Spectral distribution of particle fluence in small field detectors and its implication on small field dosimetry.

Correction factors for the relative dosimetry of narrow megavoltage photon beams have recently been determined in several publications. These corrections are required because of the several small-field effects generally thought to be caused by the lack of lateral charged particle equilibrium (LCPE) in narrow beams. Correction factors for relative dosimetry are ultimately necessary to account for the fluence perturbation caused by the detector. For most small field detectors the perturbation depends on field size, resulting in large correction factors when the field size is decreased. In this work, electron and photon fluence differential in energy will be calculated within the radiation sensitive volume of a number of small field detectors for 6 MV linear accelerator beams. The calculated electron spectra will be used to determine electron fluence perturbation as a function of field size and its implication on small field dosimetry analyzed. Fluence spectra were calculated with the user code PenEasy, based on the PENELOPE Monte Carlo system. The detectors simulated were one liquid ionization chamber, two air ionization chambers, one diamond detector, and six silicon diodes, all manufactured either by PTW or IBA. The spectra were calculated for broad (10 cm × 10 cm) and narrow (0.5 cm × 0.5 cm) photon beams in order to investigate the field size influence on the fluence spectra and its resulting perturbation. The photon fluence spectra were used to analyze the impact of absorption and generation of photons. These will have a direct influence on the electrons generated in the detector radiation sensitive volume. The electron fluence spectra were used to quantify the perturbation effects and their relation to output correction factors. The photon fluence spectra obtained for all detectors were similar to the spectrum in water except for the shielded silicon diodes. The photon fluence in the latter group was strongly influenced, mostly in the low-energy region, by photoabsorption in the high-Z shielding material. For the ionization chambers and the diamond detector, the electron fluence spectra were found to be similar to that in water, for both field sizes. In contrast, electron spectra in the silicon diodes were much higher than that in water for both field sizes. The estimated perturbations of the fluence spectra for the silicon diodes were 11-21% for the large fields and 14-27% for the small fields. These perturbations are related to the atomic number, density and mean excitation energy (I-value) of silicon, as well as to the influence of the "extracameral"' components surrounding the detector sensitive volume. For most detectors the fluence perturbation was also found to increase when the field size was decreased, in consistency with the increased small-field effects observed for the smallest field sizes. The present work improves the understanding of small-field effects by relating output correction factors to spectral fluence perturbations in small field detectors. It is shown that the main reasons for the well-known small-field effects in silicon diodes are the high-Z and density of the "extracameral" detector components and the high I-value of silicon relative to that of water and diamond. Compared to these parameters, the density and atomic number of the radiation sensitive volume material play a less significant role.

Benmakhlouf H ，Andreo P 《-》

Output correction factors for nine small field detectors in 6 MV radiation therapy photon beams: a PENELOPE Monte Carlo study.

To determine detector-specific output correction factors,[Formula: see text], in 6 MV small photon beams for air and liquid ionization chambers, silicon diodes, and diamond detectors from two manufacturers. Field output factors, defined according to the international formalism published byAlfonso et al. [Med. Phys. 35, 5179-5186 (2008)], relate the dosimetry of small photon beams to that of the machine-specific reference field; they include a correction to measured ratios of detector readings, conventionally used as output factors in broad beams. Output correction factors were calculated with the PENELOPE Monte Carlo (MC) system with a statistical uncertainty (type-A) of 0.15% or lower. The geometries of the detectors were coded using blueprints provided by the manufacturers, and phase-space files for field sizes between 0.5 × 0.5 cm(2) and 10 × 10 cm(2) from a Varian Clinac iX 6 MV linac used as sources. The output correction factors were determined scoring the absorbed dose within a detector and to a small water volume in the absence of the detector, both at a depth of 10 cm, for each small field and for the reference beam of 10 × 10 cm(2). The Monte Carlo calculated output correction factors for the liquid ionization chamber and the diamond detector were within about ± 1% of unity even for the smallest field sizes. Corrections were found to be significant for small air ionization chambers due to their cavity dimensions, as expected. The correction factors for silicon diodes varied with the detector type (shielded or unshielded), confirming the findings by other authors; different corrections for the detectors from the two manufacturers were obtained. The differences in the calculated factors for the various detectors were analyzed thoroughly and whenever possible the results were compared to published data, often calculated for different accelerators and using the EGSnrc MC system. The differences were used to estimate a type-B uncertainty for the correction factors. Together with the type-A uncertainty from the Monte Carlo calculations, an estimation of the combined standard uncertainty was made, assigned to the mean correction factors from various estimates. The present work provides a consistent and specific set of data for the output correction factors of a broad set of detectors in a Varian Clinac iX 6 MV accelerator and contributes to improving the understanding of the physics of small photon beams. The correction factors cannot in general be neglected for any detector and, as expected, their magnitude increases with decreasing field size. Due to the reduced number of clinical accelerator types currently available, it is suggested that detector output correction factors be given specifically for linac models and field sizes, rather than for a beam quality specifier that necessarily varies with the accelerator type and field size due to the different electron spot dimensions and photon collimation systems used by each accelerator model.

Benmakhlouf H ，Sempau J ，Andreo P 《-》

Determination of the KQclinfclin,Qmsr fmsr correction factors for detectors used with an 800 MU/min CyberKnife(®) system equipped with fixed collimators and a study of detector response to small photon beams using a Monte Carlo method.

Moignier C ，Huet C ，Makovicka L 《-》

Monte Carlo investigation of electron fluence perturbation in MRI-guided radiotherapy beams using six commercial radiation detectors.

With the integration of treatments with MRI-linacs to the clinical workflow, the understanding and characterization of detector response in reference dosimetry in magnetic fields are required. The external magnetic field perturbs the electron fluence. The degree of perturbation depends on the irradiation conditions and on the detector type. The purpose of this study is to evaluate the magnetic field impact on the electron fluence spectra in several detectors to provide a deeper understanding of detector response in these conditions. Monte Carlo calculations of the electron fluence are performed in six detectors (solid-state: PTW60012 and PTW60019, ionization chambers: PTW30013, PTW31010, PTW31021, and PTW31022) in water and irradiated by a 7 MV FFF photon beam with a small and a reference field, at 0 and 1.5 T. Three chamber axis orientations are investigated: parallel or perpendicular (either the Lorentz force pointing towards the stem or the tip) to the magnetic field and always perpendicular to the photon beam. One orientation for the solid-state detector is studied: parallel to the photon beam and perpendicular to the magnetic field. Additionally, electron fluence spectra are calculated in modified detector geometries to identify the underlying physical mechanisms behind the fluence perturbations. The total electron fluence in the Farmer chamber varies up to 1.24% and 5.12% at 1.5 T, in the parallel and perpendicular orientation, respectively. The interplay between the gyration radius and the Farmer chamber cavity length significantly affects the electron fluence in the perpendicular orientation. For the small-cavity chambers, the maximal variation in total electron fluence is 0.19% in the parallel orientation for the reference field. Significant small-field effects occur in these chambers; the magnetic field reduces the total electron fluence (with respect to the no field case) between 9.86% and 14.50%, depending on the orientation. The magnetic field strongly impacted the solid-state detectors in both field sizes, probably due to the high-Z components and cavity density. The maximal reductions of total electron fluence are 15.06 ± 0.09% (silicon) and 16.00 ± 0.07% (microDiamond). This work provides insights into detector response in magnetic fields by illustrating the interplay between several factors causing dosimetric perturbation effects: (1) chamber and magnetic field orientation, (2) cavity size and shape, (3) extracameral components, (4) air gaps and their asymmetry, (5) electron energy. Low-energy electron trajectories are more susceptible to change in magnetic fields, and are associated with detector response perturbation. Detectors with higher density and high-Z extracameral components exhibit more significant perturbations in the presence of a magnetic field, regardless of field size.

Cervantes Y ，Duane S ，Bouchard H 《-》

A Monte Carlo study on the PTW 60019 microDiamond detector.

Data on the output correction factor for small photon beam dosimetry of the microDiamond detector manufactured by the company PTW can be found in a variety of papers. Referring either to measurements or to Monte Carlo (MC) calculations, they show substantial disagreements particularly at very small fields. This work reports results of a further MC study aiming at a better understanding of how specific properties of the microDiamond detector are influencing its output correction factor and whether this can explain at least some of the disagreements. In this study the method of a fluence-based decomposition of the dose conversion factor was used which is considered as a useful tool to understand the response of a detector in nonreference conditions. This decomposition method yields the following three factors: (a) the stopping power ratio water to diamond, (b) a perturbation factor pint taking into account all fluence changes in the transition from a small water voxel at the point of dose determination to the bare diamond detector, and (c) a perturbation factor pext taking into account all additional fluence changes in the fully simulated diamond detector caused by the material and design details outside the sensitive volume. Monte Carlo calculated output correction factors were obtained for Co-60, 6 MV and 10 MV photon beams showing that the maximum variation with field size remained in the order of 2% for quadratic field sizes larger than about 0.3 cm. For field sizes smaller than about 0.5 cm a clear under-response is obtained at all three radiation qualities in agreement with all known MC calculations, however, in contrast to some measured result. The shape of the output correction factor can be well explained by an opposite mode of action between under-response expressed by the perturbation factor pint and over-response expressed by the perturbation factor pext where the first one is mainly influenced by volume averaging, and the second one by a back scatter effect of electrons from the diamond substrate into the sensitive volume. The response of microDiamond detector can be well described under various measuring conditions by the dose conversion factor and the dependency of its fluence-based subfactors on detector characteristics. Monte Carlo simulations offer an improvement in the understanding particularly of small-field effects by relating the output correction factor to spectral fluence changes in the sensitive volume of the detector. The most significant influence factors are the finite size of the active volume and the presence of the high-density diamond substrate causing a field size-dependent backscattering. These perturbations are opposite in their effects. The diamond in the sensitive volume itself and in particular its density has almost no influence. Scattering of results at very small field sizes can be explained by different gradients of dose profiles around the beam axis at identical full width half maximum (FWHM) field size parameters and by possible deviations of the radius of the sensitive volume from the nominal radius. The backscattering effect also has an influence on the determination of profiles and for very small field sizes on the response at different rotation angles.

Hartmann GH ，Zink K 《-》