Current Dose Calculation Research Articles

A pencil beam algorithm for magnetic resonance image-guided proton therapy.

PurposeThe feasibility of magnetic resonance image (MRI)‐based proton therapy is based, among several other factors, on the implementation of appropriate extensions on current dose calculation methods. This work aims to develop a pencil beam algorithm (PBA) for dose calculation of proton beams within magnetic field regions of up to 3 T.MethodsMonte Carlo (MC) simulations using the GATE 7.1/GEANT4.9.4p02 toolkit were performed to generate calibration and benchmarking data for the PBA. Dose distributions from proton beams in the clinical required energy range 60–250 MeV impinging on a 400 × 400 × 400 mm3 water phantom and transverse magnetic fields ranging from 0 to 3 T were considered. Energy depositions in homogeneous and heterogeneous phantoms filled with water, adipose, bone, and air were evaluated for proton energies of 80, 150, and 240 MeV, combining a trajectory calculation method and look‐up tables (LUT). A novel parametrization model, using independent tailed Gauss fitting functions, was employed to describe the nonsymmetric shape of lateral beam profiles. Integrated depth‐dose curves (IDD), lateral dose profiles, and two‐dimensional dose distributions calculated with the PBA were compared with results from MC simulations to assess the performance of the algorithm. A gamma index criterion of 2%/2 mm was used for analysis.ResultsA close to perfect agreement was observed for PB‐based dose calculations in water in magnetic fields of 0.5, 1.5, and 3 T. IDD functions showed differences between the PBA and MC of less than 0.1% before the Bragg peak, and deviations of 2–8% in the distal energy falloff region. Gamma index pass rates higher than 99% and mean values lower than 0.1 were encountered for all analyzed configurations. For homogeneous phantoms, only the full bone configuration offered deviations in the Bragg peak position of up to 1.7% and overestimations of the lateral beam spot width for high‐energy protons and magnetic field intensities. An excellent agreement between PBA and MC dose calculation was also achieved using slab‐like and lateral heterogeneous phantoms, with gamma index passing rates above 98% and mean values between 0.1 and 0.2. As expected, agreement reduced for high‐energy protons and high‐intensity magnetic fields, although results remained good enough to be considered for future implementation in clinical practice.ConclusionsThe proposed pencil beam algorithm for protons can accurately account for dose distortion effects induced by external magnetic fields. The application of an analytical model for dose estimation and corrections reduces the calculation times considerably, making the presented PBA a suitable candidate for integration in a treatment planning system.

47. MRI-only radiotherapy, benefits and remaining challenges

Introduction The aim of current presentation is to provide an overview of the current status of MRI-only treatment planning and MRI-only radiotherapy in general. It is well known that MRI provides an optimal soft tissue differentiation and allows more accurate target volume delineation in modern radiotherapy. Current dose calculation algorithms require electron density though, thus MRI and CT data need to be registered, increasing uncertainty. A direct dose calculation on MRI images can reduce the uncertainty and simplifies the treatment planning process. Recently introduced radiotherapy specific MRI systems allow positioning the patient in the actual treatment position and image distortions are corrected at a 1 mm level. Methods and materials The main focus of current presentation will be an overview of the different methods described in literature to calculate dose on MRI images, by conversion of MRI to pseudo CT. These conversion algorithms were originally introduced for attenuation correction calculations on PET/MRI systems, and later fine-tuned for treatment planning purposes. The simplest method consists of assigning so-called bulk densities. For each delineated organ a unique density is defined. The main disadvantages of this method are the need of delineation (sometimes semi-automatically) and the fact that inhomogeneities within organs are ignored. In spite of the high level of approximation, this method has proven to lead to an acceptable dosimetrical precision for a number of clinical indications, such as prostate cases. A second popular method is based on deformable image registration using reference images (atlas-based methods). Depending on the quality of the deformation algorithm, this method provides excellent results for the majority of treatment plans, and can be applied without any user interaction (no contouring required). On the other hand, when the patient geometry is very specific, deformations cannot always reconstruct the geometry correctly. A third method is based on a direct conversion of the MRI grey values and mostly demands dedicated MRI sequences, having T1 and T2 weighting removed, such as UTE or ZTE sequences. Some authors have also considered combining some of the methods described above. Other aspects, important for MRI-only radiotherapy, such as MRI simulation, MRI compatible positioning tools, optimization of radiotherapy specific MRI antennas and the impact of image distortions will be discussed as well. Conclusion Listing all these challenges might seem discouraging to go towards MRI-only radiotherapy, but we probably do not have a choice of treatment precision needs to be further improved. Currently, when using the most advanced radiotherapy techniques, all having a sub-millimeter precision regarding positioning and tracking, the remaining uncertainty is in de definition of the GTV, which demands the most advanced imaging modalities.

SU‐F‐T‐142: An Analytical Model to Correct the Aperture Scattered Dose in Clinical Proton Beams

Purpose:Apertures or collimators are used to laterally shape proton beams in double scattering (DS) delivery and to sharpen the penumbra in pencil beam (PB) delivery. However, aperture‐scattered dose is not included in the current dose calculations of treatment planning system (TPS). The purpose of this study is to provide a method to correct the aperture‐scattered dose based on an analytical model.Methods:A DS beam with a non‐divergent aperture was delivered using a single‐room proton machine. Dose profiles were measured with an ion‐chamber scanning in water and a 2‐D ion chamber matrix with solid‐water buildup at various depths. The measured doses were considered as the sum of the non‐contaminated dose and the aperture‐scattered dose. The non‐contaminated dose was calculated by TPS and subtracted from the measured dose. Aperture scattered‐dose was modeled as a 1D Gaussian distribution. For 2‐D fields, to calculate the scatter‐dose from all the edges of aperture, a sum of weighted distance was used in the model based on the distance from calculation point to aperture edge. The gamma index was calculated between the measured and calculated dose with and without scatter correction.Results:For a beam with range of 23 cm and aperture size of 20 cm, the contribution of the scatter horn was ∼8% of the total dose at 4 cm depth and diminished to 0 at 15 cm depth. The amplitude of scatter‐dose decreased linearly with the depth increase. The 1D gamma index (2%/2 mm) between the calculated and measured profiles increased from 63% to 98% for 4 cm depth and from 83% to 98% at 13 cm depth. The 2D gamma index (2%/2 mm) at 4 cm depth has improved from 78% to 94%.Conclusion:Using the simple analytical method the discrepancy between the measured and calculated dose has significantly improved.

Assessing the Clinical Impact of Approximations in Analytical Dose Calculations for Proton Therapy

To assess the impact of approximations in current analytical dose calculation methods (ADCs) on tumor control probability (TCP) in proton therapy. Dose distributions planned with ADC were compared with delivered dose distributions as determined by Monte Carlo simulations. A total of 50 patients were investigated in this analysis with 10 patients per site for 5 treatment sites (head and neck, lung, breast, prostate, liver). Differences were evaluated using dosimetric indices based on a dose-volume histogram analysis, a γ-index analysis, and estimations of TCP. We found that ADC overestimated the target doses on average by 1% to 2% for all patients considered. The mean dose, D95, D50, and D02 (the dose value covering 95%, 50% and 2% of the target volume, respectively) were predicted within 5% of the delivered dose. The γ-index passing rate for target volumes was above 96% for a 3%/3 mm criterion. Differences in TCP were up to 2%, 2.5%, 6%, 6.5%, and 11% for liver and breast, prostate, head and neck, and lung patients, respectively. Differences in normal tissue complication probabilities for bladder and anterior rectum of prostate patients were less than 3%. Our results indicate that current dose calculation algorithms lead to underdosage of the target by as much as 5%, resulting in differences in TCP of up to 11%. To ensure full target coverage, advanced dose calculation methods like Monte Carlo simulations may be necessary in proton therapy. Monte Carlo simulations may also be required to avoid biases resulting from systematic discrepancies in calculated dose distributions for clinical trials comparing proton therapy with conventional radiation therapy.

A modified dose calculation formalism for electronic brachytherapy sources.

To propose a modification of the current dose calculation formalism introduced in the Task Group No. 43 Report (TG-43) to accommodate an air-kerma rate standard for electronic brachytherapy sources as an alternative to an air-kerma strength standard. Electronic brachytherapy sources are miniature x-ray tubes emitting low energies with high-dose-rates. The National Institute of Standards and Technology (NIST) has introduced a new primary air-kerma rate standard for one of these sources, in contrast to air-kerma strength. A modification of the TG-43 protocol for calculation of dose-rate distributions around electronic brachytherapy sources including sources in an applicator is presented. It cannot be assumed that the perturbations from sources in an applicator are negligible, and thus, the applicator is incorporated in the formalism. The modified protocol mimics the fundamental methodology of the original TG-43 formalism, but now incorporates the new NIST-traceable source strength metric of air-kerma rate at 50cm and introduces a new subscript, i, to denote the presence of an applicator used in treatment delivery. Applications of electronic brachytherapy sources for surface brachytherapy are not addressed in this Technical Note since they are well documented in other publications. A modification of the AAPM TG-43 protocol has been developed to accommodate an air-kerma rate standard for electronic brachytherapy sources as an alternative to an air-kerma strength standard. The modified TG-43 formalism allows dose calculations to be performed using a new NIST-traceable source strength metric and introduces the concept of applicator-specific formalism parameters denoted with subscript, i.

Poster — Thur Eve — 46: Monte Carlo model of the Novalis Classic 6MV stereotactic linear accelerator using the GATE simulation platform

Monte Carlo (MC) simulation is accepted as the most accurate method to predict dose deposition when compared to other methods in radiation treatment planning. Current dose calculation algorithms used for treatment planning can become inaccurate when small radiation fields and tissue inhomogeneities are present. At our centre the Novalis Classic linear accelerator (linac) is used for Stereotactic Radiosurgery (SRS). The first MC model to date of the Novalis Classic linac was developed at our centre using the Geant4 Application for Tomographic Emission (GATE) simulation platform. GATE is relatively new, open source MC software built from CERN's Geometry and Tracking 4 (Geant4) toolkit. The linac geometry was modeled using manufacturer specifications, as well as in‐house measurements of the micro MLC's. Among multiple model parameters, the initial electron beam was adjusted so that calculated depth dose curves agreed with measured values. Simulations were run on the European Grid Infrastructure through GateLab. Simulation time is approximately 8 hours on GateLab for a complete head model simulation to acquire a phase space file. Current results have a majority of points within 3% of the measured dose values for square field sizes ranging from 6×6 mm2 to 98×98 mm2 (maximum field size on the Novalis Classic linac) at 100 cm SSD. The x‐ray spectrum was determined from the MC data as well. The model provides an investigation into GATE'S capabilities and has the potential to be used as a research tool and an independent dose calculation engine for clinical treatment plans.

MO‐FF‐A1‐06: Quantitative Light Dosimetry in Air Using Spherical Optical Fiber Detector

Purpose: To quantitatively evaluate current clinical dose calculations delivered by cylindrical diffusers used in superficial Photodynamic Therapy (PDT) treatments. Method and Materials: A setup was developed to investigate light irradiance patterns in air ranging from 2–50mm of Source to Detector Distances (SDD), distances customarily used in superficial PDT treatments. A 630 nm PDT diode laser light source was used in junction with diffusers ranging from: 10, 20, 25, and 50 mm. The detector was mounted on an optical bench along varying SDDs. The irradiance was then measured using a spherical optical fiber detector (1.8‐mm diameter), which was calibrated against a 1cm×1cm window photodiode detector (Ophir PD300‐3W traceable to NIST) connected to a power meter. Finally, sets of predicted irradiance values were calculated under typical clinical conditions by dividing the total power by the cylinder area bounded by the SDD. Results: The measured and predicted values were plotted and compared. For the 10mm diffuser, at approximately 50mm SDD, the predicted value was nearly fourteen times greater than that of the measured. For the larger diffusers, the disparity between the predicted and measured results narrowed. For example, the 50mm diffuser, at the same distance as that of the 10mm, predicted an intensity of only two times greater than that of the measured. Conclusion: Agreement between measured and predicted irradiance values is expected to occur for a hypothetical infinite length diffuser or for small SDD. The results however, revealed that at large distances from the source, with respect to the diffuser size, the theoretical results diverge from those obtained experimentally. Therefore, the current dose calculation should be reviewed for clinical PDT, as the understanding of the irradiance in air is paramount for the correct calculation of light dose delivered in PDT treatments.

Boron distribution in the normal rat brain after intravenous injection of boronophenylalanine-fructose

Boron neutron capture therapy (BNCT) is an experimental form of radiation therapy for malignant brain tumors and peripheral melanoma. The micro-distribution of the boron compound is critical to determine the radiation effects for both tumors and normal tissue. In the current dose calculation of BNCT, normal brain tissue is considered to have a homogeneous boron concentration. The purpose of this study was to examine the structure-specific boron concentration in normal rat neural tissue. At 10, 30 and 60 min after intravenous injection of 300 mg/kg boronophenylalanine-fructose to 10-week-old CD Fisher rats, neural tissue and blood were collected. Various neural structures were anatomically and histologically identified and specific boron concentrations were analyzed using high-resolution quantitative autoradiography. At 60 min after the injection, only the pituitary gland showed a higher boron concentration than that in blood, with the former being threefold higher. All other neural structures showed lower boron concentrations than that in blood. The present study thus demonstrated an extremely high boron concentration in the pituitary gland following intravenous injection of boronophenylalanine-fructose. In clinical trials of BNCT using an epithermal neutron beam, the radiation dose to the pituitary gland should be carefully evaluated.

Determination of parameters for a multiple-source model of megavoltage photon beams using optimization methods

Accurate modelling of the radiation output of a medical linear accelerator is important for radiotherapy treatment planning. The major challenge is the adjustment of the model to a specific treatment unit. One approach is to use a multiple-source model containing a set of physical parameters. In this work, the parameters were derived from standard beam data measurements using optimization methods. The source model used includes sub-sources for bremsstrahlung radiation from the target, extra-focal photon radiation and electron contamination. The cost function includes a gamma error measure between measurements and current dose calculations. The procedure was applied to six beam data sets (6 MV to 23 MV) measured with accelerators from three vendors, but the results focus primarily on Varian accelerators. The obtained average gamma error (1%, 1 mm) between dose calculations and measurements used in optimization was smaller than 0.7 for each studied treatment beam and field size, and a minimum of 83% of measurement points passed the γ < 1 criterion. For experiments made at different SSDs and for asymmetric fields, the average gamma errors were smaller than 1.1. For irregularly shaped MLC apertures, the differences in point doses were smaller than 1.0%. This work demonstrates that the source model parameters can be automatically derived from simple measurements using optimization methods. The developed procedure is applicable to a wide range of accelerators, and has an acceptable accuracy and processing time.

Optimizer convergence and local minima errors and their clinical importance

Two of the errors common in the inverse treatment planning optimization have been investigated. The first error is the optimizer convergence error, which appears because of non-perfect convergence to the global or local solution, usually caused by a non-zero stopping criterion. The second error is the local minima error, which occurs when the objective function is not convex and/or the feasible solution space is not convex. The magnitude of the errors, their relative importance in comparison to other errors as well as their clinical significance in terms of tumour control probability (TCP) and normal tissue complication probability (NTCP) were investigated. Two inherently different optimizers, a stochastic simulated annealing and deterministic gradient method were compared on a clinical example. It was found that for typical optimization the optimizer convergence errors are rather small, especially compared to other convergence errors, e.g., convergence errors due to inaccuracy of the current dose calculation algorithms. This indicates that stopping criteria could often be relaxed leading into optimization speed-ups. The local minima errors were also found to be relatively small and typically in the range of the dose calculation convergence errors. Even for the cases where significantly higher objective function scores were obtained the local minima errors were not significantly higher. Clinical evaluation of the optimizer convergence error showed good correlation between the convergence of the clinical TCP or NTCP measures and convergence of the physical dose distribution. On the other hand, the local minima errors resulted in significantly different TCP or NTCP values (up to a factor of 2) indicating clinical importance of the local minima produced by physical optimization.

Dose calculation of anticancer drugs: a review of the current practice and introduction of an alternative.

To review the current dose-calculation practice and propose a non-body-surface area (BSA)-based dose-calculation method. Data that supported the introduction of BSA-based dose calculation in the late 1950s were reviewed. Data on 18 drugs that correlated pharmacokinetic (PK) variables for cytotoxic drugs with BSA were examined. Other methods of dose calculation, such as therapeutic drug monitoring (TDM) and adaptive control, were also examined. The BSA-based method of dose calculation was adopted without adequate investigation of its accuracy. BSA fails to standardize the marked interpatient variation in PK for most cytotoxic drugs. A definite correlation was found between PK variables and BSA for only one drug (docetaxel). PK parameters correlate with toxicity, as well as response in some tumors, but do not completely explain the variation in drug effect between individuals. The complexities of TDM may make its universal use impractical. A non-BSA-based dose calculation method is proposed that defines three mandatory steps: prime dose, modified dose, and toxicity-adjusted dose (PMT dosing). Prime dose is the fixed dose of a drug used alone or in combination and is derived from the reanalysis of phase I/II studies and from clinical practice. Modified dose is an adjustment of the prime dose before administration, based on dose-adjustment guidelines that predict the drug-handling ability of an individual. Population pharmacodynamic studies may be used for the development of these guidelines. Subsequent doses are adjusted in each patient according to a target toxicity, such as nadir neutrophil count or other objective toxicity, that serves as a surrogate marker for potential antitumor effect (toxicity-adjusted dose). Patients who are predicted to have very abnormal drug handling should be excluded from such a dosing scheme and TDM may be more suitable. The routine use of BSA for dose calculation should be reevaluated. Other methods of dose calculation should be investigated. TDM may be impractical in all patients and remains unvalidated. PMT dosing ensures that the condition of each individual is considered, to predict drug effects better. Clinical dose-calculation systems such as PMT dosing should be evaluated prospectively.

Implementation of the ETAR method for 3D inhomogeneity correction using FFT.

The equivalent tissue-air-ratio (ETAR) method employs three-dimensional (3D) CT pixel information to approximate scatter dose contribution for inhomogeneity correction. In general, the calculation provides better agreement with measurements than the one-dimensional (1D) methods typically used in commercial treatment planning systems. In its original implementation, the 3D formulation of the ETAR method is modified empirically as a 2D calculation in order to reduce computation time. The modification compromises the use of the method in several treatment geometries. An examination of the ETAR formulation shows that the calculation can be expressed as a convolution and thus can be performed in 3D using fast Fourier transform (FFT) techniques. The algorithm has been implemented as a 3D FFT convolution. Making use of the symmetric properties of the FFT, the new approach shows significant savings in computation time without excessive memory requirement. Despite its fundamental limitations when applied to regions of electronic disequilibrium, the ETAR method offers a practical solution to improving current dose calculation in 3D treatment planning, particularly when the more advanced scatter ray-tracing dose calculation algorithms remain impractical for clinical use. Recent work to extend the method to approximate electron transport is also encouraging.

Dose in bone and tissue near bone-tissue interface from electron beam

This work has quantitatively studied the variation of dose both within bone and in unit density tissue near bone-tissue interfaces. Dose upstream of a bone-tissue interface is increased because of an increase in the backscattered electrons from the bone. The magnitude of this effect was measured using a thin parallel-plate ionization chamber upstream of a polymethyl methacrylate (PMMA)-hard bone interface. The electron backscatter factor (EBF) increased rapidly with bone thickness until a full EBF was achieved. This occurred at approximately 3.5 mm at 2 MeV and 6 mm at 13.1 MeV. The full EBF at the interface ranged from approximately 1.018 at 13.1 MeV to 1.05 at 2 MeV. It was also observed that the EBF had a dependence on the energy spectrum at the interface. The penetration of the backscattered electrons in the upstream direction of PMMA was also measured. The dose penetration fell off rapidly in the upstream direction of the interface. Dose enhancement to unit density tissue in bone was measured for an electron beam by placing thermoluminescent dosimeters (TLDs) in a PMMA-bone-PMMA phantom. The maximum dose enhancement in bone was approximately 7% of the maximum dose in water. However, the pencil-beam algorithm of Hogstrom et al. predicted an increase of only 1%, primarily owing to the inverse-square correction. Film was also used to measure the dose enhancement in bone. The film plane was aligned either perpendicular or parallel to the central axis of the beam. The film data indicated that the maximum dose enhancement in bone was approximately 8 % for the former film alignment (which was similarly predicted by the TLD measurements) and 13% for the latter film alignment. These results confirm that the X ray film is not suitable to be irratiated “edge on” in an inhomogeneous phantom without making perturbation corrections resulting from the film acting as a long narrow inhomogeneous cavity within the bone. In addition, the results give the radiotherapist a basis for clinical judgment when electron beams are used to treat lesions behind bone or near bony structures. We feel these data enhance the ability to recognize the shortcomings of the current dose calculation algorithm used clinically.