C Lee

Betel Chewing and Arecoline Affects Eotaxin-1, Asthma and Lung Function

Background Betel nut is commonly used in many countries. Despite evidence suggesting an association with asthma, few studies have investigated the connection between betel nut use and asthma; thus, the underlying mechanism for the association with asthma is also unclear. The aim of this study was to investigate the association between betel chewing and asthma as well as the associations of plasma arecoline (a biomarker for exposure) and eotaxin-1 (a potential mediator) with asthma and lung function. Methods We recruited 600 hospital-based asthmatic patients and 1200 age- and gender-matched community controls in southern Taiwan. To clarify the mechanism of action for eotaxin-1 in the association between betel chewing and asthma, we also designed an in vitro experiment to study the functional associations between arecoline exposure and eotaxin-1 levels. Results A significant association was found between asthma and current betel chewing (adjusted odds ratio 2.05, 95% CI = 1.12–3.76), which was independent of potential confounders but was attenuated following adjustment for eotaxin-1. Arecoline and eotaxin-1 levels were positively correlated (Spearman r = 0.303, p = 0.02), while arecoline and arecaidine were negatively correlated with lung function. Functionally, arecoline alone does not induce eotaxin-1 release in vitro from dermal and gingival fibroblasts. However, in the presence of IL-4 and TNF-alpha, arecoline at 100 μg/ml induced more eotaxin-1 release than arecoline at 0 μg/ml (2700±98 pg/ml vs 1850±142 pg/ml, p = 0.01 in dermal fibroblast cells, and 1489±78 pg/ml vs 1044±95 pg/ml, p = 0.03 in gingival fibroblast cells, respectively). Conclusion Betel chewing is associated with asthma in this population, with arecoline induction of eotaxin-1 supported as a plausible causal pathway.

SU‐FF‐T‐182: Analysis of Systematic Uncertainty in Vivo Dosimetry Using Diodes

Purpose: This study analyzes systematic uncertainty of in vivo dosimetry using diodes. Several possible sources of error are investigated to eliminate or correct systematic errors. Method and Materials: Two diodes are placed on both sides of a phantom/patient to measure entrance and exit doses. The midline dose can thus be derived from these entrance and exit doses. We selected patients with tumors in their midline and with geometrically symmetric structures Several experiments are specially designed to measure response variation of diodes (1) due to temperature change, (2) with/without mask presence, (3) with different types of tapes used to fix the diodes, (4) with the space between mask and skin, (5) with the off field edge distance, and (6) with inhomogeneity of the patients. Results: The systematic uncertainty for measurement with/without the mask is 1%. The systematic uncertainty for temperature change is 3∼4%. The systematic uncertainty from different types of tapes is 1% .The systematic uncertainty due to space between the mask and the skin is less than 2%. If the diodes were placed off edge for distances greater than one‐fourth of the field size, the systematic uncertainty is less than 2%. Conclusion: Temperature change may be an important source of error for in vivo dosimetry using diodes. Other sources of error we investigated produced uncertainties less than 2 %.

SU‐E‐T‐323: A CT‐Based Monte Carlo Dose Calculation for Correction of Metal Artifacts Due to the Henschke Applicator

Purpose:Monte Carlo method is the most accurate dose calculation system in the modern world. And we use it to evaluate the influence of computed tomography(CT) metal artifacts resulted from the Henschke applicator in intracavitary brachytherapydose distribution. Methods: The CTimages of the Plastic Water® phantom with Henschke applicator and the applicator associated metal artifacts are applied to the Monte Carlo N‐Particle Transport Code X (MCNPX) for dose calculation. The accuracy of the CTimages input is assessed by the phantom dose simulation with and without CTimages based on our previous Monte Carlo parameters optimization. To reduce the effect of artifacts, the HU number of metal artifacts and metal materials in CTimages are replaced by phantom HU number according to the redefined HU density table. Then the Henschke applicator geometry is superimposed on the corrected CTimage. This corrected CTimages are applied to MCNPX for dose calculation. The dose calculation with and without metal artifacts can be analyzed for the impact of metal artifacts. Results: This system can be implemented because the dose discrepancy with and without CTimages is below 0.05%. We analyze the dose distribution with corrected CT‐based phantom for the impact of metal artifacts. The dose distribution shows the largest dose discrepancy is 8–9% and which is located at the region above 2cm distal to ovoid edge. Conclusions: In this study, the dose at the region above 2cm distal to ovoid edge is significantly under prescription dose. The artifacts resulted from Henschke applicator leads to a certainly inaccurate dose calculation in the planning system. Thus, dose distribution must be evaluated before using new applicator. The CT‐based Monte Carlo method can be used to evaluate the influence of CT metal artifacts in intracavity brachytherapy.

SU‐E‐T‐661: Study on Rebuildup after Air Cavity with the Monte Carlo Technique and Ultra‐Thin TLD Dosimeters

Purpose: The purpose of the study is to investigate the re‐buildup after air cavity of various thicknesses for 6MV therapeuticphoton beams with different field sizes. Methods: The BEAMnrc code was chosen for Monte Carlo(MC) dose simulation. Complete simulation includes all geometry and components of the treatment head of our Varian 21EX linear accelerator and beam propagation within the water phantom. 6 MV photon beams with field sizes at 3×3 and 10×10 cm2 were studied. Air cavities with thickness 1 and 5 cm were inserted at 4 cm depth. The simulated re‐buildup dose distribution was compared to both those from ultra‐thin TLD measurement (Rexon TLD‐100H, 4.5mm × 0.1mm) and treatment planning calculation (Eclipse, Varian Medical Systems, Palo Alto, CA) using the AAA algorithm.Results: According to the MC simulation results, dose reductions at the re‐buildup surface for 10×10 cm2 fields are 4.6% and 20.5% for 1 and 5 cm air cavities, respectively. Those for the 3×3 cm2 fields are 29.0% and 77.7% for 1 and 5 cm air cavities, respectively. The differences between TLD measurement and MC simulation are within the detector uncertainty. The results indicate that the treatment planning calculation shows minimum ability to model the re‐buildup phenomenon. Conclusions: Dose re‐buildup near an air cavity is a possible cause of target underdose. Results from the current study indicate that it is more profound for large cavity thicknesses and small irradiation fields. Ultra‐thin TLD was also proven an effective detector for measuring dose distribution near re‐buildup region.

SU‐GG‐T‐221: Clinical Implementation of Monte Carlo Simulation for RapidArc Dosimetry Verification

Purpose: The RapidArc technique, which increases the treatment efficiency with reduced monitor units (MU), has been increasingly applied clinically. However, the small isolated fields and the highly irregular field shapes are challenging for conventional dose calculation algorithms employed in commercial treatment planning systems (TPS). Owing to the ability to handle heterogeneity and small fields, Monte Carlo simulation has been recognized as the benchmark method for radiotherapydose calculation. This study aims to demonstrate the performance of TPS calculation on head and neck, abdomen, and pelvis RapidArc treatments using Monte Carlo simulations.Method and Materials: To simplify the simulation process, Monte Carlodose computation was based on the reconstructed fluence distributions between control points. The dosimetric features of the MLC including rounded leaf ends, intra‐leaf transmission and the tongue and groove effect were included in our simulation. Ten RapidArc patients with different treatment sites were recruited for this retrospective study. The dose distributions and the dose volume histograms (DVH) calculated by the Monte Carlo simulations were compared with TPS calculations. Results: The isodose comparisons between TPS and Monte Carlo simulation showed good agreement except for the highly heterogeneous region and extremely complex treatment geometry. For the DVH comparison, overall, the mean dose of the PTV calculated by the Monte Carlo agreed with the TPS calculation to within 2%. The dose to the critical organs calculated by the TPS was higher than that by Monte Carlo simulations.Conclusion: This study evaluated ten patients treated by the RapidArc technique to demonstrate the performance of the TPS dose calculation for different treatment sites. It is suggested that for the treatment sites with severe heterogeneity, Monte Carlo simulations are necessary.

SU‐FF‐J‐166: The Development of a Precise Electron Irradiator for Small Animal Studies

Purpose: This study focuses on the development, validation, and evaluation of a precise MV‐electron irradiator. Compared with all other prototypes which use photons to irradiate small animals, an electron irradiator has many advantages in its shallow dose distribution. Method and Materials: Two major approaches including simulation and measurement were used to evaluate the feasibility of an electron animal irradiator. These simulations/measurements were taken in three different fields (a 6 cm square field, and 30 and 4 mm diameter circular fields) and with two different energies (6 and 18 MeV). A PTW Semiflex chamber i n a PTW‐MP3 water tank, a PTW Markus chamber type 23343, a PTW diamonddetector type 60003, and KODAK XV films were used to measure PDDs, lateral beam profiles, and output factors for either optimizing parameters of Monte Carlo simulation or to verify Monte Carlo simulation in small fields. Results: Results show good agreement for the comparisons of PDD (⩽ 2.5% for 6 MeV e; ⩽ 1.8% for 18 MeV e) and profiles (FWHM ⩽ 0.5 mm) between simulations and measurements on the 6 cm field. Greater deviation can be observed in the 4 mm field due to the partial volume effects of detectors. The output factor for the 18 MeV electron beam is 0.970 in the 30 mm field, and 0.610 in the 4 mm field; the FWHM of profiles for the 18 MeV electron beam is 32.6 mm in the 30 mm field, and 4.7 mm in the 4 mm field at the d 90. Two different digital phantoms were also constructed, including a homogeneous cylindrical water phantom and a CT‐based heterogeneous mouse phantom, and were implemented into Monte Carlo to simulate the dose distribution with different electron irradiations.Conclusion: These results ensure that our proposed electron irradiator is feasible for precise small animal irradiation.

SU‐GG‐T‐145: Dose Response of Spectral Change of AS1000 EPID

Purpose:Amorphous siliconelectronic portal imaging devices have been used as 2D dosimeters due to its excellent dose linearity response. However, dose response (cGy/signal) may be spectra‐dependent when patients are in the beam. The aim of this study was to evaluate the dosimetric sensitivity of a Varian aS1000 EPID to spectral change caused by the presence of a patient and give conclusive recommendation for clinical applications. Method and Materials: The signal‐to‐absolute‐dose response curves of the EPID were established for 0 to 40 cm thick of solid water phantoms on couch. The thickness can well cover the water equivalent thicknesses of most patients from head (11 to 19 cm) to pelvis (AP 15 to 25 cm, lateral 26 to 37 cm). Monte Carlo simulation (BEAMnrc 2006) was used to evaluate spectral variation at the EPID plane. Results: The results show that spectra at the EPID plane change dramatically for phantom thickness from 0 to 5 cm, and become more stable with further increased phantom thickness. Direct measurements have shown that large discrepancy up to 9.6% can be found for dose response within phantom thickness between 0 and 30 cm. It was also shown that the max variation between 10 to 40 cm which cover thicknesses of head and pelvis, 10 to 20 cm which cover thicknesses of head, 15 to 40 cm which cover thicknesses of pelvis, were 2.7%, 1.3%, 1.1% respectively. Conclusion: We thus concluded that for clinical absolute dose measurements, opened field dosimetry (without patient in the beam) and transit dosimetry should have identical dose response curves. From the different variations of dose response among 10 to 40 cm, 10 to 20 cm, and 15 to 40 cm, we suggest that for transit dosimetry, head and pelvis should have its own one dose response curve.

SU‐GG‐T‐261: Monte Carlo Simulation of Small Electron Fields for Small Animal Irradiation

Purpose: This work studies beam characteristics of small electron fields using Monte Carlo methods and evaluate its feasibility for small animal irradiation.Method and Materials: Small electron fields with diameters 4, 6, 14 and 30 mm were generated with 1.5‐cm‐thick Cerrobend inserts on a Varian 2100 C/D linear accelerator. Full simulations were performed using the BEAMnrc and DOSXYZ codes for geometries including the treatment head, electron cones, block inserts and water phantom. Input parameters of the initial energy, angular and spacial distrbutions at the beam exit window for 6 and 18 MeV beams were optimized to insure depth dose discrepancy with measurement to less than 2 % for a 6 × 6 cm field. Dependence of PDDs, lateral profiles and beam outputs at dmax on beam energy, beam diameter and photon jaw setting were studied. Results: Results revealed that photon jaw setting has minimum impact on lateral dose distributions except for, however, it can changes beam outputs dramatically. With photon jaws fixed at 3 × 3 cm, relative output for 4, 6, 14 and 30 mm inserts are 0.13, 0.26, 0.64, 0.88 for 6 MeV and 0.61, 0.78, 0.92, 0.97 for 18 MeV beams when normalized to the output for a open 6 × 6 cm cone. Two effects of reducing beam sizes on PDD were observed: (1) dmax shiftes toward the surface and (2) the beams become less penetrating. For beam energy selection, 18 MeV beams can generate broder area of uniform dose distribution than 6 MeV beams which makes them more favorable for small animal irradiation.Conclusion: Investigations were performed to study beam characteristics of small electron fields. We have concluded that 18 MeV beams are more favorable than 6 MeV beams for small animal irradiation.

SU‐FF‐I‐117: System Quantification for Micro CT

Purpose: To report our efforts of the development for the software tool with the latest parallel computing techniques to characterize the various physical properties for the micro CT.Method and Materials: Various physical aspects in the micro CTimager were investigated with the strategies of mathematical modeling and computer simulation. The simulated elements included were the x‐ray focal spot blurring, scattering kernel, point spread function, quantum noise, detector blurring, and system noise. The projections of the mathematical phantoms were analytically calculated using the Radon transform formalism. Simulated noises were then added to those projection images and the final images were obtained from the image reconstruction algorithms including the exact, approximate, and iterative methods. A dedicated random number generator for the noise simulation was developed to produce un‐correlated random number sequences. A randomness test of the intra‐ and inter‐correlation for the random number sequences was also developed to validate the quality of noise simulation. The Monte Carlo code, BEAMnrc06, was used to evaluate the x‐ray photon scattering kernel, point spread function, and the simulated exposed dose level. Results: Our preliminary results indicated that the various noiseproperties can be quantified by using the noise simulation and noise power spectrum calculations. The scattering kernel was determined by using the Monte Carlo method according to the actual x‐ray tube geometry and was in agreement with the experimental measurement. Lower bound of the delivered dose level was determined based on the simulated low‐dose imagesreconstructed form the higher‐dose images. The motion artifact was also quantified based on the Fourier analysis in the spatial frequency domain. Conclusion: We have developed a computer simulation package where the key substances such as the quantum noise were included. The simulated results were used to quantify the micro CTimager and the optimized system performance can be achieved.

TU‐FF‐A2‐05: Dose Verification of 2‐, 4‐, 6‐Mm Cones for Stereotactic Radiosurgery Radiotherapy

Purpose: Small field dosimetry is critical in radiosurgery. Many researchers have reported the dose verification results for cones larger than 5 mm. Cones smaller than 5 mm, though are not common used in Stereotactic Radiosurgery, have the potential in benign tumor radiosurgery and small animal irradiation. This study demonstrates dose verification results for 2‐, 4‐, and 6‐mm cones using several different detectors and determines their maximum spatial resolution for small fields without lateral electronic equilibrium. Method and Materials: Several different small‐volume detectors including a small volume ion chamber (PTW 23323), a pin‐point chamber (PTW31014), a PTW diamonddetector (faced parallel or perpendicular), KODAK XV‐film (scanned in 0.1 mm resolution), and BEAMnrc06 Monte Carlo(MC) simulation were used to study percentage depth doses, profiles, and cone factors for 2‐, 4‐, and 6‐mm cones. Results: All detectors agree well with MC simulation for larger cones (30 mm or 14 mm). Notable dose deviation between small volume and pin point ion chambers can be observed for 6 mm cone. The dose measured using diamonddetector faced perpendicular does not agree well with that measured using film, MC simulation, and diamonddetector faced parallel. Finally, neither detectors nor simulation agree with measurement for 2‐mm cone. In fact, this purported 2‐mm cone is assumed to be 3.1 mm since the PDDs, profile, and cone factor measured using film and diamonddetector faced parallel agree well with MC simulation of a 3.1‐mm cone. Conclusion: Films and the diamonddetector faced parallel are more suitable for small field dosimetry. Noting that the ECUT should be reduced to 521 keV for simulating cones smaller than 1 cm, the dose deviation of PDDs and profiles between MC and measurement (film, diamond) are less than 1.5%.