|
WESCAN 2010 Opening Remarks
Scientific Session
Title: Epp: A C++ EGSnrc user code for Monte Carlo simulation of photon transport
Authors: Jonas Lippuner, Congwu Cui, Harry Ingleby and Idris Elbakri
Institute: CancerCare Manitoba
Presenting Author: Congwu Cui
Abstract:
We developed Epp (easy particle propagation), a Monte Carlo simulation EGSnrc user code to generate images of an arbitrary geometry and to calculate dose in a voxelized volume.
Epp is based on the EGSnrc C++ class library which makes modeling particle sources and simulation geometries simpler than in DOSXYZnrc and other BEAM user codes. Epp is written in C++ and integrated photon propagation to an image plane defined by the user. Furthermore, in the image plane, Epp can produce separate images of single Compton scatter, single Rayleigh scatter, and multiple scatter. Dose calculation is a reimplementation of the function of DOSXYZnrc with new features added and some restrictions removed. Analytical or voxelized geometries can be used in Epp. Epp was validated against DOSXYZnrc by comparing simulation results with the same input and was found to agree with DOSXYZnrc within statistical errors. With its capability to use analytical geometries, Epp can be at least twice as fast as DOSXYZnrc in simulating images and can model more complex geometries.
As a user code for Monte Carlo simulation, Epp can model complex sources and geometries and be used in radiation image simulations and radiation dose calculations.
Title: DICOM JuJutsu
Authors: Iulian Badragan, PhD, MCCPM
Institute: Abbotsford Cancer Centre (BCCA
Presenting Author: Iulian Badragan
Abstract:
DICOM file format has gradually become the standard for packing and exchange of medical information in digital form and has been adopted to various degrees by virtually all equipment manufacturers.
The Radiation Therapy practice often needs the ability to manipulate DICOM files in order to support its training and research needs. These needs can be very diverse, from simply anonymizing a CT set till customizing treatment plans for QA purposes or building custom CT phantoms to name only a few things. The commercial tools available for dong this, tend to be very expensive and as a rule do not cover the entire spectrum of needs.
The present work intends to share some of the knowledge accumulated by the author in time, relative to manipulating DICOM files. It presents the typical structure of a DICOM file, points to some of the free tools available to edit it and offers some practical examples. The final aim is that the information provided will enable everybody to successfully deal with DICOM data.
Title: So You Think You Can Plan
Authors: C.Field, H.Warkentin, A.Syme, K.Powell, R.Scrimger, M.Parliament, N.Jha, M.MacKenzie
Institute: Cross Cancer Institute
Presenting Author: Colin Field
Abstract:
A spreadsheet has been developed to score radiotherapy treatment plans. The ‘evaluator’ specifies: volumes of interest (VOIs, e.g. targets, sensitive structures), dose-volume constraints (DVCs), and a relative importance for each DVC. DVCs are specified in a number of ways: target cold and hot spots, dose limits to sensitive structures using absolute or relative volumes. The evaluator also specifies incentives for doing better than the DVC and penalties for failing to achieve the DVC. Additional points are scored for the treatment delivery time (for Tomotherapy), or total MUs (for linacs), and for the elapsed time to prepare the plan. The ‘planners’ (e.g. dosimetrists, medical physicists, radiation oncologists, students, residents, etc) generate their plan recording the time spent in the planning process. The scoring spreadsheet is then populated with data from the relevant DVH curves and other pertinent information. The score for each component, and the total score, is calculated and displayed. The plan can be further optimized to increase the score. This scoring system can be used for a number of purposes, including:
Planning challenge: development of planning abilities. Multiple planners generate their best plan, and then the highest score plan gets prize and explains how the plan was arrived at.
TPS challenge: different treatment planning and delivery systems can be used to generate rival plans and then they can be ranked. Training: Students must achieve a certain score on a plan before the plan is deemed acceptable. The scoring spreadsheet will be shown and explained. It will be used to compare different plans prepared by a variety of users for a head and neck cancer patient. Specification of the scoring metrics forces a clear clinical specification of the treatment planning goals, and is the most difficult step in the process.
Title: Measurements of Dosimetric leaf gap for testing MLC leaf gap accuracy in dynamic IMRT delivery
Authors: Xaingyang Mei, Ian Nygren, and Eduardo Villarreal Barajas
Institute: Tom Baker Cancer Centre
Presenting Author: Xiangyang Mei
Abstract:
MLC leaf gap accuracy in dynamic MLC delivery is critical to accurate delivery of dynamic IMRT plans. This study provides experimental results and analysis for detecting MLC leaf gap errors from measuring dosimetric leaf gap (DLG) using ion chamber and electronic portal imaging device (EPID).
DLG was derived from measurements of sliding window (SW) fields using an ion chamber placed at central axis in a solid water phantom. These measurements were performed at different gantry angles, and using different number of MLC control points. The 2D distribution of DLG was derived from measuring these SW fields using EPID. Systematic leaf gap errors of ±0.2 mm, and ±0.5 mm were introduced to test the sensitivity of detecting MLC leaf gap variations using ion chamber. These four different errors were introduced in four isolated areas (1—5 cm2) in each SW field to test the sensitivity of detecting them using EPID.
Ion chamber measurements show that DLG varies within ±0.04mm for gantry angles in a full rotation. The ion chamber measurements detected correctly the introduced leaf gap errors of ±0.2 mm and ±0.5 mm, while EPID detected correctly the errors introduced in the four isolated off-axis areas. The DLG is ~0.5mm greater for SW fields only specified starting and ending positions (2 control points) than those with control points specified every 1 cm or less for symmetrical X jaws of 12 cm width. EPID measured DLG is consistently ~0.4mm smaller than that from ion chamber measurements. This difference was found to be mainly due to under response of EPID to low dose irradiations.
We have demonstrated that both ion chamber and EPID based DLG measurements can detect small changes of leaf gaps for dynamic MLC delivery. EPID measurements of SW fields are useful to check leaf gap consistency in 2D.
Title: Electronic compensation at extended SSD for TBI, a preliminary study
Authors: Amjad Hussain and Jose Edaurdo Villarreal-Barajas
Institute: Tom Baker Cancer Centre
Presenting Author: Jose Edaurdo Villarreal-Barajas
Abstract:
The accuracy of dose calculations at extended SSD is critical for the planning of total body irradiation (TBI). In a first step toward the implementation of electronic, multi-leaf collimator compensation for dose inhomogeneities and surface contour in TBI, we have evaluated the ability of the Eclipse AAA to accurately predict dose distributions in water at extended SSD. For this purpose, we use the Eclipse AAA algorithm, commissioned with a 6 MV photon beam data measured at standard SSD (100 cm). The model was then used to calculate dose distribution in water at extended SSD (180cm). The confidence limits derived for profiles and absolute depth dose curves were within 2% and 2 mm for 5x5, 10x10, 20x20 and 40x40 cm2 field sizes. As an extension of this work dose distribution measurements in an inhomogeneous anthropomorphic phantom (Rando®) at extended SSD (185 cm SSD) were performed using TLDs and radiochromic films. An opposed beam pair (AP-PA) was delivered at a zero gantry with a large field (55x42 cm2) to cover the head down to the lower chest. The agreement between measurements and calculations for these setup was within ±2 % except for the lung region where up to 4.9% dose overestimation was predicted. Breitman K. et al. have previously reported a similar dose overestimation of the dose to the chest region in Rando® at standard SSD. In order to achieve a more uniform dose distribution in Rando, a plan using a single large field and the irregular surface compensation as implemented in the EclipseTM AAA was used. The calculated dose homogeneity was significantly improved particularly in the chest-lung region. Overall these dose calculations agree within ±3% with TLD and Radiochromic films measurements. These results indicate the potential clinical use of the irregular surface compensation for TBI at extended SSD.
Title: Tentative: Clinical experience with EBT2 gafchromic film at CCSI
Authors: Jurgen Last, Cynthia Araujo, Alistair Baillie, Stuart Burnett, Larry Watts, Matthew Schmid
Institute: Cancer Centre of the Southern Interior (BCCA)
Presenting Author: Dr. Jurgen Last
Abstract:
At the CCSI we use Gafchromic® EBT2 film and ion chambers for patient IMRT QA. EBT2 became available in late spring 2009 and production of it's predecessor EBT ceased. Although EBT2 is advertised as a drop-in solution for the older product, reports about significantly lower gamma scores in IMRT QA and other issues soon emerged. In this paper we describe our hardware and software choices, film processing chain, calibration options and present data that highlights various steps in our IMRT QA process.
Title: Using PCXMC to calculate organ and effective doses for digital tomosynthesis
Authors: J.M. King, I.A. Elbakri
Institute: CancerCare Manitoba
Presenting Author: Jenna King
Abstract:
PCXMC is a Monte-Carlo-based software application for calculating organ and effective doses for diagnostic imaging exams. To calculate these values accurately for digital tomosynthesis (DT), where a series of discrete projection images are acquired as the x-ray tube is swept through a limited angle across the patient, we have to consider each discrete projection angle separately and sum the results, which is a complicated and time-consuming task. Completing the full sweep simulation is particularly important for determining individual organ doses, as organs might move in or out of the field of view over the course of the sweep. However, we hypothesized that neglecting the motion of the x-ray tube may have little effect on the overall effective dose calculation, and by applying the sum of the incident air kerma for all projections to a single central projection we can obtain the effective dose more efficiently. We tested this hypothesis for both a lateral lumbar spine and a posterior-anterior facial bone DT exam using a 15-year-old patient in PCXMC. We calculated and compared the organ and effective dose results using both methods. The full sweep method resulted in effective doses of 1.434+/-0.001mSv and 1.389+/-0.001mSv for the lumbar spine and facial bone exams respectively, while the quick method yielded 1.434+/-0.009mSv and 0.1385+/-0.009mSv. As expected, we observed some differences in the dose to individual organs of interest. The differences were generally less than 10% for most radiosensitive organs. For organs that experience large dose variations over the sweep, the difference between the quick method and the more accurate sweep calculation was large. By neglecting the motion of the x-ray tube during DT, the dose to an individual organ may be over- or under-estimated but it does provide an efficient method for acquiring an effective dose value for a DT exam.
Radiation Therapy Session
Title: Radiation Therapy Workflow – Treatment Planning Aspects
Authors: Mike Darud ACT, BSc
Institute: Vancouver Cancer Clinic, BC Cancer Agency
Presenting Author: Mike Darud ACT, BSc
Abstract:
Radiation Therapy, in general, has not been significantly studied to understand what the factors that impact work flow are. In this study the treatment planning aspect of radiation therapy was studied to assess which factors may have an influence on workload and resources.
Ten factors (complexity, skill of planner, number of fields, training, Radiation Oncologist, intent of treatment, standard technique, time from CT scan to start of treatment, time from CT scan to RO work being completed, time from patient being ready to treat to start of treatment) were available either through the Hospital Information System or a dedicated database. These factors were collected from September 1st, 2006 to June 30th, 2008 and analyzed with descriptive statistics and regression analysis. The main focus of study was the breast, prostate and head and neck sites.
From the descriptive statistics, plans with lower complexity take less time (average and median values) to complete than more complex plans. Plans that do not meet standard criteria take longer to plan as discussion and consultation needs to take place with the RO prior to completion of the plan. The number of fields in a plan seems to influence planning time as we see an increase in the planning time as the number of field’s increases. From the regression analysis, the skill of the planner and the number of fields play the most significant role in predicting the planning time. Three time frames captured within the dataset appear significant and indicated that efficiency could be gained by reducing the time frames.
Information such as this allows us to understand what factors in the clinical setting affect planning time. Knowing these, we are able to better plan our staffing and predict our workload.
Title: Equivalent Uniform Dose (EUD) as a planning tool for IMRT
Authors: Joe Andreas BSc, RTT, CMD
Institute: Saskatoon Cancer Centre
Presenting Author: Joe Andreas BSc, RTT, CMD
Abstract:
Equivalent Uniform Dose (EUD) is based on the concept that different dose distributions could be considered equal if the clinical effect is equal: that is to say that an equivalent uniform dose is a homogenous dose that has the same effect as a heterogeneous dose. This concept has been taken a step further as a means to quantify dose distributions. The Pinnacle treatment planning system (tps) incorporates a version of EUD which has been in use for several years at the Saskatoon Cancer Centre (SCC).
The Pinnacle tps EUD tools are intended to manipulate maximum dose, minimum dose and mean dose. Our experience shows that a significant benefit from EUD is in the area of mean dose reduction. A further advantage of using EUD is that it provides a useful tool for evaluation of complicated dose volume histogram information and helps to increase the solution space for IMRT planning (compared to other available tools).
The intent of this presentation is to explain the basic principles behind EUD (and the Pinnacle tps version) as it is used at SCC and to provide some insight into our current clinical use of EUD as a planning tool.
Title: A Prospective Evaluation of the Informational Content of Setup Verification Images for Patients Receiving Breast Radiation Therapy Tangentially With or Without Supraclavicular Treatment
Authors: Veronica Patenaude (BSc, ACT) Cindy Golling (BA, RTT) Derek Wells (PhD), Leigh McGovern, Dorothy Sayers, Dr. Elaine Wai (MD)
Institute: Vancouver Island Cancer Centre
Presenting Author: Veronica Patenaude BSc, ACT
Abstract:
Standard radiation therapy treatment for breast cancer patients consists of tangential beams treating the whole breast or chestwall to minimize the amount of healthy lung receiving radiation. Patients requiring treatment to axillary and supraclavicular nodes are treated with a mono-isocentric breast technique incorporating the tangents combined with anterior supraclavicular field and posterior axilla boost when required. Current practice at this clinic is to verify patient positioning by using electronic portal imaging (EPI) for the medial and lateral tangential fields. The mono-isocentric technique will utilize an anterior EPI to verify the supraclavicular field.
A study at the clinic analyzing digitally reconstructed radiographs of medial tangential field versus anterior reference field with deliberate translational and rotational setup errors simulated indicated the detection of setup errors improve with the use of an anterior reference image. The new imaging protocol utilizing an anterior EPI with a lateral tangential EPI appears to be a more sensitive method for detection of set-up errors than the current protocol of tangential EPIs.
The study has continued to our current trial of 200 patients receiving breast radiation therapy utilizing an anterior reference field to evaluate the proportion of patients who would benefit from the increased setup error detection. Based on preliminary data of first 50 patients with this new imaging protocol, 20 subjects have required a move because of set-up variation outside of the 5 mm tolerance limit. This is significantly higher than the current protocol in which only 1 subject was detected to be outside of the 5mm tolerance limit. Re-imaging of the subject set-up after the moves were made showed that the standard deviation of the errors was decreased in the anterior-posterior and medial-lateral directions with the new protocol, compared to the current protocol. A 1-sided paired t-test for the subjects that were re-imaged using both protocols indicated that the error reduction in both the anterior-posterior and medial lateral directions was statistically significant. The corrective moves based on the new protocol appear to reduce systematic setup uncertainty more effectively than the current protocol.
Title: Feasibility study of treatment of Whole Brain and Brain metastases with the Varian RapidArcTM
Authors: Rosemin Vellani RTT
Institute: Vancouver Cancer Clinic, BC Cancer Agency
Presenting Author: Rosemin Vellani RTT
Abstract:
Background:
Patients with Brain metastases requiring Whole Brain radiation are currently treated with a parallel pair to the Whole brain and Stereotatic RadioSurgery to the metastases. Stereotactic RadioSurgery uses many non-coplanar beams with multiple isocentres to deliver high doses to small targets within the brain. Due to the complexity of this technique, it can take anywhere from forty to ninety minutes to deliver treatment. RapidArc uses a single isocentre with one or multiple arcs with variable gantry speed, variable dose rate and Dynamic Multileaf Collimators to deliver treatment. The goal of this study was assess the feasibility of using RapidArc to produce clinically acceptable plans for these patients, thus offering a simpler and more efficient treatment delivery.
Methods:
Ten patients were planned using two 6 MV arcs to treat multiple target volumes in a single plan. The oncologist contoured all Organs At Risk (OARs) and Planning Target Volumes (PTVs) - PTV metastases, PTV metastases plus margin and PTV whole brain. The prescribed dose was 5000cGy to the PTV metastases, 4000cGy to PTV metastases plus margin and 2000cGy to PTV whole brain in five fractions. The dose to the optic chiasm and retinas was limited to less than 1% volume to receive 2500cGy, the brainstem less than 1% volume to receive 3000cGy and the lenses to a maximum dose of 1000cGy. The metastases varied in size, location and number for each patient. The plans were deemed clinically acceptable if 95% of the prescribed dose was received by 98% volume of all targets and OARs met the set dose constraints.
Results:
The RapidArc plans were clinically acceptable meeting all dose constraints for PTVs and OARs.
Conclusion:
Delivery of treatment using the RapidArc technique is more efficient than the current method of treatment. RapidArc improves patient comfort by quicker delivery and fewer visits. Planning and treatment resources are also reduced.
Title: Treatment of Nasopharyngeal cancers with Intensity Modulated Radiation Therapy and the Varian RapidArcTM
Authors: Yick Ming Fong BSc, RTT, CMD
Institute: Vancouver Cancer Clinic, BC Cancer Agency
Presenting Author: Yick Ming Fong BSc, RTT, CMD
Abstract:
To compare the treatment of Nasopharyngeal cancers(NPC) using Intensity Modulated Radiation Therapy (IMRT) and the Varian RapidArc at the British Columbia Cancer Agency, Vancouver Cancer Centre.
IMRT was implemented for the treatment of NPC in 2003 at the Vancouver Cancer Centre(VCC). IMRT has proven to be beneficial, delivering the desired tumour doses, whilst sparing the critical structures and normal tissue, thus improving the quality of life for these patients.
In March of 2008, RapidArc was introduced to VCC for β-testing. RapidArc is a revolutionary IMRT technique developed by Varian Medical Systems. A feasibility study was done to investigate the potential benefits if any, in the use of RapidArc for clinical treatment of NPC. Ten nasopharyngeal patients were planned with both conventional IMRT and a single rotation of RapidArc. The quality of the plans was then evaluated based on Planning Target Volumes (PTVs) coverage, isodose distribution, dose conformality, critical structure sparing, dose volume histograms and monitor units. The impact on planning and treatment resources was compared between RapidArc and conventional IMRT. The results showed that in six out of ten patients, an extra salivary structure was spared to a clinically relevant dose using RapidArc.
In October of 2009, a single rotation RapidArc delivery technique was implemented for the treatment of NPC. The single arc technique produced equivalent or better sparing of critical structures relative to IMRT, and the monitor units delivered were significantly lower. A preliminary investigative study using a double arcs technique has shown further improvement on PTVs coverage and dose sparing to critical structures. The results of this planning study will be presented, comparing the various delivery techniques.
Title: Decreasing Treatment Times for IMRT Treatments using “Step and Shoot” Beams
Authors: Karen L. Davis BSc(T), RTT, CMD
Institute: Saskatoon Cancer Centre
Presenting Author: Karen L. Davis BSc(T), RTT, CMD
Abstract:
IMRT (Intensity Modulated Radiation Therapy) Treatments for many sites are becoming more popular, however the time it takes to deliver an IMRT plan puts more pressure on the treatment units to get through the daily patient workload. In our study we looked at head and neck treatments and what inverse planning parameters could be used to help decrease the time for treatment while keeping the same quality of plan. We timed how long it took for treatment from “moded-up” to the last beam off on plans that we considered to be optimal and were created with our standard inverse planning parameters of, minimum MU’s of 2, Segment number of 100-120 and minimum segment size of 2cm. We then re-planned with decreased segment number, increased segment size, and increased number of minimum MU’s. We then compared both plans in the three different 2D views (axial, sagittal and coronal) and the DVH’s, to ensure that the quality of the plan was still acceptable i.e. homogeneity of dose, hot spots in the right places and normal tissue dose still within tolerance. We then started looking at prostate and brain and compared the plans to see if we could do the same with these sites. We found that we could shorten delivery times by using the new inverse planning parameters of increased minimum number of MU’s, increased segment size and decreased segments. We also found that though we could do this for head and neck, and prostate but with brain we needed the small segment size to conform to the PTV and keep other ROI (Regions of Interest) within tolerance.
Title: Comparison of Planar vs. Volumetric Image-Guidance and IntrafractionMotion in Stereotactic Lung Radiotherapy
Authors: Shannah Murland, Zsolt Gabos, Harold Lau
Institute: Cross Cancer Institute, Tom Baker Cancer Centre and Alberta Health Services
Presenting Author: Shannah Murland
Abstract:
The implementation of hypofractionated radiotherapy creates challenges in treatment accuracy and organ motion management due to the high dose-per-fraction that is administered. A stereotactic protocol for patients with early stage non-small cell lung cancers is in use with treatment schedules of 4800 cGy/4 fractions or 6000 cGy/10 fractions. These patients are treated on Varian® linear accelerators equipped with On-Board Imaging® and Real-Time Position Management® in order to apply precise image-guidance and monitor respiratory motion during imaging and treatment.
Pre-treatment imaging was changed from kilovoltage orthogonal pairs to volumetric cone-beam CT (CBCT) during phase I of this protocol. CBCT allows high quality visualization of tumour tissue and the longer acquisition time better represents tumour positions throughout the respiratory cycle and allows direct tumour matching. kV imaging uses a bone match that may not correlate with tumour position. Patients were imaged using both modalities to evaluate whether the difference in treatment accuracy justifies the increased time and patient dose with CBCT. The mean difference in isocentre position in 162 treatments was 0.54 cm, with a maximum directional change of 1.9 cm. This demonstrates the importance of soft-tissue matching in hypofractionated radiotherapy to prevent geographical miss.
Treatment times for stereotactic lung radiotherapy can be over thirty minutes, so maintaining position can be difficult. Image-guidance improves treatment accuracy but adds time that may increase intrafraction motion. A CBCT performed immediately after treatment allowed for measurement of patient movement during these lengthened treatment times. Ninety-four CBCT’s on 18 patients gave an average movement of 0.28 cm, with a maximum position change of 0.68 cm. Correlations between intrafraction motion and various immobilization devices show that patient positioning is crucial for preventing movement.
Additional imaging applied to new treatment protocols has been shown to aid in quality assurance and process development of image-guidance and immobilization techniques in stereotactic body radiotherapy, and has demonstrated the importance of evaluating new technologies before full clinical implementation.
Graduate Student Session
Title: Motion Estimation Using Cone-Beam CT Projection Images
Authors: Nathan Becker, W.L. Smith, Sarah Quirk, Ian Kay
Institute: Tom Baker Cancer Centre
Presenting Author: Nathan Becker
Abstract:
Purpose:
To estimate the trajectory of a fiducial marker that moves with respiration using cone-beam CT (CBCT) projection images.
Methods and Materials:
A CBCT of a fiducial marker with realistic respiratory motion was simulated with MATLAB®. A reconstruction algorithm was developed that first binned the CBCT projection images based on the phase of the fiducial marker motion. The average trajectory was determined by intersecting images at the same phase. To determine the complete trajectory, a 3D coordinate was estimated from each 2D projection image in the CBCT dataset. The unknown 3D position was located on a ray from the source to the fiducial coordinates in the image, and this point was estimated by choosing the point that was closest the average trajectory. This was repeated for all projection images, and the result was an estimate of the entire seed trajectory during CBCT acquisition.
Results:
Simulation testing was done on 32 realistic breathing patterns. The average root mean square errors between the true and reconstructed trajectory were 0.01 mm, 0.18 mm, and 0.24 mm in the superior-inferior, anterior-posterior, and left-right directions.
Conclusions:
There is motion information present in the raw CBCT dataset that can be exploited with the use of an implanted fiducial marker. Using a computer simulation, we have shown that average and complete trajectory of the moving fiducial marker can be reconstructed. This could provide the internal motion of a lung tumour at the treatment unit from the same dataset used for patient setup.
Title: Bench-Top Validation of GATE Simulations of Lu-176 Intrinsic Activity in LSO Detectors
Authors: B. McIntosh, A. L. Goertzen
Institute: University of Manitoba
Presenting Author: B. McIntosh
Abstract:
Lutetium-based scintillators such as LSO and LYSO have become the scintillator of choice for PET imaging in the past decade due to their high stopping power, high light output, and fast decay time. These scintillators also contain a low level of intrinsic radioactivity due to the presence of Lu-176. This activity does not measurably effect routine clinical scanning but the effects can become significant when imaging with wide energy windows or low count rates such as in cell trafficking studies involving small animal imaging. To date, no systematic validation of Monte Carlo simulations of the effects of Lu-176 activity has been performed, making it difficult to incorporate them into the design and simulation of a proposed scanner. This study seeks to validate GATE (Geant4 Application for Tomographic Emission) simulations of the Lu-176 intrinsic activity in LSO-based detectors against data gathered from a bench-top characterization of Siemens Inveon detectors. Measurements from two opposing detector modules were acquired using NIM based electronics and a PC based data acquisition (DAQ) card. The detectors were characterized by determining the count rate in coincidence mode at various distances while stepping the lower energy discriminator and by generating the energy spectra for singles and coincidence measurements for each crystal in the detector. Monte Carlo simulations were performed using GATE to reproduce the geometry of the bench-top measurements made with the two detectors, modelling the intrinsic activity of the Lu-176 as an ion source located within the scintillator crystals. Good agreement was found between the simulated and measured energy spectra, and count rates agreed within 6% or better except for cases where the energy window is set near an intrinsic gamma ray photopeak. This simulation model is now being extended to a full Inveon PET system to ensure that it holds for a complete PET scanner.
Title: A Monte Carlo Investigation of the Effects of a Novel In-vivo Transmission Detector
Authors: G. Asuni and B.M.C McCurdy
Institute: CancerCare Manitoba
Presenting Author: G. Asuni
Abstract:
Purpose:
The transmission detector, a new IMRT quality assurance tool to be used for in vivo dose measurements. As a scattering material, the device will be a source of contaminant electrons and could potentially affect prescribed dose to patients during treatment. The goals of this investigation are to characterize radiation particles in a clinical photon beam and to understand the effect of the transmission detector on typical IMRT treatment fields using Monte Carlo simulation.
Methods:
The linear accelerator head together with the transmission detector (TRD) was modeled using BEAMnrc code. Radiation particles scored at 70 cm and 100cm SSD for different field sizes (i.e. 5 x 5 cm2, 10 x 10 cm2, and 20 x 20 cm2) with and without the TRD were separated according to where they last interacted (photons) and where they were created (electrons) in the linac head. They were categorized as focal photons, extra-focal photons, and contaminant electrons. In addition, two IMRT fields with and without the TRD were simulated and their respective 2D calculated absolute dose distributions were compared.
Results:
The extra-focal photons relative to focal (primary) photons from the primary collimator with and without TRD are not field-size dependent, but those from the flattening filter are field size-dependent at 70 cm and 100 cm SSD. Without the TRD, air is the major source of contaminant electrons. When TRD is in the beam path it absorbs nearly all the incident contaminant electrons, but becomes the major source of contaminant electrons at 70 cm and 100 cm SSD. For both IMRT fields, the percentage dose differences of the calculated absolute doses with and without TRD at the isocenter were found to be comparable to the measured transmission factor for the TRD.
Conclusions:
Although a major source of contaminant electrons like the block tray (earlier studies), the TRD does not introduce excessive electron contamination into a clinical 6 MV photon beam and is therefore suitable for in vivo dose measurements if fully commissioned as an IMRT quality assurance tool.
Title: Light Collection and Spatial Resolution of long LYSO Crystals for a Compact PET Scanner
Authors: Fazal ur-Rehman, Bryan McIntosh and Andrew L. Goertzen
Institute: University of Manitoba
Presenting Author: Fazal ur-Rehman
Abstract:
Preclinical PET systems can image animal models of human disease that are used to evaluate new therapeutic strategies for the treatment of cancer and other diseases. We are investigating using dual-ended readout of axially-oriented long thin scintillator crystals in detectors for a compact geometry, small ring diameter preclinical PET system. In this work we examine the light collection, axial-positioning resolution and energy resolution of 1.5-5 × 2 × 100 mm3 LYSO scintillator crystals readout at either end by Hamamatsu R8900U-00-C12 position sensitive photomultiplier tubes (PSPMTs). An electronically collimated beam of 511 keV photons oriented perpendicular to the long axis of the crystal is used to irradiate crystals of width 1.5, 2, 3, 4 and 5 mm at locations every 1 cm from 1-9 cm from one end. The summed signal of the two PMTs is used to calculate the light collection and energy resolution for each position and the ratio of the two PMT signals (light-sharing) is used to determine the axial-positioning calibration and axial resolution in the light sharing direction. For a polished, Teflon-wrapped crystal, the light collection increases with the crystal cross-section. The mean energy resolution (range) and standard deviation of the photopeak over the nine irradiation positions was 14.5(13.4-15.6)±0.8%, 16.0(14.3-18.0)±1.2%, 14.9(14.0-16.1)±0.7%, 14.5(14.0-16.1)±0.7% and 13.7(12.9-14.8)±0.6% while the mean axial resolution (range) and standard deviation over the nine irradiation positions was 4.3(2.2-6.3)±1.5 mm, 4.7(2.5-7.0)±1.7 mm, 5.0(3.0-7.3)±1.6 mm, 4.9(2.7-7.1)±1.6 mm and 5.3(2.6-8.2)±2.1 mm for crystal sizes of 1.5, 2, 3, 4 and 5 mm, respectively. Generally, the axial resolution improves with decreasing crystal cross section due to more loss of light along the axis of the crystals. These characterization results of PSPMT-based dual-ended long LYSO crystals will be useful in the design of detector modules for a compact geometry, small ring diameter preclinical PET scanner system using this detector technology.
Title: Comparison of Organ Doses Between PCXMC and Phantom Measurements for Neonates
Authors: Hillgan Ma, Azeez Omotayo, Jenna King, Idris Elbakri
Institute: CancerCare Manitoba
Presenting Author: Hillgan Ma
Abstract:
Purpose:
Monte Carlo (MC) simulations and phantom based measurements are essential methods to estimate patient dose. We conduct a comparison of neonatal organ doses calculated by the MC-based software, PCXMC, with TLD measurements obtained using an anthropomorphic phantom.
Methods and Materials:
We used TLD-100H by Harshaw as the primary dosimeter for the phantom, calibrated under beam conditions similar to those used in the neonatal intensive care unit. Using a pediatric newborn phantom (ATOM, model 703D), TLDs were placed at organ locations provided by the phantom manufacturer, 4 TLDs per dose site. The data was collected, averaged, and then converted to organ dose values. These values were then compared to PCXMC values produced using the same exposure parameters. The parameters were chosen based on NICU images and their settings (60kVp, 0.5mAs, and 3mm Al filtration). We considered the dose to the lungs, liver, esophagus, stomach, pancreas, ovaries and uterus.
Results:
For Lungs, Liver, Esophagus, Stomach, Pancreas, Ovaries, and Uterus, we measured doses of 0.0203mGy, 0.0263mGy, 0.0138mGy, 0.0266mGy, 0.0181mGy, 0.0183mGy, and 0.0211mGy respectively. PCXMC for the same organs were 0.0217mGy, 0.0270mGy, 0.0125mGy, 0.0303mGy, 0.016mGy, 0.02mGy and 0.0195mGy respectively. This yields a percent difference of 6.6%, 2.49%, 10.25%, 12.93%, 11.95%, 8.99% and 8.30% respectively.
Conclusions:
For the organs considered, our results show good agreement between PCXMC and TLD phantom measurements. We have observed that there is usually good agreement for larger organs and when organs are fully irradiated. Our results suggest that PCXMC can be used for organ and effective dose estimation for neonatal phantoms.
Title: Are failures of patient-specific IMRT verification related to planning algorithm deficiencies?
Authors: J. Crawford, W. Ansbacher, K. Bush, A. Jirasek, and I.M. Gagne
Institute: Vancouver Island Cancer Centre (BCCA)
Presenting Author: Jason Crawford
Abstract:
Background:
Since 2006, VIC has used portal image (PI) -based 3D dose reconstruction for patient-specific IMRT QA. The rate of plans exceeding the set action levels has increased in recent years, coinciding with increased fluence modulation for H&N IMRT plans. In order to determine whether the clinical calculation algorithm (PBC) was contributing to the discrepancies between calculated and reconstructed dose distributions, QA results were compared to those obtained using more accurate calculation methods, AAA and Monte Carlo.
Methods:
24 clinical verification plans exceeding the set IMRT QA action levels were redelivered in order to assess the reproducibility of the results. Epidose, an in-house software package written in Matlab, was used to reconstruct 3D dose for each verification plan by convolving the portal images with an empirically derived scatter kernel and then projecting the dose through a virtual cylindrical water phantom. Plans reproducibly failing were recalculated using AAA (Analytic Anisotropic Algorithm) and Monte Carlo (0.5% uncertainty). Each planned dose distribution was then compared to the same reconstructed dose.
Results:
50% of the plans consistently failed when reanalyzed using EPIDOSE. A number of issues related to measurement error were identified as possible reasons for unrepeatable clinical failures. For the 12 reproducible failures, the mean reference point dose difference between EPIDOSE and PBC (AAA) was -2.5% ± 0.9% (-1.0% ± 0.9%), the mean point dose difference of the high dose region was -1.5% ± 0.5% (-0.6% ± 0.6%), and the mean percentage of voxels within tolerance was 87.4% ± 5.5% (95.7% ± 3.8%). Monte Carlo and AAA agreed for all three metrics, and all 12 plans passed in terms of these action levels.
Conclusions:
Replacing the PBC algorithm with more accurate calculation means, namely AAA and Monte Carlo, provided significant improvements to QA results for H&N IMRT plans. AAA has subsequently become the standard clinical calculation method.
Theme Day Session
Title: Using PET/CT imaging data to help guide radiotherapy: Pearls and Pitfalls
Institute: Department of Radiology, University of Manitoba
Presenting Author: Dr. S. Demeter MD, MHSc, MSc, FRCPC
Abstract:
Positron Emission Tomography (PET) imaging allows the in-vivo visualization, assessment, and quantification of tissue/organ biological activity. Although this technology is not new the use of PET derived data to influence clinical management is becoming more prevalent. Radiolabeled sugar (i.e. Fludeoxyglucose [18F] or FDG) is the only Health Canada approved PET tracer. All other tracers are used in clinical trial settings. FDG PET scanning has been demonstrated to be very helpful in staging many cancers and in assessing response to therapy in a smaller subset. The use of FDG PET imaging to inform radiotherapy is a newer phenomenon. This talk will discuss the utility and constraints of the use of FDG PET/CT data to help inform radiotherapy planning. There will also be a discussion on the use of FDG PET/CT data as a tool to assess response to therapy.
Title: Did Radiation Oncology Do Well?
Institute: Radiation Oncology, Cancercare Manitoba
Presenting Author: Dr. E. Lyn MBBChir, MRCP, FRCR, FRCPC
Abstract:
In the last decade there have been enormous advances in technical radiotherapy; multi-leaf collimation, electronic portal imaging, cone beam CT, 3D conformal radiotherapy, Intensity Modulated Radiotherapy, Image Guided Radiotherapy, Adaptive Radiotherapy. There is a myriad of immobilisation devices and devices to allow tracking of disease.
However, although the new technology was embraced, assumptions were made and uncertainties not addressed and results of treatment may be improved if they were not neglected.
Title: CLINICAL UNCERTAINTIES IN RADIATION ONCOLOGY
Institute: Radiation Oncology, Cancercare Manitoba
Presenting Author: Dr. A. Cooke MD, BSc(Med), FRCPC
Abstract:
Medical Physicists have long been aware of uncertainty in patient and tumor imaging, patient set up and dose measurement. New technologies such as IMRT, IGRT, PET, and magnetic resonance spectroscopy have, or will allow unprecedented degrees of precision in the delivery of radiation therapy. However clinical and biological uncertainty is pervasive in Oncology and is orders of magnitude greater than Physics related uncertainty. Inter-personal (host) genetic variability, inter- and even intra-tumoral genetic variability, optimal treatment dose and volume uncertainty, and even whether treatment is indicated at all are issues that clinicians struggle with daily. Until these biological uncertainties are resolved, further technical improvements in radiation therapy delivery will have little incremental impact on tumor control or toxicity.
Title: A Hitchhiker’s Guide to Oncology
Institute: Epidemiology/Population Oncology, Cancercare Manitoba
Presenting Author: Dr. Donna Turner PhD
Abstract:
Cancer is a significant public health concern for Canadians, with an estimated 171,000 new cases and 75,300 deaths occurring in 2009. The perspectives we have on cancer, what we know about it and what to do about it, can be grouped into three broad categories: basic science, clinical, and population health. This presentation will weave together information from each of these viewpoints to provide an overview of cancer as it relates to those of us who work or live with this disease – “a hitchhiker’s guide” of sorts. Highlights include an introduction to the basic biology and terminology of cancer, a basic “statistical survival kit” and what is known about its risk factors.
Title: Normal Tissue Tolerance Doses: Do We Know Our Limit(ation)s in the Modern Era
Institute: Radiation Oncology, Cancercare Manitoba
Presenting Author: Dr. Aldrich Ong, MD, MSc, FRCPC
Abstract:
When radiation therapy was initially used to treat cancer, the technology was limited in its ability to image and target radiation specifically to a tumor while limiting the dose to adjacent healthy tissues. Technology has now evolved significantly, allowing for treatments to be targeted more directly to the tumor and minimizing exposure to surrounding tissues. However, the knowledge regarding the radiation tolerance of normal tissues is limited, uncertain, and still largely based on initial recommendations published in 1991. The challenges of setting normal tissue tolerance doses for treatment planning will be explored and discussed in this presentation. The most recent and most comprehensive attempt to update recommendations for safe irradiation of normal organs will also be presented.
RapidArc Workshop
|