Improvement of the penumbra for small radiosurgical fields using flattening filter free low megavoltage beams

Monte-Carlo simulation of the Siemens Artiste linear accelerator flat 6 MV and flattening-filter-free 7 MV beam line

The aim of our work is to provide the up-to-now missing information on the Siemens Artiste FFF 7 MV beam line using a Monte-Carlo model fit to the realistic dosimetric measurements at the linear accelerator in clinical use at our department. The main Siemens Artiste 6MV and FFF 7MV beams were simulated using the Geant4 toolkit. The simulations were compared with the measurements with an ionization chamber in a water phantom to verify the validation of simulation and tuning the primary electron parameters. Hereafter, other parameters such as surface dose, spectrum, electron contamination, symmetry, flatness/unflatness, slope, and characteristic off-axis changes were discussed for both Flat and FFF mode. The mean electron energy for the FFF beam was 8.8 MeV and 7.5 MeV for Flat 6 MV, the spread energy and spot size of the selected Gaussian distribution source were 0.4 MeV and 1mm, respectively. The dose rate of the FFF beam was 2.8 (2.96) times higher than for the flattened beam for a field size of 10×10 (20×20) cm2. The electron contamination has significant contribution to the surface dose especially for the flattened beam. The penumbra, surface dose and the mean energy of photons decrease by removing the flattening filter. Finally, the results show that off-axis changes have no strong effect on the mean energy of FFF beams, while this effect was more considerable for the flattened beam.

Steeper dose gradients resulting from reduced source to target distance-a planning system independent study

Purpose

To quantify the contribution of penumbra in the improvement of healthy tissue sparing at reduced source‐to‐axis distance (SAD) for simple spherical target and different prescription isodoses (PI).

Method

A TPS‐independent method was used to estimate three‐dimensional (3D) dose distribution for stereotactic treatment of spherical targets of 0.5 cm radius based on single beam two‐dimensional (2D) film dosimetry measurements. 1 cm target constitutes the worst case for the conformation with standard Multi‐Leaf Collimator (MLC) with 0.5 cm leaf width. The measured 2D transverse dose cross‐sections and the profiles in leaf and jaw directions were used to calculate radial dose distribution from isotropic beam arrangement, for both quadratic and circular beam openings, respectively. The results were compared for standard (100 cm) and reduced SAD 70 and 55 cm for different PI.

Results

For practical reduction of SAD using quadratic openings, the improvement of healthy tissue sparing (HTS) at distances up to 3 times the PTV radius was at least 6%–12%; gradient indices (GI) were reduced by 3–39% for PI between 40% and 90%. Except for PI of 80% and 90%, quadratic apertures at SAD 70 cm improved the HTS by up to 20% compared to circular openings at 100 cm or were at least equivalent; GI were 3%–33% lower for reduced SAD in the PI range 40%–70%. For PI = 80% and 90% the results depend on the circular collimator model.

Conclusion

Stereotactic treatments of spherical targets delivered at reduced SAD of 70 or 55 cm using MLC spare healthy tissue around the target at least as good as treatments at SAD 100 cm using circular collimators. The steeper beam penumbra at reduced SAD seems to be as important as perfect target conformity. The authors argue therefore that the beam penumbra width should be addressed in the stereotactic studies.

Penumbra width determination of single beam and 201 beams of Gamma Knife machine model 4C using Monte Carlo simulation

Background

One of the stereotactic radiosurgery techniques is Gamma Knife radiosurgery, in which intracranial lesions that are inaccessible or inappropriate for surgery are treated using 201 cobalt-60 sources in one treatment session. In this conformal technique, the penumbra width, which results in out-of-field dose in tumour-adjacent normal tissues should be determined accurately. The aim of this study is to calculate the penumbra widths of single and 201 beams for different collimator sizes of Gamma Knife machine model 4C using EGSnrc/BEAMnrc Monte Carlo simulation code and comparison the results with EBT3 film dosimetry data.

Methods and materials

In this study, simulation of Gamma Knife machine model 4C was performed based on the Monte Carlo codes of EGSnrc/BEAMnrc. To investigate the physical penumbra width (80−20%), the single beam and 201 beams profiles were obtained using EGSnrc/DOSXYZnrc code and EBT3 films located at isocentre point in a spherical Plexiglas head phantom.

Results

Based on the results, the single beam penumbra widths obtained from simulation data for 4, 8, 14 and 18 mm collimator sizes alongXaxis were 0·75, 0·77, 0·90 and 0·92 mm, respectively. The data for 201 beams obtained from simulation were 2·61, 4·80, 7·92 and 9·81 mm alongXaxis and 1·31, 1·60, 1·91 and 2·14 mm alongZaxis and from film dosimetry were 3·21, 4·90, 8·00 and 10·61 mm alongXaxis and 1·22, 1·69, 2·01 and 2·25 mm alongZaxis, respectively.

Conclusion

The differences between measured and simulated penumbra widths are in an acceptable range. However, for more precise measurement in the penumbra region in which dose gradient is high, Monte Carlo simulation is recommended.

Different Dosimeters/Detectors Used in Small-Field Dosimetry: Pros and Cons

With the advent of complex and precise radiation therapy techniques, the use of relatively small fields is needed. Using such field sizes can cause uncertainty in dosimetry; therefore, special attention is required both in dose calculations and measurements. There are several challenges in small-field dosimetry such as the steep gradient of the radiation field, volume averaging effect, lack of charged particle equilibrium, partial occlusion of radiation source, beam alignment, and unable to use a reference dosimeter. Due to these challenges, special dosimeters are needed for small-field dosimetry, and this review article discusses this topic.

Application of Geant4 Monte Carlo simulation in dose calculations for small radiosurgical fields

The Geant4 toolkit was used to develop a Monte Carlo (MC)-based engine for accurate dose calculations in small radiation field sizes. The Geant4 toolkit (version 10.1.p02) was used to simulate 6-MV photon beam of a Varian2100C linear accelerator that is being used for stereotactic radiosurgery (SRS) treatment with small radiation fields. Geometric models of 3 in-house designed radiosurgical divergent cones, with the diameters of their projections at the isocenter being 10, 20, and 30 mm, were simulated. The accuracy of the MC simulation technique was examined by reproducing several different simulated dosimetric parameters of the primary beams with the experimental data. The dose distributions are first checked for single beams for each cone, then standard multiple field (SMF) techniques are applied. A sample set of DICOM files from computed tomography (CT) scan imaging of a patient's head was converted to the Geant4 geometry format to implement MC-based engine for a clinical test. To validate the accuracy of the MC-based calculations for SMF arrangements, the isodose lines from MC simulation in water phantom were compared with the measured isodose lines using EBT3 Gafchromic film in Solid Water phantoms. Agreements between measured and simulated depth dose values and beam profiles for SRS cones were generally within 2%/2 mm. For output factors, the largest discrepancy was observed for 10 mm SRS cone, which was 1.7%. For SMF techniques, in SRS cones, the MC simulation and EBT3 Gafchromic film dosimetry were in acceptable agreement (5%/5 mm). Excellent agreement between the results of the MC-based and measured dose values for both single and SMF techniques in SRS cones indicates the ability of the Geant4 toolkit to be applied as the platform for treatment planning of advanced radiotherapy techniques.

Investigating the dosimetric effects of grid size on dose calculation accuracy using volumetric modulated arc therapy in spine stereotactic radiosurgery

PURPOSE

Sharp dose gradients between the target and the spinal cord are critical to achieve dose constraints in spine stereotactic radiosurgery (SRS), however the accuracy of the doses to the spinal cord at these high dose gradients is sensitive to the how the dose is sampled across the structure using a discretized isotropic calculation grid. In this study, the effect of the grid size (GS) on the dosimetric accuracy of volumetric modulated arc therapy (VMAT) spine SRS plans was investigated.

METHODS

The Eclipse v11.0 Anisotropic Analytical Algorithm (AAA) algorithm was used for dose calculation. Plan qualities of fifty treatment plans were evaluated with a GS of 2.5 (AAA's default value), 1.5 and 1mm. All plans were prescribed to the 90% isodose line in 1 fraction. Parameters used for plan comparison included the distance-to-fall-off (DTF) between the 90% and 50% isodose levels in the axial plane, planning tumor volume (PTV) coverage to 99%, 95%, 5% and 0.03cc, dose to 10% (Cord_D10%) and 0.03cc (Cord_D0.03cc) of the spinal cord sub volume. The dosimetric accuracy was evaluated based on film dosimetry percent gamma pass rate, line profile through the cord. Calculation times between different grid sizes as well as DVH algorithm differences between two treatment planning systems (Eclipse vs Velocity) were compared. Paired t-test was used to investigate the statistical significance.

RESULTS

The DTF decreased for all plans with 1mm compared to 1.5mm and 2.5mm GS (2.52±0.54mm, 2.83±0.58mm, 3.30±0.64, p<0.001). Relative to the 1mm GS, Cord_D0.03cc and Cord_D10% increased by 6.24% and 7.81% with the 1.5mm GS, and 9.80% and 13% with the 2.5mm GS. Film analysis demonstrated higher gamma pass rates for 1.5mm GS compared to 1 and 2.5mm GS (95.9%±5.4%, 94.3%±6.0%, 93.6%±5.4%, p<0.001), however 1mm GS showed better agreement in the high dose gradient near the cord. Calculation times for 1mm GS plans increased for 1.5 and 2.5mm GS (61% and 84%, p<0.001). The average difference between the two treatment planning systems was approximately 0-1.2%. A maximum difference of 5.9% occurred for Cord_D0.03cc for the 1mm GS.

CONCLUSION

Plans calculated with a 1mm grid size resulted in the most accurate representation of the dose delivered to the cord, however resulted in less uniform dose distributions in the high dose region of the PTV. The use of a 1.5mm grid size may balance accurate cord dose and PTV coverage, while also being more practical with respect to computation time.