Small Megavoltage Photon Fields Research Articles

Properties of IBA Razor Nano Chamber in small-field radiation therapy using 6 MV FF, 6 MV FFF, and 10 MV FFF photon beams

Background Small megavoltage photon fields are increasingly used in modern radiotherapy techniques such as stereotactic radiotherapy. Therefore, it is important to study the reliability of dosimetry in the small-field conditions. The IBA Razor Nano Chamber (Nano chamber) ionization chamber is particularly intended for small-field measurements. In this work, properties of the Nano chamber were studied with both measurements and Monte Carlo (MC) simulations. Material and methods The measurements and MC simulations were performed with 6 MV, 6 MV FFF and 10 MV FFF photon beams from the Varian TrueBeam linear accelerator. The source-to-surface distance was fixed at 100 cm. The measurements and MC simulations included profiles, percentage depth doses (PDD), and output factors (OF) in square jaw-collimated fields. The MC simulations were performed with the EGSnrc software system in a large water phantom. Results The measured profiles and PDDs obtained with the Nano chamber were compared against IBA Razor Diode, PTW microDiamond and the PTW Semiflex ionization chamber. These results indicate that the Nano chamber is a high-resolution detector and thus suitable for small field profile measurements down to field sizes 2 × 2 cm2 and appropriate for the PDD measurements. The field output correction factors and field OFs were determined according to TRS-483 protocol In the 6 MV FF and FFF beams, the determined correction factors were within 1.2% for the field sizes of 1 × 1 cm2–3 × 3 cm2 and the experimental and MC defined field output factors showed good agreement. Conclusion The Nano chamber with its small cavity volume is a potential detector for the small-field dosimetry. In this study, the properties of this detector were characterized with measurements and MC simulations. The determined correction factors are novel results for the NC in the TrueBeam fields.

Monte-Carlo-computed dose, kerma and fluence distributions in heterogeneous slab geometries irradiated by small megavoltage photon fields*

Small-field dosimetry is central to the planning and delivery of radiotherapy to patients with cancer. Small-field dosimetry is beset by complex issues, such as loss of charged-particle equilibrium (CPE), source occlusion and electron-scattering effects in low-density tissues. The purpose of the present research is the elucidation of the fundamental physics of small fields through the computation of absorbed dose, kerma and fluence distributions in heterogeneous media using the Monte-Carlo (MC) method. Absorbed dose and kerma were computed using the DOSRZnrc MC user-code for beams with square field sizes ranging from 0.25 × 0.25 to 7 × 7 cm2 (for 6 MV ‘full linac’ geometry) and 0.25 × 0.25 to 16 × 16 cm2 (for 15 MV ‘full linac’ geometry). In the bone inhomogeneity the dose increases (vs. homogeneous water) for field sizes <1 × 1 cm2 at 6 MV and ⩽3 × 3 cm2 at 15 MV and decreases (vs. homogeneous water) for field sizes ⩾3 × 3 cm2 at 6 MV and ⩾5 × 5 cm2 at 15 MV. In the lung inhomogeneity there is negligible decrease in dose compared to in uniform water for field sizes >5 × 5 cm2 at 6 MV and ⩾16 × 16 cm2 at 15 MV, consistent with the Fano theorem. The near-unity value of the absorbed-dose to collision-kerma ratio, D/Kcol, at the centre of the bone and lung slabs in the heterogeneous phantom demonstrates that CPE is achieved in bone for field sizes >1 × 1 cm2 at 6 MV and ⩾5 × 5 cm2 at 15 MV; CPE is achieved in lung at field sizes >5 × 5 cm2 at 6 MV and ⩾16 × 16 cm2 at 15 MV. Electron-fluence perturbation factors for the 0.25 × 0.25 cm2 field were 1.231 and 1.403 for bone-to-water and 0.454 and 0.333 for lung-to-water at 6 and 15 MV, respectively. For field sizes large enough for quasi-CPE, the MC-derived dose-perturbation factors, lung-to-water, were close to unity; electron-fluence perturbation factors, lung-to-water, were ∼1.0, consistent with the Fano theorem. At 15 MV in the lung inhomogeneity the magnitude and also the ‘shape’ of the primary electron-fluence spectrum differ significantly from that in water. Beam penumbrae relative to water are narrower in the bone inhomogeneity and broader in the lung inhomogeneity for all field sizes.

Density compensated diodes for small field dosimetry: comprehensive testing and implications for design

Purpose. In small megavoltage photon fields, the accuracies of an unmodified PTW 60017-type diode dosimeter and six diodes modified by adding airgaps of thickness 0.6–1.6 mm and diameter 3.6 mm have been comprehensively characterized experimentally and computationally. The optimally thick airgap for density compensation was determined, and detectors were micro-CT imaged to investigate differences between experimentally measured radiation responses and those predicted computationally. Methods. Detectors were tested on- and off-axis, at 5 and 15 cm depths in 6 and 15 MV fields ≥ 0.5 × 0.5 cm2. Computational studies were carried out using the EGSnrc/BEAMnrc Monte Carlo radiation transport code. Experimentally, radiation was delivered using a Varian TrueBeam linac and doses absorbed by water were measured using Gafchromic EBT3 film and ionization chambers, and compared with diode readings. Detector response was characterized via the formalism, choosing a 4 × 4 cm2 reference field. Results. For the unmodified 60017 diode, the maximum error in small field doses obtained from diode readings uncorrected by factors was determined as 11.9% computationally at +0.25 mm off-axis and 5 cm depth in a 15 MV 0.5 × 0.5 cm2 field, and 11.7% experimentally at −0.30 mm off-axis and 5 cm depth in the same field. A detector modified to include a 1.6 mm thick airgap performed best, with maximum computationally and experimentally determined errors of 2.2% and 4.1%. The 1.6 mm airgap deepened the modified dosimeter’s effective point of measurement by 0.5 mm. For some detectors significant differences existed between responses in small fields determined computationally and experimentally, micro-CT imaging indicating that these differences were due to within-tolerance variations in the thickness of an epoxy resin layer. Conclusions. The dosimetric performance of a 60017 diode detector was comprehensively improved throughout 6 and 15 MV small photon fields via density compensation. For this approach to work well with good detector-to-detector reproducibility, tolerances on dense component dimensions should be reduced to limit associated variations of response in small fields, or these components should be modified to have more water-like densities.

Improving the accuracy of converting dose to medium to dose to water algorithms in small megavoltage photon fields in dose to medium based treatment planning systems

PurposeTo present a formalism to improve the accuracy of converting absorbed dose to medium in medium (Dm) to absorbed dose to water in medium (Dw) in small megavoltage photon fields for different human tissues in Dm-based treatment planning systems (TPS). MethodsEight kinds of real human tissues were simulated to convert Dm to Dw. Four kinds of virtual water media were deliberately designed to analyze source of deviations from the conventional Bragg-Gray theory. Mass electronic stopping powers were calculated using the ESTAR code. The phase-space data was generated by the EGSnrc/BEAMnrc Monte Carlo code. The dose deposition was calculated with the EGSnrc/DOSRZnrc code. Electron fluence spectra calculated with EGSnrc/FLURZnrc code were utilized to analyze fluence perturbations and determine fluence intensity (Φw,mint) and fluence spectral shape (Φw,mS) correction factors. ResultsLarge conversion errors of Dw using Bragg-Gray theory were observed, such as 19.65% ± 9.58% (average value ± standard deviation, type A) for inflated lung (ICRU). Fluence perturbations could be exacerbated by severe charged particle disequilibrium conditions. These deviations were caused by the synergy between tissues’ different mean excitation energies and smaller mass densities compared to those of water. Adding Φw,mint and Φw,mS correction factors to modify Bragg-Gray theory could greatly reduce Dw conversion errors, within 1.00% for all tissues studied. ConclusionsThe current clinically used Dw conversion algorithm in commercial Dm-based TPS isn’t appropriate for some human tissues in small field dosimetry. Correction factors should be exploited to improve the accuracy.

Integral small field output factor measurements using a transmission ionisation chamber.

Previous studies describe the use of a large area parallel-plate chamber, the PTW Bragg Peak chamber, for measuring dose-area product (DAP) and output factors in small megavoltage photon fields. However, in radiotherapy departments without protons, this detector would have to be purchased separately for this purpose. This work investigated the feasibility of alternatively using a large transmission ionisation chamber, the IBA round Stealth chamber (SC), for output factor measurements of stereotactic fields. This type of detector is more commonly found in radiotherapy departments as a reference chamber for water tank scanning of small fields, and hence DAP could be performed without an additional purchase. The SC's large sensitive area (diameter of 94mm) measures the integral dose, also known as DAP, over the whole two-dimensional (2D) dose distribution of the small field. The measurements were performed using a 6 MV beam from an Elekta Infinity linear accelerator. Conversion of DAP to central axis point dose was performed using 2D dose maps from Gafchromic EBT3 films. The field sizes measured ranged from side length of 5mm to 50mm (all square). The resultant output factors were compared against measurements with a stereotactic diode. The small field output factors measured using SC + film were in good agreement with the stereotactic diode (within 2% for field sizes as small as 6mm; 3% difference at 5mm). The new proposed method showed that a transmission chamber like SC is a good alternative large-area parallel plate chamber to measure DAP and derive small field OFs. Furthermore, the feasibility of using 2D reconstructed dose maps from water tank profiles and hence filmless approach was investigated. Results showed that filmless conversion of DAP to central axis point dose is feasible using profiles. However, a large number of profiles are required (i.e. 15° increments (star pattern) are required for accurate 2D dose reconstruction), and hence the water tank scanning for this approach may be prohibitively time-consuming.

Breakdown of Bragg–Gray behaviour for low-density detectors under electronic disequilibrium conditions in small megavoltage photon fields

In small photon fields ionisation chambers can exhibit large deviations from Bragg–Gray behaviour; the EGSnrc Monte Carlo (MC) code system has been employed to investigate this ‘Bragg–Gray breakdown’. The total electron (+positron) fluence in small water and air cavities in a water phantom has been computed for a full linac beam model as well as for a point source spectrum for 6 MV and 15 MV qualities for field sizes from 0.25 × 0.25 cm2 to 10 × 10 cm2. A water-to-air perturbation factor has been derived as the ratio of total electron (+positron) fluence, integrated over all energies, in a tiny water volume to that in a ‘PinPoint 3D-chamber-like’ air cavity; for the 0.25 × 0.25 cm2 field size the perturbation factors are 1.323 and 2.139 for 6 MV and 15 MV full linac geometries respectively. For the 15 MV full linac geometry for field sizes of 1 × 1 cm2 and smaller not only the absolute magnitude but also the ‘shape’ of the total electron fluence spectrum in the air cavity is significantly different to that in the water ‘cavity’. The physics of this ‘Bragg–Gray breakdown’ is fully explained, making reference to the Fano theorem. For the 15 MV full linac geometry in the 0.25 × 0.25 cm2 field the directly computed MC dose ratio, water-to-air, differs by 5% from the product of the Spencer–Attix stopping-power ratio (SPR) and the perturbation factor; this ‘difference’ is explained by the difference in the shapes of the fluence spectra and is also formulated theoretically. We show that the dimensions of an air-cavity with a perturbation factor within 5% of unity would have to be impractically small in these highly non-equilibrium photon fields. In contrast the dose to water in a 0.25 × 0.25 cm2 field derived by multiplying the dose in the single-crystal diamond dosimeter (SCDDo) by the Spencer–Attix ratio is within 2.9% of the dose computed directly in the water voxel for full linac geometry at both 6 and 15 MV, thereby demonstrating that this detector exhibits quasi Bragg–Gray behaviour over a wide range of field sizes and beam qualities.

Detector dose response in megavoltage small photon beams. I. Theoretical concepts.

To explain the reasons for significant quality correction factors in megavoltage small photon fields and clarify the underlying concepts relevant to dosimetry under such conditions. The validity of cavity theory and the requirement of charged particle equilibrium (CPE) are addressed from a theoretical point of view in the context of nonstandard beams. Perturbation effects are described into four main subeffects, explaining their nature and pointing out their relative importance in small photon fields. It is demonstrated that the failure to meet classical cavity theory requirements, such as CPE, is not the reason for significant quality correction factors. On the contrary, it is shown that the lack of CPE alone cannot explain these corrections and that what matters most, apart from volume averaging effects, is the relationship between the lack of CPE in the small field itself and the density of the detector cavity. The density perturbation effect is explained based on Fano's theorem, describing the compensating effect of two main contributions to cavity absorbed dose. Using the same approach, perturbation effects arising from the difference in atomic properties of the cavity medium and the presence of extracameral components are explained. Volume averaging effects are also discussed in detail. Quality correction factors of small megavoltage photon fields are mainly due to differences in electron density between water and the detector medium and to volume averaging over the detector cavity. Other effects, such as the presence of extracameral components and differences in atomic properties of the detection medium with respect to water, can also play an accentuated role in small photon fields compared to standard beams.

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

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

Monte Carlo modeling and image-guidance in particle therapy

Monte Carlo modeling and image-guidance in particle therapy

SU‐FF‐T‐03: 3D Characterization of Small Megavoltage Radiation Fields Using An Optical CT Scanner with Ferrous Xylenol Orange Gels

Purpose: To investigate the feasibility of using a 3D dosimetry system consisting of a radiochromic ferrous xylenol orange (FX) gel and a prototype commercial cone beam optical‐CT scanning system (Vista™, Modus Medical Devices, Inc., London, ON) to dosimetrially characterize small megavoltage photon fields. Method and Materials: An FXG gel was irradiated using 6MV x‐ray fields (30, 10 and 5mm diameter) produced via custom stereotactic collimators mounted on a Varian Clinac 2100EX accelerator. Measurements were also completed using radiochromic (Gafchromic EBT) and radiographic (Kodak EDR2) film, and a Exradin #a1SL ion chamber for comparison purposes. The gel was optically CT scanned 10, 20, 30 and 40 minutes post‐irradiation to assess the effects of xylenol orange/ferric ion complex diffusion in gel. Lateral dose profiles and percentage depth dose (PDD) curves were extracted from the 3D image data sets for analysis. Results: FX gels together with fast optical CT scanning allows for the measurement of all dosimetric parameters required as pencil beam input for treatment planning systems in a single irradiation. The experimental set‐up is simple, and the gel allows for dose measurements in a tissue‐equivalent and energy‐independent medium. Lateral beam profiles as measured in the FXG gel matched those measured using both types of film within 2 mm. PDD curves measured with gel and ion chamber were also in good agreement (within 3 mm) up to 65mm depth. Conclusion: The radiochromic FX gel and fast optical CT scanner is a promising dosimetry system for the characterization of small x‐ray fields for the purposes of commissioning treatment planning systems. The advantage is that a full 3D data set can be acquired quickly and efficiently. Conflict of Interest: Two of the authors (KJ and JB) have a licensing agreement with Modus Medical Devices, Inc. for the commercialization of the Vista™ scanner.