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Dec 2005

Volume 32, Issue 12, pp. 3507-3863

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POINT/COUNTERPOINT: Methods for image segmentation should be standardized and calibrated

Edward Chaney, Geoffrey Ibbott, and William R. Hendee, Moderator

Med. Phys. 32, 3507 (2005); http://dx.doi.org/10.1118/1.2131093 (4 pages) | Cited 5 times

Online Publication Date: 9 November 2005

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Abstract Unavailable
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87.55.-x Treatment strategy

RADIATION THERAPY PHYSICS: Benchmarking analytical calculations of proton doses in heterogeneous matter

George Ciangaru, Jerimy C. Polf, Martin Bues, and Alfred R. Smith

Med. Phys. 32, 3511 (2005); http://dx.doi.org/10.1118/1.2064887 (13 pages) | Cited 13 times

Online Publication Date: 9 November 2005

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A proton dose computational algorithm, performing an analytical superposition of infinitely narrow proton beamlets (ASPB) is introduced. The algorithm uses the standard pencil beam technique of laterally distributing the central axis broad beam doses according to the Moliere scattering theory extended to slablike varying density media. The purpose of this study was to determine the accuracy of our computational tool by comparing it with experimental and Monte Carlo (MC) simulation data as benchmarks. In the tests, parallel wide beams of protons were scattered in water phantoms containing embedded air and bone materials with simple geometrical forms and spatial dimensions of a few centimeters. For homogeneous water and bone phantoms, the proton doses we calculated with the ASPB algorithm were found very comparable to experimental and MC data. For layered bone slab inhomogeneity in water, the comparison between our analytical calculation and the MC simulation showed reasonable agreement, even when the inhomogeneity was placed at the Bragg peak depth. There also was reasonable agreement for the parallelepiped bone block inhomogeneity placed at various depths, except for cases in which the bone was located in the region of the Bragg peak, when discrepancies were as large as more than 10%. When the inhomogeneity was in the form of abutting air-bone slabs, discrepancies of as much as 8% occurred in the lateral dose profiles on the air cavity side of the phantom. Additionally, the analytical depth-dose calculations disagreed with the MC calculations within 3% of the Bragg peak dose, at the entry and midway depths in the phantom. The distal depth-dose 20%–80% fall-off widths and ranges calculated with our algorithm and the MC simulation were generally within 0.1 cm of agreement. The analytical lateral-dose profile calculations showed smaller (by less than 0.1 cm) 20%–80% penumbra widths and shorter fall-off tails than did those calculated by the MC simulations. Overall, this work validates the usefulness of our ASPB algorithm as a reasonably fast and accurate tool for quality assurance in planning wide beam proton therapy treatment of clinical sites either composed of homogeneous materials or containing laterally extended inhomogeneities that are comparable in density and located away from the Bragg peak depths.
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87.53.Bn Dosimetry/exposure assessment
87.55.K- Monte Carlo methods
87.56.Da Ancillary equipment
87.55.Qr Quality assurance in radiotherapy
87.55.-x Treatment strategy
02.50.Sk Multivariate analysis

NON-IONIZING RADIATION PHYSICS: Optimized interstitial PDT prostate treatment planning with the Cimmino feasibility algorithm

Martin D. Altschuler, Timothy C. Zhu, Jun Li, and Stephen M. Hahn

Med. Phys. 32, 3524 (2005); http://dx.doi.org/10.1118/1.2107047 (13 pages) | Cited 22 times

Online Publication Date: 9 November 2005

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The primary aim of this study was to determine whether optimized photodynamic therapy (PDT) treatment planning (seeking optimized positions, lengths, and strengths of the light sources to satisfy a given dose prescription) can improve dose coverage to the prostate and the sparing of critical organs relative to what can be achieved by the standard PDT plan. The Cimmino algorithm and search procedures based on that algorithm were tested for this purpose. A phase I motexafin lutetium (MLu)-mediated photodynamic therapy protocol is ongoing at the University of Pennsylvania. PDT for the prostate is performed with cylindrical diffusing fibers of various lengths inserted perpendicular to a base plate to obtain longitudinal coverage by a matrix of parallel catheters. The standard plan for the protocol uses sources of equal strength with equal spaced (1-cm) loading. Uniform optical properties were assumed. Our algorithms produce plans that cover the prostate and spare the urethra and rectum with less discrepancy from the dose prescription than the standard plan. The Cimmino feasibility algorithm is fast enough that changes to the treatment plan may be made in the operating room before and during PDT to optimize light delivery.
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87.50.wp Therapeutic applications

RADIATION PROTECTION PHYSICS: The UF series of tomographic computational phantoms of pediatric patients

Choonik Lee, Jonathan L. Williams, Choonsik Lee, and Wesley E. Bolch

Med. Phys. 32, 3537 (2005); http://dx.doi.org/10.1118/1.2107067 (12 pages) | Cited 27 times

Online Publication Date: 9 November 2005

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Two classes of anthropomorphic computational phantoms exist for use in Monte Carlo radiation transport simulations: tomographic voxel phantoms based upon three-dimensional (3D) medical images, and stylized mathematical phantoms based upon 3D surface equations for internal organ definition. Tomographic phantoms have shown distinct advantages over the stylized phantoms regarding their similarity to real human anatomy. However, while a number of adult tomographic phantoms have been developed since the early 1990s, very few pediatric tomographic phantoms are presently available to support dosimetry in pediatric diagnostic and therapy examinations. As part of a larger effort to construct a series of tomographic phantoms of pediatric patients, five phantoms of different ages (9‐month male, 4‐year female, 8‐year female, 11‐year male, and 14‐year male) have been constructed from computed tomography (CT) image data of live patients using an IDL-based image segmentation tool. Lungs, bones, and adipose tissue were automatically segmented through use of window leveling of the original CT numbers. Additional organs were segmented either semiautomatically or manually with the aid of both anatomical knowledge and available image-processing techniques. Layers of skin were created by adding voxels along the exterior contour of the bodies. The phantoms were created from fused images taken from head and chest-abdomen-pelvis CT exams of the same individuals (9‐month and 4‐year phantoms) or of two different individuals of the same sex and similar age (8‐year, 11‐year, and 14‐year phantoms). For each model, the resolution and slice positions of the image sets were adjusted based upon their anatomical coverage and then fused to a single head-torso image set. The resolutions of the phantoms for the 9‐month, 4‐year, 8‐year, 11‐year, and 14‐year are 0.43×0.43×3.0 mm, 0.45×0.45×5.0 mm, 0.58×0.58×6.0 mm, 0.47×0.47×6.00 mm, and 0.625×0.625×6.0 mm, respectively. While organ masses can be matched to reference values in both stylized and tomographic phantoms, side-by-side comparisons of organ doses in both phantom classes indicate that organ shape and positioning are equally important parameters to consider in accurate determinations of organ absorbed dose from external photon irradiation. Preliminary studies of external photon irradiation of the 11‐year phantom indicate significant departures of organ dose coefficients from that predicted by the existing stylized phantom series. Notable differences between pediatric stylized and tomographic phantoms include anterior-posterior (AP) and right lateral (RLAT) irradiation of the stomach wall, left lateral (LLAT) and right lateral (RLAT) irradiation of the thyroid, and AP and posterior-anterior (PA) irradiation of the urinary bladder.
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87.53.Bn Dosimetry/exposure assessment
87.59.bd Computed radiography
87.55.K- Monte Carlo methods
87.56.Da Ancillary equipment

RADIATION THERAPY PHYSICS: Target volume dose considerations in proton beam treatment planning for lung tumors

Martijn Engelsman and Hanne M. Kooy

Med. Phys. 32, 3549 (2005); http://dx.doi.org/10.1118/1.2126187 (9 pages) | Cited 36 times

Online Publication Date: 11 November 2005

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We performed a treatment planning study in order to gather basic insight in the effect of setup errors and breathing motion on the cumulative proton dose to a lung tumor. We used a simplified geometry that simulates a 50 mm diameter gross tumor volume (GTV) located centrally inside lung tissue. The GTV was expanded with a uniform 5 mm margin into a clinical target volume (CTV) and into a variety of planning target volume (PTV’s). Proton beam apertures were designed to conform the prescribed dose laterally to the PTV while the range compensator was designed to provide distal coverage of the CTV. Different smearing distances were applied to the range compensators, and the cumulative dose in the CTV was evaluated for different combinations of breathing motion and systematic setup errors. Evaluation parameters were the dose to 99% of the CTV (D99) and the equivalent uniform dose (EUD), with a surviving fraction at 2 Gy of SF2=0.5. For a single proton field designed to a 15 mm expansion of the CTV and without smearing applied to the range compensator, D99 of the CTV reduced from 96% for no tumor displacement to 41% and 13% for systematic setup errors of 5 and 10 mm, respectively. For a representative clinical combination, of 5 mm systematic error and 10 mm breathing amplitude, the EUD of the CTV was about 40 Gy (prescribed dose 70 Gy) regardless the CTV to PTV margin, and without smearing. Smearing the range compensator increases the dose to the CTV substantially with a lateral margin and smearing distance of 7.5 mm providing ample tumor coverage. In this latter case, D99 of the target volume increased to 87% for a single field treatment plan. Smearing does, however, lead to an increase in dose to normal tissues distal to the clinical target volume. Next to countering geometric mismatches due to patient setup, smearing can also be used to counter the detrimental effects of breathing motion on the dose to the clinical target volume. We show that the lateral margin and smearing distance can be substantially smaller than the maximum tumor displacement due to setup errors and patient breathing, as measured by the D99 and the EUD.
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87.55.-x Treatment strategy
87.19.X- Diseases
87.19.U- Hemodynamics
87.19.Wx Pneumodyamics, respiration
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: Correction of pixel sensitivity variation and off-axis response for amorphous silicon EPID dosimetry

Peter B. Greer

Med. Phys. 32, 3558 (2005); http://dx.doi.org/10.1118/1.2128498 (11 pages) | Cited 46 times

Online Publication Date: 11 November 2005

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The aim of this work is to determine the pixel sensitivity variation and off-axis dose response of an amorphous silicon electronic portal imaging device (EPID), and develop a correction method to improve EPID dosimetry. The uncorrected or raw pixel response of the aS500 amorphous silicon EPID shows differences in response (sensitivity) of individual pixels as well as a large off-axis differential response with respect to an ion chamber in water. Both can be corrected by division of raw images by the flood-field (FF) image. However, this leads to two problems for dosimetry: (1) the beam profile is present in both the raw image and FF image, and hence is “washed out” of the corrected image, and (2) any mismatch of EPID position between dosimetry and FF calibration means that the beam profile and off-axis response in the raw image and FF are misaligned. This causes artifacts in FF division and dosimetric errors. A method was developed to measure the off-axis response and pixel sensitivity variation separately to allow correction of images at any EPID position while retaining beam profile information. The pixel sensitivity variation is applied to the imager plane and is independent of imager position. The off-axis response depends on the imager plane position relative to the beam central axis. The pixel sensitivities were derived from multiple images of the same symmetric field acquired with the detector displaced laterally between each image. The off-axis response was measured by acquiring off-axis raw images (FF correction removed) and dividing out the off-axis beam fluence and previously determined pixel sensitivity differences. The dosimetric errors due to lateral and vertical detector displacement with the conventional FF calibration method were measured and compared to the new method. Corrected EPID profiles were then compared to beam profiles measured with ion chamber in water for open fields. The EPID was found to have a large off-axis differential response with respect to an ion chamber in water, particularly for 6 MV. This increased to 13% at 15 cm off-axis for 6 MV, and 3.5% for 18 MV at the isocenter plane. The dosimetric errors introduced by detector displacement with conventional FF calibration were found to be approximately 1% per centimeter of lateral detector displacement and 0.1% per centimeter of vertical displacement. These were reduced to less than 1% for any position with the new correction method. Corrected EPID images agreed with ion-chamber measurements to within 2% (excluding penumbra and low-dose areas outside the field) for various field sizes. The new correction method gives consistent dosimetry for any EPID position and retains beam profile information in the image.
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87.55.km Verification
87.53.Bn Dosimetry/exposure assessment

RADIATION IMAGING PHYSICS: Expandable and rigid endorectal coils for prostate MRI: Impact on prostate distortion and rigid image registration

Yongbok Kim, I-Chow J. Hsu, Jean Pouliot, Susan Moyher Noworolski, Daniel B. Vigneron, and John Kurhanewicz

Med. Phys. 32, 3569 (2005); http://dx.doi.org/10.1118/1.2122467 (10 pages) | Cited 13 times

Online Publication Date: 15 November 2005

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Endorectal coils (ERCs) are used for acquiring high spatial resolution magnetic resonance (MR) images of the human prostate. The goal of this study is to determine the impact of an expandable versus a rigid ERC on changes in the location and deformation of the prostate gland and subsequently on registering prostate images acquired with and without an ERC. Sagittal and axial T2 weighted MR images were acquired from 25 patients receiving a combined MR imaging/MR spectroscopic imaging staging exam for prostate cancer. Within the same exam, images were acquired using an external pelvic phased array coil both alone and in combination with either an expandable ERC (MedRad, Pittsburgh, PA) or a rigid ERC (USA Instruments, Aurora, OH). Rotations, translations and deformations caused by the ERC were measured and compared. The ability to register images acquired with and without the ERC using a manual rigid-body registration was assessed using a similarity index (SI). Both ERCs caused the prostate to tilt anteriorly with an average tilt of 18.5° (17.4±9.9 and 19.5±11.3°, mean±standard deviation, for expandable and rigid ERC, respectively). However, the expandable coil caused a significantly larger distortion of the prostate as compared to the rigid coil; compressing the prostate in the anterior/posterior direction by 4.1±3.0 mm vs 1.2±2.2 mm (14.5% vs 4.8%) (p<0.0001), and widening the prostate in the right/left direction by 3.8±3.7 mm vs 1.5±3.1 mm (8.3% vs 3.4%) (p=0.004). Additionally, the ability to manually align prostate images acquired with and without ERC was significantly (p<0.0001) better for the rigid coil (SI=0.941±0.008 vs 0.899±0.033, for the rigid and expandable coils, respectively). In conclusion, the manual rigid-body alignment of prostate MR images acquired with and without the ERC can be improved through the use of a rigid ERC.
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87.61.-c Magnetic resonance imaging
87.57.N- Image analysis
87.19.R- Mechanical and electrical properties of tissues and organs

RADIATION THERAPY PHYSICS: Monte Carlo simulation estimates of neutron doses to critical organs of a patient undergoing 18 MV x-ray LINAC-based radiotherapy

R. Barquero, T. M. Edwards, M. P. Iñiguez, and H. R. Vega-Carrillo

Med. Phys. 32, 3579 (2005); http://dx.doi.org/10.1118/1.2122547 (10 pages) | Cited 12 times

Online Publication Date: 15 November 2005

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Absorbed photoneutron dose to patients undergoing 18 MV x-ray therapy was studied using Monte Carlo simulations based on the MCNPX code. Two separate transport simulations were conducted, one for the photoneutron contribution and another for neutron capture gamma rays. The phantom model used was of a female patient receiving a four-field pelvic box treatment. Photoneutron doses were determinate to be higher for organs and tissues located inside the treatment field, especially those closest to the patient’s skin. The maximum organ equivalent dose per x-ray treatment dose achieved within each treatment port was 719 μSv∕Gy to the rectum (180° field), 190 μSv∕Gy to the intestine wall (0° field), 51 μSv∕Gy to the colon wall (90° field), and 45 μSv∕Gy to the skin (270° field). The maximum neutron equivalent dose per x-ray treatment dose received by organs outside the treatment field was 65 μSv∕Gy to the skin in the antero-posterior field. A mean value of 5±2 μSv∕Gy was obtained for organs distant from the treatment field. Distant organ neutron equivalent doses are all of the same order of magnitude and constitute a good estimate of deep organ neutron equivalent doses. Using the risk assessment method of the ICRP-60 report, the greatest likelihood of fatal secondary cancer for a 70 Gy dose is estimated to be 0.02% for the pelvic postero-anterior field, the rectum being the organ representing the maximum contribution of 0.011%.
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87.53.Bn Dosimetry/exposure assessment
87.55.K- Monte Carlo methods
87.56.Da Ancillary equipment

RADIATION IMAGING PHYSICS: The physics of computed radiography: Measurements of pulse height spectra of photostimulable phosphor screens using prompt luminescence

Kristina N. Watt, Kuo Yan, Giovanni DeCrescenzo, and J. A. Rowlands

Med. Phys. 32, 3589 (2005); http://dx.doi.org/10.1118/1.2122587 (10 pages) | Cited 3 times

Online Publication Date: 15 November 2005

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Computed radiography (CR) is a digital technology that employs reusable photostimulable phosphor (PSP) imaging plates (IP) to acquire radiographic images. In CR, the x-ray attenuation pattern of the imaged object is temporarily stored as a latent charge image within the PSP. The latent image is optically readout as photostimulated luminescence (PSL) when the phosphor is subsequently stimulated using a scanning laser. The multiple stages necessary to create a CR image make it difficult to investigate either experimentally or theoretically. In order to examine the performance of the CR system at a fundamental level separate measurements of the processes involved are desirable. Here pulse height spectroscopy is used to study the prompt violet light emission or prompt luminescence (PL) from commercial PSP screens. Since the mechanism by which light escapes from the phosphor is identical for PL and PSL, observations and conclusions based on the pulse height spectra (PHS) of PL are relevant to the understanding of the behavior of the PSL light emission that outputs the radiographic image in CR. The PL PHS of screens of different thickness and optical properties were measured and compared with the PHS of conventional phosphors. A new method for calibration of the PHS in terms of the absolute number of optical photons per x-ray is introduced and compared to previously established methods.
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87.59.B- Radiography
87.59.bf Digital radiography
07.68.+m Photography, photographic instruments; xerography
87.63.-d Non-ionizing radiation equipment and techniques

RADIATION PROTECTION PHYSICS: Important changes in medical x-ray imaging facility shielding design methodology. A brief summary of recommendations in NCRP Report No. 147

Benjamin R. Archer and Joel E. Gray

Med. Phys. 32, 3599 (2005); http://dx.doi.org/10.1118/1.2124587 (3 pages)

Online Publication Date: 15 November 2005

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The recently published Report No. 147 of The National Council on Radiation Protection and Measurements entitled “Structural shielding design for medical x-ray imaging facilities” provides an update of shielding recommendations for x rays used for medical imaging. The goal of this report is to ensure that the shielding in these facilities limits radiation exposures to employees and members of the public to acceptable levels. Board certified medical and health physicists, as defined in this report, are the “qualified experts” who are competent to design radiation shielding for these facilities. As such, physicists must be aware of the new technical information and the changes from previous reports that Report No. 147 supersedes. In this article we summarize the new data, models and recommendations for the design of radiation barriers in medical imaging facilities that are presented in Report No. 147.
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87.55.N- Radiation monitoring, control, and safety
87.59.-e X-ray imaging
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