy and SL25 linear accelerator (Linac), photon beam transmission measurements for 6 MV and 15 MV, acquired in narrow beam geometry with a 0.6 cm3 Farmer-type ion chamber showed that the mass attenuation coefficient µm of Nylon-12 agrees with the values of water, water-equivalent RW3 and Perspex phantom materials within 3%. For 6 MV, the µm values were 0.0477 ±â€¯0.002 cm2/g, 0.0490 ±â€¯0.003 cm2/g, 0.0482 ±â€¯0.001 cm2/g and 0.0479 ±â€¯0.002cm2/g for Nylon-12, water, RW3, and Perspex, respectively. Differences within 2% were attained between depth dose data measured in Nylon-12 slabs with Gafchromic EBT3 films and in water with a Roos ion chamber for 10 × 10 cm2 6, 12 and 20 MeV electron beams produced by the Elekta Synergy and SL25 Linacs. Also, a good agreement within 2% was obtained between percent depth doses computed by DOSXYZnrc Monte Carlo simulations in water, Nylon-12 and RW3 materials for photon spectra between 250 kV and 15 MV. The discrepancies between the ratios of average, restricted stopping powers of Nylon to air and water to air for photon spectra ranging from 2 to 45 MV are typically within 1% signifying that Nylon and water have equivalent stopping power characteristics. This study highlights that Nylon-12 can be used as a tissue-mimicking phantom material for dosimetric measurements in clinical megavoltage photon and electron beams as it exhibits good water-equivalence.

Set-up error validation with EPID images: Measurements vs Egs_cbct simulation.

AIM: In this study, the egs_cbct code's ability to replicate an electronic portal imaging device (EPID) is explored. BACKGROUND: We have investigated head and neck (H&N) setup verification on an Elekta Precise linear accelerator. It is equipped with an electronic portal imaging device (EPID) that can capture a set of projection images over different gantry angles. METHODS AND MATERIALS: Cone-beam computed tomography (CBCT) images were reconstructed from projection images of two different setup scenarios. Projections of an Anthropomorphic Rando head phantom were also simulated by using the egs_cbct Monte Carlo code for comparison with the measured projections.Afterwards, CBCT images were reconstructed from this data. Image quality was evaluated against a metric defined as the image acquisition interval (IAI). It determines the number of projection images to be used for CBCT image reconstruction. RESULTS: From this results it was established that phantom shifts could be determined within 2 mm and rotations within one degree accuracy using only 20 projection images (IAI = 10 degrees). Similar results were obtained with the simulated data. CONCLUSION: In this study it is demonstrated that a head and neck setup can be verified using substantially fewer projection images. Bony landmarks and air cavities could still be observed in the reconstructed Rando head phantom. The egs_cbct code can be used as a tool to investigate setup errors without tedious measurements with an EPID system.

Improved Monte Carlo clinical electron beam modelling.

PURPOSE: An EGSnrc based electron model was developed and validated for an Elekta Synergy® 160-leaf Agility™ linear accelerator. It was able to reproduce measured central axis (CAX) percentage depth dose (PDD) curves and off-axis profiles (OAPs) within 2%/2 mm, and relative output factors (ROFs) within 3%. METHODS: BEAMnrc component modules were used to model the accelerator accurately according to vendor supplied specifications. The electron beam focal spot size and input energy spectrum were determined through their effects on electron CAX PDDs and OAPs as benchmarked against water tank data. Phase space files were used as source input in DOSXYZnrc water phantom simulations. Dose distributions were calculated for six electron nominal energies, 11 field sizes and two source-to-surface distances. RESULTS: The full width at half maximum of the focal spot (assuming a Gaussian intensity distribution) was determined to be 1.50 mm. An asymmetrical input electron energy spectrum with a low-energy tail produced good agreement with measured data and solved the match in the build-up (BU) region for all electron energies used in this study. CONCLUSIONS: The improved input electron spectra for the electron model could predict dose distributions within 2%/2 mm of measured data. The model's success is embedded in the asymmetrical energy spectrum which provided a valuable free parameter which, by fine adjustment, improved the match in the BU region of dose distributions. Furthermore, focal spot parameters could be determined by means of iterative simulations.

Dose Shadowing and Prosthesis Involvement for Megavoltage Photon In vivo Diode Dosimetry.

AIM: The aim of the study is to investigate the photon beam perturbations induced by an in vivo diode in combination with prosthesis involvement in a human-like phantom. MATERIALS AND METHODS: Beam perturbations for 6 MV and 10 MV photons caused by an EDP-203G in vivo diode in combination with prosthesis involvement were studied in a unique water-equivalent pelvic phantom, equipped with bony structures and Ti prosthesis using single fields between 2 × 2 and 15 cm × 15 cm as well as 10 MV lateral opposing fields and a four-field plan. Dose distributions were measured with Gafchromic EBT3 films with and without the diode included in the beams on the prosthesis (prosthetic fields) and non-prosthesis (non-prosthetic fields) sides of the phantom. Differences between prosthetic and non-prosthetic field dose data were determined to assess the effect of the prosthesis on the diode-induced beam perturbations inside the phantom. RESULTS: Photon beam dose perturbations ranged from 2% to 7% and from 5% to 12% for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 4%. In addition, d50 depths ranging from 8.7 to 11.5 cm and from 11.5 to 15 cm were acquired in the phantom for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 5%. CONCLUSION: On the basis of accuracy requirements in radiotherapy noting that a small underdose to tumors could yield a decrease in the probability of tumor control, the diode-induced beam perturbations in combination with prosthesis involvement in the photon fields may affect treatment outcome, as there would be a reduction in the prescribed target dose during treatment delivery.

Monte Carlo dose in a prosthesis phantom based on exact geometry vs streak artefact contaminated CT data as benchmarked against Gafchromic film measurements.

PURPOSE: In radiotherapy, accurate calculation of patient radiation dose is very important for good clinical outcome. In the presence of metallic implants, the dose calculation accuracy could be compromised by metal artefacts generated in computed tomography (CT) images of patients. This study investigates the influence of metal-induced CT artefacts on MC dose calculations in a pelvic prosthesis phantom. METHODS: A pelvic phantom containing unilateral Ti prosthesis was CT-scanned and accurate Hounsfield unit (HU) values were assigned to known materials of the phantom as opposed to HU values produced through the artefact CT images of the phantom. Using the DOSXYZnrc MC code, dose calculations were computed in the phantom model constructed from the original CT images containing the artefacts and artefact-free images made from the exact geometry of the phantom with known materials. The dose calculations were benchmarked against Gafchromic EBT3 film measurements using 15 MeV electron and 10 MV photon beams. RESULTS: The average deviations between film and MC dose data decreased from 3 ±â€¯2% to 1 ±â€¯1% and from about 6 ±â€¯2% to 3 ±â€¯1% for the artefact and artefact-free phantom models against film data for the electron and photon fields, respectively. CONCLUSIONS: For the Ti prosthesis phantom, the presence of metal-induced CT artefacts could cause dose inaccuracies of about 3%. Construction of an artefact-free phantom model made from the exact geometry of the phantom with known materials to overcome the effect of artefacts is advantageous compared to using CT data directly of which the exact tissue composition is not well-known.

Measurement of the influence of titanium hip prosthesis on therapeutic electron beam dose distributions in a novel pelvic phantom.

PURPOSE: To investigate the degree of 18 and 22MeV electron beam dose perturbations caused by unilateral hip titanium (Ti) prosthesis. METHODS: Measurements were acquired using Gafchromic EBT2 film in a novel pelvic phantom made out of Nylon-12 slices in which a Ti-prosthesis is embedded. Dose perturbations were measured and compared using depth doses for 8×8, 10×10 and 11×11cm2 applicator-defined field sizes at 95cm source-surface-distance (SSD). Comparisons were also made between film data at 100cm SSD for a 10×10cm2 field and dose calculations made on CMS XiO treatment planning system utilizing the pencil beam algorithm. The extent of dose deviations caused by the Ti prosthesis based on film data was quantified through the dose enhancement factor (DEF), defined as the ratio of the dose influenced by the prosthesis and the unchanged beam. RESULTS: At the interface between Nylon-12 and the Ti implant on the prosthesis entrance side, the dose increased to values of 21±1% and 23±1% for 18 and 22MeV electron beams, respectively. DEFs increased with increasing electron energy and field size, and were found to fall off quickly with distance from the nylon-prosthesis interface. A comparison of film and XiO depth dose data for 18 and 22MeV gave relative errors of 20% and 25%, respectively. CONCLUSION: This study outlines the lack of accuracy of the XiO TPS for electron planning in highly heterogeneous media. So a dosimetric error of 20-25% could influence clinical outcome.

Sensitivity Analysis of the Integral Quality Monitoring System® Using Monte Carlo Simulation.

The Integral Quality Monitoring (IQM) System is a real-time beam output verifying system that validates the integrity and accuracy of patient treatment plan (TP) data during radiation treatment. The purpose of this study was to evaluate the sensitivity of the IQM to errors in segment using EGSnrc/BEAMnrc Monte Carlo (MC) codes. Sensitivity analysis (SA) techniques were applied to study the significance of small alterations of field sizes (segments) on the IQM signal response. One hundred and eighty multileaf segments were analyzed with methods that include scatter plots (SP), brute force, variance-based (VAR), and standard regression coefficient SA. The segments were altered randomly within ±1, ±2, and ±3 mm leaf steps for 10 MV photon beams. SP analysis gradient and VAR maximum index are 1.045 and 0.556 for the smallest segment while the largest segment has the value of 0.018 and 0.504, respectively. The brute force and standard regression displayed maximum sensitivity indices around the unaltered segments. These tests conclusively indicated that the IQM was more sensitive to alterations of small segments compared to larger segments. This is important since small segment variation will cause a higher dose output variation that should be picked up during online beam monitoring.

Dose comparison between Gafchromic film, XiO, and Monaco treatment planning systems in a novel pelvic phantom that contains a titanium hip prosthesis.

The presence of metallic prostheses during external beam radiotherapy of malignancies in the pelvic region has the potential to strongly influence the dose distribution to the target and to tissue surrounded by the prostheses. This study systematically investigates the perturbation effects of unilateral titanium prosthesis on 6 and 15 MV photon beam dose distributions using Gafchromic EBT2 film measurements in a novel pelvic phantom made out of a stack of nylon slices. Comparisons were also made between the film data and dose calculations made on XiO and Monaco treatment planning systems. The collapsed cone algorithm was chosen for the XiO and the Monte Carlo algorithm used on Monaco is XVMC. Transmission measurements were taken using a narrow-beam geometry to determine the mass attenuation coefficient of nylon = 0.0458 cm2 /g and for a water-equivalent RW3 phantom, it was 0.0465 cm2 /g. The perturbation effects of the prosthesis on dose distributions were investigated by measuring and comparing dose maps and profiles. The magnitude of dose perturbations was quantified by calculating dose enhancement and reduction factors using field sizes of 3 × 3, 5 × 5, 10 × 10, and 15 × 15 cm2 . For the studied beams and field sizes, dose enhancements between 21 and 30% and dose reductions between 15 and 21% were observed at the nylon-prosthesis interface on the proximal and distal sides of the prosthesis for film measurements. The dose escalation increases with beam energy, and the dose reduction due to attenuation decreases with increasing beam energy when compared to unattenuated beam data. A comparison of film and XiO depth doses for the studied fields gave relative errors between 1.1 and 23.2% at the proximal and distal interfaces of the Ti prosthesis. Also, relative errors < 4.0% were obtained between film and Monaco dose data outside the prosthesis for 6 and 15 MV lateral opposing fields.

Monte carlo electron source model validation for an Elekta Precise linac.

PURPOSE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Inclusion of compensator-induced scatter and beam filtration in pencil beam dose calculations.

Compensators can be used as beam intensity modulation devices for intensity-modulated radiation therapy applications. In contrast with multileaf collimators, compensators introduce scatter and beam hardening into the therapeutic x-ray beam. The degree of scatter and beam filtering depends on the compensator material and beam energy. Pencil beam dose calculation models can be used to derive the shape of the compensator. In this study a novel way of incorporating the effect of compensator-induced scatter and beam filtration is presented. The study was conducted using 6, 8, and 15 MV polyenergetic pencil beams (PBs). The compensator materials that were studied included wax, brass, copper, and lead. The perturbation effects of the compensators on the PB dose profiles were built in the PB dose profiles and tested for regular fields containing a step compensator and benchmarked against DOSXYZnrc Monte Carlo calculated dose profiles. These effects include compensator beam filtration and Compton-scattered photons generated in the compensator materials that influence the resulting PB dose profiles. These data were obtained from DOSXYZnrc simulations. A Gaussian function was used to model off-axis scatter and an exponential function was used to model beam hardening at any radius, r. Dose profiles were calculated under a step compensator using the method that can model beam hardening and off-axis scatter, as well as a conventional method where the PB profiles are not adjusted, but a single effective attenuation coefficient is used instead to best match the dose profiles. Both sets of data were compared to the DOSXYZnrc data. Depth and profile dose data for 10 x 10 cm2 and 20 x 20 cm2 fields indicated that at 2 cm depth in water the method that takes compensator scatter into account agrees more closely with the DOSXYZnrc data compared to the data using only an effective attenuation coefficient. Further, it was found that the effective attenuation method can only replicate the DOSXYZnrc data at 10 cm depth where it was chosen to do so. At shallower depths the effective attenuation method overestimates the dose and beyond 10 cm depth it causes an underestimation in the dose. The scatter and beam hardening inclusion method does not exhibit such properties. The exclusion of scatter can lead to dose errors of up to 4 percent with a copper compensator at 5 cm depth for a 10 X 10 cm2 field under a thickness of 5 cm at 6 MV. For materials such as lead this discrepancy could be as high as 7 to 8 percent at 6 MV. For larger fields (20 X 20 cm2) the effect of in-phantom scatter reduces the differences between the dose profiles calculated with the mentioned methods.

Characterization of megavoltage electron beams delivered through a photon multi-leaf collimator (pMLC).

A study is presented that characterizes megavoltage electron beams delivered through an existing double-focused photon multi-leaf collimator (pMLC) using film measurements in a solid water phantom. Machine output stability and linearity were evaluated as well as the effect of source-to-surface distance (SSD) and field size on the penumbra for electron energies between 6 and 18 MeV over an SSD range of 60-100 cm. Penumbra variations as a function of field size, depth of measurement and the influence of the jaws were also studied. Field abutment, field flatness and target coverage for segmented beams were also addressed. The measured field size for electrons transported through the pMLC was the same as that for an x-ray beam up to SSDs of 70 cm. At larger SSD, the lower energy electron fields deviated from the projected field. Penumbra data indicated that 60 cm SSD was the most favourable treatment distance. Backprojection of P(20-80) penumbra data yielded a virtual source position located at 98.9 cm from the surface for 18 MeV electrons. For 6 MeV electrons, the virtual source position was at a distance of 82.6 cm. Penumbra values were smaller for small beam slits and reached a near-constant value for field widths larger than 5 cm. The influence of the jaws had a small effect on the penumbra. The R90 values ranged from 1.4 to 4.8 cm between 6 and 21 MeV as measured at 60 cm SSD for a 9 x 9 cm2 field. Uniformity and penumbra improvement could be demonstrated using weighted abutted fields especially useful for small segments. No detectable electron leakage through the pMLC was observed. Bremsstrahlung measurements taken at 60 cm SSD for a 9 x 9 cm2 field as shaped by the pMLC compared within 1% to bremsstrahlung measurements taken at 100 cm SSD for a 10 x 10 cm2 electron applicator field at 100 cm SSD.

Radiological properties of a wax-gypsum compensator material.

In this paper the radiological properties of a compensator material consisting of wax and gypsum is presented. Effective attenuation coefficients (EACs) have been determined from transmission measurements with an ion chamber in a Perspex phantom. Measurements were made at 80 and 100 cm source-to-skin distance (SSD) for beam energies of 6, 8, and 15 MV, for field sizes ranging from narrow beam geometries up to 40 x 40 cm2, and at measurement depths of maximum dose build-up, 5 and 10 cm. A parametrization equation could be constructed to predict the EAC values within 4% uncertainty as a function of field size and depth of measurement. The EAC dependence on off-axis position was also quantified at each beam energy and SSD. It was found that the compensator material reduced the required thickness for compensation by 26% at 8 MV when compared to pure paraffin wax for a 10 x 10 cm2 field. Relative surface ionization (RSI) measurements have been made to quantify the effect of scattered electrons from the wax-gypsum compensator. Results indicated that for 80 cm SSD the RSI would exceed 50% for fields larger than 15 x 15 cm2. At 100 cm SSD the RSI values were below 50% for all field sizes used.

Monte Carlo calculation of effective attenuation coefficients for various compensator materials.

Effective attenuation coefficients for 6, 8, and 15 MV photon beams were derived and studied for various compensator materials for square beams with side lengths of 0.5, 1.0, 2.0, 3.0, and 5.0 cm. Calculations were based on depth dose data in water obtained from EGS4 based DOSXYZ Monte Carlo simulations. Depth dose data were calculated using different compensator materials as attenuators of variable thickness. The absorbed dose varied exponentially as a function of absorber thickness at any depth in water on the beam axis for all materials. The effective attenuation coefficient data were compared with measurements for wax, aluminum and brass with values from the literature. Theoretical narrow beam linear attenuation coefficients were calculated and compared with the Monte Carlo data. The effective attenuation coefficient data for all materials were parametrized as functions of field size and depth in water. The effective attenuation coefficient was also parametrized as a function of atomic number. It was found that the effective attenuation coefficients calculated from the DOSXYZ data using a simple source model correspond to measured data for wax, aluminum and brass and published data for lead.