|
|
Upstate New York Association of Physicists in Medicine, Inc. and
Western New York Chapter of the Health Physics Society Joint Meeting Room 2-64-24, 601 Elmwood Ave, Rochester, NY 14642 |
12:00 pm – 5:30 pm
Sponsoring Vendor Exhibits:
Vision
RT
North American Scientific
Impac
Medical Systems Upstate
Linac Services, LLC
Core Oncology LACO, Inc
IBA Dosimetry
10:30 AM-11:30 AM Business Meeting
12:00 Lunch Sponsored
by Upstate Linac Services, LLC
12:30 Refreshment
and Vendor Exhibit –
|
1:00 |
Meeting Introduction |
Walter O’Dell UNYAPM President |
|
|
Proffered Paper Session (8 minute talks, 2 min QA) |
|||
|
1:10 |
Random
walk model for predicting patterns of microscopic glioma spread using DTI: A
prospective study |
A. Krishnan, D. Davis, P. Okunieff, and W. O'Dell |
|
|
1:20 |
Fetal Dose Reduction in
Different Shielding Scenarios During Thoracic CT |
K. Greene-Donnelly, K Ogden, M. Roskopf |
|
|
1:30 |
Extracranial Dose measurements for the
Leksell Gamma Knife Model 4C using Gafchromic EBT Film |
T. Tran, C.D. Arndt, J.P. Steinman, and M.B. Podgorsak |
|
|
1:40 |
An Optical Guidance
Technique for Patient Position during Breast Radiotherapy |
J Schmitt, K Hoffmann, M Bakhtiari, D Nazareth , H
Malhotra |
|
|
1:50 |
Radiographs In
Pretreatment IMRT QA: Are Films Necessary In Addition To Electronic Quality
Assurance Methods? |
D W Bailey, S F de Boer and M B Podgorsak |
|
|
2:00 |
Effect of surface waves on the
dosimetric measurements in water tanks |
M. Bakhtiari, S. de Boer, and M. B. Podgorsak |
|
|
2:10-2:30 |
Development of an Adequate Quality Assurance
Program for Gamma Probes |
Rich Harvey |
|
|
2:30 |
Refreshment , Vendor Exhibits and Poster
Viewing*
– Sponsored by
Upstate Linac services |
||
|
3:15 |
Invited Speaker Will They Ever Learn? The Public Education Game |
Howard
Dickson President-elect of the national Health Physics Society |
|
|
4:20 |
Unaccounted Intracranial Dose during Patient Repositioning with the Gamma Knife APS Device |
T. Tran,
T.R. Stanley, H.K. Malhotra, S.F. deBoer, D. Prasad, M.B. Podgorsak, |
|
|
4:30 |
A comprehensive quality assurance procedure for
ultrasound-guided radiation therapy |
Dinko Plenkovich, Matthew B. Podgorsak, Jubei Liu |
|
|
4:40 |
Dose Perturbation from Implanted I-125
Seeds in External Beam Therapy for Prostate Cancer |
JP Steinman, M Bakhtiari, DP Nazareth, HK Malhotra |
|
|
4:50 |
Respiratory gating using online
automatic segmentation of pulmonary nodules in megavoltage electronic portal images using a level
set method |
JS Schildkraut, J Gomez, A Singh, D Nazareth, HK Malhotra |
|
|
5:00 |
Invitation for a
tour of the new URMC cancer center |
Dr. Schell |
|
ADJOURN
Driving Directions to the University of Rochester Medical
Center
From the West: New York State Thruway to Exit 47.
- Take exit #47/LEROY (RT-19)/ROCHESTER
onto I-490 E (Toll applies) - go 19.63 mi
- Take exit #9B/AIRPORT onto I-390
S - go 2.9
- Take exit #17/SCOTTSVILLE RD. -
go 0.2 mi
- Turn Left on SCOTTSVILLE RD(RT-383)
- go 0.6 mi
- Bear Right on ELMWOOD AVE - go 0.9 mi
Parking is available in the Ramp Garage.
From Rochester Airport (ROC):
- Take exit #9B/AIRPORT onto I-390
S - go 2.9
- Take exit #17/SCOTTSVILLE RD. -
go 0.2 mi
- Turn Left on SCOTTSVILLE RD(RT-383)
- go 0.6 mi
- Bear Right on ELMWOOD AVE - go 0.9 mi
From the East: New York State Thruway to Exit 46.
- Take exit #46/ROCHESTER/CORNING onto I-390
N toward ROCHESTER (Toll applies) - go 6.9mi
- Take exit #16/E HENRIETTA RD/W
HENRIETTA RD (RT-15) - go 0.2
mi
- Turn Right on E HENRIETTA RD(RT-15A)
- go 0.9 mi
- Bear Right on MT HOPE AVE(RT-15)
- go 0.2 mi
- Turn
Left on ELMWOOD AVE - go 0.3 mi
From the South: 390 North and follow the directions above when coming from the East.
UNYAPM
SPRING MEETING PROCEEDINGS
University
of Rochester Medical Center, Rochester, NY
May 15, 2009
Unaccounted Intracranial Dose during Patient Repositioning with the
Gamma Knife APS Device
T. Tran, T.R. Stanley, H.K. Malhotra, S.F. deBoer, D. Prasad,
M.B. Podgorsak,
Roswell Park Cancer Inst., Buffalo NY
Purpose: Measure unaccounted dose delivered to the target site
and its periphery from the defocus and transit beam during automatic
positioning system (APS) repositioning for Gamma Knife Radiosurgery.
Methods and
Materials: A stereotactic head-frame
was attached to a 16cm diameter spherical phantom with a calibrated ion-chamber
at its center. Using a fiducial-box to
determine the coordinates of the target, a CT scan with 1mm slice thickness was
taken of the phantom and registered in the GammaPlan TPS. 10Gy to the 50% isodose line was prescribed
to the target site for all measurements.
Plans were generated for the 18mm, 14mm and 8mm helmets with varying
number repositions for each plan to determine the relationship of measured dose
with number of repositions of the APS system and helmet size. The shot isocenter was identical in the
entire study and there was no movement of APS between various shots; this
allows for measurement of transit dose (couch moves from the focus to defocus
position and back) and the least defocus dose (at defocus couch position). The couch was suspended in the defocus
position allowing intracranial defocus dose measurements.
Results: Dose increases
with frequency of repositioning and collimator size. Overdose of up to
5.71±0.07% at target position can result from couch transfer. Dose rate of 8.81±0.41cGy/min (18mm helmet)
and 5.89±0.51cGy/min (8mm helmet) where measured. During couch transit, the target receives
more dose than peripheral regions; in the defocus position, the greatest dose
is superior on the phantom where dose rate is 4.91±0.01cGy/min.
Conclusion: APS repositioning results in additional dose to the
target site and its periphery for multi-shot runs. Doses in superior regions should be monitored
due to epilation. Consideration of
conformity index is suggested when generating a treatment plan as risk of
toxicity is a concern, especially around critical structures (such as the optic
nerve). Application of a timer error
would account for these doses during couch transit and APS repositioning; this
would also improve the accuracy of the prescription dose.
Fetal
Dose Reduction in Different Shielding Scenarios During Thoracic CT
K. Greene-Donnelly, K Ogden, M. Roskopf, Upstate Medical University, Syracuse NY Purpose: To determine the potential for reducing fetal
dose in early pregnancy during thoracic computed tomography by shielding the
patient’s abdomen and pelvis.
Method and Materials: An
anthropomorphic phantom (Rando) representing a medium sized adult was used to
measure relative tissue doses in the abdomen/pelvis during thoracic CT
scanning. TLD’s were used to measure the
dose along the central axis of the phantom from the level of the adrenals to
the location of the uterus. The phantom
was scanned on a GE Lightspeed VCT 64 slice scanner
using 120 kVp, 750 mAs, and pitch of 0.984 to increase the TLD
signal. Scans were performed with no
shielding, with shielding (lead apron) on the anterior aspect of the phantom
over the abdomen and pelvis, and with shielding on both the anterior and
posterior aspects of the phantom. Tissue doses were normalized to the in-scan
value measured at the level of the adrenals.
Results: Tissue doses decreased exponentially with
increasing distance from the bottom of the scanned anatomy. The rates of decay were -0.18 ± 0.010 cm-1,
-0.21 ± 0.0024 cm-1, and -0.23 ± 0.0018 cm-1 for the no
shielding, half shielding, and full shielding cases, respectively. The dose values at the level of the uterus
were 0.99%, 0.65%, and 0.51% of the dose at the level of the adrenals, for the
no shielding, half shielding, and full shielding cases, respectively.
Conclusion: These data show that the total absolute dose
received by a fetus early in the pregnancy may be reduce by approximately 1/2
during thoracic CT by use of shielding on the abdomen/pelvis. For a clinical technique of 120 kVp, 150 mAs,
and pitch of 1.375, this would reduce the fetal dose from an estimated 0.1 mGy
to 0.05 mGy.
Extracranial Dose measurements for the Leksell Gamma Knife Model 4C
using Gafchromic EBT Film
T. Tran, C.D. Arndt, J.P. Steinman, and M.B. Podgorsak,
Roswell Park Cancer Institute, Buffalo, NY
Objective: To obtain measurements of scatter dose using the
Rando phantom and Gafchromic film in critical, extracranial organs for patients
undergoing Gamma Knife Radiosurgery.
Method and
Materials: A stereotactic frame was
attached to the head of an anthropomorphic Rando phantom. Using a fiducial box to determine the
coordinates of the target (center of the Rando head), a CT scan was taken and
registered in the GammaPlan treatment planning system where a dose prescription
of 25Gy to the 50% isodose line was applied to the target site. The plan was generated for the 18 mm
collimator size helmet with a single shot run using the automatic positioning
system (APS). Multiple (2 to 5) 2”x2”
Gafchromic EBT films were placed between each slice of the Rando body phantom,
from the neck to pelvic region (phantom slice 8 to 31), and analysis was done
using a Vidar VXR-16 scanner with the RIT113 Version5.1 analysis software. An H&D curve was created using Gafchromic
EBT film, a calibrated 0.05cm3 ion chamber, the Keithley 35617EBS
Programmable Dosimeter, and a framed 16cm diameter sphere phantom (with film
and ion chamber inserts).
Results: The dose was 11.3±1.1 cGy to the thyroid, 8.7±1.3 cGy
to the thymus, 5.1±0.4 cGy average dose to the adrenal glands, 1.0±0.2 cGy to
the ovaries, 0.8±0.3 cGy for the testes, 3.3±0.4 cGy to the pancreas and
2.7±0.5 cGy to the colon.
Conclusion: Extracranial doses depended on total target dose and
the distance the organ was from isocenter during treatment. For all organs, greater prescription doses
lead to greater doses to extracranial organs.
Dose to the organs also decreased with increasing distance from the
focal point. The extracranial doses are
well within tissue tolerances and are comparable with other studies. Future projects will include comparisons with
the Leksell Gamma Knife PERFEXION. Doses
are low but may be considered for younger patients with longer life expectancy.
An Optical Guidance Technique for Patient
Position during Breast Radiotherapy
J Schmitta, K Hoffmannb, M Bakhtiaria,
D Nazaretha , H 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.
Radiographs In Pretreatment IMRT QA: Are Films Necessary In Addition To
Electronic Quality Assurance Methods?
D W Bailey, S F de Boer and M B Podgorsak,
Roswell Park Cancer Institute, Buffalo, NY
14263
Purpose: The use of radiographs for IMRT QA has several
disadvantages, including time-consuming and resource-demanding development,
scanning, and dose calibration. Although
radiographs offer the best resolution in measurement of dose distributions,
electronic QA methods, e.g. diode arrays and electronic portal imaging devices
(EPIDs), have become standard for efficiently and accurately verifying dose distributions. There is still a question as to whether or
not radiographs are necessary in addition
to electronic QA to qualitatively verify the geometric accuracy of delivered
fluences. However, due to the finite
ability of the human eye to compare film exposures to planning system
printouts, electronic QA methods may detect geometric inaccuracies before the
same error is recognizable on film.
This study addresses the question of whether or not the qualitative use
of films contributes significantly to the pretreatment verification
process.
Methods: An IMRT fluence was delivered on film, and errors
were systematically introduced by omitting portions of the fluence, decreasing
the number of delivered monitor units and control points proportionally. This
method simulates a communication error between planning and delivery
systems. The same modified fluence was
delivered on MapCHECK (Sun Nuclear Corporation, Melbourne FL) and compared to
the TPS verification plan using gamma evaluation of 3%, 3 mm. The process was
repeated, omitting increasingly greater portions of the fluence until the IMRT
QA failed, either by errors observed on the film or by MapCHECK gamma analysis
below 85% passing points. This procedure
was repeated on four fluences with increasing levels of modulation.
Results: For every modified fluence that failed IMRT QA, the
errors were apparent in MapCHECK before they were observed on film. Even with as much as 20% of control points
omitted, the radiographs for all fluences appear virtually unchanged to the
naked eye. Contrastingly, MapCHECK
analysis of the same fields failed due to omission of control points by as
little as 6.3% (average) from the center of the fluences, and 19.3% (average)
from the outer portions of the fluences.
Conclusions:
If a portion of an IMRT fluence is
omitted due to data transfer errors, qualitative analysis of radiographs does
not enhance the ability to detect such errors during pretreatment IMRT QA if a
diode array with acceptable resolution is utilized. However, because such errors may not be
detected by 3%, 3mm gamma evaluation until 10-20% of control points are lost,
gamma analysis of IMRT fields should always be accompanied by comparison of the
isodose distributions in measured and predicted fluences. Further experimentation must be conducted to
determine if MapCHECK eliminates the need for radiographs in the event of other
types of errors.
Effect of surface waves on the dosimetric measurements in water
tanks
M.
Bakhtiari, S. de Boer, and M. B. Podgorsak,
Roswell Park Cancer Institute, Buffalo, NY
14263.
Purpose: To study
the effect of surface water waves on the accuracy of ionization measurements in
large scanning water phantom.
Methods and Materials: Profile measurements were taken in a PTW water
tank (50cm×50cm×50cm) filled with water. The detector (ion-chamber) is attached
to a variable speed movable arm that moves in a Cartesian coordinate system.
The arms speed was varied from 1 mm/s to 50 mm/s, the dose collection time was
0.3 ms, and the spatial resolution was selected to be 1 mm. Profiles were
measured at a depth of 12.6 mm (R50) for a 10cm×10cm, 4 MeV electron beam. Two
sets of experiments were carried out; 1) after each profile measurement the arm
was left at the end point. For starting the new profile the arm had to come
back to the starting point and immediately start the new profile, 2) after each
measurement the arm was immediately brought back to the starting point and the
next profile measurements were started 2 minutes later.
Results: The amplitude of the surface water waves
increases with increasing the speed of arm. Consequently some errors in the
measurements were observed. The measurements with slow moving arms (1 mm/s)
were more reproducible and demonstrated less fluctuation. The reproducibility
decreased and fluctuations increased with increasing the speed. When the
measurements started a while after the arm was brought to starting point the
accuracy increased, otherwise even with slow moving arms some errors were
observed in the measurements.
Conclusion: The moving arms in large
water tanks can have an impact in dosimetry.
It was found the surface waves can cause errors of 3% and 8% for slow
moving and fast moving arms, respectively.
Development of an Adequate Quality Assurance Program
for Gamma Probes
Richard P. Harvey, DrPH, ABSNM, CHP, CMLSO. CLSO, LMP
Director of Radiation Safety and Radiation Safety Officer, Roswell Park Cancer Institute
Gamma
probes are a radiation detection tool used in surgery to identify tissue for
resection and their oncologic applications have become fairly common. Many surgeons and surgical departments
purchase these devices for use regardless of radiation physics assessment and
thought for Quality Assurance. Many
physicians believe the limited quality control recommended by the sales
representative is enough to provide adequate performance evaluation of gamma
probes.
Each licensee must develop an adequate Quality Assurance
Program to ensure proper function and adequate calibration of instruments used
for patient care. These methods need to
be established and communicated among licensees for the benefit of surgical
patients treated at all healthcare facilities.
Random walk model for predicting patterns of
microscopic glioma spread using DTI: A prospective study
A.
Krishnan, D. Davis, P. Okunieff,
and W. O'Dell
Depts of Biomedical Engineering and Radiation
Oncology, University of Rochester, Rochester, NY
Purpose: The current methods of determining treatment margins
needed to encompass microscopic tumor spread for Stereotactic Radiotherapy
(SRT) are often inadequate as recurrences/secondary tumors often occur at the
boundary of the treatment margin. We hypothesize that paths of elevated water
diffusion along the white matter tracts provide a preferred path for migration
of glioma cells. If our hypothesis is true, then future SRT plans would be
modified to provide elongated margins along white matter tracts from the
primary tumor, thereby targeting tissue with unseen, microscopic spread of
tumor cells and hence reducing the incidence of recurrence/secondary tumors. We
present here the pattern of glioma spread observed in follow-up MR images and
compare it with the results of anticipated tumor spread from our predictive
random walk model of cell migration based on DTI obtained prospectively.
Methods and Materials: We acquired high resolution DTI datasets of glioma
patients in a prospective study to validate the predictive power of our
hypothesis. As per standard of care the primary tumor was surgically resected
followed by SRT. For our protocol the patients were then imaged either
pre-surgically or post-surgically before SRT after the reduction of edema. Three volunteers and thirteen patients with
gliomas were imaged. Following SRT, patients were given repeated clinical MRI
follow-ups at regular intervals to identify early incidence of tumor
recurrence. Our method involved DTI acquisition and processing, followed by the
application of a constrained random walk model for cell migration. 1) The DTI datasets were reconstructed with
Camino/DTIStudio and PDD and Fractional Anisotropy (FA) were obtained. 2) The
migration of each cell from the surface voxel was simulated independently. The
uncertainty in the direction of cell migration about the PDD was determined
based on the FA value of the voxel. The PDD was given by the in-plane and
out-of-plane solid angles. The uncertainty in cell migration was ±35°, ±20° and
±10° about the PDD when the FA was 0-0.3, 0.3-0.6 and 0.6-1, respectively. 3)
At each step the direction of migration was decided randomly within the
uncertainty range. 4) When the cell was on the tumor surface it was constrained
to move away from the center of the tumor. 5) The probability of cell migration
was defined as the number of cells found in or passing through each voxel after
a fixed number of steps.
Results and Conclusions: Of the 13 patients recruited to date, six have had
recurrence/secondary tumors. Two patients had secondary tumors outside the
treatment margin and in both of these patients there was a high correlation
between the areas of high cell concentration predicted by our random walk model
and the location of secondary tumors (Figure 1). Four patients had recurrences
within the treatment margin. In one of these patients the areas of high cell
concentration from the random walk model predicted the direction of tumor
spread (Figure 2). For recurrences outside the treatment margin our hypothesis
appears to be valid.
A
comprehensive quality assurance procedure for ultrasound-guided radiation
therapy
Dinko Plenkovich, Roswell Park Cancer Institute, WCA Cancer
Treatment Center, Jamestown, NY
Matthew
B. Podgorsak, Jubei Liu, Roswell Park Cancer Institute, Buffalo, NY
Purpose: Develop a method for evaluation of the entire process
for ultrasound guidance in the targeting of cancer treatment sites. The vendor has provided only the phantoms
for: a) camera verification by registering the ultrasound coordinate system to
that of the linac room coordinate system, and b) ultrasound probe verification,
which informs the ultrasound system of the condition of the ultrasound probe,
including the integrity of ultrasound imaging and the stability of the optical
tracking array relative to the probe.
Method and Materials: An ultrasound phantom was developed and scanned on
the CT scanner. The images were
transferred to the treatment planning system and the structures in the phantom
were contoured. The treatment plan was
exported to an ultrasound system for positioning an anatomical target to the
linac isocenter for extracranial radiation therapy
Results: By tracking the probe’s position and matching
pretreatment isocenter CT image contours to image models, structure position
variances were determined and corrected by repositioning the target.
Conclusion: The phantoms provided by the vendor are not
sufficient. In one of our affiliate
institutions, the vendor-recommended morning quality assurance method failed to
detect a 2-cm difference between the actual and perceived position of the
prostate. It is necessary to evaluate
the whole process from the CT scan to the comparison of the ultrasound images
with the contours created in the treatment planning system. An appropriate phantom should be available
for this evaluation.
Dose Perturbation from Implanted I-125 Seeds in External Beam Therapy
for Prostate Cancer
JP Steinman, M Bakhtiari, DP Nazareth, HK Malhotra,
Roswell Park Cancer Institute, Buffalo, NY
Purpose: Many
times a suboptimal dose distribution resulting from I-125 seeds in prostate
brachytherapy is salvaged by giving additional radiation dose using
3DCRT/IMRT. In standard treatment
planning, the dosimetric perturbations introduced by the existing seeds are
usually ignored. Present study aims at
studying these perturbations for 6MV and 18MV beams within a phantom setting in
region immediately behind the seed.
Methods and
Materials: Three Kodak X-OmatV films were placed on top of 10cm
of Solid Water at 100cm SSD. On top of
the films a single non-radioactive (preactivated) seed was placed and aligned
parallel in the longitudinal direction under 1cm bolus and 4cm Solid Water for
a total buildup of 5cm. A 1cm x 1cm
field was setup and irradiated with 10MU of 6MV and 18MV photons. A second set of measurements was obtained
using three seeds each separated vertically by 0.5cm bolus material allowing
the study of the interseed shielding effect.
Control fields were irradiated with no seeds. All the films belonged to the same batch and
were processed simultaneously. The films
were scanned using a Vidar VXR-16 scanner and analyzed using RIT 113 Version
5.1 obtaining profiles in the transverse and longitudinal direction. Additional external beam treatment plans
were generated for a prostate patient implanted with I-125 seeds using a Monte
Carlo software [VMC++] with and without accounting for the effect of I-125
seeds for both 6 MV and 23MV.
Results: For
the single seed measurement, at about 0.5mm from the seed (top film), the
maximum change in dose from having no seed was 27.1% (6MV) and 13.4%
(18MV). The three seed measurement
revealed 24.1% (6MV) and 11.1% (18MV). Results of external beam planning using
Monte Carlo simulation (VMC++) carried out on a patient implanted with I-125
seeds will be presented.
Conclusion: The
dose perturbation caused by the I-125 seeds is significant locally around the
seed. This can be seen by the fact that
the change in the dose profile is independent of the number of seeds spaced
intermitted above the seed.
Respiratory gating using online automatic segmentation of pulmonary
nodules in megavoltage
electronic portal images using a level set method
JS Schildkraut, J Gomez, A Singh, D Nazareth, HK Malhotra,
Carestream Health, Inc. Rochester NY, SUNY Buffalo
Purpose: Tumors
of the thoracic region present unique problems during their treatment due to
the associated motion of the tumor.
Various methods like respiratory gating alone or in conjunction with
abdominal compression have been developed to either reduce the motion or to
gate the treatment. Unfortunately,
conventional respiratory gating relies on an external surrogate. Studies have shown the inadequacy of this
approach in many cases. Fortunately,
majority of the lung tumors are surrounded by low density lung tissues allowing
an easier identification even with megavoltage imaging. The present work focus on the development of
an alternative system which harnesses the density differences between the tumor
and its surroundings lung tissue directly, thereby, removing the necessity of an
external surrogate system.
Methods and
Materials: A total of 7 patients of non-small cell lung cancer
were used in this study. The study was
carried out on a Varian Trilogy unit equipped with electronic portal imaging
system [EPID] employing an AS-1000 system.
The system has a resolution of 1024x1024 and provides an active area of
30x40 cm2. During the treatment, a
custom designed image acquisition template was applied which captured the
images at every 10% of the delivered monitor units. The resultant images were exported in dicom
format. The treatment plan along with
the raw scan data and structure was also exported in DIOCM format.
Results: The
location of the pulmonary nodule is delineated in each CT slice in which it
appears. A digitally reconstructed radiograph (DRR) is calculated from the
treatment planning CT scan using the source and detector geometry of the EPID.
In the process of calculating the DRR, the projection of the nodule in the DRR
is determined. The nodule’s projection in the DRR is subsequently used as a
shape prior. Nodules are segmented in each portal image using a level set
segmentation algorithm which includes an energy term that is minimized when the
shape of the segmented region matches the shape prior. The nodule segmentation
method was tested on a series of 20 portal images of a nodule that is located
just above the diaphragm. The nodule has an average distance from the center of
the portal of 5.68 mm. Due to respiratory motion, the distance between the
nodule and the portal center has a standard deviation, minimum, and maximum
distance of 1.79, 2.44, and 9.13 mm, respectively. The distance between the
nodule and segmented region center was also measured. The average distance is
1.98 mm and the standard deviation, minimum, and maximum distance is 0.96,
0.13, and 4.12 mm, respectively. These preliminary results suggest that if the
radiation treatment system were to use the results of the nodule segmentation
algorithm to track the nodule, the average position error between the portal
and nodule center could be reduced by 35%. Also, the maximum position error can
be reduced by 45%. The segmentation algorithm has a runtime of 4 seconds on a
2.33 GHz Intel® Core™2 Duo Desktop Processor E6550 with 3.48 GB of RAM.
Conclusion: The
purposed method utilizes the tumor motion directly, thereby, eliminating an
external surrogate system and its associated inaccuracies. Efforts are in progress to harness the
massive parallel computational power of the new breed of graphic cards [GPUs] which
will enable the execution of the algorithm in real-time are in progress. This method can then control linac beam on
in standard RPM based systems. The method can be used for both amplitude based
as well as phase based respiratory gating techniques including breath-hold
technique of treatment.