Volume 36, Issue 6, June 2009
- medical physics letters
- vision 20/20
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- ultrasound physics
- tissue measurements
- books and publications
- education council symposium: ballroom b
- education general poster discussion: exhibit hall
- education symposia: room 211a
- education symposium: room 211a
- imaging continuing education course: room 210a
- imaging continuing education course: room 213a
- imaging continuing education course: room 303a
- imaging general poster discussion: exhibit hall
- imaging moderated poster session: exhibit hall ‐ area 4
- imaging scientific session: room 304a
- imaging symposium: room 210a
- industrial physics forum: frontiers in physics: ballroom c
- industrial physics forum: imaging symposium: room 210a
- industrial physics forum: joint imaging/therapy symposium: ballroom c
- industrial physics forum: joint imaging/therapy symposium: room 303a
- industrial physics forum: therapy symposium: ballroom d
- innovations in medical physics education: room 303a
- joint imaging/therapy general poster discussion: exhibit hall
- joint imaging/therapy moderated poster session: exhibit hall ‐ area 3
- joint imaging/therapy scientific session: ballroom c
- joint imaging/therapy scientific session: room 210a
- joint imaging/therapy scientific session: room 213a
- joint imaging/therapy scientific session: room 303a
- joint imaging/therapy symposium: ballroom c
- joint imaging/therapy symposium: room 303a
- practical medical physics: room 213a
- president's symposium: theme session for industrial physics forum: ballroom b
- professional general poster discussion: exhibit hall
- professional session: room 211a
- professional symposium: ballroom d
- professional symposium: room 211a
- sam imaging symposium: room 304a
- sam professional: ballroom b
- sam therapy symposium: ballroom b
- the aapm 51st annual meeting program
- therapy continuing education course: ballroom c
- therapy continuing education course: ballroom d
- therapy continuing education course: room 211a
- therapy continuing education symposium: ballroom b
- therapy general poster discussion exhibit hall
- therapy general poster discussion: exhibit hall
- therapy moderated poster session: exhibit hall ‐ area 1
- therapy moderated poster session: exhibit hall ‐ area 2
- therapy scientific session: ballroom b
- therapy scientific session: ballroom d
- therapy symposium: ballroom d
- young investigators symposium: ballroom c
Index of content:
Hypofractionation is a proven safe and effective modality for postoperative whole-breast radiotherapy for early breast cancer patients36(2009); http://dx.doi.org/10.1118/1.3116462View Description Hide Description
- MEDICAL PHYSICS LETTERS
In vivo functional chronic imaging of a small animal model using optical-resolution photoacoustic microscopy36(2009); http://dx.doi.org/10.1118/1.3137572View Description Hide Description
Optical-resolution photoacousticmicroscopy (OR-PAM) has been validated as a valuable tool for label-free volumetric microvascular imaging. More importantly, the advantages of noninvasiveness and measurement consistency suggest the use of OR-PAM for chronic imaging of intact microcirculation. Here, such chronic imaging is demonstrated for the first time by monitoring the healing process of laser-induced microvascular lesions in a small animal model in vivo. The central part of a 1 mm by 1 mm region in a nude mouse ear was treated under a continuous-wave laser to create a microvascular lesion for chronic study. The region of interest was imaged before the laser treatment, immediately after the treatment, and throughout the healing process using both the authors’ OR-PAM system and a commercial transmission-mode optical microscope. Three-dimensional microvascular morphology and blood oxygenation information were imaged simultaneously at capillary-level resolution. Transmission-mode optical microscopic images were acquired for comparison. OR-PAM has potential important applications in microcirculatory physiology or pathophysiology, tumor angiogenesis, laser microsurgery, and neuroscience.
36(2009); http://dx.doi.org/10.1118/1.3132273View Description Hide Description
In this Letter the authors introduce a wide-field transmission ultrasound approach to breast imaging based on the use of a large area acousto-optic (AO) sensor. Accompanied by a suitable acoustic source, such a detector could be mounted on a traditional mammographysystem and provide a mammographylikeultrasound projection image of the compressed breast in registration with the x-ray mammogram. The authors call the approach acoustography. The hope is that this additional information could improve the sensitivity and specificity of screening mammography. The AO sensor converts ultrasound directly into a visual image by virtue of the acousto-opticeffect of the liquid crystal layer contained in the AO sensor. The image is captured with a digital video camera for processing, analysis, and storage. In this Letter, the authors perform a geometrical resolution analysis and also present images of a multimodality breast phantom imaged with both mammography and acoustography to demonstrate the feasibility of the approach. The geometric resolution analysis suggests that the technique could readily detect tumors of diameter of 3 mm using 8.5 MHz ultrasound, with smaller tumors detectable with higher frequency ultrasound, though depth penetration might then become a limiting factor. The preliminary phantom images show high contrast and compare favorably to digital mammograms of the same phantom. The authors have introduced and established, through phantom imaging, the feasibility of a full-field transmission ultrasounddetector for breast imaging based on the use of a large area AO sensor. Of course variations in attenuation of connective, glandular, and fatty tissues will lead to images with more cluttered anatomical background than those of the phantom imaged here. Acoustic coupling to the mammographically compressed breast, particularly at the margins, will also have to be addressed.
- VISION 20/20
36(2009); http://dx.doi.org/10.1118/1.3120285View Description Hide Description
Tomosynthesis is a decades-old technique for section imaging that has seen a recent upsurge in interest due to its promise to provide three-dimensional information at lower dose and potentially lower cost than CT in certain clinical imaging situations. This renewed interest in tomosynthesis began in the late 1990s as a new generation of flat-panel detectors became available; these detectors were the one missing piece of the picture that had kept tomosynthesis from enjoying significant utilization earlier. In the past decade, tomosynthesisimaging has been investigated in a variety of clinical imaging situations, but the two most prominent have been in breast and chest imaging.Tomosynthesis has the potential to substantially change the way in which breast cancer and pulmonary nodules are detected and managed. Commercial tomosynthesis devices are now available or on the horizon. Many of the remaining research activities with tomosynthesis will be translational in nature and will involve physicist and clinician alike. This overview article provides a forward-looking assessment of the translational questions facing tomosynthesisimaging and anticipates some of the likely research and clinical activities in the next five years.
36(2009); http://dx.doi.org/10.1118/1.3125136View Description Hide Description
Brachytherapy is a mature treatment modality that has benefited from technological advances. Treatment planning has advanced from simple lookup tables to complex, computer-based dose-calculation algorithms. The current approach is based on the AAPM TG-43 formalism with recent advances in acquiring single-source dose distributions. However, this formalism has clinically relevant limitations for calculating patient dose.Dose-calculation algorithms are being developed based on Monte Carlo methods, collapsed cone, and solving the linear Boltzmann transport equation. In addition to improved dose-calculation tools, planning systems and brachytherapytreatment planning will account for material heterogeneities, scatter conditions, radiobiology, and image guidance. The AAPM, ESTRO, and other professional societies are working to coordinate clinical integration of these advancements. This Vision 20/20 article provides insight into these endeavors.
- RADIATION THERAPY PHYSICS
An approach to using conventional brachytherapy software for clinical treatment planning of complex, Monte Carlo-based brachytherapy dose distributionsa)36(2009); http://dx.doi.org/10.1118/1.3121510View Description Hide Description
Certain brachytherapydose distributions, such as those for LDR prostate implants, are readily modeled by treatment planning systems (TPS) that use the superposition principle of individual seed dose distributions to calculate the total dose distribution. However, dose distributions for brachytherapy treatments using high- shields or having significant material heterogeneities are not currently well modeled using conventional TPS. The purpose of this study is to establish a new treatment planning technique (Tufts technique) that could be applied in some clinical situations where the conventional approach is not acceptable and dose distributions present cylindrical symmetry. Dose distributions from complex brachytherapy source configurations determined with Monte Carlo methods were used as input data. These source distributions included the 2 and 3 cm diameter Valencia skin applicators from Nucletron, 4–8 cm diameter AccuBoost peripheral breast brachytherapy applicators from Advanced Radiation Therapy, and a 16 mm COMS-based eye plaque using , , and seeds. Radial dose functions and 2D anisotropy functions were obtained by positioning the coordinate system origin along the dose distribution cylindrical axis of symmetry. Origin:tissue distance and active length were chosen to minimize TPS interpolation errors. Dosimetry parameters were entered into the PINNACLE TPS, and dose distributions were subsequently calculated and compared to the original Monte Carlo-derived dose distributions. The new planning technique was able to reproduce brachytherapydose distributions for all three applicator types, producing dosimetric agreement typically within 2% when compared with Monte Carlo-derived dose distributions. Agreement between Monte Carlo-derived and planned dose distributions improved as the spatial resolution of the fitted dosimetry parameters improved. For agreement within 5% throughout the clinical volume, spatial resolution of dosimetry parameter data was required, and the virtual brachytherapy source data set included over 5000 data points. On the other hand, the lack of consideration for applicator heterogeneity effect caused conventional dose overestimates exceeding an order of magnitude in regions of clinical interest. This approach is rationalized by the improved dose estimates. In conclusion, a new technique was developed to incorporate complex Monte Carlo-based brachytherapydose distributions into conventional TPS. These results are generalizable to other brachytherapy source types and other TPS.
36(2009); http://dx.doi.org/10.1118/1.3120286View Description Hide Description
The numerical calculation of dose is central to treatment planning in radiation therapy and is at the core of optimization strategies for modern delivery techniques. In a clinical environment, dose calculation algorithms are required to be accurate and fast. The accuracy is typically achieved through the integration of patient-specific data and extensive beam modeling, which generally results in slower algorithms. In order to alleviate execution speed problems, the authors have implemented a modern dose calculation algorithm on a massively parallel hardware architecture. More specifically, they have implemented a convolution-superposition photon beam dose calculation algorithm on a commodity graphics processing unit (GPU). They have investigated a simple porting scenario as well as slightly more complex GPU optimization strategies. They have achieved speed improvement factors ranging from 10 to 20 times with GPU implementations compared to central processing unit (CPU) implementations, with higher values corresponding to larger kernel and calculation grid sizes. In all cases, they preserved the numerical accuracy of the GPU calculations with respect to the CPU calculations. These results show that streaming architectures such as GPUs can significantly accelerate dose calculation algorithms and let envision benefits for numerically intensive processes such as optimizing strategies, in particular, for complex delivery techniques such as IMRT and arc therapy.
36(2009); http://dx.doi.org/10.1118/1.3121506View Description Hide Description
Quasidiscrete scanning is a delivery strategy for proton and ion beam therapy in which the beam is turned off when a slice is finished and a new energy must be set but not during the scanning between consecutive spots. Different scanning paths lead to different dose distributions due to the contribution of the unintended transit dose between spots. In this work an algorithm to optimize the scanning path for quasidiscrete scanned beams is presented. The classical simulated annealing algorithm is used. It is a heuristic algorithm frequently used in combinatorial optimization problems, which allows us to obtain nearly optimal solutions in acceptable running times. A study focused on the best choice of operational parameters on which the algorithm performance depends is presented. The convergence properties of the algorithm have been further improved by using the next-neighbor algorithm to generate the starting paths. Scanning paths for two clinical treatments have been optimized. The optimized paths are found to be shorter than the back-and-forth, top-to-bottom (zigzag) paths generally provided by the treatment planning systems. The gamma method has been applied to quantify the improvement achieved on the dose distribution. Results show a reduction of the transit dose when the optimized paths are used. The benefit is clear especially when the fluence per spot is low, as in the case of repainting. The minimization of the transit dose can potentially allow the use of higher beam intensities, thus decreasing the treatment time. The algorithm implemented for this work can optimize efficiently the scanning path of quasidiscrete scanned particle beams. Optimized scanning paths decrease the transit dose and lead to better dose distributions.
Investigation of intracranial peripheral dose arising from the treatment of large lesions with Leksell GammaKnife® Perfexion™36(2009); http://dx.doi.org/10.1118/1.3125133View Description Hide Description
This investigation involves quantifying the extent of intracranial peripheral dose arising from simulated targets situated in the skull-base or upper-spine region using the Leksell GammaKnife® Perfexion™ treatment unit. For each of three spherical target volumes—denoted as , , and —three treatment plans were manually generated, one for each of the three collimator sizes—4, 8, and 16 mm. Each of the plans was delivered to a spherical dosimetry phantom with an insert containing EBT Gafchromic film. The total dose at 70 mm from the targets’ edges, , was measured as a function of elevation angle and expressed as a percentage of the prescription dose. The film insert was placed centered in the median sagittal plane (Leksell ) and was measured for the angular range from 0° (superior/along axis) to 90° (anterior/along axis). For a given collimator, the irradiation time to treat a spherical target of volume using the 50% isodose line was observed to follow a power-law relationship of the form where was the maximum dose divided by collimatordose rate and was the volume encompassed by the 50% isodose line for a single shot. The mean value of was 0.61 (range: 0.61–0.62). Along the superior direction and up to angles of around 30°, the was always highest for the 4 mm plans, followed by the 8 mm, followed by the 16 mm. In this angular range, the maximum measured was 1.7% of the prescription dose. The intracranial peripheral dose along the superior direction (combined scatter and leakage dose) resulting from irradiation of upper-spine or base-of-skull lesions is measured to be less than 2% of the prescription dose, even for very large targets. The results of this study indicate that, for a given target volume, treatment plans consisting of only 4 mm shots yield larger peripheral dose in the superior direction than 8 mm shot only plans, which in turn yield larger peripheral dose than 16 mm shot only plans.
36(2009); http://dx.doi.org/10.1118/1.3125134View Description Hide Description
It has been a challenge to perform accurate 2D or 3D dosimetry in the surface region with steep dose gradient for megavoltage photon beams. We developed a dosimetry method in the superficial buildup region for the 6 and 15 MV photon beams using a radiochromic EBT film stack. Eight radiochromic EBT film strips stacked together formed a 3D dosimeter. The film stack was positioned above a polystyrene phantom and surrounded by Solid Water slabs (0.2 cm) with the top film layer at 100 cm SSD. A open field was used to irradiate the film stack with 1000 MU. All films were scanned using Epson 4870 flatbed scanner with transmission mode, 48 bit color, and 150 dpi (0.017 cm pixel resolution). The pixel values were converted to doses using an established calibration curve. This method allowed dose measurement for depths from 0.012 to 0.18 cm with fine spatial resolution (0.017 cm horizontally and 0.024 cm vertically). For each energy modality, we obtained both the central axis percent depth doses and the beam profiles along the central line covering the primary field and peripheral region at each depth. The primary field doses varied steeply with depth, while those in the peripheral region were weakly dependent on depth. For the 6 MV and 15 MV photon beams, (1) the central axis percent depth doses in the eight film layers ranged from 22% to 66% and from 15% to 44%, respectively; (2) the extrapolated percent depth doses at were 15% and 14%, respectively. Agreement with the previously reported central axis percent depth doses in this region using parallel plate thin window ion chamber and ultrathin TLD was observed. The percent depth doses and beam profiles data can be incorporated in the treatment planning system for more accurate assessment of the doses to skin and shallow tumors to accomplish more accurate calculation results in the clinical usage.
First MR images obtained during megavoltage photon irradiation from a prototype integrated linac-MR system36(2009); http://dx.doi.org/10.1118/1.3125662View Description Hide Description
The authors report the first magnetic resonance(MR)images produced by their prototype MR system integrated with a radiation therapy source. The prototype consists of a 6 MV linac mounted onto the open end of a biplanar 0.2 T permanent MR system which has 27.9 cm pole-to-pole opening with flat gradients (40 mT/m) running under a TMX NRC console. The distance from the magnet isocenter to the linac target is 80 cm. The authors’ design has resolved the mutual interferences between the two devices such that the MRmagnetic field does not interfere with the trajectory of the electron in the linac waveguide, and the radiofrequency (RF) signals from each system do not interfere with the operation of the other system. Magnetic and RF shielding calculations were performed and confirmed with appropriate measurements. The prototype is currently on a fixed gantry; however, in the very near future, the linac and MRmagnet will rotate in unison such that the linac is always aimed through the opening in the biplanar magnet.MRimaging was found to be fully operational during linac irradiation and proven by imaging a phantom with conventional gradient echo sequences. Except for small changes in SNR,MRimages produced during irradiation were visually and quantitatively very similar to those taken with the linac turned off. This prototype system provides proof of concept that the design has decreased the mutual interferences sufficiently to allow the development of real-time MR-guided radiotherapy. Low field-strength systems (0.2–0.5 T) have been used clinically as diagnostic tools. The task of the linac-MR system is, however, to provide MR guidance to the radiotherapy beam. Therefore, the 0.2 T field strength would provide adequate image quality for this purpose and, with the addition of fast imaging techniques, has the potential to provide 4D soft-tissue visualization not presently available in image-guidedradiotherapy systems. The authors’ initial design incorporates a permanent magnet; however, other types of magnets and field strengths could also be incorporated. Usable MRimages were obtained during linac irradiation from the linac-MR prototype. The authors’ prototype design can be used as the functional starting point in developing real-time MR guidance offering soft-tissue contrast that can be coupled with tumor tracking for real-time adaptive radiotherapy.
36(2009); http://dx.doi.org/10.1118/1.3125137View Description Hide Description
Photoelectric-enhanced radiation therapy is a bimodal therapy, consisting of the administration of highly radiation-absorbing substances into the tumor area and localized regional irradiation with orthovoltage x-rays. Irradiation can be performed by a modified computed tomography(CT) unit equipped with an additional x-ray optical module which converts the polychromatic, fan-shaped CT beam into a monochromatized and focused beam for energy-tuned photoelectric-enhanced radiotherapy. A dedicated x-ray optical module designed for spatial collimation, focusing, and monochromatization was mounted at the exit of the x-ray tube of a clinical CT unit. Spectrally resolved measurements of the resulting beam were performed using an energy-dispersive detection system calibrated by synchrotron radiation. The spatial photon fluence was determined by film dosimetry. Depth-dose measurements were performed and compared to the polychromatic CT and a therapeutic 6 MV beam. The spatial dose distribution in phantoms using a rotating radiation source (quasi-monochromatic CT and 6 MV, respectively) was investigated by geldosimetry. The photoelectric dose enhancement for an iodine fraction of 1% in tissue was calculated and verified experimentally. The x-ray optical module selectively filters the energy of the tungsten emission line with an FWHM of 5 keV. The relative photon fluence distribution demonstrates the focusing characteristic of the x-ray optical module. A beam width of about 3 mm was determined at the isocenter of the CT gantry. The depth-dose measurements resulted in a half-depth value of approximately 36 mm for the CT beams (quasi-monochromatic, polychromatic) compared to 154 mm for the 6 MV beam. The rotation of the radiation source leads to a steep dose gradient at the center of rotation; the geldosimetry yields an entrance-to-peak dose ratio of 1:10.8 for the quasi-monochromatic CT and 1:37.3 for a 6 MV beam of the same size. The photoelectric dose enhancement factor increases from 2.2 to 2.4 by using quasi-monochromatic instead of polychromatic radiation. An additional increase in the radiationdose by a factor of 1.4 due to the focusing characteristic of the x-ray optical module was calculated. Photoelectric-enhanced radiation therapy based on a clinical CT unit combined with an x-ray optical module is a novel therapy option in radiationoncology. The optimized quasi-monochromatic radiation is strongly focused and ensures high photoelectric dose enhancement for iodine.
36(2009); http://dx.doi.org/10.1118/1.3121489View Description Hide Description
Protonradiotherapy centers that currently use passively scattered proton beams do field specific calibrations for a non-negligible fraction of treatment fields, which is time and resource consuming. Our improved understanding of the passive scattering mode of the IBA universal nozzle, especially of the current modulation function, allowed us to re-commission our treatment control system for accurate delivery of SOBPs of any range and modulation, and to predict the output for each of these fields. We moved away from individual field calibrations to a state where continued quality assurance of SOBP field delivery is ensured by limited system-wide measurements that only require one hour per week. This manuscript reports on a protocol for generation of desired SOBPs and prediction of dose output.
Four-dimensional inverse treatment planning with inclusion of implanted fiducials in IMRT segmented fields36(2009); http://dx.doi.org/10.1118/1.3121425View Description Hide Description
The purpose of this study is to develop a 4D inverse planning strategy capable of controlling the appearance of the implanted fiducial(s) in segmented IMRT fields for cine MV or combined MV/kV image-guidedIMRT. This work is focused on enhancing the visibility of the implanted fiducial(s) in 4D IMRT inverse planning, whose goal is to derive a set of time-resolved (or phase-tagged) MLC segments to cater for the motion of the patient anatomy extracted from the emerging 4D images. The task is to optimize the shapes and weights of all the segments for each incident beam, with the fiducial(s) being forced/encouraged to be inside the segmented fields. The system is modeled by a quadratic objective function with inclusion of a hard/soft constraint characterizing the authors’ level of preference for the fiducial(s) to be included in the segmented fields. A simulated annealing algorithm is employed to optimize the system. The proposed technique is demonstrated using two clinical cases. A segment-based inverse planning framework for 4D radiation therapy, capable of providing tempospatially optimized IMRT plans, has been established. Furthermore, using the described 4D optimization approach, it is demonstrated that the MLC blockage of the implanted fiducial(s) during the segmented delivery is avoided without severely compromising the final dose distribution. The visibility of implanted fiducials in 4D IMRT can be improved without significantly deteriorating final dose distribution. This is a foundation for the authors to use cine MV or combined MV/KV to effectively guide the 4D IMRT delivery.
Development of an irradiation method with lateral modulation of SOBP width using a cone-type filter for carbon ion beams36(2009); http://dx.doi.org/10.1118/1.3130021View Description Hide Description
Passive irradiation methods deliver an extra dose to normal tissues upstream of the target tumor, while in dynamic irradiation methods, interplay effects between dynamic beam delivery and target motion induced by breathing or respiration distort the dose distributions. To solve the problems of those two irradiation methods, the authors have developed a new method that laterally modulates the spread-out Bragg peak (SOBP) width. By reducing scanning in the depth direction, they expect to reduce the interplay effects. They have examined this new irradiation method experimentally. In this system, they used a cone-type filter that consisted of 400 cones in a grid of 20 cones by 20 cones. There were five kinds of cones with different SOBP widths arranged on the frame two dimensionally to realize lateral SOBP modulation. To reduce the number of steps of cones, they used a wheel-type filter to make minipeaks. The scanning intensity was modulated for each SOBP width with a pair of scanning magnets. In this experiment, a stepwise dose distribution and spherical dose distribution of 60 mm in diameter were formed. The nonflatness of the stepwise dose distribution was 5.7% and that of the spherical dose distribution was 3.8%. A 2 mm misalignment of the cone-type filter resulted in a nonflatness of more than 5%. Lateral SOBP modulation with a cone-type filter and a scanned carbonion beam successfully formed conformal dose distribution with nonflatness of 3.8% for the spherical case. The cone-type filter had to be set to within 1 mm accuracy to maintain nonflatness within 5%. This method will be useful to treat targets moving during breathing and targets in proximity to important organs.
36(2009); http://dx.doi.org/10.1118/1.3121508View Description Hide Description
In treatment planning of brachytherapy, absorbed dose is calculated by superposing predetermined distributions of absorbed dose to water in water for the single source according to the irradiation pattern [i.e., placement of the source(s) or dwelling position(s)]. Single-source reference water data are derived from Monte Carlo(MC) simulations and/or experiments. For reasons of positional accuracy, experimental brachytherapydosimetry is most often performed in plastic phantoms. This work investigates the water equivalence of phantoms made from polystyrene, PMMA, and solid water for dosimetry. The EGSnrc MC code is used to simulate radial absorbed dose distributions in cylindrical phantoms of dimensions ranging in size from diameter and height of 20 cm to diameter and height of 40 cm. Water equivalence prevails if the absorbed dose to water in the plastic phantom is the same as the absorbed dose to water in a water phantom at equal distances from the source. It is shown that water equivalence at a specified distance from the source depends not only on the size of the plastic phantom but also on the size of the water phantom used for comparison. Compared to equally sized water phantoms, phantoms of polystyrene are less water equivalent than phantoms of PMMA and solid water but compared to larger water phantoms they are the most water equivalent. Although phantom dimension is the most important single factor influencing the dose distributions around sources, the effect of material properties is non-negligible and becomes increasingly important as phantom dimensions increase. The importance of knowing the size of the water phantom whose data underlies treatment planning systems, when using such data as a reference in, e.g., detector evaluation studies, is discussed. To achieve the highest possible accuracy in experimental dosimetry, phantom-specific correction factors should be used.
Evaluation of a lithium formate EPR dosimetry system for dose measurements around brachytherapy sources36(2009); http://dx.doi.org/10.1118/1.3110068View Description Hide Description
A dosimetry system using lithium formate monohydrate as detector material and electron paramagnetic resonance(EPR)spectroscopy for readout has been used to measure absorbed dose distributions around clinical sources. Cylindrical tablets with diameter of 4.5 mm, height of 4.8 mm, and density of were manufactured. Homogeneity test and calibration of the dosimeters were performed in a 6 MV photon beam. irradiations were performed in a PMMA phantom using two different source models, the GammaMed Plus HDR and the microSelectron PDR-v1 model. Measured absorbed doses to water in the PMMA phantom were converted to the corresponding absorbed doses to water in water phantoms of dimensions used by the treatment planning systems (TPSs) using correction factors explicitly derived for this experiment. Experimentally determined absorbed doses agreed with the absorbed doses to water calculated by the TPS to within ±2.9%. Relative standard uncertainties in the experimentally determined absorbed doses were estimated to be within the range of 1.7%–1.3% depending on the radial distance from the source, the type of source (HDR or PDR), and the particular absorbed doses used. This work shows that a lithium formate dosimetry system is well suited for measurements of absorbed dose to water around clinical HDR and PDR sources. Being less energy dependent than the commonly used thermoluminescent lithium fluoride (LiF) dosimeters,lithium formate monohydrate dosimeters are well suited to measure absorbed doses in situations where the energy dependence cannot easily be accounted for such as in multiple-source irradiations to verify treatment plans. Their wide dynamic range and linear dose response over the dose interval of 0.2–1000 Gy make them suitable for measurements on sources of the strengths used in clinical applications. The dosimeter size needs, however, to be reduced for application to single-source dosimetry.
Image-guided respiratory-gated lung stereotactic body radiotherapy: Which target definition is optimal?36(2009); http://dx.doi.org/10.1118/1.3129161View Description Hide Description
In stereotactic body radiotherapy(SBRT), the respiratory tumor motion makes target definition very important to achieve optimal clinical results for treatment of early stage lungcancer. In this article, the authors quantitatively evaluated the influence of different target definition strategies on image-guided respiratory-gated SBRT for lungcancer. Twelve lungcancer patients with 4D CT estimated target motion of were selected for this retrospective study. An experienced physician contoured gross target volumes (GTVs) at each 4D CT phase for all patients. Three types of internal target volumes (ITVs) were generated based on the contoured GTVs:(1) : GTV contoured on deep expiration breath-hold (BH) CT with an isotropic internal margin (IM) of ; (2) : GTV contoured at the end-expiration (50%) phase with an isotropic IM of ; (3) : Composite volume of all GTVs within the gating window, defined as several phases around phase 50% with residual target motion of . Planning target volumes (PTVs) were generated by adding isotropic setup error margin to ITVs. Three treatment plans, namely, , , and , were created based on the three PTVs. Identical beam settings and planning constraints were used for all three plans for each patient. The prescription dose was in three fractions. The potential toxicities to the critical organs were quantified by mean lungdose (MLD), lung volume receiving (V20), mean heartdose(MHD), and spinal cord dose (SCD). It is shown that the tumor volume and dose coverage are comparable for and . On average, are 38% less than . Although for most patients encompasses the entire , up to (6%) of is outside . Compared to , prescribed percentage is about 2% higher for , and the average dose decreases in critical organs are for MLD, 1.02% for V20, for MHD and for maximum SCD. For the cases receiving high lung and heartdose with , the dose reduction is for MLD and for MHD with . Our preliminary results show that a patient-specific ITV, defined as the composite volume of all GTVs within the gating window, may be used to define PTV in image-guided respiratory-gated SBRT. This approach potentially reduces the irradiated volume of normal tissue further without sacrificing target dose coverage and thus may minimize the risk of treatment-related toxicities.
An overview of the comprehensive proton therapy machine quality assurance procedures implemented at The University of Texas M. D. Anderson Cancer Center Proton Therapy Center–Houston36(2009); http://dx.doi.org/10.1118/1.3120288View Description Hide Description
The number of proton and carbon ion therapy centers is increasing; however, since the publication of the International Commission on Radiation Units and Measurements report, there has been no dedicated report dealing with proton therapy quality assurance. The purpose of this article is to describe the quality assurance procedures performed on the passively scattered proton therapy beams at The University of Texas M. D. Anderson Cancer Center Proton Therapy Center in Houston. The majorities of these procedures are either adopted from procedures outlined in the American Association of Physicists in Medical Task Group (TG) 40 report or are a modified version of the TG 40 procedures. In addition, new procedures, which were designed specifically to be applicable to the synchrotron at the author’s center, have been implemented. The authors’ procedures were developed and customized to ensure patient safety and accurate operation of synchrotron to within explicit limits. This article describes these procedures and can be used by others as a guideline for developing QA procedures based on particle accelerator specific parameters and local regulations pertinent to any new facility.
36(2009); http://dx.doi.org/10.1118/1.3132422View Description Hide Description
In proton therapydelivered with range modulated beams, the energy spectrum of protons entering the delivery nozzle can affect the dose uniformity within the target region and the dose gradient around its periphery. For a cyclotron with a fixed extraction energy, a rangeshifter is used to change the energy but this produces increasing energy spreads for decreasing energies. This study investigated the magnitude of the effects of different energy spreads on dose uniformity and distal edge dose gradient and determined the limits for controlling the incident spectrum. A multilayerFaraday cup (MLFC) was calibrated against depth dose curves measured in water for nonmodulated beams with various incident spectra. Depth dose curves were measured in a water phantom and in a multilayerionization chamber detector for modulated beams using different incident energy spreads. Some nozzle entrance energy spectra can produce unacceptable dose nonuniformities of up to over the modulated region. For modulated beams and small beam ranges, the width of the distal penumbra can vary by a factor of 2.5. When the energy spread was controlled within the defined limits, the dose nonuniformity was less than . To facilitate understanding of the results, the data were compared to the measured and Monte Carlo calculated data from a variable extraction energy synchrotron which has a narrow spectrum for all energies. Dose uniformity is only maintained within prescription limits when the energy spread is controlled. At low energies, a large spread can be beneficial for extending the energy range at which a single range modulator device can be used. An MLFC can be used as part of a feedback to provide specified energy spreads for different energies.