Dose Perturbation Effects Research Articles

Dose perturbation effects by metal hip prosthesis in gynaecological 192Ir HDR brachytherapy

The present study was conducted to elucidate the effects of hip prostheses in 192Ir HDR brachytherapy and determine dose uncertainties introduced by the treatment planning. A gynaecological phantom irradiated using Nucletron 192Ir microSelectron HDR source was modeled using MCNP5 code. Three hip materials considered in this study were water, bone, and metal prosthesis. According to the obtained results, a dose perturbation was observed within the medium with a higher atomic number, which reduced the dose to the nearby region.

A phantom-based experimental and Monte Carlo study of the suitability of in-vivo diodes and TLD for entrance in-vivo dosimetry in small-to-medium sized 6 MV photon fields

In-vivo dosimetry is recommended for radiotherapy treatment verification. The main purpose of the study is to investigate the suitability of various dosimeters for performing in-vivo measurements in small 6 MV photon fields by measurement as well as Monte Carlo simulation. Two types of diode dosimeters designed for placement on skin for the purpose of in-vivo dosimetry (EDD-5 and EDP-10), 3.1 × 3.1 × 0.89 mm3 LiF TLDs, and an electron field diode (EFD) were used for dosimetric measurements in a custom-made PMMA phantom. An Elekta accelerator’s treatment head was Monte Carlo modeled in detail, which was used together with validated Monte Carlo models of the diodes. Dose perturbation effects of the dosimeters and their suitability for in-vivo measurements were investigated. In fields ≥2 × 2 cm2, differences in the measured response of EDD-5, EDP-10 and TLD from the Monte Carlo gold-standard were <2%. For the 1 × 1 cm2 field, however, the differences from the Monte Carlo gold-standard were all greater than 2% due to the influence of dosimeter size as well as any misalignment and set-up uncertainties. In fields ≥1 × 1 cm2, EDD-5, EDP-10, and TLD produced 2.2%–3.2%, 5.3%–6.6% and 0.8–1.1% dose perturbation (≤10 cm depths), respectively. A better than 2% dosimetric accuracy was not achieved by these diodes for field sizes smaller than 2 × 2 cm2. Routine patient in-vivo dosimetry is feasible with the EDD-5 diode and TLD for field sizes down to 2 × 2 cm2 without exceeding the 5% tolerance level for overall uncertainty. But the EDP-10 diode produces a higher perturbation making it much less suitable for this purpose.

The Dosimetric Effect of MRI Positive-Contrast Markers vs. Negative-Contrast Fiducial Markers on Proton Radiation

The usage of magnetic resonance imaging (MRI) has been limited in prostate cancer radiation because a lack of effective positive-signal MRI fiducial markers (FM). The FM used in our prostate clinic (gold anchor [GAFM] for photons; carbon [CFM] for protons) are not reliably localized with MRI. The uncertainty of FM localization using MRI affects spatial accuracy of the target volume planning and limits optimum patient treatment positioning. Current FM generate 10-38% proton dose perturbation. We hypothesize that the novel multimodality positive-signal FM (C4 Nova [C4NFM], are visible on kilovoltage, computed tomography, and MR images, with a reduced imaging artifact versus GAFM or CFM) can potentially improve the precision and accuracy of radiotherapy and present more acceptable proton dose perturbation. Herein, dose perturbation of different FM was compared.The negative- (GAFM [0.38 × 8.4 mm], CFM [0.59-0.88 × 3 mm]) and positive-signal (C4NFM-5 [1.0 × 5.5 mm], C4NFM-10 [1.0 × 9.8 mm]) MRI FM were tested. Markers were arranged inside plastic water material at the center of a passively scattered spread-out Bragg peak (160 MeV, range 16 cm, SOBP 10 cm). The dose was measured using radiochromic film placed at 0, 2, 5, 10, 20, 30, and 40 mm downstream from the markers. The FM were oriented perpendicularly (PER) or parallel (PAR) relative to the beam axis. The films were scanned using Epson 11000XL flatbed scanning in 300 depi resolution and were further analyzed using imageJ software.The proton dose perturbation is associated with marker type, orientation, and the depth of measurement (Table). Proton dose perturbation increased with the depth away from the FM before it gradually decreased to zero. Less dose perturbations were observed at the PER vs. PAR orientation. GAFM induced higher dose perturbation versus CFM and C4NFM in both the maximum value and the perturbation depth. Compared to CFM, C4NFM produced a lower maximum dose perturbation (1.4% vs. 3.0% at PER; 5.7% vs. 7.9% at PAR), and shallower perturbation depth (5 vs. 20 mm at PER). Notably, both the longer C4NFM-10 and shorter C4NFM-5 produced the same dose perturbation in both maximum value (5.7% at PER) and the depth (20 mm).C4 Nova MRI positive-contrast fiducial markers have the least proton dose perturbation effect compared with gold and carbon MRI negative-contrast markers. These data may serve as evidence for future clinical trials to study the benefit of MRI from treatment planning to delivery using positive MRI signal C4 Nova FM in preserving tumor control while decrease radiation related toxicities in patients with prostate cancer.

Dosimetric effects of supine immobilization devices on the skin in intensity-modulated radiation therapy for breast cancer: a retrospective study

BackgroundThe dose perturbation effect of immobilization devices is often overlooked in intensity-modulated radiation therapy (IMRT) for breast cancer (BC). This retrospective study assessed the dosimetric effects of supine immobilization devices on the skin using a commercial treatment planning system.MethodsForty women with BC were divided into four groups according to the type of primary surgery: groups A and B included patients with left and right BC, respectively, who received 50 Gy radiotherapy in 25 fractions after radical mastectomy, while groups C and D included patients with left and right BC, respectively, who received breast-conservation surgery (BCS) and 40.05 Gy in 15 fractions as well as a tumor bed simultaneous integrated boost to 45 Gy. A 0.2-cm thick skin contour and two sets of body contours were outlined for each patient. Dose calculations were conducted for the two sets of contours using the same plan. The dose differences were assessed by comparing the dose-volume histogram parameter results and by plan subtraction.ResultsThe supine immobilization devices for BC resulted in significantly increased skin doses, which may ultimately lead to skin toxicity. The mean dose increased by approximately 0.5 and 0.45 Gy in groups A and B after radical mastectomy and by 2.7 and 3.25 Gy in groups C and D after BCS; in groups A–D, the percentages of total normal skin volume receiving equal to or greater than 5 Gy (V5) increased by 0.54, 1.15, 2.67, and 1.94%, respectively, while the V10 increased by 1.27, 1.83, 1.36, and 2.88%; the V20 by 0.85, 1.87, 2.76, and 4.86%; the V30 by 1.3, 1.24, 10.58, and 11.91%; and the V40 by 1.29, 0.65, 10, and 10.51%. The dose encompassing the planning target volume and other organs at risk, showed little distinction between IMRT plans without and with consideration of immobilization devices.ConclusionsThe supine immobilization devices significantly increased the dose to the skin, especially for patients with BCS. Thus, immobilization devices should be included in the external contour to account for dose attenuation and skin dose increment.Trial registrationThis study does not report on interventions in human health care.

An avoidance method to minimize dose perturbation effects in proton pencil beam scanning treatment of patients with small high-Z implants

To implement a multi-field-optimization (MFO) technique for treating patients with high-Z implants in pencil beam scanning proton-therapy and generate treatment plans that avoids small implants. Two main issues were addressed: (i) the assessment of the optimal CT acquisition and segmentation technique to define the dimension of the implant and (ii) the distance of pencil beams from the implant (avoidance margin) to assure that it does not affect dose distribution. Different CT reconstruction protocols (by O-MAR or standard reconstruction and by 12 bit or 16 bit dynamic range) followed by thresholding segmentation were tested on a phantom with lead spheres of different sizes. The proper avoidance margin was assessed on a dedicated phantoms of different materials (copper/tantalum and lead), shape (square slabs and spheres) and detectors (two-dimensional array chamber and radio-chromic films). The method was then demonstrated on a head-and-neck carcinoma patient, who underwent carotid artery embolization with a platinum coil close to the target. Regardless the application of O-MAR reconstruction, the CT protocol with a full 16 bit dynamic range allowed better estimation of the sphere volumes, with maximal error around −5% in the greater sphere only. Except the configuration with a shallow target (which required a pre-absorber), particularly with a retracted snout, an avoidance margin of around 0.9–1.3 cm allowed to keep the difference between planned and measured dose below 5–10%. The patient plan analysis showed adequate plan quality and confirmed effective implant avoidance. Potential target under-dosage can be produced by patient misalignment, which could be minimized by daily alignment on the implant, identifiable on orthogonal kilovolt images. By implant avoidance MFO it was possible to minimize potential dose perturbation effects produced by small high-Z implants. An advantage of such approach lies in its potential applicability for any type of implant, regardless the precise knowledge of its composition.

Comparison of In Vitro Dose Perturbation Effects of Amalgam, Composite Resin, Glass Ionomer Cement and Ketac Silver During Head and Neck Radiation with Photon Beam

The most common oral manifestation from head and neck radiotherapy is mucositis. A part of complications is backscattering from metallic dental materials in radiation field, resulting in a dosage enhancement at the tissue-metal interface. The proper management of the patient’s restorative materials must be performed preoperatively to reduce these complications. This research compared the relative dose enhancement (RDE) in the different restorative materials (Amalgam, Composite resin, Glass ionomer cement and Ketac Silver®), the different tooth preparations (Class I, Class I other, Class II MOD, Cusp Tipping and Core build-up) and the location of ionizing chamber (buccal, occlusal, lingual surface). The dimension of controlled 20 teeth are as followed lower third molar, buccolingual width (10.41±0.82mm), mesiodistal width (11.28±0.72mm), dentin and enamel buccolingual width (1.5±0.5mm). The experiment is set using a modular radiation beam analyzer with at least 2 cm water as soft tissue equivalent material, 3 times irradiated with LINAC 6MV 200MU. The backscattering occurred in the amalgam filled tooth at the buccal and occlusal aspect in every preparation. The highest RDE occurring at the buccal aspect of cusp tipping amalgam filled tooth was 2.7%. In the lingual aspect, every preparation of amalgam filled tooth demonstrated lower RDE. The composite resin and glass ionomer cement produced low backscattering, while Ketac Silver® produced higher RDE. For any amalgam restorations in the radiation field, we recommended using the one-layer glass ionomer technique, which is suitable in preparing patient ahead of head and neck radiation. The recommended filling technique may decrease mucositis the adjacent area and improve the patient’s quality of life.

Effects of Selenium Species on the Belousov-Zhabotinsky Reaction.

The effects of selenium species on the Belousov-Zhabotinsky (B-Z) reaction were investigated by adding them to the system before and during oscillation. When selenium species were added into the system before oscillation, sodium selenite prolonged the induction period, whose effect was strong as sodium selenite could consume malonic acid to prohibit the accumulation of bromomalonic acid. For selenomethionine and selenocystine, their effects were derived from their reaction with •CH2COOH and •Br2- producing a radical cation of selenoamino acids, which prohibited the accumulation of bromomalonic acid. Here, the selenium atoms in selenoamino acids, as reactive centers, took part in the redox reaction. As a result, the induction period was prolonged. However, as a diselenide, selenocystine can reduce bromate in acidic medium, which led to shortening of the induction period. Therefore, the effect of selenocystine on the induction period was the result of two opposite effects. Nanoselenium shortened the induction period in a certain concentration range because bromate was directly reduced by nanoselenium and the accumulation of bromomalonic acid was promoted. Furthermore, the dose perturbation effect was investigated by the injection of nanoselenium during oscillation. It was found that the amplitude was increased or decreased in a dose-dependent fashion when nanoselenium was injected at peak or trough of the time-dependent redox potential curve.

A comparison of Monte Carlo, anisotropic analytical algorithm (AAA) and Acuros XB algorithms in assessing dosimetric perturbations during enhanced dynamic wedged radiotherapy deliveries in heterogeneous media

AbstractBackgroundA comparison of anisotropic analytical algorithm (AAA) and Acuros XB (AXB) dose calculation algorithms with Electron Gamma Shower (EGSnrc) Monte Carlo (MC) for modelling lung and bone heterogeneities encountered during enhanced dynamic wedged (EDWs) radiotherapy dose deliveries was carried out.Materials and methodsIn three heterogenous slab phantoms: water–bone, lung–bone and bone–lung, wedged percentage depth doses with EGSnrc, AAA and AXB algorithms for 6 MV photons for various field sizes (5×5, 10×10 and 20×20 cm2) and EDW angles (15°, 30°, 45° and 60°) have been scored.ResultsFor all the scenarios, AAA and AXB results were within ±1% of the MC in the pre-inhomogeneity region. For water–bone AAA and AXB deviated by 6 and 1%, respectively. For lung–bone an underestimation in lung (AAA: 5%, AXB: 2%) and overestimation in bone was observed (AAA: 13%, AXB: 4%). For bone–lung phantom overestimation in bone (AAA: 7%, AXB: 1%), a lung underdosage (AAA: 8%, AXB: 5%) was found. Post bone up to 12% difference in the AAA and MC results was observed as opposed to 6% in case of AXB.ConclusionThis study demonstrated the limitation of the AAA (in certain scenarios) and accuracy of AXB for dose estimation inside and around lung and bone inhomogeneities. The dose perturbation effects were found to be slightly dependent on the field size with no obvious EDW dependence.

Dosimetric Effects of Head and Neck Immobilization Devices on Multi-field Intensity Modulated Radiation Therapy for Nasopharyngeal Carcinoma.

Background: In practice, the dose perturbation effect of head and neck immobilization devices is often overlooked in intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC). Purpose of this study is to verify and analyze the dosimetric effect of head and neck immobilization devices on NPC multi-field IMRT.Methods: Ten patients with nasopharyngeal carcinoma were randomly selected. Two sets of body contours were established for each patient. One set of body contours did not contain the immobilization device, and the other contour set included the immobilization device. For each patient, dose calculations were conducted for the two sets of contours using the same 9-field IMRT plan, which were recorded as Plan- and Plan+. The dose difference caused by the head and neck immobilization devices was assessed by comparing the dose-volume histogram (DVH) parameter results and by plan subtraction. The gafchromic EBT3 film and anthropomorphic phantom were used to verify the calculated doses.Results: The target coverage and average dose of Plan+ were lower than those of Plan- : the prescription dose coverage rates for PTVnx, PTVnd, PTV1 and PTV2 decreased by 2.4%, 9.9%, 1.5%, and 3.6%, respectively, and the mean doses were reduced by 0.9%, 1.9%, 1.1%, and 1.5%, respectively. Doses in the organs at risk showed no significant differences or slight reductions (the maximum reduction in mean dose was 1.7%). From the EBT3 measurements, the skin dose on the posterior neck was increased by approximately 53%.Conclusion: The attenuation and bolus effects of the head and neck immobilization device reduce dose coverage rate and average dose of the planning target volumes in nasopharyngeal carcinoma and lead to an increase in the skin dose. During treatment planning and dose calculation, the immobilization device should be included within body contour to account for the dose attenuation and skin dose increment.

Investigation of the dose perturbation effect for therapeutic beams with the presence of a 1.5 T transverse magnetic field in magnetic resonance imaging-guided radiotherapy.

Magnetic resonance imaging (MRI)-guided radiotherapy is a promising image-guided cancer radiotherapy method. For MRI-guided radiotherapy, the proper energy of a therapeutic beam is important for beam-designing processes, and the magnetic-induced dose perturbation would be mainly influenced, especially the perturbation surrounding the tissue-air or air-tissue interfaces. Thus, it was necessary to investigate the impact of beam energy from photon, proton, and carbon ion beams on the magnetic-induced dose perturbations. Using a phantom of a water-air-water structure, the dose distributions were calculated with or without the presence of a 1.5 T uniform magnetic field through GEANT4. Based on the calculated doses, magnetic-induced dose perturbations were then obtained. For investigating the effects of beam energies on magnetic-induced dose perturbations, low-, middle-, and high-beam energies were adopted for each beam type. For photon beams, the dose perturbations were increased as the beam energies increased. At the up water-air interface, the maximum perturbations exceeded 50%. Near the edge of the radiation field, perturbations of 5%-20% were achieved. For proton and carbon ion beams, their Bragg peaks were shifted from original positions, and the shifting distances were increased with the increased beam energies. However, no evident magnetic-induced dose perturbations were noted at the up water-air interface and bottom air-water interface for all the beam energies. To some extent, this study provided references for assessing the effects of beam energies on magnetic-induced dose perturbations, especially the perturbations around the air cavities inside cancer patients. In MRI-guided cancer radiotherapy, the dose perturbation effects for therapeutic beams are relatively obvious, and the beam energies of therapeutic beams have large impacts on the magnetic-induced dose perturbations with the presence of a 1.5 T transverse magnetic field.

Perturbation effects of the carbon fiber‐PEEK screws on radiotherapy dose distribution

Radiation therapy, in conjunction with surgical implant fixation, is a common combined treatment in cases of bone metastases. However, metal implants generally used in orthopedic implants perturb radiation dose distributions. Carbon‐Fiber Reinforced Polyetheretherketone (CFR‐PEEK) material has been recently introduced for production of intramedullary nails and plates. The purpose of this work was to investigate the perturbation effects of the new CFR‐PEEK screws on radiotherapy dose distributions and to evaluate these effects in comparison with traditional titanium screws. The investigation was performed by means of Monte Carlo (MC) simulations for a 6 MV photon beam. The project consisted of two main stages. First, a comparison of measured and MC calculated doses was performed to verify the validity of the MC simulation results for different materials. For this purpose, stainless steel, titanium, and CFR‐PEEK plates of various thicknesses were used for attenuation and backscatter measurements in a solid water phantom. For the same setup, MC dose calculations were performed. Next, MC dose calculations for titanium, CFR‐PEEK screws, and CFR‐PEEK screws with ultrathin titanium coating were performed. For the plates, the results of our MC calculations for all materials were found to be in good agreement with the measurements. This indicates that the MC model can be used for calculation of dose perturbation effects caused by the screws. For the CFR‐PEEK screws, the maximum dose perturbation was less than 5%, compared to more than 30% perturbation for the titanium screws. Ultrathin titanium coating had a negligible effect on the dose distribution. CFR‐PEEK implants have good prospects for use in radiotherapy because of minimal dose alteration and the potential for more accurate treatment planning. This could favorably influence treatment efficiency and decrease possible over‐ and underdose of adjacent tissues. The use of such implants has potential clinical advantages in the treatment of bone metastases.

Backscatter dose effects for high atomic number materials being irradiated in the presence of a magnetic field: A Monte Carlo study for the MRI linac.

To quantify and explain the backscatter dose effects for clinically relevant high atomic number materials being irradiated in the presence of a 1.5 T transverse magnetic field. Interface effects were investigated using Monte Carlo simulation techniques. We used gpumcd (v5.1) and geant4 (v10.1) for this purpose. gpumcd is a commercial software written for the Elekta AB, MRI linac. Dose was scored using gpumcd in cubic voxels of side 1 and 0.5 mm, in two different virtual phantoms of dimensions 20 × 20 × 20 cm and 5 × 5 × 13.3 cm, respectively. A photon beam was generated from a point 143.5 cm away from the isocenter with energy distribution sampled from a histogram representing the true Elekta, MRI linac photon spectrum. A slab of variable thickness and position containing either bone, aluminum, titanium, stainless steel, or one of the two different dental filling materials was inserted as an inhomogeneity in the 20 × 20 × 20 cm phantom. The 5 × 5 × 13.3 cm phantom was used as a clinical test case in order to explain the dose perturbation effects for a head and neck cancer patient. The back scatter dose factor (BSDF) was defined as the ratio of the doses at a given depth with and without the presence of the inhomogeneity. Backscattered electron fluence was calculated at the inhomogeneity interface using geant4. A 1.5 T magnetic field was applied perpendicular to the direction of the beam in both phantoms, identical to the geometry in the Elekta MRI linac. With the application of a 1.5 T magnetic field, all the BSDF's were reduced by 12%-47%, compared to the no magnetic field case. The corresponding backscattered electron fluence at the interface was also reduced by 45%-64%. The reduction in the BSDF at the interface, due to the application of the magnetic field, is manifested in a different manner for each material. In the case of bone, the dose drops at the interface contrary to the expected increase when no magnetic field is applied. In the case of aluminum, the dose at the interface is the same with and without the presence of the aluminum. For all of the other materials the dose increases at the interface. The reduction in dose at the interface, in the presence of the magnetic field, is directly related to the reduction in backscattered electron fluence. This reduction occurs due to two different reasons. First, the electron spectrum hitting the interface is changed when the magnetic field is turned on, which results in changes in the electron scattering probability. Second, some electrons that have curved trajectories due to the presence of the magnetic field are absorbed by the higher density side of the interface and no longer contribute to the backscattered electron fluence.

On the accuracy of dose prediction near metal fixation devices for spine SBRT.

The metallic fixations used in surgical procedures to support the spine mechanically usually consist of high‐density materials. Radiation therapy to palliate spinal cord compression can include prophylactic inclusion of potential tumor around the site of such fixation devices. Determination of the correct density and shape of the spine fixation device has a direct effect on the dose calculation of the radiation field. Even with the application of modern computed tomography (CT), under‐ or overestimation of dose, both immediately next to the device and in the surrounding tissues, can occur due to inaccuracies in the dose prediction algorithm. In this study, two commercially available dose prediction algorithms (Eclipse AAA and ACUROS), EGSnrc Monte Carlo, and GAFchromic film measurements were compared for a clinical spine SBRT case to determine their accuracy. An open six‐field plan and a clinical nine‐field IMRT plan were applied to a phantom containing a metal spine fixation device. Dose difference and gamma analysis were performed in and around the tumor region adjacent to the fixation device. Dose calculation inconsistency was observed in the open field plan. However, in the IMRT plan, the dose perturbation effect was not observed beyond 5 mm. Our results suggest that the dose effect of the metal fixation device to the spinal cord and the tumor volume is not observable, and all dose calculation algorithms evaluated can provide clinically acceptable accuracy in the case of spinal SBRT, with the tolerance of 95% for gamma criteria of 3%/3 mm.PACS number(s): 87.53.bn, 87.53.Ly, 87.55.kd

An investigation on the dosimetric impact of hip prosthesis in radiotherapy

The objectives of this study are to evaluate the dosimetric impact of hip prosthesis using ELEKTA linac X-rays and compare the dose perturbations of metallic and nonmetallic hip prostheses. The local dose distributions of typical hip prostheses were calculated for 4, 8, and \(15\,\hbox {MV}\) beams by Geant4. Three prosthesis materials were selected in calculation to reveal the relation between material type and local dose perturbations of prostheses. Furthermore, the effect of nominal energy on prosthesis perturbation was also discussed and analyzed. Taking the calculated dose to the hip joint as reference, considerable differences were observed between prostheses and hip joints. In the prosthesis shadow region, the relative dose decreasing was up to 36, 21, and 16 % for the Co–Cr–Mo alloy, titanium alloy, and ceramic prostheses, respectively. In backscattering region, the relative dose increasing was about 1–7 %. Overall, the results show that the dose perturbation effect of prostheses was mainly determined by material type, nominal energy, and density. Among these typical hip prostheses, ceramic prosthesis introduces the lowest dose perturbations.

Dose perturbation effect of metallic spinal implants in proton beam therapy.

The purpose of this study was to investigate the effect of dose perturbations for two metallic spinal screw implants in proton beam therapy in the perpendicular and parallel beam geometry. A 5.5 mm (diameter) by 45 mm (length) stainless steel (SS) screw and a 5.5 mm by 35 mm titanium (Ti) screw commonly used for spinal fixation were CT‐scanned in a hybrid phantom of water and solid water. The CT data were processed with an orthopedic metal artifact reduction (O‐MAR) algorithm. Treatment plans were generated for each metal screw with a proton beam oriented, first parallel and then perpendicular, to the longitudinal axis of the screw. The calculated dose profiles were compared with measured results from a plane‐parallel ion chamber and Gafchromic EBT2 films. For the perpendicular setup, the measured dose immediately downstream from the screw exhibited dose enhancement up to 12% for SS and 8% for Ti, respectively, but such dose perturbation was not observed outside the lateral edges of the screws. The TPS showed 5% and 2% dose reductions immediately at the interface for the SS and Ti screws, respectively, and up to 9% dose enhancements within 1 cm outside of the lateral edges of the screws. The measured dose enhancement was only observed within 5 mm from the interface along the beam path. At deeper depths, the lateral dose profiles appeared to be similar between the measurement and TPS, with dose reduction in the screw shadow region and dose enhancement within 1–2 cm outside of the lateral edges of the metals. For the parallel setup, no significant dose perturbation was detected at lateral distance beyond 3 mm away from both screws. Significant dose discrepancies exist between TPS calculations and ion chamber and film measurements in close proximity of high‐Z inhomogeneities. The observed dose enhancement effect with proton therapy is not correctly modeled by TPS. An extra measure of caution should be taken when evaluating dosimetry with spinal metallic implants.PACS number: 87.50.sj

SU‐E‐T‐378: Evaluation of An Analytical Model for the Inter‐Seed Attenuation Effect in 103‐Pd Multi‐Seed Implant Brachytherapy

Purpose:Brachytherapy treatment planning systems based on TG‐43 protocol calculate the dose in water and neglects the heterogeneity effect of seeds in multi‐seed implant brachytherapy. In this research, the accuracy of a novel analytical model that we propose for the inter‐seed attenuation effect (ISA) for 103‐Pd seed model is evaluated.Methods:In the analytical model, dose perturbation due to the ISA effect for each seed in an LDR multi‐seed implant for 103‐Pd is calculated by assuming that the seed of interest is active and the other surrounding seeds are inactive. The cumulative dosimetric effect of all seeds is then summed using the superposition principle. The model is based on pre Monte Carlo (MC) simulated 3D kernels of the dose perturbations caused by the ISA effect. The cumulative ISA effect due to multiple surrounding seeds is obtained by a simple multiplication of the individual ISA effect by each seed, the effect of which is determined by the distance from the seed of interest. This novel algorithm is then compared with full MC water‐based simulations (FMCW).Results:The results show that the dose perturbation model we propose is in excellent agreement with the FMCW values for a case with three seeds separated by 1 cm. The average difference of the model and the FMCW simulations was less than 8%±2%.Conclusion:Using the proposed novel analytical ISA effect model, one could expedite the corrections due to the ISA dose perturbation effects during permanent seed 103‐Pd brachytherapy planning with minimal increase in time since the model is based on multiplications and superposition. This model can be applied, in principle, to any other brachytherapy seeds. Further work is necessary to validate this model on a more complicated geometry as well.

Poster — Thur Eve — 21: Off‐axis dose perturbation effects in water in a 5 × 5 cm2 18 MV photon beam for the PTW microLion and Exradin A1SL ionization chambers

A PTW microLion liquid ionization chamber and an Exradin A1SL air‐filled ionization chamber have been modeled using the egs_chamber user code of the EGSnrc system to determine their perturbation effects in water in a 5 × 5 cm2 18 MV photon beam. A model of the Varian CL21EX linear accelerator was constructed using the BEAMnrc Monte Carlo code, and was validated by comparing measured PDDs and profiles from the microLion and A1SL chambers to calculated results that included chamber models. Measured PDDs for a 5 × 5 cm2 field for the microLion chamber agreed with calculations to within 1.5% beyond a depth of 0.5 cm, and the A1SL PDDs agreed within 1.0% beyond 1.0 cm. Measured and calculated profiles at 10 cm depth agreed within 1.0% for both chambers inside the field, and within 4.0% near the field edge. Local percent differences increased up to 15% at 4 cm outside the field. The ratio of dose to water in the absence of the chamber relative to dose in the chamber's active volume as a function of off‐axis distance was calculated using the egs_chamber correlated sampling technique. The dose ratio was nearly constant inside the field and consistent with the stopping power ratios of water to detector material, but varied up to 3.3% near the field edge and 5.2% at 4 cm outside the field. Once these perturbation effects are fully characterized for more field sizes and detectors, they could be applied to clinical water tank measurements for improved dosimetric accuracy.

SU‐E‐T‐280: Reconstructed Rectal Wall Dose Map‐Based Verification of Rectal Dose Sparing Effect According to Rectum Definition Methods and Dose Perturbation by Air Cavity in Endo‐Rectal Balloon

Purpose:Dosimetric effect and discrepancy according to the rectum definition methods and dose perturbation by air cavity in an endo‐rectal balloon (ERB) were verified using rectal‐wall (Rwall) dose maps considering systematic errors in dose optimization and calculation accuracy in intensity‐modulated radiation treatment (IMRT) for prostate cancer patients.Methods:When the inflated ERB having average diameter of 4.5 cm and air volume of 100 cc is used for patient, Rwall doses were predicted by pencil‐beam convolution (PBC), anisotropic analytic algorithm (AAA), and AcurosXB (AXB) with material assignment function. The errors of dose optimization and calculation by separating air cavity from the whole rectum (Rwhole) were verified with measured rectal doses. The Rwall doses affected by the dose perturbation of air cavity were evaluated using a featured rectal phantom allowing insert of rolled‐up gafchromic films and glass rod detectors placed along the rectum perimeter. Inner and outer Rwall doses were verified with reconstructed predicted rectal wall dose maps. Dose errors and extent at dose levels were evaluated with estimated rectal toxicity.Results:While AXB showed insignificant difference of target dose coverage, Rwall doses underestimated by up to 20% in dose optimization for the Rwhole than Rwall at all dose range except for the maximum dose. As dose optimization for Rwall was applied, the Rwall doses presented dose error less than 3% between dose calculation algorithm except for overestimation of maximum rectal dose up to 5% in PBC. Dose optimization for Rwhole caused dose difference of Rwall especially at intermediate doses.Conclusion:Dose optimization for Rwall could be suggested for more accurate prediction of rectal wall dose prediction and dose perturbation effect by air cavity in IMRT for prostate cancer.This research was supported by the Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT &amp; Future Planning (MSIP) (Grant No. 200900420)

Experimental and Monte Carlo evaluation of Eclipse treatment planning system for effects on dose distribution of the hip prostheses

The present study aimed to investigate the effects of titanium, titanium alloy, and stainless steel hip prostheses on dose distribution based on the Monte Carlo simulation method, as well as the accuracy of the Eclipse treatment planning system (TPS) at 6 and 18MV photon energies. In the present study the pencil beam convolution (PBC) method implemented in the Eclipse TPS was compared to the Monte Carlo method and ionization chamber measurements. The present findings show that if high-Z material is used in prosthesis, large dose changes can occur due to scattering. The variance in dose observed in the present study was dependent on material type, density, and atomic number, as well as photon energy; as photon energy increased back scattering decreased. The dose perturbation effect of hip prostheses was significant and could not be predicted accurately by the PBC method for hip prostheses. The findings show that for accurate dose calculation the Monte Carlo-based TPS should be used in patients with hip prostheses.

SU-E-T-502: Dose Perturbation Effects Near Implant Surfaces Caused by Secondary Electron Transport in Photon-Beam Therapy.

To investigate the dose perturbation effects at interfaces between water and a Titanium implant, attributable to secondary electron transport across the interface, during high energy photon radiotherapy. While dose enhancement is characteristic for the proximal interface of a high-atomic number implant, the dose perturbation at the distal interface varies from reduction to enhancement, requiring proper computation of secondary electron transport effects. The backward and forward perturbation factors pb and pf will be calculated. Using DOSRZnrc, depth dose curves were computed in a water phantom using photon spectra of nominal energies 4, 6, 10, 15, 24 MV for conditions (i) homogeneous water without any insert, (ii) alternatively with Titanium inserts of thicknesses 3 and 5 cm placed at 10 cm water depth. Backscatter factor pb was computed as the ratio of the dose with implant against that without implant, whereas pf was calculated by first accounting for photon attenuation in the implant and then taking the ratio of the dose with implant against that without implant. At the front interface, pb is independent of the material thickness and varies slightly with beam energy and incident angle. On consideration of photon attenuation in the implant, pf was also found to be independent on material thickness, but strongly varying with energy, including change of sign. For 4-24 MV photon beams the maximum spread of the dose perturbation effect remains within only a few millimeters from the interface, with pb values ranging from 1.18-1.22, while factor pf ranges from 0.9-1.21 at normal incidence, indicating the extent to which planning systems may over- or underestimate the doses near implant interfaces. At inclined beam incidence the dose perturbation effects even increase, and for instance pb (1.24-1.25) and pf (0.85-1.32) were determined for 6 MV and 24 MV beams at 45° incidence.