An independent dose calculation algorithm for MLC-based radiotherapy including the spatial dependence of MLC transmission

Universal evaluation of MLC models in treatment planning systems based on a common set of dynamic tests

PURPOSE

To demonstrate the feasibility of characterising MLCs and MLC models implemented in TPSs using a common set of dynamic beams.

MATERIALS AND METHODS

A set of tests containing synchronous (SG) and asynchronous sweeping gaps (aSG) was distributed among twenty-five participating centres. Doses were measured with a Farmer-type ion chamber and computed in TPSs, which provided a dosimetric characterisation of the leaf tip, tongue-and-groove, and MLC transmission of each MLC, as well as an assessment of the MLC model in each TPS. Five MLC types and four TPSs were evaluated, covering the most frequent combinations used in radiotherapy departments.

RESULTS

Measured differences within each MLC type were minimal, while large differences were found between MLC models implemented in clinical TPSs. This resulted in some concerning discrepancies, especially for the HD120 and Agility MLCs, for which differences between measured and calculated doses for some MLC-TPS combinations exceeded 10%. These large differences were particularly evident for small gap sizes (5 and 10 mm), as well as for larger gaps in the presence of tongue-and-groove effects. A much better agreement was found for the Millennium120 and Halcyon MLCs, differences being within ± 5% and ± 2.5%, respectively.

CONCLUSIONS

The feasibility of using a common set of tests to assess MLC models in TPSs was demonstrated. Measurements within MLC types were very similar, but TPS dose calculations showed large variations. Standardisation of the MLC configuration in TPSs is necessary. The proposed procedure can be readily applied in radiotherapy departments and can be a valuable tool in IMRT and credentialing audits.

Selection of the most sensitive configuration of strip array detectors for x-ray beam monitoring in radiotherapy of cancer utilizing singular value decomposition

We propose a concise mathematical framework in order to compare detector configurations efficiently for x-ray beam monitoring in radiotherapy of cancer. This framework consists of the singular value decomposition (SVD) of the system matrix and the definition of an effective information threshold based on the relative error inequality utilizing the condition number of a matrix. The goal of this paper is to present the mathematical argument as well as to demonstrate its use for modeling the best detector configuration for monitoring x-ray beams in external beam therapy. This analysis depends neither on specific measurements of a given set of x-ray beams, nor does it depend in specific reconstruction algorithms of the beam shape, and therefore represents a configuration meta-analysis. In the results section, we compare three possible detector designs, each leading to a highly underdetermined system, and are able to determine their effective information content relative to each other. Furthermore, by changing design parameters, such as the geometric detector configuration, number of detectors, detector pixel size, and the x-ray beam blur, deeper insight in this challenging inverse problem is achieved and the most sensitive monitoring scheme is determined. Graphical Abstract Illustration of the general approach for performing configuration meta-analysis.

Strip detector array (SDA) for beam monitoring in radiotherapy: reconstruction of MLC parameters from multiple projections of flux

Objective:In this paper we propose and investigate a new detector with multiple strip detector arrays (SDA) for monitoring MLC shaped x-ray beams for radiotherapy treatment.Approach:Each SDA measures 1D dose profiles equivalent to dose projections. The goal of such a detector is to determine individual MLC leaf positions as well as the Monitor Units (MU) per MLC segment during radiotherapy. In the present work we investigate an optimal SDA detector configuration and reconstruction algorithm. We determine the accuracy of SDA for different treatment sites (spine, pelvis, retroperitoneum, prostate, brain SRT, SRS, lung and head and neck). We perform a simulation study accounting for different type of MLC leaf positional errors: random MLC leaf, systematic for the whole leaf bank and systematic for an individual leaf. In a similar fashion, we also account for errors in Monitor Units per segment.Main results:We demonstrate that for a broad range of IMRT treatment plans a robust reconstruction of errors is achievable with only 3 projections (3 sets of SDA oriented at at 0◦, 45◦ and 135◦). The SDA is capable of capturing both systematic errors in leaf banks and individual leaves as well as random errors sufficient for practical clinical purposes.Significance:These features of the SDA detector makes it suitable for real-time Quality Control of MLC collimated linac output.

Remote sensing array (RSA) for linac beam monitoring

The purpose of the present work is to evaluate the feasibility of a novel real-time beam monitoring device for medical linacs which remotely senses charge carriers produced in air by the beam without intersecting and attenuating the beamline. The primary goal is to elaborate a theoretical concept of a possible detector geometry and underlying physical model that allows for determination of clinically relevant beam data in real time, namely MLC leaf positions and dose rate. The detector consists of two opposing electrode arrays arranged in two possible orientations around the beamline. Detection of charge carriers is governed by electromagnetic principles described by Shockley–Ramo theorem. Ions produced by ionization of the air column upstream of patient move laterally in an external electric field. According to the method of images, mirror charges and mirror currents are formed in the strip electrodes. Determination of MU rate and MLC positions using the measured signal requires solution of an inverse problem. In the present work we adopted a Least-Square approach and characterized detector response and sensitivity to detection of beam properties for different electrode geometries and MLC shapes. Results were dependent on MLC field shape and the leaf position within the active volume. The accuracy of determination of leaf positions were in the sub-mm range (up to 0.25–1 mm). Additionally, detector sensitivity was quantified by simulating ions/pulse delivered with a radiation transport deterministic computation in 1D in CEPXS/ONEDANT. For a 6 MV linac pulse, signal amplitude per pulse was estimated to be in the lower pA to fA range. We computationally demonstrated feasibility of the remote sensing detector capable of measuring beam parameters such as MLC leaf positions and dose range for each pulse. Future work should focus on optimizing the electrode geometry to increase sensitivity and better reconstruction algorithms to provide more accurate solutions of the inverse problem.

Identification of a potential source of error for 6FFF beams delivered on an AgilityTM multileaf collimator

Purpose

The performance of the Agility^TM multileaf collimator was investigated with a focus on dynamic, small fields for flattening filter free (FFF) beams.

Methods

In this study we have developed a simple tool to test the robustness of the control mechanisms during dynamic beam delivery for Elekta’s VersaHD linear accelerator with Integrity 4.0.4 control software. We have programed the planning system to calculate dose for delivery of sweeping gaps. These sweeping gaps have a constant speed, constant size, and are delivered at a constant dose rate. Therefore they specifically identify delivery problems in dynamic mode.

Results

The Elekta Agility^TM control mechanism fails to maintain accurate delivery for small, dynamic sweeping gaps. For small gap sizes, the Agility^TM control mechanism delivers a field that is more than four times the size of the planned field width without generating an interlock. This has dosimetric implications: The discrepancy between calculated and measured doses increases with decreasing gap size and exceeds 10% and 60% at isocenter for a 3.5 mm and 1 mm gap size, respectively.

Conclusion

A deficiency of the Agility^TM control system was identified in this study. This deficiency is a potential source of error for volumetric modulated arc therapy fields and could therefore contribute to relatively high failure rates in quality assurance measurements, especially for FFF beams.

MLC parameters from static fields to VMAT plans: an evaluation in a RT-dedicated MC environment (PRIMO)

Background

PRIMO is a graphical environment based on PENELOPE Monte Carlo (MC) simulation of radiotherapy beams able to compute dose distribution in patients, from plans with different techniques. The dosimetric characteristics of an HD-120 MLC (Varian), simulated using PRIMO, were here compared with measurements, and also with Acuros calculations (in the Eclipse treatment planning system, Varian).

Materials and methods

A 10 MV FFF beam from a Varian EDGE linac equipped with the HD-120 MLC was used for this work. Initially, the linac head was simulated inside PRIMO, and validated against measurements in a water phantom. Then, a series of different MLC patterns were established to assess the MLC dosimetric characteristics. Those tests included: i) static fields: output factors from MLC shaped fields (2 × 2 to 10 × 10 cm²), alternate open and closed leaf pattern, MLC transmitted dose; ii) dynamic fields: dosimetric leaf gap (DLG) evaluated with sweeping gaps, tongue and groove (TG) effect assessed with profiles across alternate open and closed leaves moving across the field. The doses in the different tests were simulated in PRIMO and then compared with EBT3 film measurements in solid water phantom, as well as with Acuros calculations. Finally, MC in PRIMO and Acuros were compared in some clinical cases, summarizing the clinical complexity in view of a possible use of PRIMO as an independent dose calculation check.

Results

Static output factor MLC tests showed an agreement between MC calculated and measured OF of 0.5%. The dynamic tests presented DLG values of 0.033 ± 0.003 cm and 0.032 ± 0.006 cm for MC and measurements, respectively. Regarding the TG tests, a general agreement between the dose distributions of 1–2% was achieved, except for the extreme patterns (very small gaps/field sizes and high TG effect) were the agreement was about 4–5%. The analysis of the clinical cases, the Gamma agreement between MC in PRIMO and Acuros dose calculation in Eclipse was of 99.5 ± 0.2% for 3%/2 mm criteria of dose difference/distance to agreement.

Conclusions

MC simulations in the PRIMO environment were in agreement with measurements for the HD-120 MLC in a 10 MV FFF beam from a Varian EDGE linac. This result allowed to consistently compare clinical cases, showing the possible use of PRIMO as an independent dose calculation check tool.

Optimization of MLC parameters for TPS calculation and dosimetric verification: application to single isocenter radiosurgery of multiple brain lesions using VMAT

Linac and MLC-based stereotactic radiosurgery (SRS) using single-isocenter-multiple-target (SIMT) VMAT has become increasingly popular in the management of multi-focal cranial metastases. However, significant geometrical and dosimetric challenges exist due to the typically small target volumes and in most cases, non-isocentric locations. To the best of our knowledge, there hasn't been a study in the optimization of MLC parameters, in the context of SIMT SRS, to ensure TPS calculation accuracy. In this work, we set out to optimize the dosimetric leaf gap (DLG) for the HD MLC installed on dedicated stereotactic Varian STx systems using a diverse group of 21 clinical SRS and SBRT plans. These plans featured a broad range of target sizes and target-to-isocenter distances that are typical of the stereotactic cases treated on these systems. Dose discrepancies between TPS calculations and verification measurements using a previously validated diode array Delta⁴ (ScandiDos) were minimized in a balanced manner to accommodate the variety of stereotactic plans. A DLG of 0.6 mm was found to be 'optimal' for the HD MLC and for the 'typical' plans treated on our STx systems. The finding was independently verified using commercially available 3D polymer gel dosimeter CrystalBall^TM (MGS Research Inc.). 3D verification for 6 SIMT SRS plans, consisted of 5 to 15 targets, achieved an average gamma score of 97.3% (σ = 2.0%) on 3%/2 mm criteria with a cutoff isodose level of 20%. We further examined the practice of routine dosimetric verifications including the selection of appropriate detectors and optimal gamma parameters. We found that the commonly used standard 3%/3 mm criteria would have resulted in all but 4 (out of 2840) clinical plans achieving a gamma score of 95% or better, and therefore, losing sensitivity to detect potential dosimetric discrepancies. Based on the characteristics of stereotactic plans, a more stringent distance-to-agreement parameter is needed.

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

The dosimetric leaf gap (DLG) and tongue-and-groove (T&G) effects are critical aspects in the modeling of multileaf collimators (MLC) in the treatment planning system (TPS). In this study, we investigated the dosimetric impact of limitations associated with the T&G modeling in stereotactic plans and its relationship with the need for tuning the DLG in the Eclipse TPS. Measurements were carried out using Varian TrueBeam STx systems from two different institutions. Test fields presenting MLC patterns with several MLC gap sizes (meanGap) and different amounts of T&G effect (TGi) were first evaluated. Secondly, dynamic conformal arc (DCA) and volumetric modulated arc therapy (VMAT) deliveries of stereotactic cases were analyzed in terms of meanGap and TGi. Two DLG values were used in the TPS: the measured DLG (DLGmeas ) and an optimal DLG (DLGopt ). Measured and calculated doses were compared according to dose differences and gamma passing rates (GPR) with strict local gamma criteria of 2%/2 mm. The discrepancies were analyzed for DLGmeas and DLGopt , and their relationships with both TGi and meanGap were investigated. DCA arcs involved significantly lower TGi and larger meanGap than VMAT arcs (P < 0.0001). By using DLGmeas in the TPS, the dose discrepancies increased as TGi increased and meanGap decreased for both test fields and clinical plans. Dose discrepancies dramatically increased with the ratio TGi/meanGap. Adjusting the DLG value was then required to achieve acceptable calculations and configuring the TPS with DLGopt led to an excellent agreement with median GPRs (2%/2 mm) > 99% for both institutions. We also showed that DLGopt could be obtained from the results of the test fields. We demonstrated that the need for tuning the DLG is due to the limitations of the T&G modeling in the Eclipse TPS. A set of sweeping gap tests modified to incorporate T&G effects can be used to determine the optimal DLG value.

A new method for modelling the tongue-and-groove in treatment planning systems

Commercial TPSs typically model the tongue-and-groove (TG) by extending the projections of the leaf sides by a certain constant width. However, this model may produce discrepancies of as much as 7%-10% in the calculated average doses, especially for the High Definition multi-leaf collimator (MLC) (Hernandez et al 2017 Phys. Med. Biol. 62 6688-707). The purpose of the present study is to introduce and validate a new method for modelling the TG that uses a non constant TG width. We provide the theoretical background and a detailed methodology to determine the optimal shape of this TG width from measurements and we fit an empirical function to the TG width that depended on two parameters [Formula: see text] and [Formula: see text]. Parameter [Formula: see text] represents the TG width and [Formula: see text] introduces a curvature correction in the width near the leaf tip end. The new TG model was implemented in MATLAB and when the curvature correction was zero ([Formula: see text]) it caused the same discrepancies as the constant width model used by the Eclipse TPS. On the other hand, when the experimentally determined [Formula: see text] was used the new model's calculations were in close agreement with measurements, with all differences in average doses [Formula: see text]1%. Additionally, film dosimetry was used to successfully validate the potential of the new TG model to recreate the fine spatial details associated to TG effects. We also showed that the parameters [Formula: see text], [Formula: see text] depend solely on the MLC design by evaluating three different linear accelerators for each MLC model considered, namely Varian's High Definition and Millennium120 MLCs. In conclusion, a new method was presented that greatly improves the TG modelling. The present method can be easily implemented in commercial TPSs and has the potential to further increase their accuracy, especially for MLCs with rounded leaf ends.

Commissioning of the tongue-and-groove modelling in treatment planning systems: from static fields to VMAT treatments

Adequate modelling of the multi-leaf collimator (MLC) by treatment planning systems (TPS) is essential for accurate dose calculations in intensity-modulated radiation-therapy. For this reason modern TPSs incorporate MLC characteristics such as the leaf end curvature, MLC transmission and the tongue-and-groove. However, the modelling of the tongue-and-groove is often neglected during TPS commissioning and it is not known how accurate it is. This study evaluates the dosimetric consequences of the tongue-and-groove effect for two different MLC models using both film dosimetry and ionisation chambers. A set of comprehensive tests are presented that evaluate the ability of TPSs to accurately model this effect in (a) static fields, (b) sliding window beams and (c) VMAT arcs. The tests proposed are useful for the commissioning of TPSs and for the validation of major upgrades. With the ECLIPSE TPS, relevant differences were found between calculations and measurements for beams with dynamic MLCs in the presence of the TG effect, especially for the High Definition MLC, small gap sizes and the 1 mm calculation grid. For this combination, dose differences as high as 10% and 7% were obtained for dynamic MLC gaps of 5 mm and 10 mm, respectively. These differences indicate inadequate modelling of the tongue-and-groove effect, which might not be identified without the proposed tests. In particular, the TPS tended to underestimate the calculated dose, which may require tuning of other configuration parameters in the TPS (such as the dosimetric leaf gap) in order to maximise the agreement between calculations and measurements in clinical plans. In conclusion, a need for better modelling of the MLC by TPSs is demonstrated, one of the relevant aspects being the tongue-and-groove effect. This would improve the accuracy of TPS calculations, especially for plans using small MLC gaps, such as plans with small target volumes or high complexities. Improved modelling of the MLC would also reduce the need for tuning parameters in the TPS, facilitating a more comprehensive configuration and commissioning of TPSs.

Beam quality and dose perturbation of 6 MV flattening-filter-free linac

The aim of this study is twofold: (a) determination of the spectral differences for flattening-filter-free (FFF) versus standard (STD) linac under various clinical conditions, (b) based on an extensive list of clinically important beam configurations, identification of clinical scenarios that lead to higher macroscopic dose perturbations due to the presence of high-Z material. The focus is on dose enhancement due to contrast agents including high-Z elements such as gold or gadolinium. EGSnrc was used to simulate clinical beams under various irradiation conditions: open/IMRT/spit-IMRT fields, in/out-off-field areas, different depths and field sizes. Spectra were calculated and analyzed for about 80 beams and for a total of 480 regions. Quantitative differential effects in beam quality were characterized using energy-dependent and cumulative dose perturbation metrics. Analysis of the spectral database showed that even though the general trends for both linacs (FFF/STD) were the same, there were crucial differences. In general, the relative changes between different conditions were smaller for FFF spectra. This was because of the higher component of low-energy photons of the FFF linac, which already lead to higher dose enhancement than for the STD linac (photon energies were more "uniformly" distributed for FFF spectra and henceforth their perturbation resulted in lesser relative changes). For out-of-field FFF spectra and split-IMRT fields the strongest enhancement were observed (∼25 and ∼5 respectively). Different spectral scenarios lead to different dose enhancements, however, they scale with the higher effective-Z of the materials and were directly related to the lower range of the spectra (<200 keV).

A modification of flattening filter free linac for IMRT

PURPOSE

This study investigates the benefits of a modified flattening filter free (FFF) linac over the standard (STD) linac equipped with the flattening filter. Energy and angular spread of the electron beam of the FFF linac were modified. Modification of FFF beam parameters is explored to maximize the monitor unit efficiency and to minimize the head scatter in IMRT delivery for large target volumes or targets lying away from the central axis.

METHODS

The EGSnrc code is used to model FFF and STD linacs and study basic beam properties for both linac types in various beam configurations. Increasing energy of FFF linac results in similar beam attenuation properties and maximized dose rate compared to STD linac. Matching beam attenuation properties allows a more direct exploration of beam flatness of FFF linac in regard to IMRT delivery, especially away from the central axis where the effective dose rate is considerably smaller than the one at the central axis. Flatness of open beam dose profile of FFF linac is improved by increasing the angular spread of the electron beam. The resulting dose rate within the treatment field and outside of the field (peripheral dose) are characterized and compared to the unmodified FFF and STD linacs,

RESULTS

In order to match beam penetration properties, the energy of FFF is adjusted from 6.5 to 8.0 MeV for small to medium field sizes and from 6.5 to 8.5 MeV for larger ones. Dose rate of FFF vs STD linac increased by a factor of 1.9 (6.5 MeV) and 3.4-4.1 (8.0-8.5 MeV). Adjusting the mean angular spread of the electron beam from 0 degrees to 5 degrees-10 degrees resulted in complete flattening of photon beam for field sizes between 10 x 10 cm2 and 15 x 15 cm2 and partial flattening for field sizes from 15 x 15 cm2 to 30 x 30 cm2. Values of angular spread > or =14 degrees are not recommended as they exceed the opening of the primary collimator, affecting the area at the edges of the field. FFF fields of sizes smaller than 6 x 6 cm2 are already flat and beam flattening is not necessary. Overall, the angular spread of 5 degrees-10 degrees is sufficient and can satisfactorily flatten open beam dose profiles even for larger field sizes. Increasing the electron beam angular spread amounts to a slight decrease of dose rate of FFF linac. However, for angular spread, 5 degrees-10 degrees dose rate factor of FFF vs STD is still about 1.6-2.6, depending on the field size (and the adjusted energy). Similarly, in case of peripheral dose, a moderate increase in dose can be observed for angular spread of 5 degrees-10 degrees and for field sizes 10 x 10 cm2 to 30 x 30 cm2. Lastly, beam flatness of not modified FFF linac can be conveniently described by an analytical function representing a ratio of STD vs FFF doses: 1 + b|r|(n).

CONCLUSIONS

A modified FFF beamline with increased energy and electron beam angular spread results in satisfactory flattened beam and high dose rate within the field. Peripheral dose remaining at similar (or smaller) level than that of STD linac for the same delivered dose within the treatment field.

Sensitivity study to evaluate the dosimetric impact of off-axis ratio profiles misalignment on TomoTherapy second dose validation

Accurate dose planning and delivery are very important in the intensity modulated radiation therapy. For helical TomoTherapy dose validation, a TomoTherapy second check software, called MU-Tomo, has been developed using archived patient documents, initial coordinates and planned dose of the point of calculation, and common dosimetric functions. Based on this software, sensitivity studies on 50 patient cases have been evaluated to show the impact of off-axis ratio profile misalignment on point dose calculation. Off-axis ratio is defined as the dose profile normalized to its maximum dose value. Sensitivity studies were done for three scenarios: oscillating the fluctuation regions of two off-axis profiles, shifting the profiles, and rotating the profiles. The result of the oscillation trial is linear along the change of longitudinal off-axis ratio (OARy), while oscillating the lateral off-axis ratio (OARx) has little influence on the dose calculation. For shifting, the variation in the percentage difference from the non-shifting value is about 15 times larger in OARy modification than in OARx modification. Rotating OARx by +/- 6' gave less than 1.5% +/- 0.20% difference compared to the non-rotating value. Rotating OARy by +/- 1' changes the result more than 5% +/- 2.69%. Therefore, for helical TomoTherapy dose validation, commissioned OARy profiles are more sensitive than OARx to oscillation, shifting and rotating. As a result, different tolerances for OARx and OARy may be required for annual quality assurance.