Mm Of Distance Research Articles

Effect of Screw Insertion Torque on Push‐Out and Cantilever Bending Properties of Five Different Angle‐Stable Systems

To compare the screw push-out strength and resistance to cantilever bending of 5 different angle-stable systems using 4 different insertion torque values to tighten locking screws. In vitro mechanical testing of 5 screw-plate constructs. Screw plate constructs (n = 60) were tested; 12 of each design, 3 for each torque value. To compare push-out strength, screws were loaded in axial direction on the screw tip until loosening of the locking mechanism was recorded. For cantilever bending test, screws were loaded perpendicularly to their longitudinal axis at 2 mm of distance from the under surface of the plate. Load was applied in displacement control at 1 mm/min. There was a significant difference between the 5 different angle-stable systems regarding both push-out and cantilever bending strength. There was an influence of insertion torque value on push-out strength for 2 systems and insertion torque value influenced cantilever bending behavior only in 1 locking system. Locking mechanisms using "thread in thread" principle provided a stronger screw push-out behavior. Screws materials and core diameter of the different screws were directly related to cantilever bending strength.

Feasibility study on dosimetry verification of volumetric‐modulated arc therapy‐based total marrow irradiation

The purpose of this study was to develop dosimetry verification procedures for volumetric‐modulated arc therapy (VMAT)‐based total marrow irradiation (TMI). The VMAT based TMI plans were generated for three patients: one child and two adults. The planning target volume (PTV) was defined as bony skeleton, from head to mid‐femur, with a 3 mm margin. The plan strategy similar to published studies was adopted. The PTV was divided into head and neck, chest, and pelvic regions, with separate plans each of which is composed of 2–3 arcs/fields. Multiple isocenters were evenly distributed along the patient's axial direction. The focus of this study is to establish a dosimetry quality assurance procedure involving both two‐dimensional (2D) and three‐dimensional (3D) volumetric verifications, which is desirable for a large PTV treated with multiple isocenters. The 2D dose verification was performed with film for gamma evaluation and absolute point dose was measured with ion chamber, with attention to the junction between neighboring plans regarding hot/cold spots. The 3D volumetric dose verification used commercial dose reconstruction software to reconstruct dose from electronic portal imaging devices (EPID) images. The gamma evaluation criteria in both 2D and 3D verification were 5% absolute point dose difference and 3 mm of distance to agreement. With film dosimetry, the overall average gamma passing rate was 98.2% and absolute dose difference was 3.9% in junction areas among the test patients; with volumetric portal dosimetry, the corresponding numbers were 90.7% and 2.4%. A dosimetry verification procedure involving both 2D and 3D was developed for VMAT‐based TMI. The initial results are encouraging and warrant further investigation in clinical trials.PACS number: 87.55.Qr

The Accuracy of the Swallowing Kinematic Analysis at Various Movement Velocities of the Hyoid and Epiglottis

ObjectiveTo evaluate the accuracy of the swallowing kinematic analysis.MethodsTo evaluate the accuracy at various velocities of movement, we developed an instrumental model of linear and rotational movement, representing the physiologic movement of the hyoid and epiglottis, respectively. A still image of 8 objects was also used for measuring the length of the objects as a basic screening, and 18 movie files of the instrumental model, taken from videofluoroscopy with different velocities. The images and movie files were digitized and analyzed by an experienced examiner, who was blinded to the study.ResultsThe Pearson correlation coefficients between the measured and instrumental reference values were over 0.99 (p<0.001) for all of the analyses. Bland-Altman plots showed narrow ranges of the 95% confidence interval of agreement between the measured and reference values as follows: 0.14 to 0.94 mm for distances in a still image, -0.14 to 1.09 mm/s for linear velocities, and -1.02 to 3.81 degree/s for angular velocities.ConclusionOur findings demonstrate that the distance and velocity measurements obtained by swallowing kinematic analysis are highly valid in a wide range of movement velocity.

Retinoblastoma external beam photon irradiation with a special ‘D’-shaped collimator: a comparison between measurements, Monte Carlo simulation and a treatment planning system calculation

Retinoblastoma is the most common eye tumour in childhood. According to the available long-term data, the best outcome regarding tumour control and visual function has been reached by external beam radiotherapy. The benefits of the treatment are, however, jeopardized by a high incidence of radiation-induced secondary malignancies and the fact that irradiated bones grow asymmetrically. In order to better exploit the advantages of external beam radiotherapy, it is necessary to improve current techniques by reducing the irradiated volume and minimizing the dose to the facial bones. To this end, dose measurements and simulated data in a water phantom are essential. A Varian Clinac 2100 C/D operating at 6 MV is used in conjunction with a dedicated collimator for the retinoblastoma treatment. This collimator conforms a ‘D’-shaped off-axis field whose irradiated area can be either 5.2 or 3.1 cm2. Depth dose distributions and lateral profiles were experimentally measured. Experimental results were compared with Monte Carlo simulations’ run with the penelope code and with calculations performed with the analytical anisotropic algorithm implemented in the Eclipse treatment planning system using the gamma test. penelope simulations agree reasonably well with the experimental data with discrepancies in the dose profiles less than 3 mm of distance to agreement and 3% of dose. Discrepancies between the results found with the analytical anisotropic algorithm and the experimental data reach 3 mm and 6%. Although the discrepancies between the results obtained with the analytical anisotropic algorithm and the experimental data are notable, it is possible to consider this algorithm for routine treatment planning of retinoblastoma patients, provided the limitations of the algorithm are known and taken into account by the medical physicist and the clinician. Monte Carlo simulation is essential for knowing these limitations. Monte Carlo simulation is required for optimizing the treatment technique and the dedicated collimator.

MO-F-213AB-05: Commissioning of Gated RapidArc Radiotherapy for Treatment of Moving Targets.

To commission and evaluate gated RapidArc radiotherapy of a linear accelerator (Varian TrueBeam) for treatment of moving targets using a programmable dynamic phantom. The phantom used had different dosimetry inserts for measurement of dose and dose distribution. It could be programmed to move in the anterior-posterior and superior-inferior axes with different motion patterns, amplitudes and frequencies to simulate lung motions of patients. A set of 4D CT images was acquired with the aid of a Varian RPM system. Images acquired at the 40, 50 and 60% of the motion cycle were selected and transferred to a treatment planning system (Varian Eclipse) for planning. A two-arc RapidArc treatment plan was generated for a C-shaped target volume with a conformity index of 1.49 and transferred to the TrueBeam for treatment delivery. Dose and dose distribution measurements were performed using a 0.057 cc ionization chamber and radiochromic films, respectively and compared with the TPS calculations. Five treatment fractions were given in three days with two different target motion patterns to assess the consistency of the dose delivery. Agreement between TPS calculation and measurement were within 1.64% for dose and 3% or 3mm in distance to agreement for dose distribution. Repeatability of dose delivery between treatments was within 0.1% (1SD) in the five treatment fractions delivered in three days. The time required to deliver a dose of 2 Gy to a moving C-shaped target using gated RapidArc technique with two gantry rotations was about 15 minutes. The geometric and dosimetric accuracy and consistency of gated RapidArc radiotherapy had been verified. Our study indicated that the accuracy and consistency of the treatment modality were acceptable for clinical implementation.

Clinical validation of the Acuros XB photon dose calculation algorithm, a grid-based Boltzmann equation solver

Introduction. A new algorithm that uses a grid-based technique to solve the linear Boltzmann transport equation (LBTE) has been developed to improve the accuracy and speed of external photon beam treatment planning calculations. The aim of this study was to test the accuracy of this algorithm in both heterogeneous and homogeneous media. Material and methods. Output factors, depth dose curves and profiles for symmetric fields were measured in water using diamond and ionization chamber detectors. Furthermore, asymmetric fields, fields collimated with the multi-leaf collimator, enhanced dynamic wedge fields as well as fields with different source-skin distances were measured. Various test plans were created on a CIRS thorax phantom including tissue-equivalent inserts and corresponding dose distributions within the phantom were measured with radiochromic films. The new grid-based LBTE solver, Acuros XB (Eclipse version 10.0, Varian Medical Systems, CA, USA) was used to calculate dose distributions for all field configurations and plans, for both 6 MV and 15 MV photons. Calculations were also performed with AAA, a standard convolution algorithm. Results. Compared to measurements, the output factors were within 1% for Acuros XB. For the depth doses, the average deviations were within 1% in dose and 1 mm in distance to agreement (DTA). For the profiles, the deviations were within 2%/1 mm except near the penumbra. Similar results were obtained for the other field configurations. Good agreement with AAA was also found. For the plans calculated on the CIRS phantom, the number of points meeting the gamma criterion of 3% in dose and 3 mm in DTA was higher with Acuros XB (98% for 6 MV; 100% for 15 MV) than with AAA (94% for 6 MV; 96% for 15 MV). Conclusion. Dose calculations with the Acuros XB algorithm in homogeneous media are in good agreement with both measurements and the AAA algorithm. In heterogeneous media, the Acuros XB algorithm is superior to AAA in both lung and bony material.

The use of intensity-modulated radiation therapy photon beams for improving the dose uniformity of electron beams shaped with MLC

Electrons are ideal for treating shallow tumors and sparing adjacent normal tissue. Conventionally, electron beams are collimated by cut-outs that are time-consuming to make and difficult to adapt to tumor shape throughout the course of treatment. We propose that electron cut-outs can be replaced using photon multileaf collimator (MLC). Two major problems of this approach are that the scattering of electrons causes penumbra widening because of a large air gap, and available commercial treatment planning systems (TPSs) do not support MLC-collimated electron beams. In this study, these difficulties were overcome by (1) modeling electron beams collimated by photon MLC for a commercial TPS, and (2) developing a technique to reduce electron beam penumbra by adding low-energy intensity-modulated radiation therapy (IMRT) photons (4 MV). We used blocks to simulate MLC shielding in the TPS. Inverse planning was used to optimize boost photon beams. This technique was applied to a parotid and a central nervous system (CNS) clinical case. Combined photon and electron plans were compared with conventional plans and verified using ion chamber, film, and a 2D diode array. Our studies showed that the beam penumbra for mixed beams with 90 cm source to surface distance (SSD) is comparable with electron applicators and cut-outs at 100 cm SSD. Our mixed-beam technique yielded more uniform dose to the planning target volume and lower doses to various organs at risk for both parotid and CNS clinical cases. The plans were verified with measurements, with more than 95% points passing the gamma criteria of 5% in dose difference and 5 mm for distance to agreement. In conclusion, the study has demonstrated the feasibility and potential advantage of using photon MLC to collimate electron beams with boost photon IMRT fields.

Migration of EndoButton After Anatomic Double-Bundle Anterior Cruciate Ligament Reconstruction

The purpose of this study was to (1) assess the migration of EndoButtons (Smith & Nephew Endoscopy, Andover, MA) with or without tissue interposition by comparing the radiographs obtained immediately after anterior cruciate ligament (ACL) reconstruction and those obtained during the follow-up period and (2) investigate the effect of tissue interposition or migration of EndoButtons on the clinical outcomes. One hundred one patients underwent anatomic double-bundle ACL reconstruction with EndoButtons for femoral fixation. Anteroposterior and lateral radiographs were taken immediately postoperatively and at 1 week, 1 month, 3 months, and 1 year. The distance between the EndoButton and lateral femoral cortex was measured on the radiograph obtained immediately postoperatively, and more than 1 mm in distance was defined as positive tissue interposition. Moreover, the locations of the EndoButtons on the radiographs obtained at each follow-up visit were compared with those obtained immediately postoperatively, and EndoButton migration was considered to be present when the EndoButton had moved more than 1 mm or had rotated by more than 5°. The relations between clinical outcomes at 1 year and tissue interposition or migration of EndoButtons were also evaluated. Tissue interposition was found for 51 EndoButtons and migration was observed for 71 EndoButtons, and there was a significant difference in the incidence of migration between anteromedial and posterolateral grafts. Of the 51 EndoButtons with tissue interposition, 32 migrated within 1 year, and 39 of 151 EndoButtons without tissue interposition migrated, whereas the EndoButtons with tissue interposition migrated significantly more frequently. On the other hand, there were no significant differences in clinical outcomes between the cases with or without tissue interposition and between the cases with or without EndoButton migration. Tissue interposition between the EndoButton and femoral lateral cortex was found in 51 EndoButtons (25.2%) on the radiographs obtained immediately after ACL reconstruction, whereas EndoButtons with tissue interposition migrated more frequently than those without it 1 year after ACL reconstruction. However, neither tissue interposition nor migration of the EndoButton affected the clinical outcomes. Level IV, prognostic case series.

In Reply:

We thank De Bonis et al for their interest in our article.1 In our series of 17 patients who underwent decompressive hemicraniectomy (DH) for cerebrovascular accident-related, medically refractory increased intracranial pressure, a full standard hemicraniectomy was performed in 13 patients, whereas only 4 received large craniectomy flaps. Clinically significant hydrocephalus necessitating cerebrospinal fluid (CSF) diversion developed in none of the 14 survivors. Unfortunately, we are unable to provide the authors with the information that they request because exact measurements of the hemicraniectomy flaps, particularly the distance between their medial edge and the superior sagittal sinus, were difficult to determine in our retrospective study. However, it has been our standard practice to perform wide bony decompressions extending from the floor of the middle fossa laterally to within 1 to 2 cm of the sagittal suture medially and from the floor of the anterior fossa anteriorly to approximately 5 to 6 cm behind the pinna posteriorly.2 Therefore, we would expect that the vast majority of patients in our series had hemicraniectomy flaps extending to within 2.5 cm of the superior sagittal sinus. When analyzing the incidence of hydrocephalus after DH, it is important to distinguish between complications of the underlying cerebral insult and those resulting from the surgical procedure itself. This is particularly true in patients with head trauma and those with subarachnoid and/or intraventricular hemorrhage who are naturally prone to the development of hydrocephalus, regardless of whether DH is performed. For this reason, we have strived to eliminate these major confounding factors and exclude such patients from our analysis. We read the authors' recent article on posttraumatic hydrocephalus after decompressive craniectomy in patients with traumatic brain injury.3 Although we find the authors' hypothesis interesting in general, there are nonetheless several issues that merit discussion. Their patient cohort was particularly small, with only 26 survivors included in the final analysis. Such a small sample size is unlikely to have a high enough statistical power to allow firm conclusions regarding prognostic factors, particularly when a 6-variable multivariate analysis is performed. Moreover, numerical variables were not analyzed as such and instead were dichotomized using arbitrary cutoff values (30 d for cranioplasty, 40 yr for age, 162 cm2 for cranioplasty, and 25 mm for distance from midline). This raises significant concerns about the validity of their statistical analysis. Clinical examination was not included in the definition of hydrocephalus for many comatose patients, which may have significantly affected their results. In our series, in 2 patients, asymptomatic extra-axial CSF collections developed after DH, which resolved spontaneously after cranioplasty. We believe that such collections are the result of increased intracranial compliance after the removal of a large piece of cranium rather than an actual mismatch between CSF production and absorption.1 Most importantly, the presence of a statistically significant association between a DH extending close to midline and hydrocephalus is in no way proof of a causal relationship, particularly in severely head-injured patients. It is noteworthy that all but 2 of their patients had evidence of subarachnoid and/or intraventricular blood on computed tomography (Fisher grade 2). A major confounding factor was thus present in the absolute majority of patients, which makes it difficult to attribute the development of hydrocephalus to the surgical procedure itself. Is it possible that patients who underwent DH closer to the midline had more severe structural damage to the cerebral hemisphere on CT, which may account for their increased rate of posttraumatic hydrocephalus? This is definitely a possibility in the setting of a retrospective study that may be subject to all kinds of inherent biases. In conclusion, to establish a cause-and-effect relationship between DH and hydrocephalus, one has to study the incidence of the latter in patients undergoing DH for conditions not known to be associated with hydrocephalus (eg, ischemic stroke). Alternatively, a randomized, controlled trial comparing the incidence of hydrocephalus in severe traumatic brain injury survivors who underwent DH and those who did not may provide the answer. Until then, we have no reason to believe that DH is a risk factor for hydrocephalus, independently of the patient's underlying cerebral insult. Michel Bojanowski Ralph Rahme Montreal, Quebec

Cardiac Motion During Deep-Inspiration Breath-Hold: Implications for Breast Cancer Radiotherapy

Many patients with left-sided breast cancer receive adjuvant radiotherapy during deep-inspiration breath hold (DIBH) to minimize radiation exposure to the heart. We measured the displacement of the left anterior descending artery (LAD) and heart owing to cardiac motion during DIBH, relative to the standard tangential fields for left breast cancer radiotherapy. A total of 20 patients who had undergone computed tomography-based coronary angiography with retrospective electrocardiographic gating were randomly selected for the present study. The patients underwent scanning during DIBH to control the influence of respiration on cardiac motion. Standard medial and lateral tangential fields were placed, and the LADs were contoured on the systolic- and diastolic-phase computed tomography data sets by the clinicians. Displacement of the LAD during cardiac contractions was calculated in three directions: toward the posterior edge of the treatment fields, left-right, and anteroposterior. Displacement of the entire heart was measured on the maximal and minimal intensity projection computed tomography images. The mean displacement of the LAD from cardiac contraction without the influence of respiration for 20 patients was 2.3 mm (range, 0.7-3.8) toward the posterior edge of the treatment fields, 2.6 mm (range, 1.0-6.8) in the left-right direction, and 2.3 mm (range, 0.6-6.5) in the anteroposterior direction. At least 30% of the LAD volume was displaced >5 mm in any direction in 2 patients (10%), and <10% of the LAD volume was displaced >5 mm in 10 patients (50%). The extent of displacement of the heart periphery during cardiac motion was negligible near the treatment fields. Displacement of the heart periphery near the treatment fields was negligible during DIBH; however, displacement of the LAD from cardiac contraction varied substantially between and within patients. We recommend maintaining ≥ 5 mm of distance between the LAD and the field edge for patients undergoing breast cancer radiotherapy during DIBH.

SU-GG-T-231: Respiratory Organ Motional Effect on the Dose Distribution in Tomotherapy

Purpose: The respiratory organ motional effect on the dosimetric accuracy in Tomotherapy was analyzed according to the range and cycle of organ motion combined with the effect of field width in Tomotherapy plan. Method and Materials: The three Tomotherapy plans according to the different field widths (1.0cm, 2.5cm, 3.0cm) were developed for the treatment of lungtumor. The DQA (Delivery Quality Assurance) plans were prepared with the CT data of MapCHECK inserted in the MapPHAN (SunNuclear, USA) phantom. The phantom system which can simulate one dimensional respiratory organ motion was used and two motional ranges (1.3cm, 3.0cm) and two motional cycles (3.0sec, 6.0sec) were applied to the measurement of dose variation in the delivery of each Tomotherapy plan. The coronal dose distributions were evaluated in total fifteen Tomotherapy deliveries and the criteria for the determination of pass in MapCHECK were 3% of dose and 3mm of distance. Results: A considerable dosimetric error was occurred as increase of organ motional ranges. The effect of field width on the dosimetric accuracy in the condition of organ motion could not be analogized with a definite trend although the large field width (5.0cm) showed the a little higher average pass rate compared with other field widths. The analysis results of the effect of organ motional cycle showed some consistent trend which indicates a degraded dose accuracy as an increase of the cycle. Conclusion: Based on these results, we could verify that the proper factors should be applied in Tomotherapy planning process in order to reduce the organ motional effect and the effective data should be accumulated from the experiments which can simulate various patients' organ motional features to find the optimized planning factors.

SU-GG-T-107: Feasibility Study of Total Marrow Irradiation Using RapidArc

Purpose: Total body irradiation (TBI) is commonly used in conjunction with chemotherapy as a preconditioning regimen prior to bone marrow transplantation in patients with hematological malignancies. It is associated with significant toxicities due to irradiation of large volume of normal tissue. The purpose of this work is to investigate feasibility of using RapidArc for targeted total marrow irradiation (TMI) while minimizing dose to critical organs.Method and Materials: The CTimage volumes with 5 mm thickness of a pediatric patient treated with other disease was used for this planning study in an Eclipse treatment planning system (Varian Medical Systems, CA). The target volume was defined as the skeletal bone, excluding mandible and maxillary bones. The technique used three separate treatment plans targeting the head‐neck region, chest and pelvis. Each plan consists of one clockwise and one counter‐clockwise full arcs. The chest plan was initially optimized and subsequently used as the based plan for both head‐neck and pelvic plans optimization. For dosimetry quality assurance (QA), the calculated plan was delivered on a Varian 23 iX Clinac linear accelerator equipped with RapidArc (Varian Medical Systems, CA). The dose accuracy was verified with volumetric portal images, using Dosimetry Check (Math resolutions, LLC) program. Results: Average dose reduction of the RapidArc based TMI, compared to conventional TBI, are 16% for lung, 23% for kidney, 29% for liver, and 74% for lenses while more than 95% of the PTV is covered by 97% of the prescribed dose. Each plan passed dosimetry QA when the gamma evaluation criteria of 5% absolute point dose difference and 5 mm of distance to agreement (DTA) was selected. Conclusion: The adequate target dose coverage and reduced normal organdose were observed. Patient‐specific QA is achieved using the Dosimetry Check system. Initial results are encouraging and warrant further investigation.

Shelf-life of Almond Pastry Cookies with Different Types of Packaging and Levels of Temperature

Almond pastries are typical cookies of the south of Italy. Introduction of new packaging for this kind of cookies requires shelf-life assessments. This study, related to different types of packaging under various storage conditions of time and temperature, identifies critical parameters, as color and texture, to track during storage studies and to extend the shelf-life. The cookies were packed in three different ways and stored at two different temperatures. The pastries were separately stored: (1) in polyvinylchloride film; (2) in aluminum foil (ALL); (3) with modified atmosphere (MAP) in plastic vessels sealed into a polyamide/ polyethylene film; and (4) in vessels without any polymeric film. The storage temperatures were 20 and 30 °C. Evolution of texture, water activity, dry matter and color was assessed. Texture was evaluated by a texture analyzer with a puncturing test. Indices for hardening were the area under the curve (N × mm) up to 10 mm of distance, and the maximum force (N) corresponding to the crust fracture. The best results were obtained with ALL packaging and MAP condition, and above all, in all the trials a temperature of 30 °C reduced the crust hardness.

Radiographic inspection of porosity in pure titanium dumbbell castings

Titanium frameworks are frequently indicated for implant supported prostheses; however, voids are usually encountered inside cast titanium. This study aimed to confirm the efficacy of a radiographic technique for inspection of porosity in commercially pure titanium castings with different diameter. Sixty dumbbell rods (n=20) with a central 1.5, 2.0 and 3.5mm diameter were prepared by lost-wax casting. Cast specimens were finished and polished and submitted to radiographic examination (90kV, 15mA, 0.6s and 10-13mm of distance) using periapical film. The radiographs were visually analysed for the presence of porosity in the extension of the dumbbell or in the central portion of the rods. Data were submitted to Pearson Chi-square test (5%). The tested radiographic method proved to be suitable for the evaluation of cast frameworks. Internal porosities were observed in most of the specimens (91.7%) (p=0.0005); however, only 20% occurred on the central portion of the rods (p=0.612). Internal porosities can be visualised through radiographs and occur mostly in small diameter structures. The radiographic evaluation of metal structures can improve the quality of frameworks and thereby potentially increase the longevity of the rehabilitation.

Comparison of film dosimetry techniques used for quality assurance of intensity modulated radiation therapy

Accurate dosimetry is essential to ensure the quality of advanced radiation treatments, such as intensity modulated radiation therapy (IMRT). Therefore, a comparison study was conducted to assess the accuracy of various film dosimetry techniques that are widely used in clinics. A simulated IMRT plan that produced an inverse pyramid dose distribution in a perpendicular plane of the beam axis was designed with 6 MV x rays to characterize the large contribution of scattered photons to low dose regions. Three film dosimetry techniques, EDR2, EDR2 with low-energy photon absorption lead filters (EDR2 WF), and GafChromic EBT, were compared to ionization chamber measurements as well as Monte Carlo (MC) simulations. The accuracy of these techniques was evaluated against the ionization chamber data. Two-dimensional comparisons with MC simulation results were made by computing the gamma index (gamma) with criteria ranging from 2% of dose difference or 2 mm of distance to agreement (2%/2 mm) to 4%/4 mm on the central vertical plane (20 x 20 cm2) of a square solid water phantom. Depth doses and lateral profiles at depths of 5, 10, and 15 cm were examined to characterize the deviation of film measurements and MC predictions from ionization chamber measurements. In depth dose comparisons, the deviation between the EDR2 films was 9% in the low dose region and 5% in high dose region, on average. With lead filters, the average deviation was reduced to -1.3% and -0.3% in the low dose and high dose regions, respectively. EBT film results agreed within 1.5% difference on average with ionization chamber measurements in low and high dose regions. In two-dimensional comparisons with MC simulation, EDR2 films passed gamma tests with a 2%/2 mm criterion only in the high dose region (gamma < or = 1, total of 63.06% of the tested region). In the low dose region, EDR2 films passed gamma tests with 3%/3 mm criterion (gamma < or = 1, total of 98.4% of the tested region). For EDR2 WF and GafChromic EBT films, gamma tests with a 2% /2 mm criterion (gamma < or = 1) in the tested area was 97.3% and 96.8% of the tested region, respectively. The EDR2 film WF and GafChromic EBT film achieved an average accuracy level of 1.5% against an ionization chamber. These two techniques agreed with the MC prediction in 2%/2mm criteria evaluated by the gamma index, whereas EDR2 without filters achieved an accuracy level of 3%/3 mm with the decision criteria of agreement greater than 95% of the tested region. The overall results will provide a useful quantitative reference for IMRT verifications.

Automated beam model optimization

The beam model in a three dimensional treatment planning system (TPS) defines virtually the mechanical and dosimetric characteristics of a treatment unit. The manual optimization of a beam model during commissioning can be a time consuming task due to its iterative nature. Furthermore, the quality of the beam model commissioning depends on the user's ability to manage multiple parameters and assess their impact on the agreement between measured and calculated dose. The objective of this work is to develop and validate the performance of an automated beam model optimization system (ABMOS) based on intensity modulated radiotherapy (IMRT) beam measurements to improve beam model accuracy while streamlining the commissioning process. The ABMOS was developed to adjust selected TPS beam model parameters iteratively to maximize the agreement between measured and calculated 2D dose maps obtained for an IMRT beam pattern. A 2D diode array with high spatial resolution detectors was used to sample the entire IMRT beam pattern in a single dose measurement. The use of an IMRT beam pattern with large number of monitor units was selected to highlight the difference between planned and delivered dose and improve the signal to noise ratio in the low dose regions. ABMOS was applied to the optimization of a beam model for an Elekta Synergy S treatment unit. The optimized beam model was validated for two anatomical sites (25 paraspinal and 25 prostate cases) using two independent patient-specific IMRT quality control (QC) methods based on ion chamber and 2D diode array measurements, respectively. The conventional approach of comparing calculated and measured beam profiles and percent-depth dose curves was also used to assess improvement in beam model after ABMOS optimization. Elements of statistical process control were applied to the process of patient-specific QC performed with the ion chamber and the 2D array to complement the model comparison. After beam model optimization with ABMOS, improvement in planned to delivered dose agreement was demonstrated with both patient-specific IMRT QC methods and the calculated to measured profile comparison. In terms of ion chamber measurements, the largest improvement was observed for the paraspinal cases with the mean measured to calculated dose difference at the low dose points decreasing from - 13.8% to 2.0% with the optimized beam model. The 2D diode array patient-specific QC also demonstrated clearly the improvement in beam model for both paraspinal and prostate cases with, on average, more than 96% of the diodes satisfying tolerances of 3% of dose difference or 2 mm of distance to agreement after ABMOS optimization. The capability index (C(pk)) for both patient-specific QC methods also increased with the optimized beam model. In this work, ABMOS was developed to use 2D diode array measurements of an IMRT beam pattern for the automated multivariable optimization of a TPS beam model. Based on the observed improvements in patient-specific QC results for 25 paraspinal and 25 prostate plans, optimization of the remaining clinical beam models using ABMOS is now ongoing in the institution.

Laser Radar Receiver Channel With Timing Detector Based on Front End Unipolar-to-Bipolar Pulse Shaping

An integrated receiver channel for a pulsed time-of-flight laser range finder is presented based on a timing discrimination principle in which the incoming unipolar detector current pulse is converted to a bipolar pulse at the front end of the receiver channel. Thus no optical or electrical gain control is needed within the dynamic range of the receiver, which according to measurements is 1:3000 with a timing walk error of plusmn 55 ps (plusmn 8 mm in distance). The minimum detectable input signal current is about 1.3 muA at an SNR of 10 with a bandwidth of 200 MHz. The circuit is realized in a 0.35 mum SiGe BiCMOS process and consumes 220 mW of power.

The determination of the initial straight length in root canals of mandibular premolars -an in vitro study

The aim of the present roentgenographic in vitro study was to determine the initial straight length from the cemento-enamel junction (CEJ) to the appearance of a root canal curvature in human mandibular premolars. A total number of 282 mandibular premolars were examined. Exclusion criteria comprised root caries, extensive restorations and endodontically treated teeth. The teeth were fixed and digitally radiographed by means of a specially developed fixation device with standardized and reproducible distances with the parallel technique (Heliodent MD; Merlin 2.1). The distances from the CEJ to the first curvature (> 5°) (distance I), from the first curvature to a second curvature (distance II), and from the first or second curvature to the radiological apex (distance III) were recorded. The lengths of the initial straight distance and the appearance of a curvature were statistically analyzed and related to each other. In first mandibular premolars, the mean value for distance I was 10.9 mm, for distance II 3.7 mm and for distance III 3.04 mm. The mean values for the second mandibular premolars were 1.1 mm in distance I, 4.3 mm in distance II and 3.1 mm in distance III. No statistically significant differences between left and right mandibular premolars could be observed regarding the canal curvature location and the angle values. The results of this investigation show that curvatures are increasingly observed 9 to 12 mm from the original path in mandibular premolars.

Implementation and experimental validation of the high dose rate stereotactic treatment mode at Varian accelerators.

Background. Treating small tumours in the brain requires usage of small fields. However, it is a challenge for all treatment planning systems (TPS) to modulate small fields (≤3×3 cm2) for which lateral electronic equilibrium does not exits. Material and methods. Open field beam data was measured for the stereotactic mode at our Varian Trilogy accelerator and was implemented in the Eclipse TPS for both the Pencil Beam (PB) and the Anisotropic Analytical Algorithm (AAA). Additional output factors, profiles and percentage depth dose curves were measured in a water phantom for asymmetric fields and fields defined by the Varian 120 MLC and the BrainLAB m3 MLC. Dose distributions in transverse sections were measured with gafchromic films in a homogeneous phantom. These were compared to those calculated by the Eclipse TPS for 11 stereotactic treatment plans. The plans included non-coplanar, dynamic and arc fields. All comparisons for the film measurements in the phantom were made with a Gamma criterion of 2 mm in distance and 3% in dose. Results. The AAA and the PB algorithms broadened the penumbra of the profiles by approximately 1.5 mm and 2.0 mm, respectively. For static fields defined by the Varian 120 MLC, the measured field size was 1.0 mm broader than the calculated. The output factors were typically within 2% for field sizes ≥ 2×2 cm2. More than 95% of the points passed the Gamma criterion. Up to 50% of the points in the low dose regions failed the criterion, but overall the match was satisfactory. Conclusion. The accuracy of calculations performed by the Eclipse TPS for the SRS mode was shown to be within a few percent for most clinical fields. AAA was found to be superior to the PB algorithm in modelling the dose in the penumbra region.

SU‐GG‐T‐336: A Track Repeating Algorithm for Fast Proton Dose Calculations

Purpose: Monte Carlo tracking programs are utilized for accurate dose calculations in proton radiation therapy. They are superior to commonly used analytical dose calculations in accuracy, however, they are highly demanding computer resources. The aim of this work is to characterize fast algorithm for accurate dose calculations based on a track repeating Monte Carlo approach. Method and Materials: A few hundred thousand proton trajectories in water, generated with GEANT4, were stored in a database in discrete steps. The trajectories were utilized to calculate dose in various materials by rescaling the step length and angles between consecutive steps. The extrapolation is applied to a variety of materials, including bone and metals, which are important for patient with prostheses. Results: The track repeating approach reduced the CPU time required for a complete dose calculation in a patient by more than two orders of magnitude. The track repeating approach reproduced the results from the traditional Monte Carlo approaches within 4% in dose and within 1 mm in distance to agreement. Conclusion: A fast track repeating algorithm based on GEANT4 provided a substantial reduction in CPU time with very little loss in accuracy. The approach is not limited to GEANT4 and the developed tool can be utilized with other Monte Carlo programs.