Volume 38, Issue 3, March 2011
- medical physics letters
- vision 20/20
- radiation therapy physics
- radiation imaging physics
- radiation measurement physics
- magnetic resonance physics
- nuclear medicine physics
- ultrasound physics
- thermotherapy physics
- radiation protection physics
- books and publications
- task group report
Index of content:
PDT is better than alternative therapies such as brachytherapy, electron beams, or low-energy x rays for the treatment of skin cancers38(2011); http://dx.doi.org/10.1118/1.3512802View Description Hide Description
- MEDICAL PHYSICS LETTERS
38(2011); http://dx.doi.org/10.1118/1.3549762View Description Hide DescriptionPurpose:
The binding of nanoparticles toin vivo targets impacts their use for medical imaging, therapy, and the study of diseases and disease biomarkers. Though an array of techniques can detect binding in vitro, the search for a robust in vivo method continues. The spectral response of magnetic nanoparticles can be influenced by a variety of changes in their physical environment including viscosity and binding. Here, the authors show that nanoparticles in these different environmental states produce spectral responses, which are sufficiently unique to allow for simultaneous quantification of the proportion of nanoparticles within each state.Methods:
The authors measured the response to restricted Brownian motion using an array of magnetic nanoparticle designs. With a chosen optimal particle type, the authors prepared particle samples in three distinct environmental states. Various combinations of particles within these three states were measured concurrently and the authors attempted to solve for the quantity of particles within each physical state.Results:
The authors found the spectral response of the nanoparticles to be sufficiently unique to allow for accurate quantification of up to three bound states with errors on the order of 1.5%. Furthermore, the authors discuss numerous paths for translating these measurements toin vivo applications.Conclusions:
Multiple nanoparticle environmental states can be concurrently quantified using the spectral response of the particles. Such an ability, if translated to thein vivo realm, could provide valuable information about the fate of nanoparticlesin vivo or improve the efficacy of nanoparticle based treatments.
Full-field 3D photoacoustic imaging based on plane transducer array and spatial phase-controlled algorithm38(2011); http://dx.doi.org/10.1118/1.3555036View Description Hide DescriptionPurpose:
Photoacoustic imaging(PAI) used for noninvasive imaging of biological tissue has been reported in many literature. However, there are still some disadvantages in the novel technique, such as the poor efficiency of imaging. In the current PAI, multiple excitation of laser and multiple acquisition of signal are necessary for image reconstruction. In this case, laser pulses may injure biological tissue due to energy accumulation. To popularize PAI in clinical applications, it is necessary to develop a new imaging approach to increase the efficiency of PAI.Methods:
A spatial phase-controlled algorithm is presented for full-field three-dimensional (3D) image reconstruction. By using the algorithm, photoabsorption sources at different depths can be reconstructed using just one set of data acquired in single laser shot. Unfocused plane transducer array and parallel data-acquisition (PDA) equipment are used for real-time photoacoustic (PA) signal detection and acquisition.Results:
The spatial resolution of the 3D PAI system was analyzed. Two graphite rods at various positions in a simulation model and two bifurcate vessels in the ear of rabbit were imaged. In addition, the motion trace of one particle flowing at constant velocity was captured dynamically. Experimental results showed that spatial phase-controlled algorithm based on plane transducer array and PDA system was capable of static and dynamic 3D PAI.Conclusions:
Spatial phase-controlled algorithm is introduced for 3D image reconstruction. The PA signals are collected by plane transducer array and PDA system in single pulse excitation. The acquired volumetric data are sufficient for 3D image reconstruction. Therefore, tissue can avoid the long-term exposure to light source and it is safer than the current PAI forin vivoimaging. With an increase in the repetition rate of laser pulse and speed of image display, the imaging method will realize real-time 3D imaging, which will be significant in clinical detection and medical diagnosis.
- VISION 20/20
38(2011); http://dx.doi.org/10.1118/1.3554643View Description Hide DescriptionPurpose:
Flattening filters (FFs) have been considered as an integral part of the treatment head of a medical accelerator for more than 50 years. The reasons for the longstanding use are, however, historical ones. Advanced treatment techniques, such as stereotactic radiotherapy or intensity modulated radiotherapy have stimulated the interest in operating linear accelerators in a flattening filter free (FFF) mode. The current manuscript reviews treatment head physics of FFF beams, describes their characteristics and the resulting potential advantages in their medical use, and closes with an outlook.Methods:
A number of dosimetric benefits have been determined for FFF beams, which range from increased dose rate and dose per pulse to favorable output ratio in-air variation with field size, reduced energy variation across the beam, and reduced leakage and out-of-field dose, respectively. Finally, the softer photon spectrum of unflattened beams has implications on imaging strategies and radiation protection.Results:
The dosimetric characteristics of FFF beams have an effect on treatment delivery, patient comfort, dose calculation accuracy, beam matching, absorbed dose determination, treatment planning, machine specific quality assurance, imaging, and radiation protection. When considering conventional C-arm linacs in a FFF mode, more studies are needed to specify and quantify the clinical advantages, especially with respect to treatment plan quality and quality assurance.Conclusions:
New treatment units are already on the market that operate without a FF or can be operated in a dedicated clinical FFF mode. Due to the convincing arguments of removing the FF, it is expected that more vendors will offer dedicated treatment units for advanced photon beam therapy in the near future. Several aspects related to standardization, dosimetry,treatment planning, and optimization need to be addressed in more detail in order to facilitate the clinical implementation of unflattened beams.
- RADIATION THERAPY PHYSICS
38(2011); http://dx.doi.org/10.1118/1.3551996View Description Hide DescriptionPurpose:
Collapsed-cone convolution/superposition (CCCS) dose calculation is the workhorse for IMRTdose calculation. The authors present a novel algorithm for computing CCCS dose on the modern graphic processing unit (GPU).Methods:
The GPU algorithm includes a novel TERMA calculation that has no write-conflicts and has linear computation complexity. The CCCS algorithm uses either tabulated or exponential cumulative-cumulative kernels (CCKs) as reported in literature. The authors have demonstrated that the use of exponential kernels can reduce the computation complexity by order of a dimension and achieve excellent accuracy. Special attentions are paid to the unique architecture of GPU, especially the memory accessing pattern, which increases performance by more than tenfold.Results:
As a result, the tabulated kernel implementation in GPU is two to three times faster than other GPU implementations reported in literature. The implementation of CCCS showed significant speedup on GPU over single core CPU. On tabulated CCK, speedups as high as 70 are observed; on exponential CCK, speedups as high as 90 are observed.Conclusions:
Overall, the GPU algorithm using exponential CCK is 1000–3000 times faster over a highly optimized single-threaded CPU implementation using tabulated CCK, while the dose differences are within 0.5% and 0.5 mm. This ultrafast CCCS algorithm will allow many time-sensitive applications to use accurate dose calculation.
The use of a silicon strip detector dose magnifying glass in stereotactic radiotherapy QA and dosimetry38(2011); http://dx.doi.org/10.1118/1.3549759View Description Hide DescriptionPurpose:
Stereotactic radiosurgery/therapy (SRS/SRT) is the use of radiation ablation in place of conventional surgical excision to remove or create fibrous tissue in small target volumes. The target of the SRT/SRS treatment is often located in close proximity to critical organs, hence the requirement of high geometric precision including a tight margin on the planning target volume and a sharp dose fall off. One of the major problems with quality assurance (QA) of SRT/SRS is the availability of suitable detectors with the required spatial resolution. The authors present a novel detector that they refer to as the dose magnifying glass (DMG), which has a high spatial resolution (0.2 mm) and is capable of meeting the stringent requirements of QA and dosimetry in SRS/SRT therapy.Methods:
The DMG is an array of 128 phosphor implanted strips on a -type Si wafer. The sensitive area defined by a single strip is . The Si wafer is thick. It is mounted on a 0.12 mm thick Kapton substrate. The authors studied the dose per pulse (dpp) and angular response of the detector in a custom-made SRS phantom. The DMG was used to determine the centers of rotation and positioning errors for the linear accelerator’s gantry, couch, and collimator rotations. They also used the DMG to measure the profiles and the total scatter factor of the SRS cones. Comparisons were made with the EBT2 film and standard values. The DMG was also used for dosimetric verification of a typical SRStreatment with various noncoplanar fields and arc treatments when applied to the phantom.Results:
The dose per pulse dependency of the DMG was found to be for a dpp change of 7.5 times. The angular response of the detector was investigated in the azimuthal and polar directions. The maximum polar angular response was 13.8% at the gantry angle of 320°, which may be partly due to the phantom geometry. The maximum azimuthal angular response was 15.3% at gantry angles of 90° and 270°. The angular response at the gantry angle of 180° was 6.3%. A correction function was derived to correct for the angular dependence of the detector, which takes into account the contribution of the azimuthal and polar angular response at different treatment couch positions. The maximum positioning errors due to collimator, gantry, and couch rotation were , , and , respectively. The SRS cone agrees very well with the standard data with an average difference of . Comparison of the relative intensity profiles of the DMG and EBT2 measurements for a simulated SRStreatment shows a maximum difference of 2.5%.Conclusions:
The DMG was investigated for dose per pulse and angular dependency. Its application to SRS/SRT delivery verification was demonstrated. The DMG with its high spatial resolution and real time capability allows measurement of dose profiles for cone applicators down to 5 mm in diameter, both accurately and rapidly as required in typical SRS/SRT deliveries.
Experience of micromultileaf collimator linear accelerator based single fraction stereotactic radiosurgery: Tumor dose inhomogeneity, conformity, and dose fall offa)38(2011); http://dx.doi.org/10.1118/1.3549764View Description Hide DescriptionPurpose:
Sharp dose fall off outside a tumor is essential for high dose single fraction stereotactic radiosurgery(SRS) plans. This study explores the relationship among tumordose inhomogeneity, conformity, and dose fall off in normal tissues for micromultileaf collimator (mMLC) linear accelerator(LINAC) based cranial SRS plans.Methods:
Between January 2007 and July 2009, 65 patients with single cranial lesions were treated with LINAC-based SRS. Among them, tumors had maximum diameters: 31; between 20 and 30 mm: 21; and : 13. All patients were treated with 6 MV photons on a Trilogy®linear accelerator (Varian Medical Systems, Palo Alto, CA) with a tertiary m3® high-resolution mMLC (Brainlab, Feldkirchen, Germany), using either noncoplanar conformal fixed fields or dynamic conformal arcs. The authors also created retrospective study plans with identical beam arrangement as the treated plan but with different tumordose inhomogeneity by varying the beam margins around the planning target volume (PTV). All retrospective study plans were normalized so that the minimum PTV dose was the prescription dose (PD). Isocenter dose, mean PTV dose, RTOG conformity index (CI), RTOG homogeneity index (HI), dose gradient index (defined as the difference between equivalent sphere radius of 50% isodose volume and prescription isodose volume), and normal tissue volume (as a ratio to PTV volume) receiving 50% prescription dose were calculated.Results:
HI was inversely related to the beam margins around the PTV. CI had a “V” shaped relationship with HI, reaching a minimum when HI was approximately 1.3. Isocenter dose and mean PTV dose (as percentage of PD) increased linearly with HI. and initially declined with HI and then reached a plateau when HI was approximately 1.3. These trends also held when tumors were grouped according to their maximum diameters. The smallest tumor group (maximum diameters ) had the most HI dependence for dose fall off. For treated plans, CI averaged with HI ; the average was , , and , respectively, for tumors, between 20 and 30 mm, and .Conclusions:
Tumordose inhomogeneity can be used as an important and convenient parameter to evaluate mMLC LINAC-based SRS plans. Sharp dose fall off in the normal tissue is achieved with sufficiently high tumordose inhomogeneity. By adjusting beam margins, a homogeneity index of approximately 1.3 would provide best conformity for the authors’ SRS system.
38(2011); http://dx.doi.org/10.1118/1.3547722View Description Hide DescriptionPurpose:
Traditionally, the tongue-and-groove effect due to the multileaf collimator architecture in intensity-modulated radiation therapy(IMRT) has typically been deferred to the leaf sequencing stage. The authors propose a new direct aperture optimization method for IMRTtreatment planning that explicitly incorporates dose calculation inaccuracies due to the tongue-and-groove effect into the treatment plan optimization stage.Methods:
The authors avoid having to accurately estimate the dosimetric effects of the tongue-and-groove architecture by using lower and upper bounds on the dose distribution delivered to the patient. They then develop a model that yields a treatment plan that is robust with respect to the corresponding dose calculation inaccuracies.Results:
Tests on a set of ten clinical head-and-neck cancer cases demonstrate the effectiveness of the new method in developing robust treatment plans with tight dose distributions in targets and critical structures. This is contrasted with the very loose bounds on the dose distribution that are obtained by solving a traditional treatment plan optimizationmodel that ignores tongue-and-groove effects in the treatment planning stage.Conclusions:
A robust direct aperture optimization approach is proposed to account for the dosimetric inaccuracies caused by the tongue-and-groove effect. The experiments validate the ability of the proposed approach in designing robust treatment plans regardless of the exact consequences of the tongue-and-groove architecture.
38(2011); http://dx.doi.org/10.1118/1.3552925View Description Hide DescriptionPurpose:
A patient-specific quality assurance (QA) method was developed to verify gantry-specific individual multileaf collimator(MLC) apertures (control points) in volumetric modulated arc therapy (VMAT) plans using an electronic portal imaging device(EPID).Methods:
VMAT treatment plans were generated in an Eclipse treatment planning system (TPS). DICOM images from a Varian EPID (aS1000) acquired in continuous acquisition mode were used for pretreatment QA. Each cine image file contains the grayscale image of the MLC aperture related to its specific control point and the corresponding gantry angle information. The TPS MLC file of this RapidArc plan contains the leaf positions for all 177 control points (gantry angles). In-house software was developed that interpolates the measured images based on the gantry angle and overlays them with the MLC pattern for all control points. The 38% isointensity line was used to define the edge of the MLC leaves on the portal images. The software generates graphs and tables that provide analysis for the number of mismatched leaf positions for a chosen distance to agreement at each control point and the frequency in which each particular leaf mismatches for the entire arc.Results:
Seven patients plans were analyzed using this method. The leaves with the highest mismatched rate were found to be treatment plan dependent.Conclusions:
This in-house software can be used to automatically verify the MLC leaf positions for all control points of VMAT plans using cine images acquired by an EPID.
38(2011); http://dx.doi.org/10.1118/1.3549765View Description Hide DescriptionPurpose:
The fraction duration of roboticradiosurgerytreatments can be reduced by generating more time-efficient treatment plans with a reduced number of node positions, beams, and monitor units (MUs). Node positions are preprogramed locations where the robot can position the focal spot of the x-ray beam. As the time needed for the robot to travel between node positions takes up a large part of the treatment time, the aim of this study was to develop and evaluate a node reduction technique in order to reduce the treatment time per fraction for roboticradiosurgery.Methods:
Node reduction was integrated into the inverse planning algorithm, developed in-house for the roboticradiosurgery modality. It involved repeated inverse optimization, each iteration excluding low-contribution node positions from the planning and resampling new candidate beams from the remaining node positions. Node reduction was performed until the exclusion of a single node position caused a constraint violation, after which the shortest treatment plan was selected retrospectively. Treatment plans were generated with and without node reduction for two lung cases of different complexity, one oropharyngeal case and one prostate case. Plan quality was assessed using the number of node positions, beams and MUs, and the estimated treatment time per fraction. All treatment plans had to fulfill all clinical dose constraints. Extra constraints were added to maintain the low-dose conformality and restrict skindoses during node reduction.Results:
Node reduction resulted in 12 residual node positions, on average (reduction by 77%), at the cost of an increase in the number of beams and total MUs of 28% and 9%, respectively. Overall fraction durations (excluding patient setup) were shortened by 25% (range of 18%–40%), on average. Dose distributions changed only little and dose in low-dose regions was effectively restricted by the additional constraints.Conclusions:
The fraction duration of roboticradiosurgerytreatments can be reduced considerably by node reduction with minimal changes in dosimetrical plan quality. Additional constraints are required to guarantee low-dose conformality and to avoid unacceptable skindose.
Dosimetric verification of surface and superficial doses for head and neck IMRT with different PTV shrinkage margins38(2011); http://dx.doi.org/10.1118/1.3553406View Description Hide DescriptionPurpose:
Dosimetric uncertainty in the surface and superficial regions is still a major concern for radiation therapy and becomes more important when using the inverse planning algorithm for IMRT. The purpose of this study was to measure dose distributions and to evaluate the calculation accuracy in the superficial region for different planning target volume (PTV) shrinkage methods for head and neck IMRT plans.Methods:
A spherical polystyrene phantom 160 mm in diameter (ball phantom) was used to simulate the shape of the head. Strips of superflab bolus with thicknesses of 3.5 and 7.0 mm were spread on the surface of the ball phantom. Three sets of CTimages were acquired for the ball phantom without and with the bolus. The hypothetical clinical target volume (CTV) and critical structures (spinal cord and parotid glands) were outlined on each set of CTimages. The PTVs were initially created by expanding an isotropic 3 mm margin from the CTV and then margins of 0, 3, and 5 mm were shrunk from the phantom surface for dosimetric analysis. Seven-field IMRT plans with a prescribed dose of 180 cGy and same dose constraints were designed using an Eclipse treatment planning system. Superficial doses at depths of 0, 3.5, and 7.0 mm and at seven beam axis positions (gantry angles of 0°, 30°, 60°, 80°, 330°, 300°, and 280°) were measured for each PTV shrinkage margin using 0.1 mm ultrathin thermoluminescent dosimeters. For each plan, the measured doses were compared to the calculated doses.Results:
The PTV without shrinkage had the highest intensity and the steepest dose gradient in the superficial region. The mean measured doses for different positions at depths of 0, 3.5, and 7.0 mm were, , and , respectively. For a PTV with 3 mm shrinkage, the mean measured doses were , , and . For a PTV with 5 mm shrinkage, the mean measured doses were , , and . The comparisons indicated that more than 73.3% of the calculated points are with doses lower than the measured points and the difference of the dose becomes more significant in the shallower region. At 7.0 mm depth, the average difference between calculations and measurements was 2.5% (maximum 5.5%).Conclusions:
Application of the PTV shrinkage method should take into account the calculation inaccuracy, tumor coverage, and possible skin reaction. When the tumor does not invade the superficial region, an adequate shrinkage margin from the surface is helpful for reducing the skin reaction. As the tumor invades the superficial region, adding a bolus is a method better than only contouring PTV with skin inclusion.
Real-time verification of multileaf collimator-driven radiotherapy using a novel optical attenuation-based fluence monitor38(2011); http://dx.doi.org/10.1118/1.3549766View Description Hide DescriptionPurpose:
Multileaf collimator (MLC)-driven conformal radiotherapy modalities [e.g., such as intensity-modulated radiotherapy(IMRT), intensity-modulated arc therapy, and stereotactic body radiotherapy] are more subject to delivery errors and dose calculation inaccuracies than standard modalities. Fluence monitoring during treatment delivery could reduce such errors by allowing an independent interface to quantify and assess measured difference between the delivered and planned treatment administration. We developed an optical attenuation-baseddetector to monitor fluence for the on-line quality control of radiotherapy delivery. The purpose of the current study was to develop the theoretical background of the invention and to evaluate the detector’s performance both statistically and in clinical situations.Methods:
We aligned 60 27-cm scintillating fibers coupled to a photodetector via clear optical fibers in the direction of motion of each of the 60 leaf pairs of a 120 leaves Millenium MLC on a Varian Clinac iX. We developed a theoretical model to predict the intensity of light collected on each side of the scintillating fibers when placed under radiation fields of varying sizes, intensities, and positions. The model showed that both the central position of the radiation field on the fiber and the integral fluence passing through the fiber could be assessed independently in a single measurement. We evaluated the performance of the prototype by (1) measuring the intrinsic variation of the measured values of and , (2) measuring the impact on the measured values of and of random leaf positioning errors introduced into IMRT fields, and (3) comparing the predicted values of and calculated with the treatment planning software to the measured values of and in order to assess the predictive effectiveness of the developed theoretical model.Results:
We observed a very low intrinsic dispersion, dominated by Poisson statistics, for both (standard deviations of less than 1 mm) and (standard deviations of less than 0.20%). When confronted with random leaf positioning errors from IMRT segments, was highly sensitive to single leaf positioning errors as small as 1 mm at isocenter, while was sensitive to leaf pair translation errors of at least 2 mm at isocenter. Owing to the uncertainties in the doses calculated in regions of high perpendicular dose gradients, the measured values of and deviated from the predicted values of and by a mean of 1.3 mm and 2.6%, respectively.Conclusion:
Our study showed that an optical attenuation-baseddetector can be used to effectively monitor integral fluence during radiotherapy delivery. The performance of such a system would enable real-time quality control of the incident fluence in current MLC-driven treatments such as IMRT and future adaptive radiotherapy procedures where new treatment plans will have to be delivered without passing thru the current standard quality control chain.
The influence of the field setup on the dosimetry of abutted fields in single-isocenter half-beam techniquesa)38(2011); http://dx.doi.org/10.1118/1.3557882View Description Hide DescriptionPurpose:
To study the influence of the field setup on the dosimetry at the junction in single-isocenter half-beam techniques.Methods:
The dosimetry at the junction for a two-field setup with the gantry at zero was first evaluated with radiochromic films. A three-field setup, with an anterior field and two opposed lateral fields, was also analyzed for two different relative positions of the fields involved. In all cases, the dose increase at the central axis, called the junction dose, was measured.Results:
Junction doses varied greatly with the setup. For the three-field setup, the junction dose differed from that obtained with the two-field setup, and it greatly depended on the relative position of the fields. When the anterior field was closer to the gantry than the lateral fields, a field gap occurred and the junction dose was negative. When the anterior field was farther from the gantry than the lateral fields, a field overlap was obtained and the junction dose was positive. The difference in the junction dose between the three-field setups was around 18% for the three accelerators evaluated.Conclusions:
Having a uniform dose distribution for two fields at gantry 0° does not guarantee a uniform distribution at other gantry angles. Junction doses are largely affected by the relative position of the radiation fields, which may have an impact in clinical practice. Therefore, any method aiming to assess or to optimize the dose homogeneity at the junction should take this effect into account.
Treatment planning of a skin-sparing conical breast brachytherapy applicator using conventional brachytherapy software38(2011); http://dx.doi.org/10.1118/1.3552921View Description Hide DescriptionPurpose:
AccuBoost is a noninvasive image-guided technique for the delivery of partial breast irradiation to the tumor bed and currently serves as an alternate to conventional electron beam boost. To irradiate the target volume while providing dose sparing to the skin, the round applicator design was augmented through the addition of an internally truncated conical shield and the reduction of the source to skin distance.Methods:
Brachytherapydose distributions for two types of conical applicators were simulated and estimated using Monte Carlo(MC) methods for radiation transport and a conventional treatment planning system (TPS). MC-derived and TPS-generated dose volume histograms (DVHs) and dose distribution data were compared for both the conical and round applicators for benchmarking purposes.Results:
Agreement using the gamma-index test was ⩾99.95% for distance to agreement and dose accuracy criteria of 2 mm and 2%, respectively. After observing good agreement, TPS DVHs and dose distributions for the conical and round applicators were obtained and compared. Brachytherapydose distributions generated using Pinnacle3 for ten CTdata sets showed that the parallel-opposed beams of the conical applicators provided similar PTV coverage to the round applicators and reduced the maximum dose to skin, chest wall, and lung by up to 27%, 42%, and 43%, respectively.Conclusions:
Brachytherapydose distributions for the conical applicators have been generated using MC methods and entered into the Pinnacle3 TPS via the Tufts technique. Treatment planning metrics for the conical AccuBoost applicators were significantly improved in comparison to those for conventional electron beam breast boost.
The difference of scoring dose to water or tissues in Monte Carlo dose calculations for low energy brachytherapy photon sources38(2011); http://dx.doi.org/10.1118/1.3549760View Description Hide DescriptionPurpose:
The goal of this work is to compare(radiation transported in medium; dose scored in medium) and (radiation transported in medium; dose scored in water) obtained from Monte Carlo(MC) simulations for a subset of human tissues of interest in low energy photonbrachytherapy. Using low dose rate seeds and an electronic brachytherapy source (EBS), the authors quantify the large cavity theory conversion factors required. The authors also assess whether applying large cavity theory utilizing the sources’ initial photonspectra and average photon energy induces errors related to spatial spectral variations. First, ideal spherical geometries were investigated, followed by clinical brachytherapy LDR seed implants for breast and prostate cancer patients.Methods:
Two types of dose calculations are performed with theGEANT4MC code. (1) For several human tissues,dose profiles are obtained in spherical geometries centered on four types of low energy brachytherapy sources: , , and seeds, as well as an EBS operating at 50 kV. Ratios of over are evaluated in the 0–6 cm range. In addition to mean tissue composition, compositions corresponding to one standard deviation from the mean are also studied. (2) Four clinical breast (using ) and prostate (using ) brachytherapy seed implants are considered. MCdose calculations are performed based on postimplant CT scans using prostate and breast tissue compositions. PTV values are compared for and .Results:
(1) Differences of −3% to 70% are observed for the investigated tissues. For a given tissue, is similar for all sources within 4% and does not vary more than 2% with distance due to very moderate spectral shifts. Variations of tissue composition about the assumed mean composition influence the conversion factors up to 38%. (2) The ratio of over for clinical implants matches at 1 cm from the single point sources.Conclusions:
Given the small variation with distance, using conversion factors based on the emitted photonspectrum (or its mean energy) of a given source introduces minimal error. The large differences observed between scoring schemes underline the need for guidelines on choice of media for dose reporting. Providing such guidelines is beyond the scope of this work.
38(2011); http://dx.doi.org/10.1118/1.3555299View Description Hide DescriptionPurpose:
Radiation-induced damage, such as inflammation and fibrosis, can compromise ventilation capability of local functional units (alveoli) of the lung. Ventilation function as measured with ventilation images, however, is often complicated by the underlying mechanical variations. The purpose of this study is to present a 4DCT-based method to measure the regional ventilation capability, namely, regional compliance, for the evaluation of radiation-induced lung damage.Methods:
Six 4DCT images were investigated in this study: One previously used in the generation of a POPI model and the other five acquired at Henry Ford Health System. A tetrahedral geometrical model was created and scaled to encompass each of the 4DCT image domains. Image registrations were performed on each of the 4DCT images using a multiresolution Demons algorithm. The images at the end of exhalation were selected as a reference. Images at other exhalation phases were registered to the reference phase. For the POPI-modeled patient, each of these registration instances was validated using 40 landmarks. The displacement vector fields (DVFs) were used first to calculate the volumetric variation of each tetrahedron, which represents the change in the air volume. The calculated results were interpolated to generate 3D ventilation images. With the computed DVF, a finite element method (FEM) framework was developed to compute the stress images of the lungtissue. The regional compliance was then defined as the ratio of the ventilation and stress values and was calculated for each phase. Based on iterative FEM simulations, the potential range of the mechanical parameters for the lung was determined by comparing the model-computed average stress to the clinical reference value of airway pressure. The effect of the parameter variations on the computed stress distributions was estimated using Pearson correlation coefficients.Results:
For the POPI-modeled patient, five exhalation phases from the start to the end of exhalation were denoted by, , respectively. The average lung volume variation relative to the reference phase was reduced from 18% at to 4.8% at . The average stress at phase was 1.42, 1.34, 0.74, and 0.28 kPa, and the average regional compliance was 0.19, 0.20, 0.20, and 0.24 for , respectively. For the other five patients, their average value at the end-inhalation phase was 21.1%, 19.6%, 22.4%, 22.5%, and 18.8%, respectively, and the regional compliance averaged over all six patients is 0.2. For elasticity parameters chosen from the potential parameter range, the resultant stress distributions were found to be similar to each other with Pearson correlation coefficients greater than 0.81.Conclusions:
A 4DCT-based mechanical model has been developed to calculate the ventilation and stress images of the lung. The resultant regional compliance represents the lung’s elasticityproperty and is potentially useful in correlating regions of lung damage with radiationdose following a course of radiation therapy.
On the relationships between electron spot size, focal spot size, and virtual source position in Monte Carlo simulations38(2011); http://dx.doi.org/10.1118/1.3556560View Description Hide DescriptionPurpose:
Every year, new radiotherapy techniques including stereotactic radiosurgery using linear accelerators give rise to new applications of Monte Carlo(MC) modeling. Accurate modeling requires knowing the size of the electron spot, one of the few parameters to tune in MC models. The resolution of integrated megavoltage imaging systems, such as the tomotherapy system, strongly depends on the photon spot size which is closely related to the electron spot. The aim of this article is to clarify the relationship between the electron spot size and the photon spot size (i.e., thefocal spot size) for typical incident electron beam energies and target thicknesses.Methods:
Three electron energies (3, 5.5, and 18 MeV), four electron spot sizes (, 0.5, 1, and 1.5 mm), and two tungsten target thicknesses (0.15 and 1 cm) were considered. The formation of the photon beam within the target was analyzed through electron energy deposition with depth, as well as photon production at several phase-space planes placed perpendicular to the beam axis, where only photons recorded for the first time were accounted for. Photon production was considered for “newborn” photons intersecting a plane at the isocenter (85 cm from source). Finally, virtual source position and “effective” focal spot size were computed by backprojecting all the photons from the bottom of the target intersecting a plane. The virtual source position and focal spot size were estimated at the plane position where the latter is minimal.Results:
In the relevant case of considering only photons intersecting the plane, the results unambiguously showed that the effective photon spot is created within the first 0.25 mm of the target and that electron and focal spots may be assumed to be equal within 3–4%.Conclusions:
In a good approximation photon spot size equals electron spot size for high energy X-ray treatments delivered by linear accelerators.
A novel lateral disequilibrium inclusive (LDI) pencil-beam based dose calculation algorithm: Evaluation in inhomogeneous phantoms and comparison with Monte Carlo calculations38(2011); http://dx.doi.org/10.1118/1.3557952View Description Hide DescriptionPurpose:
Pencil-beam (PB) based dose calculation for treatment planning is limited by inaccuracies in regions of tissue inhomogeneities, particularly in situations with lateral electron disequilibrium as is present at tissue/lung interfaces. To overcome these limitations, a new “lateral disequilibrium inclusive” (LDI) PB based calculation algorithm was introduced. In this study, the authors evaluated the accuracy of the new model by film and ionization chamber measurements and Monte Carlo simulations.Methods:
To validate the performance of the newLDI algorithm implemented in Corvus 09®, eight test plans were generated on inhomogeneous thorax and pelvis phantoms. In addition, three plans were calculated with a simple effective path length (EPL) algorithm on the inhomogeneous thorax phantom. To simulate homogeneous tissues, four test plans were evaluated in homogeneous phantoms (homogeneous dose calculation).Results:
The mean pixel pass rates and standard deviations of the gamma 4%/4 mm test for the film measurements were for the plans calculated with LDI, for the plans calculated with EPL, and for the homogeneous plans. Ionization chamber measurements and Monte Carlo simulations confirmed the high accuracy of the new algorithm (dose deviations ; gamma 3%/3 mm )Conclusions:
LDI represents an accurate and fast dose calculation algorithm for treatment planning.
38(2011); http://dx.doi.org/10.1118/1.3553404View Description Hide DescriptionPurpose:
To identify the most informative methods for reporting results of treatment planning comparisons.Methods:
Seven articles from the past year ofInternational Journal of RadiationOncology Biology Physics reported on comparisons of treatment plans for IMRT and IMAT. The articles were reviewed to identify methods of comparisons. Decision theoretical concepts were used to evaluate the study methods and highlight those that provide the most information.Results:
None of the studies examined the correlation between objectives. Statistical comparisons provided some information but not enough to provide support for a robust decision analysis.Conclusions:
The increased use of treatment planning studies to evaluate different methods in radiation therapy requires improved standards for designing the studies and reporting the results.
38(2011); http://dx.doi.org/10.1118/1.3555298View Description Hide DescriptionPurpose:
To evaluate the robustness of TG119-based quality assurance metrics for an IMRT system.Methods:
Four planners constructed treatment plans for the five IMRT test cases described in TG119. All plans were delivered to a solid water phantom in one treatment session in order to minimize session-dependent variation from phantom setup, film quality, machine performance, etc. Composite measurements utilized film and an ionization chamber. Per-field measurements were collected using a diode array device at an effective depth of 5 cm. All data collected were analyzed using the TG119 specifications to determine the confidence limit values for each planner separately and then compared.Results:
The mean variance of ion chamber measurements for each planner was within 1.7% of the planned dose. The resulting confidence limits were 3.13%, 1.98%, 3.65%, and 4.39%. Confidence limit values determined by composite film analysis were 8.06%, 13.4%, 9.30%, and 16.5%. Confidence limits from per-field measurements were 1.55%, 0.00%, 0.00%, and 2.89%.Conclusions:
For a single IMRT system, the accuracy assessment provided by TG119-based quality assurance metrics showed significant variations in the confidence limits between planners across all composite and per-field evaluations. This observed variation is likely due to the different levels of modulation between each planner’s set of plans. Performing the TG119 evaluation using plans produced by a single planner may not provide an adequate estimation of IMRT system accuracy.