Upstate New York Association of Physicists in Medicine, Inc.
(A Chapter of the AAPM)
Fall Meeting – Monday  November  17, 2008
TWIG Auditorium, Rochester General Hospital,

1425 Portland Ave, Rochester, NY 14621

Major Meeting Sponsor: Upstate Linac Services, LLC

12:00 pm – 5:00 pm    Sponsoring Vendor Exhibits:

Elekta Inc.                             Varian         Phillips                         Accuray      TomoTherapy, Inc.

Upstate Linac Services, LLC         LACO          Sun Nuclear                                               

                       

10:00

Business Meeting

11:30

Lunch                                                                                 Sponsored by Phillips

12:00

Refreshments and Vendor Exhibits – Sengupta Room        Sponsored by Sun Nuclear

12:30

Meeting Introduction

Shivaji Deore, Ph.D., DABMP, UNYAPM President

Vendor Session

12:40

Static Gantry/Fixed Angle with Sliding Couch Delivery and Machine Digital QA for TomoTherapy HiART System

 

Lou Sesto, TomoTherapy Inc.

12:55

Rotational IMRT QA Now and the Future

James Ernsberger, Sun Nuclear.

Proffered Paper Session

1:10

A Proposed Correction Method For Portal Dosimetry Errors Near The Detector Edge

D W Bailey, L Kumaraswamy, and M B Podgorsak

Roswell Park Cancer Institute (RPCI), Buffalo, NY

1:25

Monte Carlo Dose Calculations on the GPU: A Feasibility Study

J P Steinman, M Bakhtiari, H K Malhotra, V Chaudhary, D P Nazareth, RPCI, Buffalo, NY

1:40

On the Beam Orientation Optimization using Genetic Algorithm

M Bakhtiari, M D Jones, H K Malhotra, M B Podgorsak, and D Nazareth, RPCI, Buffalo, NY

1:55

Refreshments and Vendor Exhibits – Sengupta Room        Sponsored by Sun Nuclear

2:30

A Guidance System for Optical Patient Alignment During Breast Radiotherapy

J. Schmitt, K R Hoffmann, M Bakhtiari, D Nazareth , H Malhotra, RPCI, Buffalo, NY

 

2:45

Dosimetric Analysis of the Effect of Tungsten Shields in a Fletcher-suit Delclos Applicator in High- Dose-Rate brachytherapy using Gafchromic Film,         

 

T R Stanley, W Jaggernauth, H K Malhotra, RPCI, Buffalo, NY

3:00

Target Dose during Defocused Patient Repositioning with the Gamma Knife Automatic Positioning System (APS)

T Tran, T R Stanley, H K Malhotra, D Prasad, M B Podgorsak, RPCI, Buffalo, NY

3:15

Algorithm for Hyperfast GPU-Based Cone Beam Computed Tomography.

P B. Noël, A M Walczak, K R Hoffmann , J Xu, J J Corso, S Schafer, University at Buffalo, Buffalo, NY

3:30

Rotational micro-CT on a clinical C-arm gantry system

V. Patel, K. R. Hoffmann, C. N. Ionita, C. Keleshis, D. R. Bednarek, S. Rudin, University at Buffalo, Buffalo, NY

 

3:45

Automated Calibration of an Angiographic Imaging System for the Reconstruction of 3D Vessel Centerlines

A M Walczak, K R. Hoffmann, V Singh, N Dashkoff, M Kassab, V S Iyer, University at Buffalo, Buffalo, NY

4:00

Refreshments and Vendor Exhibits – Sengupta Room        Sponsored by Sun Nuclear

                                                          Invited Speaker Session

4:20

Endovascular Image-Guided Interventions: Current and Future

S Rudin, University at Buffalo, Buffalo, NY

4:40

Life time achievement award winner Introduction

Steve de Boer, MS, UNYAPM Past President

4:45

"Medical Physics: Then and Now"

Lawrence N. Rothenberg, Ph.D.

5:30

Award Presentation

Shivaji Deore, Ph.D., DABMP, UNYAPM President

 

 


Driving Directions to Rochester General Hospital:

 

Directions (to Rochester General Hospital):

 

From the West:  New York State Thruway to Exit 47.  490 East to 390 North to 104 (Ridge Road) East to Carter Street exit.  Follow service road to Hospital entrance.  Parking is available in the Ramp Garage.

 

From Rochester Airport (ROC):  390 North to 104 (Ridge Road) East to Carter Street exit.  Follow service road to Hospital entrance.  Parking is available in the Ramp Garage.

 

From the East:  New York State Thruway to Exit 45. 490 West to 590 North to 104 West to Goodman Street/Portland Avenue exit. Follow service road to Portland Avenue and turn left. Hospital is on the right. Parking is available in the Ramp Garage.

 

From the South:  390 North to 590 North to 104 West to Goodman Street/Portland Avenue exit. Follow service road to Portland Avenue and turn left. Hospital is on the right. Parking is available in the Ramp Garage.

 

 

Parking is available on 3rd and 4th floors. After parking, please take Elevator to Hospital Main Entrance. The TWIG auditorium is located right after front desk.

 

 

 

  

  

 

                                               

 

 

 

 

 

UNYAPM SPRING MEETING PROCEEDINGS

Rochester General Hospital, Rochester, NY

November 17, 2008

 

Static Gantry/Fixed Angle with Sliding Couch Delivery and Machine Digital QA for TomoTherapy HiART System

Lou Sesto

TomoTherapy Inc.

 

Objective:  Present the rationale, clinical indications and implications for productivity by implementing Static Gantry/Fixed Angle beams with moving couch technique.

 

Present the rationale, scope and practical implications for implementation of automated digital quality assurance on a daily, monthly, yearly and as needed basis.

                                                                                                                              

 

A Proposed Correction Method For Portal Dosimetry Errors Near The Detector Edge

D W Bailey, L Kumaraswamy, and M B Podgorsak

Roswell Park Cancer Institute, Buffalo, NY 14263

 

Purpose: Portal dosimetric images acquired with an electronic portal imaging device (EPID) may be used for IMRT pre-treatment verification by comparing the acquired portal dose images of IMRT fields to their respective portal dose predictions created by a treatment planning system (TPS). However, it has been reported that portal images from IMRT fields near the portal dose detector edges may result in dose values as much as 10-15% higher than those predicted by the TPS. Verification using other IMRT QA methods (e.g. ion chambers, films, etc.) confirms that these fields are in fact delivered accurately, and that the high field-edge dose values are due to over-response of the portal dosimetry system. In this study, a method is proposed and demonstrated which corrects for the dose errors near the edge of the portal dose detector, resulting in high conformity between the acquired and predicted portal dose images for IMRT fields near the edges of the detector.

Method and Materials: The procedures for this study were conducted using the Varian Portal Dosimetry System (Varian Medical Systems, Palo Alto CA) and a Varian Trilogy accelerator equipped with the Varian PortalVision aS1000 imager. In converting an IMRT fluence image into a digital dose matrix which can be analyzed via Portal Dosimetry, PortalVision utilizes a number of calibration files, one of which is a 40cm x 40cm diagonal profile (measured in water at dmax) which accounts for variation in beam output at off-axis distances. This diagonal profile is accessed by PortalVision in the form of a text file, selected by the user while calibrating the detector for absolute dose before each use. For the purposes of this study, a simple IMRT fluence was created via the Eclipse TPS which spanned one entire half of the PortalVision detector. By comparing and analyzing the acquired and predicted images for this fluence, correction factors were determined to adjust the diagonal profile text file in order to precisely lower the relative dose readings at the edges of the detector. For verification purposes, several IMRT fluences were created in close proximity to the detector edges, varying in field-size and position. Portal images were then acquired for these fluences, using both the corrected and the uncorrected diagonal profiles for comparison purposes, and analyzed by dose-profile comparison and gamma evaluation. Similar comparisons were made between uncorrected portal images and corrected portal images for several past delivered IMRT treatments (some that previously failed verification because of their proximity to the detector edges, and some that passed, being near the center of the detector) in order to verify the agreement between Portal Dosimetry and PortalVision in all regions of the detector due to the corrected diagonal profile.

Results: The corrected 40cm x 40cm diagonal profile results in portal images that agree very well with their respective Portal Dosimetry predictions for all areas of the detector: treatment-toplan comparisons show dose profile improvements of as much as 20% and gamma evaluation improvements of up to 40% for fields near the detector edges. Meanwhile, Portal Dosimetry analysis of IMRT fields in the central region of the detector agrees within 1% with analysis of the same fields made with the original diagonal profile, based upon number of points that successfully pass a gamma evaluation of 3mm, 3%.

Conclusion: A precise method is proposed for alleviating the problem of PortalVision dose errors near the edges of the portal imager. Further investigation is needed to determine the actual source of the error, whether it is in the actual images acquired by PortalVision or in the algorithm used by Portal Dosimetry to calculate dose distributions in predicted portal images.

 

 

 

                                                                                                                              

 

Monte Carlo Dose Calculations on the GPU: A Feasibility Study

J P Steinman1, M Bakhtiari1, H K Malhotra1, V Chaudhary2, D P Nazareth1

1Roswell Park Cancer Institute, Buffalo, NY 14263

2Center for Computational Research, University at Buffalo, Buffalo, NY 14203

 

Purpose:  Monte Carlo (MC) simulation has been the gold standard for accurate radiation dose calculations, but because of its long calculation times, it is not always clinically feasible for radiotherapy treatment planning when run on a standard CPU.  Graphics Processor Units (GPU) have recently demonstrated computational throughput far greater than traditional CPUs and have been implemented for many scientific computational applications.  We have developed a simple MC simulation on a GPU platform to perform radiation dose calculations.  Our method currently considers the direct dose due to photons.

Method and Materials: 

We developed our program in the CUDA language a variation of C designed for GPU implementation.  The hardware employed was a GeForce 8800 Ultra GPU installed on a standard PC.  Additional routines were written to communicate with and initialize the GPU. The algorithm is a simple MC routine which simulates monoenergetic x-rays propagating through a 3D water-equivalent medium.  Each spatial step involves querying for a photon’s energy reduction and dose deposition.  As a first step towards implementing it on the GPU, we neglect dose contributions from secondary electrons, since this would create multiple threads that are not easily handled in the GPU architecture. The program was also run on a CPU, and the execution times were benchmarked for comparison.

Results:  For our simple MC implementation on the GPU and CPU, we observed a dose reduction as a function of depth, similar to that of a percent depth dose curve.  However, since secondary electrons were neglected, the dose did not exhibit a peak at dmax.  Separate simulations involving 10e5, 10e6, and 10e7 histories indicated that the GPU provided a speedup by an average factor of 23.

Conclusion:  The GPU provides tremendous increase in computational speed, and can potentially be used for MC dose calculations in radiotherapy, as indicated by agreement of depth-dose results with those of the CPU.  Future work includes considering the dose from secondary electrons using hybrid methods such as combined MC and convolution/superposition algorithms  

                                                                                                                              

 

On the Beam Orientation Optimization using Genetic Algorithm

M Bakhtiari1, M D Jones2, H K Malhotra1, M B Podgorsak1, and D Nazareth1.

1Roswell Park Cancer Institute, Buffalo, NY 14263

2Center for Computational Research, University at Buffalo, Buffalo, NY 14203

 

Purpose:  Currently, the method of selecting suitable beam angles for 3D conformal therapy [3DCRT] is highly subjective, and depends to a large extent on the experience of the planner. In the present study, we explore the development of an efficient mathematical model which can improve radiotherapy treatment planning by automatically selecting optimal coplanar beam angles for a given number of beams in a 3DCRT treatment plan. The optimal set of beam angles corresponds to the lowest value of a constraint-based objective function. Due to the complexity of the problem and the large search space involved, the selection of beam angles and the optimization of beam weights are treated as two separate processes and implemented iteratively. A genetic algorithm (GA) is employed to select suitable beam orientations. The GA incorporates four parameters: selection, elitism, crossover and mutation. We investigated the dependence of the results on the ratio of the crossover and mutation values.

Method and Materials:  The PTV and critical structures were contoured on a 3D CT dataset of a prostate case.  An open-source software package, CERR, was used to set up the beam geometry. A Monte Carlo program, VMC++, was used to perform dose calculation. The beam weights were optimized using the Nelder-Mead downhill simplex technique, a multidimensional unconstrained nonlinear minimization algorithm.  Each generation involved 40 plans.  The initial generation was produced randomly, and each plan’s objective function was evaluated.  The GA then were proceeded by performing crossover and mutation procedures on the plans having the lowest (best) scores, and the resulting plans were used to form the next generation.  About 16 generations were produced, and the best overall plan was recorded.  This procedure was repeated for many values of the crossover/mutation fractions.  The calculations were performed using the computational resources of the Center for Computational Research, an academic supercomputing facility.

Results: It was found that the performance of the GA in beam orientation optimization strongly depends on the fraction of crossover and mutation. The best performance was obtained with 80% crossover and 20% mutation.

Conclusion: The crossover rate determines how deeply the GA can explore each promising region it encounters in the search space.  The mutation rate controls how extensively the GA can search the entire space.  Having a small crossover fraction makes it unlikely to obtain a global solution, because the GA has less opportunity to improve the plans.  Therefore, a crossover rate of 80% is effective.  This allows the mutation fraction to be smaller, but still significant.  Using these parameters, the GA can optimize the beam angles and produce a plan superior to a standard clinical plan.

                                                                                                                              

 

A Guidance System for Optical Patient Alignment During Breast Radiotherapy

Jonathan Schmitta, Kenneth Hoffmannb, Mohammad Bakhtiaria, Daryl Nazaretha , Harish Malhotraa

a.) Roswell Park Cancer Institute b.) Toshiba Stroke Center, University at Buffalo

 

Purpose: Breast radiotherapy, particularly IMRT, involves large dose gradients and difficult patient positioning problems.  A critical requirement for successful treatment is accurate reproduction of the patient’s position assumed during CT simulation and planning.  Solving this problem using a simple optical system requires careful imaging geometry calibration.  We have developed an optical image-guided technique, which assists in accurately and reproducibly positioning the patient, by displaying her real-time optical image superimposed on a perspective projection image of her 3D CT data.

Methods. The Single Projection Technique (SPT) accurately determines the 3-D position and orientation of a camera from a single image acquired of a known model.  A calibration jig, composed of ten identifiable reflecting spheres, was constructed and CT imaged to provide this model. The coordinates of each point were determined with respect to a fiducial marker.  To implement our method, a digital photograph of the jig is acquired, and a centroid-finding technique is applied to this image.  The two-dimensional coordinates of each sphere, along with its 3D coordinates serves as input to the SPT program, which calculates the coordinates and orientation of the camera.  Using this information, 3D CT patient data is projected onto the camera’s imaging plane, and is displayed on a monitor, superimposed on the real-time patient image. This enables the therapist to view both the patient’s current and desired positions, and guide proper patient positioning. 

Results: The SPT can determine the position and orientation of the camera to an accuracy of 0.2 cm and 0.3°, respectively.  Investigations are ongoing to determine the accuracy and reproducibility of our method, based on film measurements performed on a breast phantom.

Conclusion: We have developed a method to calibrate an optical camera system and superimpose a perspective projection of a CT image on a patient’s real-time optical image.  Displaying this visual information will assist in accurate setup during breast radiotherapy.  Future work will enable us to quantify the setup and dose delivery accuracy of this technique.

                                                                                                                              

 

Target Dose during Defocused Patient Repositioning with the Gamma Knife Automatic Positioning System (APS)

T Tran, T R Stanley, H K Malhotra, D Prasad, M B Podgorsak, Roswell Park Cancer Institute, Buffalo, NY

 

Purpose: To measure dose delivered to the target site from the defocused beam as a function of the number of repositions with the automatic repositioning system (APS) for Gamma Knife Radiosurgery. 

Methods and Materials: A stereotactic head frame was attached to a 16 cm diameter spherical phantom with a 0.05 cubic centimeter ion chamber at its center.  Using a fiducial box to determine the coordinates of the target (the ion chamber), a CT scan with 1 mm slice thickness was taken of the phantom.  The CT fiducial box was registered in the Gamma Plan treatment planning system and a dose prescription of 10 Gray to the 50% isodose line was applied to the target site [center of the ion-chamber].  Plans were created for the 18 mm and 14 mm collimator size helmets with varying repositions for each plan.  Each helmet had treatment plans with 50, 35, 20, 10, 5 and 1 shot to determine the relationship between measured dose and number of repositions of the APS system.  Even though the numbers of shots were different between various plans, the shot isocenter was identical in the entire study and there was no movement of APS between various shots.  Such an arrangement allowed measurement of radiation dosage to the center of ion-chamber during the defocus state of Gamma Knife.  To measure the defocused dose rate, the time of each run was individually measured using a stop-watch.  The leakage charge of the Keithley electrometer during the entire measurement cycle was found to be negligible.  Measured charge in nano-coulombs was corrected for every reading for the variation in the temperature and pressure.  Measured charge was converted to absorbed dose using standard formalism. The Leksell Gamma Knife Model 4C was used to deliver radiation to the framed phantom.

Results: Single shot runs for the 14 mm and 18 mm helmets gave similar dose measurements (19.427 ± 0.006 Gy and 19.450 ± 0.003 Gy, respectively).  Measured dose increases with frequency of repositioning for both helmets.  For the 18 mm helmet, an increase in dose of 0.12% (or 0.024 Gy) was observed with each additional shot delivered; this gives a range of 1.2 Gy between the single shot plan and the 50 shot plan.  For the 14 mm helmet, there was an increase in dose of 0.1% (or 0.0196 Gy) per additional shot delivered; this is a range of 0.96 Gy.  The percent dose difference between helmets with similar repositioning frequency increases with increasing number of shots; thus, there is a diverging trend for the measured dose versus the number of repositioning between the two helmets.  The 14 mm helmet had a defocused dose rate 6.13 ± 0.51 cGy per minute and the 18 mm helmet had a defocused dose rate of 8.81 cGy ± 0.41 cGy per minute (that is 2.72% and 3.91%, respectively, of the focused dose rate of 2.254 Gy per minute for the day of measurement). 

Conclusion: The automatic repositioning system for the Leksell Gamma Knife Model 4C results in additional dose to the target site when repositioning is required between various shots in the same run.  This dose increases with the number of shots required in a run and the defocused dose rate increases with the increase in the collimator size.  Because the plans were designed such that no physical repositioning with the APS system occurred, the measured doses are the minimum that a patient may get while the patient is in defocus state.  In an actual patient, the exact extra dose to the patient will also depend upon the time needed for the APS to go to the next treatment position.

                                                                                                                              

 

Algorithm for Hyperfast GPU-Based Cone Beam Computed Tomography.

Peter B. Noël 1,2, Alan M. Walczak 2, Kenneth R. Hoffmann 1,2 , Jinhui Xu 1, Jason J. Corso 1,

and Sebastian Schafer 2

1Department of Computer Science and Engineering, The State University of New York at Buffalo

2Toshiba Stroke Research Center, The State University of New York at Buffalo

 

The use of cone beam computed tomography (CBCT) is growing in the clinical arena due to its ability to provide 3-D information during interventions, its high diagnostic quality (submillimeter resolution), and its short scanning times (60 seconds). In many situations, the short scanning time of CBCT is followed by a time consuming 3-D reconstruction. The standard reconstruction algorithm for CBCT data is the filtered backprojection, which for a volume of size 2563 takes up to 25 minutes on a standard system. Recent developments in the area of Graphic Processing Units (GPUs) make it possible to have access to high performance computing solutions at a low cost, allowing for use in applications to many scientific problems. We have implemented an algorithm for 3-D reconstruction of CBCT data using the Compute Unified Device Architecture (CUDA) provided by NVIDIA (NVIDIA Cor., Santa Clara, California), which was executed on a NVIDIA GeForce 280GTX. Our implementation results in improved reconstruction times from on the order of minutes, and perhaps hours, to a matter of seconds, while also giving the clinician the ability to view 3-D volumetric data at higher resolutions. We evaluated our implementation on ten clinical data sets and one phantom data set to observe differences that can occur between CPU and GPU based reconstructions. By using our approach, the computation time for 2563 is reduced from 25 minutes on the CPU to 3.3 seconds on the GPU. The GPU reconstruction time for 5123 is 8.7 seconds, and 10243 is 41.2 seconds.

                                                                                                                              

 

Rotational micro-CT on a clinical C-arm gantry system

V. Patel, K. R. Hoffmann, C. N. Ionita, C. Keleshis, D. R. Bednarek, S. Rudin

 

Purpose

Rotational angiography (RA) is commonly used to obtain 3D data but suffers from limited resolution. Higher-resolution data can be obtained using cone-beam micro-computed tomography (CBmCT) systems, but these small-bore or rotating-object systems cannot be used for patients. We have implemented a CBmCT system on a clinical RA system, creating a rotational micro-angiography (RMA) system for clinical use.

Method and Materials

A new custom-made, high-sensitivity micro-angiographic fluoroscope (MAF) (35 mm pixels) was affixed to a RA C-arm gantry and used to acquire high-resolution data within a region-of-interest (ROI) containing a coronary stent in a rabbit. Low-resolution, full field-of-view data were acquired using a commercial flat-panel detector (FPD) (194 mm pixels) on the same gantry at a lower dose compared to MAF acquisition. MAF and lower-dose FPD data were spatially registered using cross-correlation, and pixel values were matched using linear regression. For reconstruction, corrected lower-dose FPD data were used outside the ROI, and MAF data were used inside the ROI. A 512-cubed volume (25 mm voxel) was reconstructed. Full widths at half maximum (FWHMs) were measured for several stent struts (100 mm diameter) in various axial slices.

Results

The new RMA system provided greater detail in the reconstructed volume than did the standard dose FPD RA system. No truncation artifacts were visible. The average FWHMs were 192+21 and 313+38 mm for RMA and standard RA reconstruction, respectively, in agreement with values computed from the point spread functions of the detectors and stent width. An integral dose reduction of 54% was achieved using our system compared to standard-dose RA.  

Conclusion

A new RMA system has been successfully created by mounting a high-resolution detector on a RA C-arm system.  By coupling RMA imaging with lower-dose RA acquisitions, we are able to obtain improved high-resolution reconstructions for a ROI while retaining usable image quality outside the ROI and reducing integral dose to the patient as compared to a standard RA system.  Resolution and dose reduction may be further improved by optimizing the gantry setup and the exposure parameters.

                                                                                                                              

 

 

Automated Calibration of an Angiographic Imaging System for the Reconstruction of Three

Dimensional Vessel Centerlines

Alan M. Walczak, Kenneth R. Hoffmann, Vikas Singh, Neil Dashkoff, Monica Kassab, Vijay S. Iyer

 

Purpose

Three dimensional (3D) vessel reconstructions can be useful in assisting clinicians with diagnosing and treating vascular disease, providing information about vessel diameter, length, and tortuosity. Reconstruction of 3D vessels from two views requires knowledge or calibration of the geometry relating the two imaging systems. We introduce a geometry calibration technique that performs this calibration using only a single indicated vessel segment in the angiograms.

Methods

Vessel segments of interest were indicated in two angiographic views. The initial imaging geometry is estimated using each view’s gantry information, and corresponding points along the vessels in both views are determined using epipolar constraints. The 3D position of the vessel is determined by triangulation of the found corresponding points. The geometry is corrected by varying nine of the imaging system’s parameters using the Nelder-Mead Downhill Simplex Method, with an objective function that minimizes the distance between the reprojection of the reconstructed 3D vessel centerlines and the 2D indicated vessel centerlines in both views. Results were compared with the enhanced-Metz-Fencil (EMF) geometry correction technique, which requires identification of additional corresponding points in both views.

Results

Variations in the shapes obtained from our single vessel technique (SVT) and the EMF were comparable, median RMS of 0.47 and 0.34 mm, respectively, with magnification variations of 2.2% and 0.7%, respectively. Median errors in 2D reprojections of the 3D data for our technique and the EMF were both 0.12 mm, indicating very good agreement with the 2D indicated centerlines.

Conclusion

We have developed an imaging geometry correction technique for two views based on alignment of reprojected 3D data with its respective 2D image information using only a single indicated vessel segment. This technique is reliable and comparable to other geometry correction techniques requiring additional user input.

 

                                                                                                                              

 

Dosimetric Analysis of the Effect of Tungsten Shields in a Fletcher-suit Delclos Applicator in High-Dose-Rate brachytherapy using Gafchromic Film

Thomas R. Stanley, Wainwright Jaggernauth & H K Malhotra

Roswell Park Cancer Institute, Buffalo, NY 14263.

 

Purpose: To study the effect of tungsten shields in the radiation dose delivery in a Fletcher-Suit Delclos applicator in high-dose-rate (HDR) brachytherapy using gafchromic film dosimetry. 

Methods and Materials: A gadget for rigidly and reproducibly mounting a Fletcher-Suit Delclos (FSD) tandem and ovoid applicator along with an attachment to hold a set of gafchromic films in relation to the applicator in a conventional water phantom was designed and fabricated.  The gadget allowed placing 14 films anterior to the tandem and another 14 posterior to the ovoids at a distance of 6.025 mm from each other.  The gadget has a provision of 5 fiducial marks per film for spatial registration with the orthogonal films acquired in a simulator.  A treatment plan delivering 700 cGy to the pseudo point A was designed.  The plan does not account for tungsten shields in the ovoids.  After the films were put in place, lasers were used as guides to mark the central axis of each film with respect to the tandem (to establish spatial coordination between both).  Once the films were properly aligned, the water phantom was filled with water.  The applicator was connected to the Micorselectron HDR treatment unit and the treatment plan was delivered.  Each gafchromic film was removed, marked for its location in the pack, dried, and scanned using a Vidar VXR-16 scanner for analysis using RIT software.  Using a measured H&D curve for calibration of the gafchromic films, the dose distributions on each film was evaluated and compared to the corresponding distributions produced from the treatment planning system. 

Results:  An analysis of the data revealed a reduction in dose measured by the gafchromic film over the calculated values from the treatment planning system in the area covered by the solid angle subtended by the tungsten shields in the ovoids.  This seems to follow logically considering the treatment planning algorithm does not account for the tungsten shields within each ovoid.  The details of the results will be presented. 

Conclusion:  The growing trend in brachytherapy procedures of this nature is to use CT/MR compatible tandem and ovoid applicators which do not provide any shielding for the bladder and rectum within the ovoids.  Thus, it is very important to understand the true radiation doses being delivered to these critical structures when the original treatment has used a shielded applicator.

 

________________________________________________________________________________________

 

Endovascular Image-Guided Interventions: Current and Future

S Rudin

 

In a recent Medical Physics “Vision 2020” paper (Medical Physics 35(1): 301-309, Jan 2008), we reviewed the state of endovascular image-guided interventions (EIGI) and offered some predictions for the future. First, endovascular devices (such as clot busting tools, stents and their catheter delivery systems, and blood flow modifiers) are becoming finer, more complex, and are enabling the replacement of invasive surgical procedures with minimally invasive EIGI procedures. Innovative methods of actuating motion at the catheter tip, such as the use of external magnetic fields, are being introduced. Second, along with improvements in devices, imaging systems that provide real-time high-resolution image guidance are being developed including a Solid State X-ray Image Intensifier based on electron multiplying charge coupled devices (EMCCDs) that provide large on-chip gain to overcome instrumentation noise such as that characteristic of current flat panel detectors. SSXIIs also have very high resolution capable of exceeding 10 lp/mm yet with no lag or ghosting. Third, the new high-resolution region-of-interest (ROI) detectors can be used in combination with large conventional detectors for dual-detector cone-beam computer tomography (CB-CT) to visualize ROIs within larger objects yet with minimal truncation artifact and with reduced integral dose. Fourth, during an interventional procedure, limited projection views can be taken to generate full 3D representations of the vasculature with accurate determination of vessel lumen morphology to enable computer fluid dynamic (CFD) calculations which in turn can be used to plan further EIGI treatment within the patient treatment time. Finally, as EIGI procedures become more complex, the consequent patient dose especially where improved image quality is implemented must be more carefully monitored. For example, we found that patient dose actually increased for certain electro-physiology (EP) procedures performed in our EP Lab following replacement of a mobile c-arm with a fixed unit capable of generating improved image quality. In conclusion, while progress is being made toward fulfilling the predictions in the Vision 2020 paper, EIGI remains open to continuing exciting advancements.