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Upstate |
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
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10:00 |
Business Meeting |
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11:30 |
Lunch Sponsored by
Phillips |
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12:00 |
Refreshments and Vendor Exhibits – Sengupta Room Sponsored by Sun Nuclear |
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12:30 |
Meeting
Introduction |
Shivaji Deore,
Ph.D., DABMP, UNYAPM President |
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Vendor Session |
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12:40 |
Static
Gantry/Fixed Angle with Sliding Couch Delivery and Machine Digital QA for TomoTherapy HiART System |
Lou Sesto, TomoTherapy Inc. |
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12:55 |
Rotational
IMRT QA Now and the Future |
James Ernsberger, Sun Nuclear. |
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Proffered Paper Session |
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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 |
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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 |
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|
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 |
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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 |
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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 |
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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 |
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4:00 |
Refreshments and Vendor Exhibits – Sengupta Room Sponsored by Sun
Nuclear |
|
|
Invited Speaker Session |
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4:20 |
Endovascular
Image-Guided Interventions: Current and Future |
S
Rudin, University at Buffalo, Buffalo, NY |
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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 |
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Directions (to
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UNYAPM
SPRING MEETING PROCEEDINGS
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.