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Jul 2013

Volume 40, Issue 7 (partial)

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POINT/COUNTERPOINT: Patient-specific QA for IMRT should be performed using software rather than hardware methods

Ramon Alfredo C. Siochi, Ph.D., Andrea Molineu, M.S., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 40, 070601 (2013); http://dx.doi.org/10.1118/1.4794929 (3 pages)

Online Publication Date: 31 May 2013

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Abstract Unavailable
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87.55.Qr Quality assurance in radiotherapy
87.85.-d Biomedical engineering

RADIATION THERAPY PHYSICS: Shielding implications for secondary neutrons and photons produced within the patient during IMPT

J. DeMarco, P. Kupelian, A. Santhanam, and D. Low

Med. Phys. 40, 071701 (2013); http://dx.doi.org/10.1118/1.4807089 (8 pages)

Online Publication Date: 31 May 2013

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Purpose: Intensity modulated proton therapy (IMPT) uses a combination of computer controlled spot scanning and spot-weight optimized planning to irradiate the tumor volume uniformly. In contrast to passive scattering systems, secondary neutrons and photons produced from inelastic proton interactions within the patient represent the major source of emitted radiation during IMPT delivery. Various published studies evaluated the shielding considerations for passive scattering systems but did not directly address secondary neutron production from IMPT and the ambient dose equivalent on surrounding occupational and nonoccupational work areas. Thus, the purpose of this study was to utilize Monte Carlo simulations to evaluate the energy and angular distributions of secondary neutrons and photons following inelastic proton interactions within a tissue-equivalent phantom for incident proton spot energies between 70 and 250 MeV.
Methods: Monte Carlo simulation methods were used to calculate the ambient dose equivalent of secondary neutrons and photons produced from inelastic proton interactions in a tissue-equivalent phantom. The angular distribution of emitted neutrons and photons were scored as a function of incident proton energy throughout a spherical annulus at 1, 2, 3, 4, and 5 m from the phantom center. Appropriate dose equivalent conversion factors were applied to estimate the total ambient dose equivalent from secondary neutrons and photons.
Results: A reference distance of 1 m from the center of the patient was used to evaluate the mean energy distribution of secondary neutrons and photons and the resulting ambient dose equivalent. For an incident proton spot energy of 250 MeV, the total ambient dose equivalent (3.6 × 10−3 mSv per proton Gy) was greatest along the direction of the incident proton spot (0°–10°) with a mean secondary neutron energy of 71.3 MeV. The dose equivalent decreased by a factor of 5 in the backward direction (170°–180°) with a mean energy of 4.4 MeV. An 8 × 8 × 8 cm3 volumetric spot distribution (5 mm FWHM spot size, 4 mm spot spacing) optimized to produce a uniform dose distribution results in an ambient dose equivalent of 4.5 × 10−2 mSv per proton Gy in the forward direction.
Conclusions: This work evaluated the secondary neutron and photon emission due to monoenergetic proton spots between 70 and 250 MeV, incident on a tissue equivalent phantom. Example calculations were performed to estimate concrete shield thickness based upon appropriate workload and shielding design assumptions. Although lower than traditional passive scattered proton therapy systems, the ambient dose equivalent from secondary neutrons produced by the patient during IMPT can be significant relative to occupational and nonoccupational workers in the vicinity of the treatment vault. This work demonstrates that Monte Carlo simulations are useful as an initial planning tool for studying the impact of the treatment room and maze design on surrounding occupational and nonoccupational work areas.
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87.55.N- Radiation monitoring, control, and safety
87.53.Ay Biophysical mechanisms of interaction
87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
87.55.kh Applications

RADIATION THERAPY PHYSICS: On the use of biomathematical models in patient-specific IMRT dose QA

Heming Zhen, Benjamin E. Nelms, and Wolfgang A. Tomé

Med. Phys. 40, 071702 (2013); http://dx.doi.org/10.1118/1.4805105 (10 pages)

Online Publication Date: 31 May 2013

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Purpose: To investigate the use of biomathematical models such as tumor control probability (TCP) and normal tissue complication probability (NTCP) as new quality assurance (QA) metrics.
Methods: Five different types of error (MLC transmission, MLC penumbra, MLC tongue and groove, machine output, and MLC position) were intentionally induced to 40 clinical intensity modulated radiation therapy (IMRT) patient plans (20 H&N cases and 20 prostate cases) to simulate both treatment planning system errors and machine delivery errors in the IMRT QA process. The changes in TCP and NTCP for eight different anatomic structures (H&N: CTV, GTV, both parotids, spinal cord, larynx; prostate: CTV, rectal wall) were calculated as the new QA metrics to quantify the clinical impact on patients. The correlation between the change in TCP/NTCP and the change in selected DVH values was also evaluated. The relation between TCP/NTCP change and the characteristics of the TCP/NTCP curves is discussed.
Results: ΔTCP and ΔNTCP were summarized for each type of induced error and each structure. The changes/degradations in TCP and NTCP caused by the errors vary widely depending on dose patterns unique to each plan, and are good indicators of each plan's “robustness” to that type of error.
Conclusions: In this in silico QA study the authors have demonstrated the possibility of using biomathematical models not only as patient-specific QA metrics but also as objective indicators that quantify, pretreatment, a plan's robustness with respect to possible error types.
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87.55.Qr Quality assurance in radiotherapy
87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: A method for removing arm backscatter from EPID images

Brian W. King and Peter B. Greer

Med. Phys. 40, 071703 (2013); http://dx.doi.org/10.1118/1.4807640 (9 pages)

Online Publication Date: 3 June 2013

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Purpose: To develop a method for removing the support arm backscatter from images acquired using current Varian electronic portal imaging devices (EPIDs).
Methods: The effect of arm backscatter on EPID images was modeled using a kernel convolution method. The parameters of the model were optimized by comparing on-arm images to off-arm images. The model was used to develop a method to remove the effect of backscatter from measured EPID images. The performance of the backscatter removal method was tested by comparing backscatter corrected on-arm images to measured off-arm images for 17 rectangular fields of different sizes and locations on the imager. The method was also tested using on- and off-arm images from 42 intensity modulated radiotherapy (IMRT) fields.
Results: Images generated by the backscatter removal method gave consistently better agreement with off-arm images than images without backscatter correction. For the 17 rectangular fields studied, the root mean square difference of in-plane profiles compared to off-arm profiles was reduced from 1.19% (standard deviation 0.59%) on average without backscatter removal to 0.38% (standard deviation 0.18%) when using the backscatter removal method. When comparing to the off-arm images from the 42 IMRT fields, the mean γ and percentage of pixels with γ < 1 were improved by the backscatter removal method in all but one of the images studied. The mean γ value (1%, 1 mm) for the IMRT fields studied was reduced from 0.80 to 0.57 by using the backscatter removal method, while the mean γ pass rate was increased from 72.2% to 84.6%.
Conclusions: A backscatter removal method has been developed to estimate the image acquired by the EPID without any arm backscatter from an image acquired in the presence of arm backscatter. The method has been shown to produce consistently reliable results for a wide range of field sizes and jaw configurations.
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87.85.Pq Biomedical imaging
87.53.Jw Therapeutic applications, including brachytherapy
87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)

RADIATION THERAPY PHYSICS: Automated generation of IMRT treatment plans for prostate cancer patients with metal hip prostheses: Comparison of different planning strategies

Peter W. J. Voet, Maarten L. P. Dirkx, Sebastiaan Breedveld, and Ben J. M. Heijmen

Med. Phys. 40, 071704 (2013); http://dx.doi.org/10.1118/1.4808117 (7 pages)

Online Publication Date: 5 June 2013

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Purpose: To compare IMRT planning strategies for prostate cancer patients with metal hip prostheses.
Methods: All plans were generated fully automatically (i.e., no human trial-and-error interactions) using iCycle, the authors’ in-house developed algorithm for multicriterial selection of beam angles and optimization of fluence profiles, allowing objective comparison of planning strategies. For 18 prostate cancer patients (eight with bilateral hip prostheses, ten with a right-sided unilateral prosthesis), two planning strategies were evaluated: (i) full exclusion of beams containing beamlets that would deliver dose to the target after passing a prosthesis (IMRTremove) and (ii) exclusion of those beamlets only (IMRTcut). Plans with optimized coplanar and noncoplanar beam arrangements were generated. Differences in PTV coverage and sparing of organs at risk (OARs) were quantified. The impact of beam number on plan quality was evaluated.
Results: Especially for patients with bilateral hip prostheses, IMRTcut significantly improved rectum and bladder sparing compared to IMRTremove. For 9-beam coplanar plans, rectum V60Gy reduced by 17.5% ± 15.0% (maximum 37.4%, p = 0.036) and rectum Dmean by 9.4% ± 7.8% (maximum 19.8%, p = 0.036). Further improvements in OAR sparing were achievable by using noncoplanar beam setups, reducing rectum V60Gy by another 4.6% ± 4.9% (p = 0.012) for noncoplanar 9-beam IMRTcut plans. Large reductions in rectum dose delivery were also observed when increasing the number of beam directions in the plans. For bilateral implants, the rectum V60Gy was 37.3% ± 12.1% for coplanar 7-beam plans and reduced on average by 13.5% (maximum 30.1%, p = 0.012) for 15 directions.
Conclusions: iCycle was able to automatically generate high quality plans for prostate cancer patients with prostheses. Excluding only beamlets that passed through the prostheses (IMRTcut strategy) significantly improved OAR sparing. Noncoplanar beam arrangements and, to a larger extent, increasing the number of treatment beams further improved plan quality.
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87.53.Jw Therapeutic applications, including brachytherapy
87.55.D- Treatment planning
87.53.Bn Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: Magnetic tracking for TomoTherapy systems: Gradiometer based methods to filter eddy-current magnetic fields

John E. McGary, Zubiao Xiong, and Ji Chen

Med. Phys. 40, 071705 (2013); http://dx.doi.org/10.1118/1.4808148 (14 pages)

Online Publication Date: 5 June 2013

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Purpose: TomoTherapy systems lack real-time, tumor tracking. A possible solution is to use electromagnetic markers; however, eddy-current magnetic fields generated in response to a magnetic source can be comparable to the signal, thus degrading the localization accuracy. Therefore, the tracking system must be designed to account for the eddy fields created along the inner bore conducting surfaces. The aim of this work is to investigate localization accuracy using magnetic field gradients to determine feasibility toward TomoTherapy applications.
Methods: Electromagnetic models are used to simulate magnetic fields created by a source and its simultaneous generation of eddy currents within a conducting cylinder. The source position is calculated using a least-squares fit of simulated sensor data using the dipole equation as the model equation. To account for field gradients across the sensor area (∼25 cm2), an iterative method is used to estimate the magnetic field at the sensor center. Spatial gradients are calculated with two arrays of uniaxial, paired sensors that form a gradiometer array, where the sensors are considered ideal.
Results: Experimental measurements of magnetic fields within the TomoTherapy bore are shown to be 1%–10% less than calculated with the electromagnetic model. Localization results using a 5 × 5 array of gradiometers are, in general, 2–4 times more accurate than a planar array of sensors, depending on the solenoid orientation and position. Simulation results show that the localization accuracy using a gradiometer array is within 1.3 mm over a distance of 20 cm from the array plane. In comparison, localization errors using single array are within 5 mm.
Conclusions: The results indicate that the gradiometer method merits further studies and work due to the accuracy achieved with ideal sensors. Future studies should include realistic sensor models and extensive numerical studies to estimate the expected magnetic tracking accuracy within a TomoTherapy system before proceeding with prototype development.
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87.53.Jw Therapeutic applications, including brachytherapy
02.60.-x Numerical approximation and analysis

RADIATION THERAPY PHYSICS: Results from a multicenter prostate IMRT dosimetry intercomparison for an OCOG-TROG clinical trial

B. Healy, J. Frantzis, R. Murry, J. Martin, A. Plank, M. Middleton, C. Catton, and T. Kron

Med. Phys. 40, 071706 (2013); http://dx.doi.org/10.1118/1.4808151 (8 pages)

Online Publication Date: 7 June 2013

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Purpose: A multi-institution dosimetry intercomparison has been undertaken of prostate intensity modulated radiation therapy (IMRT) delivery. The dosimetry intercomparison was incorporated into the quality assurance for site credentialing for the Trans-Tasman Radiation Oncology Group Prostate Fractionated Irradiation Trial 08.01 clinical trial.
Methods: An anthropomorphic pelvic phantom with realistic anatomy was used along with multiplanar dosimetry tools for the assessment. Nineteen centers across Australia and New Zealand participated in the study.
Results: In comparing planned versus measured dose to the target at the isocenter within the phantom, all centers were able to achieve a total delivered dose within 3% of planned dose. In multiplanar analysis with radiochromic film using the gamma analysis method to compare delivered and planned dose, pass rates for a 5%/3 mm criterion were better than 90% for a coronal slice through the isocenter. Pass rates for an off-axis coronal slice were also better than 90% except for one instance with 84% pass rate.
Conclusions: Strengths of the dosimetry assessment procedure included the true anthropomorphic nature of the phantom used, the involvement of an expert from the reference center in carrying out the assessment at every site, and the ability of the assessment to detect and resolve dosimetry discrepancies.
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87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy

RADIATION THERAPY PHYSICS: Critical assessment of intramodality 3D ultrasound imaging for prostate IGRT compared to fiducial markers

Skadi van der Meer, Esther Bloemen-van Gurp, Jolanda Hermans, Robert Voncken, Denys Heuvelmans, Carol Gubbels, Davide Fontanarosa, Peter Visser, Ludy Lutgens, Francis van Gils, and Frank Verhaegen

Med. Phys. 40, 071707 (2013); http://dx.doi.org/10.1118/1.4808359 (11 pages)

Online Publication Date: 7 June 2013

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Purpose: A quantitative 3D intramodality ultrasound (US) imaging system was verified for daily in-room prostate localization, and compared to prostate localization based on implanted fiducial markers (FMs).
Methods: Thirteen prostate patients underwent multiple US scans during treatment. A total of 376 US-scans and 817 matches were used to determine the intra- and interoperator variability. Additionally, eight other patients underwent daily prostate localization using both US and electronic portal imaging (EPI) with FMs resulting in 244 combined US-EPI scans. Scanning was performed with minimal probe pressure and a correction for the speed of sound aberration was performed. Uncertainties of both US and FM methods were assessed. User variability of the US method was assessed.
Results: The overall US user variability is 2.6 mm. The mean differences between US and FM are: 2.5 ± 4.0 mm (LR), 0.6 ± 4.9 mm (SI), and −2.3 ± 3.6 mm (AP). The intramodality character of this US system mitigates potential errors due to transducer pressure and speed of sound aberrations.
Conclusions: The overall accuracy of US (3.0 mm) is comparable to our FM workflow (2.2 mm). Since neither US nor FM can be considered a gold standard no conclusions can be drawn on the superiority of either method. Because US imaging captures the prostate itself instead of surrogates no invasive procedure is required. It requires more effort to standardize US imaging than FM detection. Since US imaging does not involve a radiation burden, US prostate imaging offers an alternative for FM EPI positioning.
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87.63.dh Ultrasonographic imaging
87.53.Jw Therapeutic applications, including brachytherapy

RADIATION THERAPY PHYSICS: Use of plan quality degradation to evaluate tradeoffs in delivery efficiency and clinical plan metrics arising from IMRT optimizer and sequencer compromises

Joel R. Wilkie, Martha M. Matuszak, Mary Feng, Jean M. Moran, and Benedick A. Fraass

Med. Phys. 40, 071708 (2013); http://dx.doi.org/10.1118/1.4808118 (10 pages)

Online Publication Date: 10 June 2013

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Purpose: Plan degradation resulting from compromises made to enhance delivery efficiency is an important consideration for intensity modulated radiation therapy (IMRT) treatment plans. IMRT optimization and/or multileaf collimator (MLC) sequencing schemes can be modified to generate more efficient treatment delivery, but the effect those modifications have on plan quality is often difficult to quantify. In this work, the authors present a method for quantitative assessment of overall plan quality degradation due to tradeoffs between delivery efficiency and treatment plan quality, illustrated using comparisons between plans developed allowing different numbers of intensity levels in IMRT optimization and/or MLC sequencing for static segmental MLC IMRT plans.
Methods: A plan quality degradation method to evaluate delivery efficiency and plan quality tradeoffs was developed and used to assess planning for 14 prostate and 12 head and neck patients treated with static IMRT. Plan quality was evaluated using a physician's predetermined “quality degradation” factors for relevant clinical plan metrics associated with the plan optimization strategy. Delivery efficiency and plan quality were assessed for a range of optimization and sequencing limitations. The “optimal” (baseline) plan for each case was derived using a clinical cost function with an unlimited number of intensity levels. These plans were sequenced with a clinical MLC leaf sequencer which uses >100 segments, assuring delivered intensities to be within 1% of the optimized intensity pattern. Each patient's optimal plan was also sequenced limiting the number of intensity levels (20, 10, and 5), and then separately optimized with these same numbers of intensity levels. Delivery time was measured for all plans, and direct evaluation of the tradeoffs between delivery time and plan degradation was performed.
Results: When considering tradeoffs, the optimal number of intensity levels depends on the treatment site and on the stage in the process at which the levels are limited. The cost of improved delivery efficiency, in terms of plan quality degradation, increased as the number of intensity levels in the sequencer or optimizer decreased. The degradation was more substantial for the head and neck cases relative to the prostate cases, particularly when fewer than 20 intensity levels were used. Plan quality degradation was less severe when the number of intensity levels was limited in the optimizer rather than the sequencer.
Conclusions: Analysis of plan quality degradation allows for a quantitative assessment of the compromises in clinical plan quality as delivery efficiency is improved, in order to determine the optimal delivery settings. The technique is based on physician-determined quality degradation factors and can be extended to other clinical situations where investigation of various tradeoffs is warranted.
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87.55.D- Treatment planning
87.55.de Optimization

RADIATION THERAPY PHYSICS: Maintaining tumor targeting accuracy in real-time motion compensation systems for respiration-induced tumor motion

Kathleen Malinowski, Thomas J. McAvoy, Rohini George, Sonja Dieterich, and Warren D. D’Souza

Med. Phys. 40, 071709 (2013); http://dx.doi.org/10.1118/1.4808119 (5 pages)

Online Publication Date: 11 June 2013

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Purpose: To determine how best to time respiratory surrogate-based tumor motion model updates by comparing a novel technique based on external measurements alone to three direct measurement methods.
Methods: Concurrently measured tumor and respiratory surrogate positions from 166 treatment fractions for lung or pancreas lesions were analyzed. Partial-least-squares regression models of tumor position from marker motion were created from the first six measurements in each dataset. Successive tumor localizations were obtained at a rate of once per minute on average. Model updates were timed according to four methods: never, respiratory surrogate-based (when metrics based on respiratory surrogate measurements exceeded confidence limits), error-based (when localization error ≥3 mm), and always (approximately once per minute).
Results: Radial tumor displacement prediction errors (mean ± standard deviation) for the four schema described above were 2.4 ± 1.2, 1.9 ± 0.9, 1.9 ± 0.8, and 1.7 ± 0.8 mm, respectively. The never-update error was significantly larger than errors of the other methods. Mean update counts over 20 min were 0, 4, 9, and 24, respectively.
Conclusions: The same improvement in tumor localization accuracy could be achieved through any of the three update methods, but significantly fewer updates were required when the respiratory surrogate method was utilized. This study establishes the feasibility of timing image acquisitions for updating respiratory surrogate models without direct tumor localization.
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87.19.Wx Pneumodyamics, respiration
87.57.N- Image analysis
87.59.B- Radiography
02.50.-r Probability theory, stochastic processes, and statistics
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