Off-axis Dose Distributions Research Articles

Electron beam scattering device for FLASH preclinical studies with 6-MeV LINAC

In this study, an electron-scattering device was fabricated to practically use the ultra-high dose rate electron beams for the FLASH preclinical research in Dongnam Institute of Radiological and Medical Sciences. The Dongnam Institute of Radiological and Medical Sciences has been involved in the investigation of linear accelerators for preclinical research and has recently implemented FLASH electron beams. To determine the geometry of the scattering device for the FLASH preclinical research with a 6-MeV linear accelerator, the Monte Carlo N-particle transport code was exploited. By employing the fabricated scattering device, the off-axis and depth dose distributions were measured with radiochromic films. The generated mean energy of electron beams via the scattering device was 4.3 MeV, and the symmetry and flatness of the off-axis dose distribution were 0.11% and 2.33%, respectively. Finally, the doses per pulse were obtained as a function of the source to surface distance (SSD); the measured dose per pulse varied from 4.0 to 0.2 Gy/pulse at an SSD range of 20–90 cm. At an SSD of 30 cm with a 100-Hz repetition rate, the dose rate was 180 Gy/s, which is sufficient for the preclinical FLASH studies.

Off-axis dose distribution with stand-in and stand-off configurations for superficial radiotherapy treatments.

Current practice when delivering dose for superficial skin radiotherapy is to adjust the monitor units so that the prescribed dose is delivered to the central axis of the superficial unit applicator. Variations of source‐to‐surface distance due to patient’s anatomy protruding into the applicator or extending away from the applicator require adjustments to the monitor units using the inverse square law. Off‐axis dose distribution varies significantly from the central axis dose and is not currently being quantified. The dose falloff at the periphery of the field is not symmetrical in the anode–cathode axis due to the heel effect. This study was conducted to quantify the variation of dose across the surface being treated and model a simple geometric shape to estimate a patient’s surface with stand‐in and stand‐off. Isodose plots and color‐coded dose distribution maps were produced from scans of GAFChromic EBT‐3 film irradiated by a Gulmay D3300 orthovoltage x‐ray therapy system. It was clear that larger applicators show a greater dose falloff toward the periphery than smaller applicators. Larger applicators were found to have a lower percentage of points above 90% of central axis dose (SA90). Current clinical practice does not take this field variation into account. Stand‐in can result in significant dose falloff off‐axis depending on the depth and width of the protrusion, while stand‐off can result in a flatter field due to the high‐dose region near the central axis being further from the source than the peripheral regions. The central axis also received a 7% increased or decreased dose for stand‐in or stand‐off, respectively.

Independent 3D dose calculation for IMRT based on simplification of beam model and collapsed-cone convolution/superposition algorithm

Objective To realize independent 3D dose calculation for intensity-modulated radiotherapy (IMRT) by building a two-source beam model of medical linear accelerator combined with a collapsed-cone convolution/superposition (CCCS) algorithm. Methods Two-source beam models of medical linear accelerators (Varian and Elekta) were built to calculate the 3D dose distributions using the CCCS algorithm. Scp, percent depth dose, and off-axis dose distribution were compared with the scanning data of ion chamber to confirm the calculation model. Twelve intensity-modulated treatment plans from each accelerator (a total of 24 plans) were selected for comparison. The calculation results of treatment planning system (TPS) were independently validated, and further compared with the measurement results of detector matrix. Results The dose deviations at the center of rectangle fields were lower than 1%, the deviation between doses at the same position in the field was not higher than 1%, and the positional deviation in the penumbra region was not higher than 1 mm. Gamma analysis based on 3%/3 mm standard was used to compare the results calculated by detectors and TPS.The pass rates were higher than 90%. Conclusions The independent 3D dose calculation for IMRT based on two-source beam model combined with CCCS algorithm has been successfully set up. The comparison between regular field and IMRT plan indicates that this method and calculation model can be used for independent 3D dose calculation of clinical plan. Key words: Clinical linear accelerators beam model; Independent dose calculation; Quality assurance

A Monte Carlo evaluation of carbon and lithium ions dose distributions in water

Purpose: To compare dose distributions on the central- and off-axis for 12C and 7Li ion beams simulated by the codes SHIELD-HIT (Heavy Ion Transport) and FLUKA (FLUKtuierende KAskade), and compare with experimental data for 300 MeV/u 12C and 185 MeV/u 7Li ion beams.Materials and methods: The general purpose Monte Carlo codes, SHIELD-HIT10 and FLUKA 2008.3d.1 were used for the ion dose distribution calculations. SHIELD-HIT transports hadrons and atomic nuclei of arbitrary charge and mass number in an energy range from 1 keV/u up to 1 GeV/u. Similarly, FLUKA transports charged hadrons in an energy range from 100 keV up to 20 TeV. Neutrons are transported down to thermal energies in both codes. Inelastic nuclear interactions are modelled in SHIELD-HIT by the Many Stage Dynamical Model (MSDM), whereas in FLUKA the Pre-Equilibrium Approach to Nuclear Thermalisation (PEANUT) package which includes a Generalized Intra-Nuclear Cascade model was used.Results: The dose distributions in water irradiated with 300 MeV/u 12C and 185 MeV/u 7Li ion beams were simulated with the two codes. Studies were performed of the energy deposition both on the central axis and at lateral distances up to 10 cm off-axis. The dose distributions calculated by SHIELD-HIT and FLUKA were compared with published experimental data. The dose mean lineal energy , frequency mean lineal energy , dose mean specific energy , and frequency mean specific energy were calculated with the ion track-structure code PITS99 (Positive Ion Track Structure 99), coupled with the electron code KURBUC for the primary and secondary ions average energies at 1 mm before the Bragg peak.Conclusion: The Monte Carlo codes show good agreement with experimental results for off-axis dose distributions. The disagreements in the Bragg peak region for the central-axis dose distributions imply that further improvements especially in the nuclear interaction models are required to increase the accuracy of the codes.

Off-axis dose distribution for rectangle proton beam

This paper modifies an analytical algorithm originally developed for electron dose calculations to evaluate the off-axis dose distribution of rectangle proton beam. This spatial distribution could be described by Fermi–Eyges theory since a proton undergoes small-angle scattering when it passes through medium. Predictions of the algorithm for relative off-axis dose distribution by a 6 cm * 6 cm initial monoenergetic proton beam are compared with the results from the published Monte Carlo simulations. The excellent level of agreement between the results of these two methods of dose calculation (< 2%) demonstrates that the off-axis dose distribution from rectangle proton beam may be computed with high accuracy using this algorithm. The results also prompts the necessity to consider the off-axis distribution when the proton is applied to clinical radiotherapy since the penumbra is significant at the distal of its range (about 0.6 cm at the Bragg-peak depth).

Optimization of photon beam flatness for radiation therapy

In this work, we investigate the relation between lateral fluence/dose distributions and photon beam uniformity, possibly identifying ways to improve these characteristics. The calculations included treatment head scatter properties associated with three common types of linear accelerators in order to study their impact on the results. For 6 and 18 MV photon beams the lateral fluence distributions were optimized with respect to the resulting calculated flatness, as defined by the International Electrotechnical Commission (IEC), at 10 cm depth in six different field sizes. The limits proposed by IEC for maximum dose ratios (‘horns’) at the depth of dose maximum have also been accounted for in the optimization procedure. The conclusion was that typical head scatter variations among different types of linear accelerators have a very limited effect on the optimized results, which implies that the existing differences in measured off-axis dose distributions are related to non-equivalent optimization objectives. Finally, a comparison between the theoretically optimized lateral dose distributions and corresponding dose measurements for the three investigated accelerator types was performed. Although the measured data generally fall within the IEC requirements the optimized distributions show better results overall for the evaluated uniformity parameters, indicating that there is room for improved flatness performance in clinical photon beams.

Design of a computer-controlled multileaf collimator for advanced electron radiotherapy

A multileaf collimator for electrons (eMLC) has been designed that fulfils the technical requirements for providing advanced irradiation techniques with electrons. In the present work, the basic design parameters of leaf material, leaf height, leaf width and number of leaves as well as leaf overtravel and leaf shape were determined such that an eMLC with motorized leaves can be manufactured by a company specialized in MLC technology. For this purpose, a manually driven eMLC with variable source-to-collimator distance (SCD) was used to evaluate the chosen leaf specification and investigate the impact of the SCD on the off-axis dose distribution. In order to select the final SCD of the eMLC, a compromise had to be found between maximum field size, minimum beam penumbra and necessary distance between eMLC and isocentre to eliminate patient realignments during gantry rotation. As a result, the eMLC is placed according to the target position at 72 and 84 cm SCD, respectively. This feature will be achieved by interchangeable distance holders. At these SCDs, the corresponding maximum field sizes at 100 cm source-to-isocentre distance are 20 × 20 cm and 17 × 17 cm, respectively. Finally, the off-axis dose distribution at the maximum opening of the eMLC was improved by fine-tuning the settings of the accelerator jaws and introducing trimmer bars above the eMLC. Following this optimization, a prototype eMLC consisting of 2 × 24 computer-controlled brass leaves is manufactured by 3D Line Medical Systems.

Does 4 MV perform better compared to 6 MV in the presence of air cavities in the head and neck region?

Background and purpose The underdose near air cavities in the head and neck region at photon energies of 4 MV and 6 MV was studied in search for clinical advantages of the 4 MV over 6 MV treatments. Materials and methods The on-axis and off-axis dose distributions were measured with a parallel-plate ionization chamber and films in polystyrene phantoms containing an air cavity of appropriate size based on the results of computed tomography scans. Results Although most results are similar for both energies, the 4 MV photon beams give a somewhat smaller underdose effect and a faster re-build up than the 6 MV. For both energies a significant underdose effect was observed at the edge of the field in the larynx phantom. This proved to be true for small and large fields, for smaller and larger cavities, for one-beam as well as parallel-opposed beams. Conclusion For most clinically relevant situations there is no remarkable benefit in the use of either of the two energies.

Dosimetric analysis of intact breast irradiation in off-axis planes

Purpose/Objective : The purpose of this investigation is to quantify dose inhomogeneity of intact breast irradiation in off-axis planes, and determine how dose inhomogeneity varies according to patient breast size and anatomical region of the breast. Methods and Materials : Eleven patients treated with intact breast radiation underwent a treatment-ptanning computer tomography (CT) scan with 1-cm slices through the entire breast. The area of breast tissue was defined on each CT slice. Treatment planning with lung correction factors was performed using a two-dimensional treatment-planning system that calculates off-axis dose distributions on a slice-by-slice basis. Each plan utilized tangential beams with matched nondivergent posterior borders and with collimator rotation to match the chest wall slope. Dose inhomogeneity within the central plane was minimized during treatment planning by the use of a wedge on the lateral tangent field and by the differential weighting of fields. Dose was normalized at the breast and pectoratis major interface at midseparation in the central plane. Off-axis dose inhomogeneity was not considered in the optimization of the treatment plan. Dose distributions were plotted for each 1-cm slice, and the area of each isodose curve within the breast on each CT slice was calculated. The results of each slice were summed to give an approximation of dose-volume relationships. Results : For the entire population, an average of 10% of the breast volume (range 1–40%) received 110% or greater of the prescribed dose. Increasing dose inhomogeneity was positively correlated with increasing breast sizes ( r = 0.72, p = 0.01—Spearmen rank test). Analysis of dose as a function of location within the breast, revealed that the greatest dose inhomogeneity occurred in the lower anatomical quadrants of the breast ( p = 0.003—Kruskal-Wallis test). For the group, the mean breast volume that received a 110% or greater dose was: 30% at 6 cm 3 below central axis, 14% at 4 cm 3 below central axis, 6% at central axis, 5% at 4 cm 3 above central axis, and 7% at 6 em above central axis. Conclusion : Our study demonstrates that a significant volume of breast tissue receives 110% or greater of the prescribed dose. This inhomogeneity is greatest in women with larger breast sizes, providing a possible explanation for the poorer cosmetic result seen in this subset of patients compared to women with small breast sizes. In addition, our results show the greatest dose inhomogeneity in the lower quadrants of the breast. Off-axis dose inhomogeneity should be considered in the planning of tumor bed boosts in women with lower quadrant tumors.

Beam data requirements of the Clinac 4 for three-dimensional treatment planning with wedged fields.

To calculate dose distributions in off-axis planes for wedged fields, some commercial radiotherapy treatment planning systems use the Milan and Bentley method and simply modify the transverse principal-plane profile in the wedge direction with the value of its orthogonal open-field profile at the off-axis distance. For the Clinac 4 this simple modification may introduce serious distortions in off-axis dose distributions. The error can be considerably reduced if the basic beam data are supplemented with longitudinal principal-plane profiles and depth doses measured for the largest wedged field (14 X 32 cm2). These added data enable the determination of position-dependent effective attenuation coefficients of the wedges and hence, with the model proposed here, the calculation of transverse off-axis and longitudinal profiles as required. Even for very large field sizes and off-axis distances, the maximum deviation of generated profiles from measured ones varies from about 1% to the worst case of slightly above 4%. This procedure requires little extra computer time and data storage. The computed profiles are of sufficient accuracy for 3-dimensional treatment planning.

Comparison of static and dynamic treatment modes for the pion therapy beam at LAMPF

Focused beams of negative pions have been developed for dynamic treatments at the Biomedical Facility in Los Alamos. In dynamic treatment, the patient is scanned under a focused pion beam. Using measured depth-dose distributions and off-axis dose distributions, we are able to find a scanning function that allows the best coverage of the target volume. Calculation of dose distributions for dynamic beam treatment plans, both in a water phantom and in a patient, illustrates the superiority of dynamic over static treatments that use broad, essentially parallel beams in shaping the dose distribution to the target volume.

Calculation of pion dose distributions in water.

Techniques for calculating negative pion beam depth and off-axis dose distributions in a water phantom have been developed at the Clinton P. Anderson Meson Physics Facility in Los Alamos. The superposition of the unmodulated depth-dose curve produced modulated depth-dose curves. The addition of the collimator neutron dose, which has been shown to depend on field size, to the modulated depth-dose curve yields the collimated depth-dose distributions. Off-axis dose distributions under a collimator are produced by calculating the distortion of the uncollimated beam caused by multiple Coulomb scattering and beam phase space. Several comparisons of calculated and measured distributions are shown with agreement of normally +/- 3% of peak dose of +/- 3 mm for a particular dose contour. These distribution are them modified by computerized tomographic data to give patient isodose distributions.