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Jun 2013

Volume 40, Issue 6 (partial)

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POINT/COUNTERPOINT: The future of IMRT/SBRT lies in the use of unflattened x-ray beams

Chihray Liu, Ph.D., Michael G. Snyder, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 40, 060601 (2013); http://dx.doi.org/10.1118/1.4793410 (3 pages)

Online Publication Date: 3 May 2013

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Abstract Unavailable
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99.10.Cd Errata
87.55.D- Treatment planning
87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
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MEDICAL PHYSICS LETTER: Progressive cone beam CT dose control in image-guided radiation therapy

Hao Yan, Xin Zhen, Laura Cerviño, Steve B. Jiang, and Xun Jia

Med. Phys. 40, 060701 (2013); http://dx.doi.org/10.1118/1.4804215 (7 pages)

Online Publication Date: 13 May 2013

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Purpose: Cone beam CT (CBCT) in image-guided radiotherapy (IGRT) offers a tremendous advantage for treatment guidance. The associated imaging dose is a clinical concern. One unique feature of CBCT-based IGRT is that the same patient is repeatedly scanned during a treatment course, and the contents of CBCT images at different fractions are similar. The authors propose a progressive dose control (PDC) scheme to utilize this temporal correlation for imaging dose reduction.
Methods: A dynamic CBCT scan protocol, as opposed to the static one in the current clinical practice, is proposed to gradually reduce the imaging dose in each treatment fraction. The CBCT image from each fraction is processed by a prior-image based nonlocal means (PINLM) module to enhance its quality. The increasing amount of prior information from previous CBCT images prevents degradation of image quality due to the reduced imaging dose. Two proof-of-principle experiments have been conducted using measured phantom data and Monte Carlo simulated patient data with deformation.
Results: In the measured phantom case, utilizing a prior image acquired at 0.4 mAs, PINLM is able to improve the image quality of a CBCT acquired at 0.2 mAs by reducing the noise level from 34.95 to 12.45 HU. In the synthetic patient case, acceptable image quality is maintained at four consecutive fractions with gradually decreasing exposure levels of 0.4, 0.1, 0.07, and 0.05 mAs. When compared with the standard low-dose protocol of 0.4 mAs for each fraction, an overall imaging dose reduction of more than 60% is achieved.
Conclusions: PINLM-PDC is able to reduce CBCT imaging dose in IGRT utilizing the temporal correlations among the sequence of CBCT images while maintaining the quality.
Show PACS
87.55.dk Dose-volume analysis
87.53.Bn Dosimetry/exposure assessment
87.55.K- Monte Carlo methods
87.63.lm Image enhancement
87.57.Q- Computed tomography
87.55.D- Treatment planning
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RADIATION THERAPY PHYSICS: Effect of MLC leaf width on treatment adaptation and accuracy for concurrent irradiation of prostate and pelvic lymph nodes

Qingyang Shang, Peng Qi, Samah Ferjani, and Ping Xia

Med. Phys. 40, 061701 (2013); http://dx.doi.org/10.1118/1.4803499 (9 pages)

Online Publication Date: 6 May 2013

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Purpose: The aim of the study was to evaluate the impact of multileaf collimator (MLC) leaf width on treatment adaptation and delivery accuracy for concurrent treatment of the prostate and pelvic lymph nodes with intensity modulated radiation therapy (IMRT).
Methods: Seventy-five kilovoltage cone beam CTs (KV-CBCT) from six patients were included for this retrospective study. For each patient, three different IMRT plans were created based on a planning CT using three different MLC leaf widths of 2.5, 5, and 10 mm, respectively. For each CBCT, the prostate displacement was determined by a dual image registration. Adaptive plans were created by shifting selected MLC leaf pairs to compensate for daily prostate movements. To evaluate the impact of MLC leaf width on the adaptive plan for each daily CBCT, three MLC shifted plans were created using three different leaf widths of MLCs (a total of 225 adaptive treatment plans). Selective dosimetric endpoints for the tumor volumes and organs at risk (OARs) were evaluated for these adaptive plans. Using the planning CT from a selected patient, MLC shifted plans for three hypothetical longitudinal shifts of 2, 4, and 8 mm were delivered on the three linear accelerators to test the deliverability of the shifted plans and to compare the dose accuracy of the shifted plans with the original IMRT plans.
Results: Adaptive plans from 2.5 and 5 mm MLCs had inadequate dose coverage to the prostate (D99 < 97%, or Dmean < 99% of the planned dose) in 6%–8% of the fractions, while adaptive plans from 10 mm MLC led to inadequate dose coverage to the prostate in 25.3% of the fractions. The average V56Gy of the prostate over the six patients was improved by 6.4% (1.6%–32.7%) and 5.8% (1.5%–35.7%) with adaptive plans from 2.5 and 5 mm MLCs, respectively, when compared with adaptive plans from 10 mm MLC. Pelvic lymph nodes were well covered for all MLC adaptive plans, as small differences were observed for D99, Dmean, and V50.4Gy. Similar OAR sparing could be achieved for the bladder and rectum with all three MLCs for treatment adaptation. The MLC shifted plans can be accurately delivered on all three linear accelerators with accuracy similar to their original IMRT plans, where gamma (3%/3 mm) passing rates were 99.6%, 93.0%, and 92.1% for 2.5, 5, and 10 mm MLCs, respectively. The percentages of pixels with dose differences between the measurement and calculation being less than 3% of the maximum dose were 85.9%, 82.5%, and 70.5% for the original IMRT plans from the three MLCs, respectively.
Conclusions: Dosimetric advantages associated with smaller MLC leaves were observed in terms of the coverage to the prostate, when the treatment was adapted to account for daily prostate movement for concurrent irradiation of the prostate and pelvic lymph nodes. The benefit of switching the MLC from 10 to 5 mm was significant (p ≪ 0.01); however, switching the MLC from 5 to 2.5 mm would not gain significant (p = 0.15) improvement. IMRT plans with smaller MLC leaf widths achieved more accurate dose delivery.
Show PACS
87.55.dk Dose-volume analysis
87.57.Q- Computed tomography
87.53.Bn Dosimetry/exposure assessment
87.53.Jw Therapeutic applications, including brachytherapy
29.20.Ej Linear accelerators
87.19.rs Movement

RADIATION THERAPY PHYSICS: Mapping motion from 4D-MRI to 3D-CT for use in 4D dose calculations: A technical feasibility study

Dirk Boye, Tony Lomax, and Antje Knopf

Med. Phys. 40, 061702 (2013); http://dx.doi.org/10.1118/1.4801914 (11 pages)

Online Publication Date: 7 May 2013

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Purpose: Target sites affected by organ motion require a time resolved (4D) dose calculation. Typical 4D dose calculations use 4D-CT as a basis. Unfortunately, 4D-CT images have the disadvantage of being a “snap-shot” of the motion during acquisition and of assuming regularity of breathing. In addition, 4D-CT acquisitions involve a substantial additional dose burden to the patient making many, repeated 4D-CT acquisitions undesirable. Here the authors test the feasibility of an alternative approach to generate patient specific 4D-CT data sets.
Methods: In this approach motion information is extracted from 4D-MRI. Simulated 4D-CT data sets [which the authors call 4D-CT(MRI)] are created by warping extracted deformation fields to a static 3D-CT data set. The employment of 4D-MRI sequences for this has the advantage that no assumptions on breathing regularity are made, irregularities in breathing can be studied and, if necessary, many repeat imaging studies (and consequently simulated 4D-CT data sets) can be performed on patients and/or volunteers. The accuracy of 4D-CT(MRI)s has been validated by 4D proton dose calculations. Our 4D dose algorithm takes into account displacements as well as deformations on the originating 4D-CT/4D-CT(MRI) by calculating the dose of each pencil beam based on an individual time stamp of when that pencil beam is applied. According to corresponding displacement and density-variation-maps the position and the water equivalent range of the dose grid points is adjusted at each time instance.
Results: 4D dose distributions, using 4D-CT(MRI) data sets as input were compared to results based on a reference conventional 4D-CT data set capturing similar motion characteristics. Almost identical 4D dose distributions could be achieved, even though scanned proton beams are very sensitive to small differences in the patient geometry. In addition, 4D dose calculations have been performed on the same patient, but using 4D-CT(MRI) data sets based on variable breathing patterns to show the effect of possible irregular breathing on active scanned proton therapy. Using a 4D-CT(MRI), including motion irregularities, resulted in significantly different proton dose distributions.
Conclusions: The authors have demonstrated that motion information from 4D-MRI can be used to generate realistic 4D-CT data sets on the basis of a single static 3D-CT data set. 4D-CT(MRI) presents a novel approach to test the robustness of treatment plans in the circumstance of patient motion.
Show PACS
87.55.dk Dose-volume analysis
87.57.Q- Computed tomography
87.85.gj Movement and locomotion
87.59.-e X-ray imaging
87.61.-c Magnetic resonance imaging
87.50.cm Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: Feasibility of an image planning system for kilovoltage image-guided radiation therapy

Bishnu B. Thapa and Janelle A. Molloy

Med. Phys. 40, 061703 (2013); http://dx.doi.org/10.1118/1.4803508 (10 pages)

Online Publication Date: 8 May 2013

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Purpose: Image guidance has become a standard of care for many treatment scenarios in radiation therapy. This is most typically accomplished by use of kV x-ray devices mounted onto the linear accelerator (Linac) gantry that yield planar, fluoroscopic, and cone-beam computed tomography (CBCT) images. Image acquisition parameters are chosen via preset techniques that rely on broad categorizations in patient anatomy and imaging goal. However, the optimal imaging technique results in detectability of the features of interest while exposing the patient to minimum dose. Herein, the authors present an investigation into the feasibility of developing an image planning system (IPS) for radiotherapy.
Methods: In this first phase, the authors focused on developing an algorithm to predict tissue contrast produced by a common radiotherapy planar imaging chain. Input parameters include a CT dataset and simulated planar imaging technique settings that include kV and mAs. Energy-specific attenuation through each voxel of the CT dataset was calculated in the algorithm to derive a net transmitted intensity. The response of the flat panel detector was integrated into the image simulation algorithm. Verification was conducted by comparing simulated and measured images using four phantoms. Comparisons were made in both high and low contrast settings, as well as changes in the geometric appearance due to image saturation.
Results: The authors studied a lung nodule test object to assess the planning system's ability to predict object contrast and detectability. Verification demonstrated that the slope of the pixel intensities is similar, the presence of the nodule is evident, and image saturation at high mAs values is evident in both images. The appearance of the lung nodule is a function of the image detector saturation. The authors assessed the dimensions of the lung nodule in measured and simulated images. Good quantitative agreement affirmed the algorithm's predictive capabilities. The invariance of contrast with kVp and mAs prior to saturation was predicted, as well as the gradual loss of object detectability as saturation was approached. Small changes in soft tissue density were studied using a mammography step wedge phantom. Data were acquired at beam qualities of 80 and 120 kVp and over exposure values ranging from 0.04 to 500 mAs. The data showed good agreement in terms of the absolute value of pixel intensities predicted, as well as small variations across the step wedge pattern. The saturation pixel intensity was consistent between the two beam qualities studied. Boney tissue contrast was assessed using two abdominal phantoms. Measured and calculated values agree in terms of predicting the mAs value at which detector saturation, and subsequent loss of contrast occurs. The lack of variation in contrast over mAs values lower than 10 suggests that there is wide latitude for minimizing patient dose.
Conclusions: The authors developed and tested an algorithm that can be used to assist in kV imaging technique selection during localization for radiotherapy. Phantom testing demonstrated the algorithm's predictive accuracy for both low and high contrast imaging scenarios. Detector saturation with subsequent loss of imaging detail, both in terms of object size and contrast were accurately predicted by the algorithm.
Show PACS
87.57.cj Contrast
87.57.Q- Computed tomography
87.59.bd Computed radiography
29.20.Ej Linear accelerators

RADIATION THERAPY PHYSICS: A study of IMRT planning parameters on planning efficiency, delivery efficiency, and plan quality

Kathryn Mittauer, Bo Lu, Guanghua Yan, Darren Kahler, Arun Gopal, Robert Amdur, and Chihray Liu

Med. Phys. 40, 061704 (2013); http://dx.doi.org/10.1118/1.4803460 (13 pages)

Online Publication Date: 13 May 2013

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Purpose: To improve planning and delivery efficiency of head and neck IMRT without compromising planning quality through the evaluation of inverse planning parameters.
Methods: Eleven head and neck patients with pre-existing IMRT treatment plans were selected for this retrospective study. The Pinnacle treatment planning system (TPS) was used to compute new treatment plans for each patient by varying the individual or the combined parameters of dose/fluence grid resolution, minimum MU per segment, and minimum segment area. Forty-five plans per patient were generated with the following variations: 4 dose/fluence grid resolution plans, 12 minimum segment area plans, 9 minimum MU plans, and 20 combined minimum segment area/minimum MU plans. Each plan was evaluated and compared to others based on dose volume histograms (DVHs) (i.e., plan quality), planning time, and delivery time. To evaluate delivery efficiency, a model was developed that estimated the delivery time of a treatment plan, and validated through measurements on an Elekta Synergy linear accelerator.
Results: The uncertainty (i.e., variation) of the dose-volume index due to dose calculation grid variation was as high as 8.2% (5.5 Gy in absolute dose) for planning target volumes (PTVs) and 13.3% (2.1 Gy in absolute dose) for planning at risk volumes (PRVs). Comparison results of dose distributions indicated that smaller volumes were more susceptible to uncertainties. The grid resolution of a 4 mm dose grid with a 2 mm fluence grid was recommended, since it can reduce the final dose calculation time by 63% compared to the accepted standard (2 mm dose grid with a 2 mm fluence grid resolution) while maintaining a similar level of dose-volume index variation. Threshold values that maintained adequate plan quality (DVH results of the PTVs and PRVs remained satisfied for their dose objectives) were 5 cm2 for minimum segment area and 5 MU for minimum MU. As the minimum MU parameter was increased, the number of segments and delivery time were decreased. Increasing the minimum segment area parameter decreased the plan MU, but had less of an effect on the number of segments and delivery time. Our delivery time model predicted delivery time to within 1.8%.
Conclusions: Increasing the dose grid while maintaining a small fluence grid allows for improved planning efficiency without compromising plan quality. Delivery efficiency can be improved by increasing the minimum MU, but not the minimum segment area. However, increasing the respective minimum MU and/or the minimum segment area to any value greater than 5 MU and 5 cm2 is not recommended because it degrades plan quality.
Show PACS
87.55.D- Treatment planning
87.55.dk Dose-volume analysis
87.55.Qr Quality assurance in radiotherapy
87.56.bd Accelerators

RADIATION THERAPY PHYSICS: Lung sparing and dose escalation in a robust-inspired IMRT planning method for lung radiotherapy that accounts for intrafraction motion

Claire McCann, Thomas Purdie, Andrew Hope, Andrea Bezjak, and Jean-Pierre Bissonnette

Med. Phys. 40, 061705 (2013); http://dx.doi.org/10.1118/1.4805101 (10 pages)

Online Publication Date: 16 May 2013

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Purpose: To test the efficacy of a simple, robust-inspired intensity modulated radiotherapy (IMRT) planning strategy for lung radiotherapy designed to reduce lung dose and escalate tumor dose using realistic dose accumulation tools.
Methods: A deformable image registration tool was used to plan and accumulate dose over all phases of the breathing cycle for conventional and robust-inspired IMRT strategies of eight nonsmall cell lung cancer patients exhibiting peak-to-peak respiratory motion with amplitudes ranging from 1 to 2 cm in the craniocaudal direction. The authors’ robust-inspired plans were designed to have smaller beam apertures based on target location during exhale, combined with edge-enhanced intensity maps to ensure target coverage during inspiration. For these, a new planning target volume defined as the rPTV was generated from a 5-mm isotropic expansion of the clinical target volume (CTV) on end-exhale combined with a boost volume, set to 110% of the prescription dose. Plans were evaluated in terms of (i) lung sparing and (ii) dose escalation for mean lung dose (MLD) isotoxicity. CTV and planning target volumes (PTV) coverage and lung dose were compared to the conventional IMRT approach.
Results: Robust-inspired plans showed potential lung dose reductions in seven out of eight patients. For non-GTV lung, percent reductions of 3%–14% in MLD and 6%–15% in V20 were observed. For seven of eight cases, the robust-like approach yielded increased accumulated doses to CTV. Isotoxicity studies for MLD showed increased dose to the CTV and the rPTV, in the range of 104%–118% and 95%–114% of prescription dose, respectively.
Conclusions: A 4D dose calculation based on deformable image registration was used to evaluate a robust-inspired planning strategy for lung radiotherapy. This method offers notable reductions to lung dose while improving tumor coverage through the use of reduced geometric margins combined with edge enhancements.
Show PACS
87.55.dk Dose-volume analysis
87.57.nj Registration
87.19.Wx Pneumodyamics, respiration
47.63.Ec Pulmonary fluid mechanics
87.19.-j Properties of higher organisms
87.17.-d Cell processes
87.19.xj Cancer

RADIATION THERAPY PHYSICS: A generalized 2D pencil beam scaling algorithm for proton dose calculation in heterogeneous slab geometries

David C. Westerly, Xiaohu Mo, Wolfgang A. Tomé, Thomas R. Mackie, and Paul M. DeLuca, Jr.

Med. Phys. 40, 061706 (2013); http://dx.doi.org/10.1118/1.4804055 (13 pages)

Online Publication Date: 20 May 2013

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Purpose: Pencil beam algorithms are commonly used for proton therapy dose calculations. Szymanowski and Oelfke [“Two-dimensional pencil beam scaling: An improved proton dose algorithm for heterogeneous media,” Phys. Med. Biol. 47, 3313–3330 (2002)10.1088/0031-9155/47/18/304] developed a two-dimensional (2D) scaling algorithm which accurately models the radial pencil beam width as a function of depth in heterogeneous slab geometries using a scaled expression for the radial kernel width in water as a function of depth and kinetic energy. However, an assumption made in the derivation of the technique limits its range of validity to cases where the input expression for the radial kernel width in water is derived from a local scattering power model. The goal of this work is to derive a generalized form of 2D pencil beam scaling that is independent of the scattering power model and appropriate for use with any expression for the radial kernel width in water as a function of depth.
Methods: Using Fermi-Eyges transport theory, the authors derive an expression for the radial pencil beam width in heterogeneous slab geometries which is independent of the proton scattering power and related quantities. The authors then perform test calculations in homogeneous and heterogeneous slab phantoms using both the original 2D scaling model and the new model with expressions for the radial kernel width in water computed from both local and nonlocal scattering power models, as well as a nonlocal parameterization of Molière scattering theory. In addition to kernel width calculations, dose calculations are also performed for a narrow Gaussian proton beam.
Results: Pencil beam width calculations indicate that both 2D scaling formalisms perform well when the radial kernel width in water is derived from a local scattering power model. Computing the radial kernel width from a nonlocal scattering model results in the local 2D scaling formula under-predicting the pencil beam width by as much as 1.4 mm (21%) at the depth of the Bragg peak for a 220 MeV proton beam in homogeneous water. This translates into a 32% dose discrepancy for a 5 mm Gaussian proton beam. Similar trends were observed for calculations made in heterogeneous slab phantoms where it was also noted that errors tend to increase with greater beam penetration. The generalized 2D scaling model performs well in all situations, with a maximum dose error of 0.3% at the Bragg peak in a heterogeneous phantom containing 3 cm of hard bone.
Conclusions: The authors have derived a generalized form of 2D pencil beam scaling which is independent of the proton scattering power model and robust to the functional form of the radial kernel width in water used for the calculations. Sample calculations made with this model show excellent agreement with expected values in both homogeneous water and heterogeneous phantoms.
Show PACS
87.53.Bn Dosimetry/exposure assessment
87.55.dk Dose-volume analysis
87.53.Jw Therapeutic applications, including brachytherapy

RADIATION THERAPY PHYSICS: Comparison of methods for the measurement of radiation dose distributions in high dose rate (HDR) brachytherapy: Ge-doped optical fiber, EBT3 Gafchromic film, and PRESAGE® radiochromic plastic

A. L. Palmer, P. Di Pietro, S. Alobaidli, F. Issa, S. Doran, D. Bradley, and A. Nisbet

Med. Phys. 40, 061707 (2013); http://dx.doi.org/10.1118/1.4805100 (11 pages)

Online Publication Date: 20 May 2013

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Purpose: Dose distribution measurement in clinical high dose rate (HDR) brachytherapy is challenging, because of the high dose gradients, large dose variations, and small scale, but it is essential to verify accurate treatment planning and treatment equipment performance. The authors compare and evaluate three dosimetry systems for potential use in brachytherapy dose distribution measurement: Ge-doped optical fibers, EBT3 Gafchromic film with multichannel analysis, and the radiochromic material PRESAGE® with optical-CT readout.
Methods: Ge-doped SiO2 fibers with 6 μm active core and 5.0 mm length were sensitivity-batched and their thermoluminescent properties used via conventional heating and annealing cycles. EBT3 Gafchromic film of 30 μm active thickness was calibrated in three color channels using a nominal 6 MV linear accelerator. A 48-bit transmission scanner and advanced multichannel analysis method were utilized to derive dose measurements. Samples of the solid radiochromic polymer PRESAGE®, 60 mm diameter and 100 mm height, were analyzed with a parallel beam optical CT scanner. Each dosimetry system was used to measure the dose as a function of radial distance from a Co-60 HDR source, with results compared to Monte Carlo TG-43 model data. Each system was then used to measure the dose distribution along one or more lines through typical clinical dose distributions for cervix brachytherapy, with results compared to treatment planning system (TPS) calculations. Purpose-designed test objects constructed of Solid Water and held within a full-scatter water tank were utilized.
Results: All three dosimetry systems reproduced the general shape of the isolated source radial dose function and the TPS dose distribution. However, the dynamic range of EBT3 exceeded those of doped optical fibers and PRESAGE®, and the latter two suffered from unacceptable noise and artifact. For the experimental conditions used in this study, the useful range from an isolated HDR source was 5–40 mm for fibers, 3–50 mm for EBT3, and 4–21 mm for PRESAGE®. Fibers demonstrated some over-response at very low dose levels, suffered from volume averaging effects in the dose distribution measurement, and exhibited up to 9% repeatability variation over three repeated measurements. EBT3 demonstrated excellent agreement with Monte Carlo and TPS dose distributions, with up to 3% repeatability over three measurements. PRESAGE® gave promising results, being the only true 3D dosimeter, but artifacts and noise were apparent.
Conclusions: The comparative response of three emerging dosimetry systems for clinical brachytherapy dose distribution measurement has been investigated. Ge-doped optical fibers have excellent spatial resolution for single-direction measurement but are currently too large for complex dose distribution assessment. The use of PRESAGE® with optical-CT readout gave promising results in the measurement of true 3D dose distributions but further development work is required to reduce noise and improve dynamic range for brachytherapy dose distribution measurements. EBT3 Gafchromic film with multichannel analysis demonstrated accurate and reproducible measurement of dose distributions in HDR brachytherapy. Calibrated dose measurements were possible with agreement within 1.5% of TPS dose calculations. The suitability of EBT3 as a dosimeter for 2D quality control or commissioning work has been demonstrated.
Show PACS
87.55.dk Dose-volume analysis
06.20.fb Standards and calibration
87.10.Rt Monte Carlo simulations
87.55.kh Applications
87.57.Q- Computed tomography
87.50.wj Dosimetry/exposure assessment

RADIATION THERAPY PHYSICS: Quantification of dose perturbations induced by external and internal accessories in ocular proton therapy and evaluation of their dosimetric impact

A. Carnicer, G. Angellier, J. Thariat, W. Sauerwein, J. P. Caujolle, and J. Hérault

Med. Phys. 40, 061708 (2013); http://dx.doi.org/10.1118/1.4807090 (12 pages)

Online Publication Date: 22 May 2013

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Purpose: Proton scattering on beam shaping devices and protons slowing down on media with different densities within the treatment volume may produce dose perturbations and range variations that are not predicted by treatment planning systems. The aim of this work was to assess the dosimetric impact of elements present in ocular proton therapy treatments that may disturb the prescribed treatment plan. Both distal beam shaping devices and intraocular elements were considered.
Methods: A wedge filter, tantalum fiducial marker, hemispherical compensator, two intraocular endotamponades (densities 0.97 and 1.92 g cm−3) and an intraocular eye lens (IOL) were considered in the study. For these elements, longitudinal dose distributions were measured and/or calculated in water in beam alignment for a clinical spread-out Bragg peak. Under the same conditions, the unperturbed dose distributions were similarly measured and/or calculated in the absence of the element. The dosimetric impact was assessed by comparison of unperturbed and perturbed dose distributions. Measurements and calculations were carried out with two methods. Measurements are based on EBT3 films with dedicated software, which makes use of a calibration curve and correction for the quenching effect. Calculations are based on the Monte Carlo (MC) code MCNPX and reproduce the experimental conditions. Both dose maps are obtained with a resolution of 300 dpi.
Results: The degree of disturbance of distal beam shaping devices is low for the wedge filter (2% overdose ripple all along the central axis) and moderate for the hemispherical compensator (two bands of variable overdose of up to 10% downstream the compensator lateral edges and −5% underdose on the plateau at off-axis distance of 5 cm). Tantalum clips produce important dose shadows (−20% behind the clip parallel to the beam and range reduction of 1.1 mm) and bands of overdose (15%). The presence of endotamponades modifies the dose distribution very significantly (−5% underdose on the plateau and 3 mm range prolongation for the tamponade with density 0.97 g cm−3 and −15% underdose on plateau and 8 mm range reduction for that with density 1.92 g cm−3). No dose perturbations were found for the IOL. The high performance of EBT3 film and MC tools used was confirmed and good agreement was found between them (percentage of pixels passing the gamma test >87%).
Conclusions: The degree of disturbance by external beam shaping devices remains low in ocular proton therapy and can be reduced by bringing accessories closer to the eye. Tantalum fiducial markers must be located in such a way that dose perturbation is not projected on the tumor. The treatment of patients with intraocular endotamponades must be carefully managed. It is fundamental that radiation oncologists and medical physicists are informed about the presence of such substances prior to the treatment.
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87.55.dk Dose-volume analysis
42.66.Ct Anatomy and optics of eye
87.55.K- Monte Carlo methods
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