Inhomogeneous Phantom Research Articles

SU-F-T-361: Dose Enhancement Due to Nanoparticle Addition in Skin Radiotherapy: A Monte Carlo Study Using Kilovoltage Photon Beams

Purpose: This study investigated the dose enhancement due to addition of nanoparticles with different types and concentrations in skin radiotherapy using kilovoltage photon beams. Methods: An inhomogeneous water phantom (15×15×10 cm3) having the skin target layer (0.5–5 mm), added with different concentrations (3–40 mg/ml) of nanoparticles (Au, Pt, I, Ag and Fe2O3), was irradiated by the 105 and 220 kVp photon beams produced by a Gulmay D3225 Orthovoltage unit. The circular cone of 5-cm diameter and source-to-surface distance of 20 cm were used. Doses in the skin target layer with and without adding the nanoparticles were calculated using Monte Carlo simulation (the EGSnrc code) through the macroscopic approach. Dose enhancement ratio (DER), defined as the ratio of dose at the target with nanoparticle addition to the dose without addition, was calculated for each type and concentration of nanoparticle in different target thickness. Results: For Au nanoparticle, DER dependence on target thickness for the 220 kVp photon beams was not significant. However, DER for Au nanoparticle was found decreasing with an increase of target thickness when the nanoparticle concentration was increased from 18 to 40 mg/ml using the 105 kVp photon beams. For nanoparticle concentration of 40 mg/ml, DER variation with target thickness was not significant for the 220 kVp photon beams, but DEF was found decreasing with the target thickness when lower energy of photon beam (105 kVp) was used. DEF was found increasing with an increase of nanoparticle concentration. The higher the DEF increasing rate, the higher the atomic number of the nanoparticle except I and Ag for the same target thickness. Conclusion: It is concluded that nanoparticle addition can result in dose enhancement in kilovoltage skin radiotherapy. Moreover, the DER is related to the photon beam energy, target thickness, atomic number and concentration of nanoparticles.

SU‐F‐T‐449: Dosimetric Comparison of Acuros XB, Adaptive Convolve in Intensity Modulated Radiotherapy for Head and Neck Cancer

Purpose:There have been several publications focusing on dose calculation in lung for a new dose calculation algorithm of Acuros XB (AXB). AXB could contribute to dose calculation for high‐density media for bone and dental prosthesis rather than in lung. We compared the dosimetric performance of AXB, Adaptive Convolve (AC) in head and neck IMRT plans.Methods:In a phantom study, the difference in depth profile between AXB and AC was evaluated using Kodak EDR2 film sandwiched with tough water phantoms. 6 MV x‐ray using the TrueBeam was irradiated. In a patient study, 20 head and neck IMRT plans had been clinically approved in Pinnacle3 and were transferred to Eclipse. Dose distribution was recalculated using AXB in Eclipse while maintaining AC‐calculated monitor units and MLC sequence planned in Pinnacle. Subsequently, both the dose‐volumetric data obtained using the two different calculation algorithms were compared.Results:The results in the phantom evaluation for the shallow area ahead of the build‐up region shows over‐dose for AXB and under‐dose for AC, respectively. In the patient plans, AXB shows more hot spots especially around the high‐density media than AC in terms of PTV (Max difference: 4.0%) and OAR (Max. difference: 1.9%). Compared to AC, there were larger dose deviations in steep dose gradient region and higher skin‐dose.Conclusion:In head and neck IMRT plans, AXB and AC show different dosimetric performance for the regions inside the target volume around high‐density media, steep dose gradient regions and skin‐surface. There are limitations in skin‐dose and complex anatomic condition using even inhomogeneous anthropomorphic phantom Thus, there is the potential for an increase of hot‐spot in AXB, and an underestimation of dose in substance boundaries and skin regions in AC.

SU‐F‐T‐148: Are the Approximations in Analytic Semi‐Empirical Dose Calculation Algorithms for Intensity Modulated Proton Therapy for Complex Heterogeneities of Head and Neck Clinically Significant?

Purpose:Evaluate the differences in dose distributions between the proton analytic semi‐empirical dose calculation algorithm used in the clinic and Monte Carlo calculations for a sample of 50 head‐and‐neck (H&N) patients and estimate the potential clinical significance of the differences.Methods:A cohort of 50 H&N patients, treated at the University of Texas Cancer Center with Intensity Modulated Proton Therapy (IMPT), were selected for evaluation of clinical significance of approximations in computed dose distributions. H&N site was selected because of the highly inhomogeneous nature of the anatomy. The Fast Dose Calculator (FDC), a fast track‐repeating accelerated Monte Carlo algorithm for proton therapy, was utilized for the calculation of dose distributions delivered during treatment plans. Because of its short processing time, FDC allows for the processing of large cohorts of patients. FDC has been validated versus GEANT4, a full Monte Carlo system and measurements in water and for inhomogeneous phantoms. A gamma‐index analysis, DVHs, EUDs, and TCP and NTCPs computed using published models were utilized to evaluate the differences between the Treatment Plan System (TPS) and FDC.Results:The Monte Carlo results systematically predict lower dose delivered in the target. The observed differences can be as large as 8 Gy, and should have a clinical impact. Gamma analysis also showed significant differences between both approaches, especially for the target volumes.Conclusion:Monte Carlo calculations with fast algorithms is practical and should be considered for the clinic, at least as a treatment plan verification tool.

Characterization of the microbunch time structure of proton pencil beams at a clinical treatment facility

Proton therapy is an advantageous treatment modality compared to conventional radiotherapy. In contrast to photons, charged particles have a finite range and can thus spare organs at risk. Additionally, the increased ionization density in the so-called Bragg peak close to the particle range can be utilized for maximum dose deposition in the tumour volume. Unfortunately, the accuracy of the therapy can be affected by range uncertainties, which have to be covered by additional safety margins around the treatment volume. A real-time range and dose verification is therefore highly desired and would be key to exploit the major advantages of proton therapy. Prompt gamma rays, produced in nuclear reactions between projectile and target nuclei, can be used to measure the proton’s range. The prompt gamma-ray timing (PGT) method aims at obtaining this information by determining the gamma-ray emission time along the proton path using a conventional time-of-flight detector setup. First tests at a clinical accelerator have shown the feasibility to observe range shifts of about 5 mm at clinically relevant doses. However, PGT spectra are smeared out by the bunch time spread. Additionally, accelerator related proton bunch drifts against the radio frequency have been detected, preventing a potential range verification. At OncoRay, first experiments using a proton bunch monitor (PBM) at a clinical pencil beam have been conducted. Elastic proton scattering at a hydrogen-containing foil could be utilized to create a coincident proton-proton signal in two identical PBMs. The selection of coincident events helped to suppress uncorrelated background. The PBM setup was used as time reference for a PGT detector to correct for potential bunch drifts. Furthermore, the corrected PGT data were used to image an inhomogeneous phantom. In a further systematic measurement campaign, the bunch time spread and the proton transmission rate were measured for several beam energies between 69 and 225 MeV as well as for variable momentum limiting slit openings. We conclude that the usage of a PBM increases the robustness of the PGT method in clinical conditions and that the obtained data will help to create reliable range verification procedures in clinical routine.

Application of dose kernel calculation using a simplified Monte Carlo method to treatment plan for scanned proton beams.

Full Monte Carlo (FMC) calculation of dose distribution has been recognized to have superior accuracy, compared with the pencil beam algorithm (PBA). However, since the FMC methods require long calculation time, it is difficult to apply them to routine treatment planning at present. In order to improve the situation, a simplified Monte Carlo (SMC) method has been introduced to the dose kernel calculation applicable to dose optimization procedure for the proton pencil beam scanning. We have evaluated accuracy of the SMC calculation by comparing a result of the dose kernel calculation using the SMC method with that using the FMC method in an inhomogeneous phantom. The dose distribution obtained by the SMC method was in good agreement with that obtained by the FMC method. To assess the usefulness of SMC calculation in clinical situations, we have compared results of the dose calculation using the SMC with those using the PBA method for three clinical cases of tumor treatment. The dose distributions calculated with the PBA dose kernels appear to be homogeneous in the planning target volumes (PTVs). In practice, the dose distributions calculated with the SMC dose kernels with the spot weights optimized with the PBA method show largely inhomogeneous dose distributions in the PTVs, while those with the spot weights optimized with the SMC method have moderately homogeneous distributions in the PTVs. Calculation using the SMC method is faster than that using the GEANT4 by three orders of magnitude. In addition, the graphic processing unit (GPU) boosts the calculation speed by 13 times for the treatment planning using the SMC method. Thence, the SMC method will be applicable to routine clinical treatment planning for reproduction of the complex dose distribution more accurately than the PBA method in a reasonably short time by use of the GPU‐based calculation engine.PACS number(s): 87.55.Gh

Comparison of selected dose calculation algorithms in radiotherapy treatment planning for tissues with inhomogeneities

Inhomogeneity correction is recommended for accurate dose calculation in radiotherapy treatment planning since human body are highly inhomogeneous with the presence of bones and air cavities. However, each dose calculation algorithm has its own limitations. This study is to assess the accuracy of five algorithms that are currently implemented for treatment planning, including pencil beam convolution (PBC), superposition (SP), anisotropic analytical algorithm (AAA), Monte Carlo (MC) and Acuros XB (AXB). The calculated dose was compared with the measured dose using radiochromic film (Gafchromic EBT2) in inhomogeneous phantoms. In addition, the dosimetric impact of different algorithms on intensity modulated radiotherapy (IMRT) was studied for head and neck region. MC had the best agreement with the measured percentage depth dose (PDD) within the inhomogeneous region. This was followed by AXB, AAA, SP and PBC. For IMRT planning, MC algorithm is recommended for treatment planning in preference to PBC and SP. The MC and AXB algorithms were found to have better accuracy in terms of inhomogeneity correction and should be used for tumour volume within the proximity of inhomogeneous structures.

A New Class of Phantom Materials for Poroelastography Imaging Techniques

Poroelastography is an elastographic technique used to image the temporal mechanical behavior of tissues. One of the major challenges in determining experimental potentials and limitations of this technique has been the lack of complex and realistic controlled phantoms that could be used to corroborate the limited number of theoretical and simulation studies available in the literature as well as to predict its performance in complex experimental situations and in a variety of conditions. In the study described here, we propose and analyze a new class of phantom materials for temporal elastography imaging. The results indicate that, by using polyacrylamide, we can generate inhomogeneous elastographic phantoms with controlled fluid content and fluid flow properties, while maintaining mechanical and ultrasonic properties similar to those of soft tissues.

Perturbation correction for alanine dosimeters in different phantom materials in high-energy photon beams

In modern radiotherapy the verification of complex treatments plans is often performed in inhomogeneous or even anthropomorphic phantoms. For dose verification small detectors are necessary and therefore alanine detectors are most suitable. Though the response of alanine for a wide range of clinical photon energies in water is well know, the knowledge about the influence of the surrounding phantom material on the response of alanine is sparse. Therefore we investigated the influence of twenty different surrounding/phantom materials for alanine dosimeters in clinical photon fields via Monte Carlo simulations. The relative electron density of the used materials was in the range up to 1.69, covering almost all materials appearing in inhomogeneous or anthropomorphic phantoms used in radiotherapy. The investigations were performed for three different clinical photon spectra ranging from 6 to 25 MV-X and Co-60 and as a result a perturbation correction depending on the environmental material was established. The Monte Carlo simulation show, that there is only a small dependence of on the phantom material and the photon energy, which is below ±0.6%. The results confirm the good suitability of alanine detectors for in-vivo dosimetry.

High frame rate and high line density ultrasound imaging for local pulse wave velocity estimation using motion matching: A feasibility study on vessel phantoms

Pulse wave imaging (PWI) is an ultrasound-based method to visualize the propagation of pulse wave and to quantitatively estimate regional pulse wave velocity (PWV) of the arteries within the imaging field of view (FOV). To guarantee the reliability of PWV measurement, high frame rate imaging is required, which can be achieved by reducing the line density of ultrasound imaging or transmitting plane wave at the expense of spatial resolution and/or signal-to-noise ratio (SNR). In this study, a composite, full-view imaging method using motion matching was proposed with both high temporal and spatial resolution. Ultrasound radiofrequency (RF) data of 4 sub-sectors, each with 34 beams, including a common beam, were acquired successively to achieve a frame rate of ∼507Hz at an imaging depth of 35mm. The acceleration profiles of the vessel wall estimated from the common beam were used to reconstruct the full-view (38-mm width, 128-beam) image sequence. The feasibility of mapping local PWV variation along the artery using PWI technique was preliminarily validated on both homogeneous and inhomogeneous polyvinyl alcohol (PVA) cryogel vessel phantoms. Regional PWVs for the three homogeneous phantoms measured by the proposed method were in accordance with the sparse imaging method (38-mm width, 32-beam) and plane wave imaging method. Local PWV was estimated using the above-mentioned three methods on 3 inhomogeneous phantoms, and good agreement was obtained in both the softer (1.91±0.24m/s, 1.97±0.27m/s and 1.78±0.28m/s) and the stiffer region (4.17±0.46m/s, 3.99±0.53m/s and 4.27±0.49m/s) of the phantoms. In addition to the improved spatial resolution, higher precision of local PWV estimation in low SNR circumstances was also obtained by the proposed method as compared with the sparse imaging method. The proposed method might be helpful in disease detections through mapping the local PWV of the vascular wall.

Discontinuous finite element space-angle treatment of the first order linear Boltzmann transport equation with magnetic fields: Application to MRI-guided radiotherapy.

The advent of magnetic resonance imaging (MRI) guided radiotherapy systems demands the incorporation of the magnetic field into dose calculation algorithms of treatment planning systems. This is due to the fact that the Lorentz force of the magnetic field perturbs the path of the relativistic electrons, hence altering the dose deposited by them. Building on the previous work, the authors have developed a discontinuous finite element space-angle treatment of the linear Boltzmann transport equation to accurately account for the effects of magnetic fields on radiotherapy doses. The authors present a detailed description of their new formalism and compare its accuracy to geant4 Monte Carlo calculations for magnetic fields parallel and perpendicular to the radiation beam at field strengths of 0.5 and 3 T for an inhomogeneous 3D slab geometry phantom comprising water, bone, and air or lung. The accuracy of the authors' new formalism was determined using a gamma analysis with a 2%/2 mm criterion. Greater than 98.9% of all points analyzed passed the 2%/2 mm gamma criterion for the field strengths and orientations tested. The authors have benchmarked their new formalism against Monte Carlo in a challenging radiation transport problem with a high density material (bone) directly adjacent to a very low density material (dry air at STP) where the effects of the magnetic field dominate collisions. A discontinuous finite element space-angle approach has been proven to be an accurate method for solving the linear Boltzmann transport equation with magnetic fields for cases relevant to MRI guided radiotherapy. The authors have validated the accuracy of this novel technique against geant4, even in cases of strong magnetic field strengths and low density air.

A regularised boundary element formulation for contactless SAR evaluations within homogeneous and inhomogeneous head phantoms

This work presents a Boundary Element Method (BEM) formulation for contactless electromagnetic field assessments. The new scheme is based on a regularized BEM approach that requires the use of electric measurements only. The regularization is obtained by leveraging on an extension of Calderon techniques to rectangular systems leading to well-conditioned problems independent of the discretization density. This enables the use of highly discretized Huygens surfaces that can be consequently placed very near to the radiating source. In addition, the new regularized scheme is hybridized with both surfacic homogeneous and volumetric inhomogeneous forward BEM solvers accelerated with fast matrix-vector multiplication schemes. This allows for rapid and effective dosimetric assessments and permits the use of inhomogeneous and realistic head phantoms. Numerical results corroborate the theory and confirms the practical effectiveness of all newly proposed formulations.

SU-E-T-285: Dose Variation at Bone in Small-Animal Irradiation: A Monte Carlo Study Using Monoenergetic Photon Beams

Purpose: The aim of this study is to investigate the variation of bone dose on photon beam energy (keV – MeV) in small-animal irradiation. Dosimetry of homogeneous and inhomogeneous phantoms as per the same mouse computed tomography image set were calculated using the DOSCTP and DOSXYZnrc based on the EGSnrc Monte Carlo code. Methods: Monte Carlo simulations for the homogeneous and inhomogeneous mouse phantom irradiated by a 360 degree photon arc were carried out. Mean doses of the bone tissue in the irradiated volumes were calculated at various photon beam energies, ranging from 50 keV to 1.25 MeV. The effect of bone inhomogeneity was examined through the Inhomogeneous Correction Factor (ICF), a dose ratio of the inhomogeneous to the homogeneous medium. Results: From our Monte Carlo results, higher mean bone dose and ICF were found when using kilovoltage photon beams compared to megavoltage. In beam energies ranging from 50 keV to 200 keV, the bone dose was found maximum at 50 keV, and decreased significantly from 2.6 Gy to 0.55 Gy, when 2 Gy was delivered at the center of the phantom (isocenter). Similarly, the ICF were found decreasing from 4.5 to 1 when the photon beam energy was increased from 50 keV to 200 keV. Both mean bone dose and ICF remained at about 0.5 Gy and 1 from 200 keV to 1.25 MeV with insignificant variation, respectively. Conclusion: It is concluded that to avoid high bone dose in the small-animal irradiation, photon beam energy higher than 200 keV should be used with the ICF close to one, and bone dose comparable to the megavoltage beam where photoelectric effect is not dominant.

SU‐E‐T‐219: Comprehensive Validation of the Electron Monte Carlo Dose Calculation Algorithm in RayStation Treatment Planning System for An Elekta Linear Accelerator with AgilityTM Treatment Head

Purpose:This study evaluated the performance of the electron Monte Carlo dose calculation algorithm in RayStation v4.0 for an Elekta machine with Agility™ treatment head.Methods:The machine has five electron energies (6–8 MeV) and five applicators (6×6 to 25×25 cm 2). The dose (cGy/MU at dmax), depth dose and profiles were measured in water using an electron diode at 100 cm SSD for nine square fields ≥2×2 cm2 and four complex fields at normal incidence, and a 14×14 cm2 field at 15° and 30° incidence. The dose was also measured for three square fields ≥4×4 cm2 at 98, 105 and 110 cm SSD. Using selected energies, the EBT3 radiochromic film was used for dose measurements in slab‐shaped inhomogeneous phantoms and a breast phantom with surface curvature. The measured and calculated doses were analyzed using a gamma criterion of 3%/3 mm.Results:The calculated and measured doses varied by <3% for 116 of the 120 points, and <5% for the 4×4 cm2 field at 110 cm SSD at 9–18 MeV. The gamma analysis comparing the 105 pairs of in‐water isodoses passed by >98.1%. The planar doses measured from films placed at 0.5 cm below a lung/tissue layer (12 MeV) and 1.0 cm below a bone/air layer (15 MeV) showed excellent agreement with calculations, with gamma passing by 99.9% and 98.5%, respectively. At the breast‐tissue interface, the gamma passing rate is >98.8% at 12–18 MeV. The film results directly validated the accuracy of MU calculation and spatial dose distribution in presence of tissue inhomogeneity and surface curvature ‐ situations challenging for simpler pencil‐beam algorithms.Conclusion:The electron Monte Carlo algorithm in RayStation v4.0 is fully validated for clinical use for the Elekta Agility™ machine. The comprehensive validation included small fields, complex fields, oblique beams, extended distance, tissue inhomogeneity and surface curvature.

SU‐E‐T‐805: Verification of the Simplified Monte Carlo Method for Simulation in An Inhomogeneous Phantom

Purpose:A simplified Monte Carlo (SMC) method has been developed to obtain fast and accurate dose calculation in heterogeneous tissues to improve the accuracy of dose calculation. We have applied the SMC method to calculation of dose kernels for the pencil beam scanning. While the SMC method tracks individual primary protons in medium, it simplifies the dose calculation by using a measured depth‐dose distribution instead of considering nuclear reactions and tracking secondary particles. To verify the accuracy of SMC calculation, we compared a dose‐calculation Result using the SMC method with that using the Full Monte Carlo (FMC) method in an inhomogeneous phantom.Methods:As a model of the inhomogeneous media, we considered a phantom complied of a rectangular acrylic block of size 150*300*200 mm 3 immersed in water within a virtual container with a size of 300*300*400 mm3.Results:We found excellent agreement of overall dose distributions between the SMC and FMC methods. As for the laterally integrated depth‐dose distributions, slight difference was found in front of the second Bragg‐Peak between the two algorithms. While it took 25717.0 s on 2.7 GHz Intel Core i7 for the FMC method to complete the calculation, it took 15.4 s for the SMC method to complete it. The SMC method can calculate the dose distribution approximately 1675 times faster than the FMC method.Conclusion:The dose distribution obtained by SMC method agreed well that obtained by the FMC method in a simple inhomogeneous phantom. In contrast, calculation time by the SMC method has been reduced by three orders in magnitude compared with the FMC method.

SU‐E‐T‐454: Impact of Air Gap On PDDs of 6 MV Photon Beam for Various Field Sizes in Inhomogeneous Phantoms

Purpose:To investigate how the shape of air gap and its size will impact the percent depth doses (PDDs) of a 6MV photon beam for various field sizes.Methods:Two in‐house phantoms were manufactured containing rectangular (phantom A) and circular (phantom B) air gaps. Both phantoms A and B were composed of same top layer (solid‐water; 30×30cm2,5cm thickness) and bottom layer (solid‐water; 30×30cm 2,8cm thickness), but middle layer was varied to observe air gap effects and scatter contribution to the measurement point. In phantom A, a rectangular shaped air gap (30×30cm2,7cm thickness) was created by placing Styrofoam blocks between top and bottom layers of the phantom. In phantom B, middle layer was replaced by “inhomogenous block”, composed of acrylic plate (30×30cm2, 4cm thickness) followed by PVC(30×30cm2,3cm thickness). Additionally, circular air gap was created by drilling a hole (diameter=2.8cm, length=7cm) at the center of “inhomogenous block”. In both phantoms, measurement readings were obtained at 13cm depth (i.e., 1cm after air gap) and depth of maximum dose(6MV energy; 100 MUs; field sizes ranged from 3×3cm2 to 10×10cm2). The PDDs at 13cm depth were compared in both phantoms.Results:The measurements in both phantoms A and B showed an almost linear increase in PDDs with increasing field size, especially for smaller field sizes (from 3×3 to 7×7cm2). For each field size, the PDD in phantom A was smaller compared to the one in phantom B. The difference in PDDs between two phantoms decreased with an increase in field size as the PDD difference decreased from 9.0% to 6.4%.Conclusion:The shape and size of air gap affect the PDD measurements in secondary build‐up region as 6 MV primary beam traverses through the center of air gap. The scatter contribution due to increase in field size was more noticeable for field sizes ≤7×7cm2.

SU‐E‐T‐467: Implementation of Monte Carlo Dose Calculation for a Multileaf Collimator Equipped Robotic Radiotherapy System

Purpose:To improve the treatment efficiency and capabilities for full‐body treatment, a robotic radiosurgery system has equipped with a multileaf collimator (MLC) to extend its accuracy and precision to radiation therapy. To model the MLC and include it in the Monte Carlo patient dose calculation is the goal of this work.Methods:The radiation source and the MLC were carefully modeled to consider the effects of the source size, collimator scattering, leaf transmission and leaf end shape. A source model was built based on the output factors, percentage depth dose curves and lateral dose profiles measured in a water phantom. MLC leaf shape, leaf end design and leaf tilt for minimizing the interleaf leakage and their effects on beam fluence and energy spectrum were all considered in the calculation. Transmission/leakage was added to the fluence based on the transmission factors of the leaf and the leaf end. The transmitted photon energy was tuned to consider the beam hardening effects. The calculated results with the Monte Carlo implementation was compared with measurements in homogeneous water phantom and inhomogeneous phantoms with slab lung or bone material for 4 square fields and 9 irregularly shaped fields.Results:The calculated output factors are compared with the measured ones and the difference is within 1% for different field sizes. The calculated dose distributions in the phantoms show good agreement with measurements using diode detector and films. The dose difference is within 2% inside the field and the distance to agreement is within 2mm in the penumbra region. The gamma passing rate is more than 95% with 2%/2mm criteria for all the test cases.Conclusion:Implementation of Monte Carlo dose calculation for a MLC equipped robotic radiosurgery system is completed successfully. The accuracy of Monte Carlo dose calculation with MLC is clinically acceptable.This work was supported by Accuray Inc.

SU‐E‐T‐800: Verification of Acurose XB Dose Calculation Algorithm at Air Cavity‐Tissue Interface Using Film Measurement for Small Fields of 6‐MV Flattening Filter‐Free Beams

Purpose:To verify the dose accuracy of Acuros XB (AXB) dose calculation algorithm at air‐tissue interface using inhomogeneous phantom for 6‐MV flattening filter‐free (FFF) beams.Methods:An inhomogeneous phantom included air cavity was manufactured for verifying dose accuracy at the air‐tissue interface. The phantom was composed with 1 and 3 cm thickness of air cavity. To evaluate the central axis doses (CAD) and dose profiles of the interface, the dose calculations were performed for 3 × 3 and 4 × 4 cm2 fields of 6 MV FFF beams with AAA and AXB in Eclipse treatment plainning system. Measurements in this region were performed with Gafchromic film. The root mean square errors (RMSE) were analyzed with calculated and measured dose profile. Dose profiles were divided into inner‐dose profile (>80%) and penumbra (20% to 80%) region for evaluating RMSE. To quantify the distribution difference, gamma evaluation was used and determined the agreement with 3%/3mm criteria.Results:The percentage differences (%Diffs) between measured and calculated CAD in the interface, AXB shows more agreement than AAA. The %Diffs were increased with increasing the thickness of air cavity size and it is similar for both algorithms. In RMSEs of inner‐profile, AXB was more accurate than AAA. The difference was up to 6 times due to overestimation by AAA. RMSEs of penumbra appeared to high difference for increasing the measurement depth. Gamma agreement also presented that the passing rates decreased in penumbra.Conclusion:This study demonstrated that the dose calculation with AXB shows more accurate than with AAA for the air‐tissue interface. The 2D dose distributions with AXB for both inner‐profile and penumbra showed better agreement than with AAA relative to variation of the measurement depths and air cavity sizes.

SU‐C‐204‐01: A Fast Analytical Approach for Prompt Gamma and PET Predictions in a TPS for Proton Range Verification

Purpose:We describe and demonstrate a fast analytical tool for prompt‐gamma emission prediction based on filter functions applied on the depth dose profile. We present the implementation in a treatment planning system (TPS) of the same algorithm for positron emitter distributions.Methods:The prediction of the desired observable is based on the convolution of filter functions with the depth dose profile. For both prompt‐gammas and positron emitters, the results of Monte Carlo simulations (MC) are compared with those of the analytical tool. For prompt‐gamma emission from inelastic proton‐induced reactions, homogeneous and inhomogeneous phantoms alongside with patient data are used as irradiation targets of mono‐energetic proton pencil beams. The accuracy of the tool is assessed in terms of the shape of the analytically calculated depth profiles and their absolute yields, compared to MC. For the positron emitters, the method is implemented in a research RayStation TPS and compared to MC predictions. Digital phantoms and patient data are used and positron emitter spatial density distributions are analyzed.Results:Calculated prompt‐gamma profiles agree with MC within 3 % in terms of absolute yield and reproduce the correct shape. Based on an arbitrary reference material and by means of 6 filter functions (one per chemical element), profiles in any other material composed of those elements can be predicted. The TPS implemented algorithm is accurate enough to enable, via the analytically calculated positron emitters profiles, detection of range differences between the TPS and MC with errors of the order of 1–2 mm.Conclusion:The proposed analytical method predicts prompt‐gamma and positron emitter profiles which generally agree with the distributions obtained by a full MC. The implementation of the tool in a TPS shows that reliable profiles can be obtained directly from the dose calculated by the TPS, without the need of full MC simulation.

SU‐E‐J‐141: Activity‐Equivalent Path Length Approach for the 3D PET‐Based Dose Reconstruction in Proton Therapy

Purpose:Ion beam therapy is sensitive to uncertainties from treatment planning and dose delivery. PET imaging of induced positron emitter distributions is a practical approach for in vivo, in situ verification of ion beam treatments. Treatment verification is usually done by comparing measured activity distributions with reference distributions, evaluated in nominal conditions. Although such comparisons give valuable information on treatment quality, a proper clinical evaluation of the treatment ultimately relies on the knowledge of the actual delivered dose. Analytical deconvolution methods relating activity and dose have been studied in this context, but were not clinically applied. In this work we present a feasibility study of an alternative approach for dose reconstruction from activity data, which is based on relating variations in accumulated activity to tissue density variations.Methods:First, reference distributions of dose and activity were calculated from the treatment plan and CT data. Then, the actual measured activity data were cumulatively matched with the reference activity distributions to obtain a set of activity‐equivalent path lengths (AEPLs) along the rays of the pencil beams. Finally, these AEPLs were used to deform the original dose distribution, yielding the actual delivered dose. The method was tested by simulating a proton therapy treatment plan delivering 2 Gy on a homogeneous water phantom (the reference), which was compared with the same plan delivered on a phantom containing inhomogeneities. Activity and dose distributions were were calculated by means of the FLUKA Monte Carlo toolkit.Results:The main features of the observed dose distribution in the inhomogeneous situation were reproduced using the AEPL approach. Variations in particle range were reproduced and the positions, where these deviations originated, were properly identified.Conclusions:For a simple inhomogeneous phantom the 3D dose reconstruction from PET‐activity induced by proton beams was shown to be feasible.

SU-D-BRB-07: Lipiodol Impact On Dose Distribution in Liver SBRT After TACE

Purpose: Stereotactic body radiotherapy (SBRT) combining transarterial chemoembolization (TACE) with Lipiodol is expected to improve local control. This study aims to evaluate the impact of Lipiodol on dose distribution by comparing the dosimetric performance of the Acuros XB (AXB) algorithm, anisotropic analytical algorithm (AAA), and Monte Carlo (MC) method using a virtual heterogeneous phantom and a treatment plan for liver SBRT after TACE. Methods: The dose distributions calculated using AAA and AXB algorithm, both in Eclipse (ver. 11; Varian Medical Systems, Palo Alto, CA), and EGSnrc-MC were compared. First, the inhomogeneity correction accuracy of the AXB algorithm and AAA was evaluated by comparing the percent depth dose (PDD) obtained from the algorithms with that from the MC calculations using a virtual inhomogeneity phantom, which included water and Lipiodol. Second, the dose distribution of a liver SBRT patient treatment plan was compared between the calculation algorithms. Results In the virtual phantom, compared with the MC calculations, AAA underestimated the doses just before and in the Lipiodol region by 5.1% and 9.5%, respectively, and overestimated the doses behind the region by 6.0%. Furthermore, compared with the MC calculations, the AXB algorithm underestimated the doses just before and in the Lipiodol region by 4.5% and 10.5%, respectively, and overestimated the doses behind the region by 4.2%. In the SBRT plan, the AAA and AXB algorithm underestimated the maximum doses in the Lipiodol region by 9.0% in comparison with the MC calculations. In clinical cases, the dose enhancement in the Lipiodol region can approximately 10% increases in tumor dose without increase of dose to normal tissue. Conclusion: The MC method demonstrated a larger increase in the dose in the Lipiodol region than the AAA and AXB algorithm. Notably, dose enhancement were observed in the tumor area; this may lead to a clinical benefit.