Machining Tolerances Research Articles

Statistical process control and verifying positional accuracy of a cobra motion couch using step-wedge quality assurance tool.

This study utilizes process control techniques to identify action limits for TomoTherapy couch positioning quality assurance tests. A test was introduced to monitor accuracy of the applied couch offset detection in the TomoTherapy Hi‐Art treatment system using the TQA “Step‐Wedge Helical” module and MVCT detector. Individual X‐charts, process capability (cp), probability (P), and acceptability (cpk) indices were used to monitor a 4‐year couch IEC offset data to detect systematic and random errors in the couch positional accuracy for different action levels. Process capability tests were also performed on the retrospective data to define tolerances based on user‐specified levels. A second study was carried out whereby physical couch offsets were applied using the TQA module and the MVCT detector was used to detect the observed variations.Random and systematic variations were observed for the SPC‐based upper and lower control limits, and investigations were carried out to maintain the ongoing stability of the process for a 4‐year and a three‐monthly period. Local trend analysis showed mean variations up to ±0.5 mm in the three‐monthly analysis period for all IEC offset measurements. Variations were also observed in the detected versus applied offsets using the MVCT detector in the second study largely in the vertical direction, and actions were taken to remediate this error. Based on the results, it was recommended that imaging shifts in each coordinate direction be only applied after assessing the machine for applied versus detected test results using the step helical module. User‐specified tolerance levels of at least ±2 mm were recommended for a test frequency of once every 3 months to improve couch positional accuracy. SPC enables detection of systematic variations prior to reaching machine tolerance levels. Couch encoding system recalibrations reduced variations to user‐specified levels and a monitoring period of 3 months using SPC facilitated in detecting systematic and random variations. SPC analysis for couch positional accuracy enabled greater control in the identification of errors, thereby increasing confidence levels in daily treatment setups.

Vibration performance evaluation of planar flexible multibody systems with joint clearance

Clearances are necessary in assemblage of mechanisms to allow the relative motion between the members. This clearance is due to machining tolerances, wear, material deformations, and imperfections, and it can worsen mechanism performance such as precision and vibration. As a new study in this topic, the effect of joint stiffness on the variation of instantaneous natural frequencies and mode shapes of a flexible four-bar mechanism with a clearance between coupler and follower is studied in this paper. To model the clearance, the continuous contact approach is used. The Lankarani’s and Nikravesh’s continuous contact force model is used to model the contact force arising from contact between journal and bearing. Finite element method is used to determine the instantaneous natural frequencies and their corresponding mode shapes. The stiffness of the clearance is modeled as a linear spring added to the assembled stiffness matrix. To validate the clearance model in rigid mechanism, the dynamic response is compared with the results in the literature. To show the validity of the formulation which calculates the instantaneous natural frequencies, two methods are used and compared with each other in the case no clearance exists. The results show that taking the joint stiffness into account has a considerable effect on the instantaneous natural frequencies and their corresponding mode shapes of a flexible multibody system.

Experimental Study of Inconel 718 Surface Treatment by Edge Robotic Deburring with Force Control

We present the results of investigations into the application of robotics for deburring and chamfering to a predefined geometric quality. The robotic application was used for a part of the manufacturing process of an aircraft engine detail. Aircraft engine diffuser machining requires manual deburring of many edges. Finishing by hand results in several non-conforming quality details for each diffuser. This paper presents the concept of edge deburring with a controlled force progression pneumatic tool. A specific methodology was used to select and optimise the edge deburring process for robotic chamfering processing to a finer machining tolerance. The investigated machining process included a measurement system for the determination of the manufactured chamfer as a function of contact forces with feed force progression. The investigation work discussed in the paper helped to identify a specific interval of processing parameters, including the contact force and TCP motion velocity at which deburring is effective and a chamfer with specific geometric tolerance is produced. The experimental part of the investigation was conducted at a preset feed force of the high-speed machining file, tool ref. FDB150. The experimental machining sample was made from poorly machinable titanium alloy (Inconel 718), a material applied in the aerospace industry. The machining process optimisation included an approximation of the chamfer width definition points. The resulting function provided a derivative, defining the chamfer value change rate and corresponding to the actual machining tool infeed. The experimental measurement results were compared to the assumed quality indicators, by which a group of suboptimal parameters was defined.

The method of selective assembly for the RV reducer based on genetic algorithm

As a precision gear reducer, the RV reducer has a low-transmission backlash (very high assembly accuracy). Therefore, the selective assembly method is the only assembly method which can guarantee the assembly precision of the RV reducer. However, the RV reducer has a complex structure; it consists of a high number of parts whose machining tolerance cannot be very low. Furthermore, there are numerous parts, the tolerances of which influence the RV reducer transmission backlash. Therefore, it is difficult to achieve high assembly accuracy by using the traditional selective assembly method. In this paper, a method of selective assembly is proposed to make the backlash of the RV reducer meet the requirements through the analysis of the characteristics of the RV reducer structure, the processing and assembly process of the parts, and the influence of manufacturing errors on the backlash. Then, a mathematical model was established for the RV reducer assembly issue. And a matching algorithm based on a genetic algorithm was developed. Finally, the algorithm was applied to the selective assembly of the RV reducer for verifying the feasibility and validity of the proposed matching method.

Electrochemical Behaviour of Titanium Based Biomaterials in Artificial Saliva

In the last decade, new titanium alloys such as Ti6Al7Nb and Ti6Al7NbN have been developed in different areas of dentistry. It is known that mechanical/electrochemical interactions accelerate corrosion in implant alloys, and these interactions can be reduced by improving the modular component machining tolerances or by improving the corrosion resistance. The aim of this study was to compare the Ti6Al7Nb and Ti6Al7NbN alloys corrosion resistance in AFNOR artificial saliva. Due to the nitrogen diffusion, the Ti6Al7NbN alloy presents more pronounced grain structure than Ti6Al7Nb. In the electrochemical estimations the polarisation data were converted into instantaneous corrosion rate values (jcorr). The obtained passivation properties were comparable for both alloys. The EIS spectra were best fitted using an equivalent circuit (EC) which was consistent with the model of a two-layer structure for the passive film. The electrochemical and corrosion behaviour of Ti6Al7NbN increased the occurrence of the implant alloy�s electrochemical degradation.

Effect of nose perturbation on asymmetric vortices over a blunt-nose body at high angle of attack

To improve the maneuverability, the modern missiles of blunt-nose slender body configuration are required to flight at high angle of attack where many research show complex asymmetric vortices flow will develop over the slender body. The effect of artificial perturbation located on the nose of a blunt slender body at high angle of attack(α=50°) on asymmetric vortices has been investigated with low speed wind tunnel test. The Reynolds number(ReD) of the experiment was 1.48×105. The experimental results are shown to be the following: Asymmetric flow over blunt-nose body was extremely sensitive to the machining tolerances of the nose and can be governed by the artificial perturbation; with the circumferential angle of artificial perturbation varied a period, asymmetric vortices present a behavior of single-period.

In Vitro Three-Dimensional Accuracy of Digital Implant Impressions: The Effect of Implant Angulation.

To compare the three-dimensional (3D) accuracy of conventional impressions (CIs) with digital scans (DSs) using an intraoral scanner (IOS) with intraoral scan bodies (SBs) and varying buccolingual interimplant angulations. A secondary aim was to measure the SB machining tolerance and height with and without application of torque. Three master models (MMs) with two implants simulating an implant-supported three-unit fixed partial denture for bone-level implants were used. The implants had buccolingual interimplant angulations of 0, 10, and 20 degrees. Test models for the CI test groups were made with impression copings and polyether impressions. SBs were attached to the MMs, tightened to 15-Ncm torque, and scanned by an IOS for the DS test groups (six test groups, n = 5). A coordinate measuring machine measured linear distortions (dx, dy, dz), 3D distortions (dR), angular distortions (dθx, dθy), and absolute angular distortions (Absdθx, Absdθy) of the physical CI test models and STL files of the DS virtual models relative to the MMs. Metrology software allowed both physical and virtual measurement of geometric targets that were comparable and allowed computation of relative displacements of implant centroids and axes. Mean dR ranged from 31 ± 14.2 to 45 ± 3.4 μm for DS and 18 ± 8.4 to 36 ± 6.5 μm for the CI test groups. Mean Absdθx ranged from 0.041 ± 0.0318 to 0.794 ± 0.2739 degrees for DS and 0.073 ± 0.0618 to 0.545 ± 0.0615 degrees for the CI test groups. Mean Absdθy ranged from 0.075 ± 0.0615 to 0.111 ± 0.0639 degrees for DS and 0.106 ± 0.0773 to 0.195 ± 0.1317 degrees for the CI test groups. Two-way analysis of variance showed that the impression technique (P = .012) and implant angulations (P = .007) had a significant effect on dR. Distortions were mostly in the negative direction for DS test groups. Perfect coaxiality of the SB with the implant was never achieved. For SB to implant machining tolerances, the mean absolute horizontal displacement ranged from 4 ± 1.2 to 7 ± 2.3 μm. The SB dz was -5 ± 3.2 μm, which increased in the negative direction to -11 ± 4.9 μm with torque application (P = .002). Distortions were found for both DS and CI test groups. The best accuracy was obtained with CIs for parallel implants. With angulated implants, conventional and DSs were not significantly different. Excessive torque application that causes negative dz for SB fit would position the virtual implant at a deeper location compared with reality, resulting in possible framework misfit.

A probabilistic-based study on fused direct and indirect methods for tracking tool flank wear of Rene-108, nickel-based alloy

Nickel-based alloys are a class of hard-to-machine materials that exhibit a unique combination of high strength at high temperature. While they are widely used in industry, the high tool wear rate of these materials makes machining them a challenging task. Furthermore, machining tolerances, residual stresses, and tool runout impose uncertainty in tracking tool wear. In this work, the current view on tracking progressive tool wear is shifted from deterministic domains into the stochastic domain to study the probability distribution of the tool wear during the process. To do so, Bayesian-based estimation was used for accurate model inference and the result was fed into the Kalman filter for tracking tool wear in end-milling Rene-108 Ni-based alloy. To improve the accuracy, a direct laser measuring system for capturing tool length change was fused with the indirect power measurement. The results show a significant improvement in accuracy over the indirect method, with less than 8 µm root mean square error. This measuring strategy improved the accuracy, while preserving the automated platform for monitoring the tool’s health.

Passive flow control valve for protein delivery

Passive flow control valves are usually intended to deliver or drain a fluid at a constant rate independently of pressure variations. Microfluidic devices made of a stack of two plates are considered here: the first plate comprises a flexible silicon membrane having through holes while the second plate is a rigid substrate with a cavity, an outlet hole and pillars aligned with the through holes of the membrane. The liquid flows through the holes etched in the membrane and through the gap between the membrane and the pillars. Each gap can be considered as a valve which progressively closes as the pressure increases. Numerical modelling of the fluid dynamics inside the device associated with FEM simulations of the membrane distortion have been performed to design a device that exhibits a constant flow rate in a specified range of pressure. To make the design more reliable, the device characteristics have been optimized using a genetic algorithm, the fitness function taking notably into account machining and alignment tolerances. This algorithm has been finally used to design flow control valves for wearable injectors dedicated to the infusion of viscous drug formulations (hyaluronic acid, adalimumab, golimumab …) at high pressure. Prototypes have been characterized using solutions of 12 and 24 cP. It has been demonstrated experimentally that this technology is suitable to passively infuse biological products at flow rates up to 1 mL/min. The numerical model has then been refined further so as to obtain a good correlation with experimental data.

Micromachined Interfaces for Metrology and Packaging Applications in the Submillimeter-Wave Band

The continued emergence of new terahertz devices has created a need for improved approaches to packaging, integration, and measurement tools for diagnostics and characterization in this portion of the spectrum. Rectangular waveguide has for many years been the primary transmission medium for terahertz and submillimeter-wave systems operating from 300 GHz to 1 THz, with the UG-387 flange the most common interface for mating waveguide components over this frequency range. Alignment of UG-387 flanges is accomplished with pins and alignment holes that are placed around the flange perimeter and, under the standard MIL SPECS tolerances, misalignments of up to 6 mils (150 microns) are possible as a result of practical milling tolerances. With the emergence of vector network analyzers operating beyond 1 THz, such misalignment of waveguide mating flanges is not negligible and is recognized as a fundamental issue limiting calibration and measurement precision at frequencies greater than 300 GHz. In response to this issue, a number of new waveguide flange concepts have been investigated to reduce flange misalignment and the P1785 IEEE Standard was recently issued to recommend designs for waveguide interfaces at frequencies above 110 GHz. Among the new flange concepts being proposed is a modified UG-387 that utilizes tighter machining tolerances and the ring-centered flange where alignment is accomplished using a precision coupling ring that fits over raised bosses that are centered on each waveguide. This paper discusses the new interface concepts that are being developed to address waveguide flange misalignment as well as emerging micromachined interconnects, calibration standards and heterogeneous integration methods that are being applied to implement low-loss and high-performance circuit architectures for the terahertz frequency range. Among the technologies that will be described are (1) design and characterization methods for the new ring-centered waveguide standard, (2) micromachined waveguide components and calibration standards for the terahertz band, (3) silicon-based micromachined probe structures for direct-contact interfacing and metrology, and (4) epitaxial transfer of III-V semiconductor material onto high-resistivity silicon to realize a low-loss platform for integration of terahertz components. Details of the processing methods used to realize these components as well as measurement techniques for assessing their performance will be described.

Characterization of the effect of joint clearance on the energy loss of flexible multibody systems with variable kinematic structure

Clearances are essential for the assemblage of mechanisms to allow the relative motion between the joined bodies. This clearance exists due to machining tolerances, wear, material deformations, and imperfections, and it can worsen the mechanism performance when the precision and smoothly-working are intended. Energy is a subject which is less paid attention in the area of clearance. The effect of the clearance on the energy of a flexible slider-crank mechanism is investigated in this paper. A clearance exists in the joint between the slider and the coupler. The contact force model is based on the Lankarani and Nikravesh model and the friction force is calculated using the modified Coulomb\'s friction law. The hysteresis damping which has been included in the contact force model dissipates energy in clearance joints. The other source for the energy loss is the friction between the journal and the bearing. Initial configuration and crank angular velocity are changed to see their effects on the energy of the system. Energy diagrams are plotted for different coefficients of friction to see its influence. Finally, considering the coupler as a flexible body, the effect of flexibility on the energy of the system is investigated.

Analysis of dynamic mechanism and global response of vehicle shimmy system with multi-clearance joints

Joints with clearance always exist in steering mechanism of vehicles due to the machining tolerances, deformations and wear, which should not be neglected in modeling and analysis of vehicle shimmy system. Based on Lagrange equation method, a six-DOF dynamic model of vehicle shimmy system with consideration of multi-clearance joints was established. The model of movement pairs with clearance was built based on Hertzian contact law. The nonlinear cornering force of pneumatic tire was described based on PAC2002 tire model, and its parameters were obtained by an identification method based on experimental data. Then numerical examples were employed to evaluate the dynamic response of the shimmy system. The influence of multi-clearance joints on the response of shimmy was analyzed, and it is compared with that of one clearance joint. The acting mechanism of contact force between movement pairs was investigated, and global dynamic analysis was carried out by using point mapping method. Moreover, the dynamic mechanism for vehicle shimmy was investigated based on energy input. The conclusions can provide theoretic evidence for attenuation of vehicle shimmy, especially for those in-service vehicles.

A method to reconstruct and apply 3D primary fluence for treatment delivery verification.

MotivationIn this study, a method is reported to perform IMRT and VMAT treatment delivery verification using 3D volumetric primary beam fluences reconstructed directly from planned beam parameters and treatment delivery records. The goals of this paper are to demonstrate that 1) 3D beam fluences can be reconstructed efficiently, 2) quality assurance (QA) based on the reconstructed 3D fluences is capable of detecting additional treatment delivery errors, particularly for VMAT plans, beyond those identifiable by other existing treatment delivery verification methods, and 3) QA results based on 3D fluence calculation (3DFC) are correlated with QA results based on physical phantom measurements and radiation dose recalculations.MethodsUsing beam parameters extracted from DICOM plan files and treatment delivery log files, 3D volumetric primary fluences are reconstructed by forward‐projecting the beam apertures, defined by the MLC leaf positions and modulated by beam MU values, at all gantry angles using first‐order ray tracing. Treatment delivery verifications are performed by comparing 3D fluences reconstructed using beam parameters in delivery log files against those reconstructed from treatment plans. Passing rates are then determined using both voxel intensity differences and a 3D gamma analysis. QA sensitivity to various sources of errors is defined as the observed differences in passing rates. Correlations between passing rates obtained from QA derived from both 3D fluence calculations and physical measurements are investigated prospectively using 20 clinical treatment plans with artificially introduced machine delivery errors.ResultsStudies with artificially introduced errors show that common treatment delivery problems including gantry angle errors, MU errors, jaw position errors, collimator rotation errors, and MLC leaf position errors were detectable at less than normal machine tolerances. The reported 3DFC QA method has greater sensitivity than measurement‐based QA methods. Statistical analysis‐based Spearman's correlations shows that the 3DFC QA passing rates are significantly correlated with passing rates of physical phantom measurement‐based QA methods.ConclusionAmong measurement‐less treatment delivery verification methods, the reported 3DFC method is less demanding than those based on full dose re‐calculations, and more comprehensive than those that solely checks beam parameters in treatment log files. With QA passing rates correlating to measurement‐based passing rates, the 3DFC QA results could be useful for complementing the physical phantom measurements, or verifying treatment deliveries when physical measurements are not available. For the past 4+ years, the reported method has been implemented at authors’ institution 1) as a complementary metric to physical phantom measurements for pretreatment, patient‐specific QA of IMRT and VMAT plans, and 2) as an important part of the log file‐based automated verification of daily patient treatment deliveries. It has been demonstrated to be useful in catching both treatment plan data transfer errors and treatment delivery problems.

Dynamic Analysis of Gas Turbine Foundation Design

Geotechnical investigation and dynamic analysis of gas turbine foundation, coupled with the present turbine foundation design codes and Standards were considered for the analytical design of gas turbine foundation. This work was carried out to establish a calculation method that will consider all the fundamental modes of vibration with calculated mass participation during gas turbine foundation design, with the aim of limiting the vibration response amplitude to a specified machine or human tolerance. This was done by creating a model for calculating fundamental modes of vibration. The dynamic analysis of the foundation was carried out through Finite element and the structural analysis program STAAD Pro 2007 and DYNA5.Pile foundation was found to be suitable for the foundation of the study area, to ensure that the vibrating amplitude and the frequencies are maintained below the threshold that may affect human and the serviceability of the gas turbine machine. The dynamic analysis results meet with the turbine foundation code of standard.

Tip clearance effects on loads and performances of semi-open impeller centrifugal pumps at different specific speeds

Relevant industrial standards or customer's specifications could strictly forbid any device adjusting the axial rotor/stator position, so that tip clearance between semi-open impeller and casing might become a result of the pump machining tolerances and assembling process, leading to big tip clearance variations compared to its nominal value. Consequently, large disparities of global performances (head, power, efficiency) and axial loads are observed with high risk of both specifications noncompliance and bearing damages. This work aims at quantifying these variations by taking into account tip clearance value and pump specific speed. Computational Fluid Dynamics is used to investigate this phenomenon by means of steady simulations led on a semi-open centrifugal pump numerical model including secondary flows, based on a k-omega SST turbulence model. Four different specific speed pump sizes are simulated (from 8 to 50, SI units), with three tip clearances for each size on a wide flow range (from 40% to 120% of the best efficiency point). The numerical results clearly show that head, power and efficiency increase as the tip clearance decreases for the whole flow range. This effect is more significant when the specific speed is low. Meanwhile, the resulting axial thrust on the impeller is very sensitive to the tip clearance and can even lead to direction inversion.

Study of on-machine error identification and compensation methods for micro machine tools

Micro machining plays an important role in the manufacturing of miniature products which are made of various materials with complex 3D shapes and tight machining tolerance. To further improve the accuracy of a micro machining process without increasing the manufacturing cost of a micro machine tool, an effective machining error measurement method and a software-based compensation method are essential. To avoid introducing additional errors caused by the re-installment of the workpiece, the measurement and compensation method should be on-machine conducted. In addition, because the contour of a miniature workpiece machined with a micro machining process is very tiny, the measurement method should be non-contact. By integrating the image re-constructive method, camera pixel correction, coordinate transformation, the error identification algorithm, and trajectory auto-correction method, a vision-based error measurement and compensation method that can on-machine inspect the micro machining errors and automatically generate an error-corrected numerical control (NC) program for error compensation was developed in this study. With the use of the Canny edge detection algorithm and camera pixel calibration, the edges of the contour of a machined workpiece were identified and used to re-construct the actual contour of the work piece. The actual contour was then mapped to the theoretical contour to identify the actual cutting points and compute the machining errors. With the use of a moving matching window and calculation of the similarity between the actual and theoretical contour, the errors between the actual cutting points and theoretical cutting points were calculated and used to correct the NC program. With the use of the error-corrected NC program, the accuracy of a micro machining process can be effectively improved. To prove the feasibility and effectiveness of the proposed methods, micro-milling experiments on a micro machine tool were conducted, and the results have demonstrated the success of the micro-machining accuracy enhancement for micro machine tools.

A Greedy reassignment algorithm for the PBS minimum monitor unit constraint

Proton pencil beam scanning (PBS) treatment plans are made of numerous unique spots of different weights. These weights are optimized by the treatment planning systems, and sometimes fall below the deliverable threshold set by the treatment delivery system. The purpose of this work is to investigate a Greedy reassignment algorithm to mitigate the effects of these low weight pencil beams. The algorithm is applied during post-processing to the optimized plan to generate deliverable plans for the treatment delivery system. The Greedy reassignment method developed in this work deletes the smallest weight spot in the entire field and reassigns its weight to its nearest neighbor(s) and repeats until all spots are above the minimum monitor unit (MU) constraint. Its performance was evaluated using plans collected from 190 patients (496 fields) treated at our facility. The Greedy reassignment method was compared against two other post-processing methods. The evaluation criteria was the γ-index pass rate that compares the pre-processed and post-processed dose distributions. A planning metric was developed to predict the impact of post-processing on treatment plans for various treatment planning, machine, and dose tolerance parameters. For fields with a pass rate of 90 ± 1% the planning metric has a standard deviation equal to 18% of the centroid value showing that the planning metric and γ-index pass rate are correlated for the Greedy reassignment algorithm. Using a 3rd order polynomial fit to the data, the Greedy reassignment method has 1.8 times better planning metric at 90% pass rate compared to other post-processing methods. As the planning metric and pass rate are correlated, the planning metric could provide an aid for implementing parameters during treatment planning, or even during facility design, in order to yield acceptable pass rates. More facilities are starting to implement PBS and some have spot sizes (one standard deviation) smaller than 5 mm, hence would require small spot spacing. While this is not the only parameter that affects the optimized plan, the perturbation due to the minimum MU constraint increases with decreasing spot spacing. This work could help to design the minimum MU threshold with the goal to keep the γ-index pass rate above an acceptable value.

TH‐CD‐209‐01: A Greedy Reassignment Algorithm for the PBS Minimum Monitor Unit Constraint

Purpose:To investigate a Greedy Reassignment algorithm in order to mitigate the effects of low weight spots in proton pencil beam scanning (PBS) treatment plans.Methods:To convert a plan from the treatment planning system's (TPS) to a deliverable plan, post processing methods can be used to adjust the spot maps to meets the minimum MU constraint. Existing methods include: deleting low weight spots (Cut method), or rounding spots with weight above/below half the limit up/down to the limit/zero (Round method). An alternative method called Greedy Reassignment was developed in this work in which the lowest weight spot in the field was removed and its weight reassigned equally among its nearest neighbors. The process was repeated with the next lowest weight spot until all spots in the field were above the MU constraint. The algorithm performance was evaluated using plans collected from 190 patients (496 fields) treated at our facility. The evaluation criteria were the γ‐index pass rate comparing the pre‐processed and post‐processed dose distributions. A planning metric was further developed to predict the impact of post‐processing on treatment plans for various treatment planning, machine, and dose tolerance parameters.Results:For fields with a gamma pass rate of 90±1%, the metric has a standard deviation equal to 18% of the centroid value. This showed that the metric and γ‐index pass rate are correlated for the Greedy Reassignment algorithm. Using a 3rd order polynomial fit to the data, the Greedy Reassignment method had 1.8 times better metric at 90% pass rate compared to other post‐processing methods.Conclusion:We showed that the Greedy Reassignment method yields deliverable plans that are closest to the optimized‐without‐MU‐constraint plan from the TPS. The metric developed in this work could help design the minimum MU threshold with the goal of keeping the γ‐index pass rate above an acceptable value.

SU-F-T-593: Technical Treatment Accuracy in a Clinic of Fractionated Stereotactic Radiosurgery

Purpose: The purpose of this study is to estimate technical treatment accuracy in fractionated stereotactic radiosurgery (fSRS) using extend system (ES) of Gamma Knife (GK). Methods: The fSRS with GK relies on a patient specific re-locatable immobilization system. The reference treatment position is estimated using a digital probe and a repositioning check tool (RCT). The “calibration values” of RCT apertures were compared with measured values on RCT-QA tool to evaluate the standard error (SE) associated with RCT measurements. A treatment plan with single “4 mm collimator shot” was created to deliver a radiation dose of 5 Gy at the predefined plane of a newly designed in-house head-neck phantom. The plan was investigated using radiochromic EBT3 films. The stereotactic CT imaging of a designed mini CT phantom and distortion study of MR imaging, were combined to calculate imaging SE. The focal precision check for GK machine tolerance was performed using a central diode test tool. Results: Twenty observations of RCT and digital probe, shown the SE of +/−0.0186mm and +/−0.0002mm respectively. A mean positional shift of 0.2752mm (σ=0.0696mm) was observed for twenty similar treatment settings of head-neck phantom. The difference between radiological and predefined exposure point was 0.4650mm and 0.4270mm; for two independent experiments. The imaging studies showed a combined SE of +/− 0.1055mm. Twenty frequent runs of a diode test tool showed the tolerance SE of +/−0.0096mm. If, the measurements are considered to be at 95% of confidence level, an expanded uncertainty was evaluated as +/− 0.2371mm with our system. The positional shift, when combined with an expanded uncertainty, a trivial variation of 0.07mm (max) was observed in comparing resultant radiological precision through film investigations. Conclusion: The study proposes an expression of “technical treatment accuracy” within “known uncertainties” is rational in the estimation of routine fSRS quality. The research work is supported by the research section of “All India Institute of Medical Sciences” - New Delhi, India under project no A-247.

Changes in Patterns of Intensity-modulated Radiotherapy Verification and Quality Assurance in the UK

AimsBetween 2012 and 2014 the number of patients treated in the UK with intensity-modulated radiotherapy (IMRT) techniques increased significantly. One reason for this was the radiotherapy innovation fund for the centres in England. Before the announcement of the fund, a survey of radiotherapy centres was carried out in 2012 which collected data on IMRT uptake, obstacles to implementation, equipment used, delivery techniques and verification methods. A repeat survey was carried out in 2014 to identify key changes to IMRT quality assurance and verification practices. Materials and methodsAn online questionnaire was sent out to all 65 UK radiotherapy centres in the summer of 2012 and again in the summer of 2014. Questions covered background and equipment, machine tolerance and quality assurance, machine-based verification, software-based verification and future plans. ResultsThere have been significant changes in the delivery techniques used for IMRT, with more than twice as many centres reporting the use of volumetric-modulated arc therapy techniques in 2014 compared with 2012. This has been combined with an increase in Monte Carlo-based algorithms in treatment planning systems. In 2012 all centres reported the need to carry out machine-based measurements for IMRT plan verification, dropping to 93% in 2014. Nineteen per cent of centres now report making only one measurement per month for prostate plans and 8% of breast plans never have physical measurements. Most centres use detector arrays for quality assurance measurement (86% in 2012 and 91% in 2014), but a significant number still use film and/or ionisation chambers (51% and 41%). In the analysis of these measurements there has been an increase in the use of tighter criteria. There has been a significant increase in the use of software for verification from 63% in 2012 to 95% in 2014. All centres reported that they needed further resources in order to efficiently achieve the quality assurance required for the number of patients planned to be treated in their centre. ConclusionsThe increased numbers of patients being treated with IMRT has meant that there have been significant changes in the way that quality assurance is carried out. These have been mainly in the reduction of measurements and the increase in software-based verification. However, quality assurance is still a significant burden and still has an effect on the numbers of patients who can be treated with IMRT.