Intensity-modulated Radiation Therapy Treatment Plans Research Articles

Dosimetric effect of small bowel oral contrast on conventional radiation therapy, linear accelerator–based intensity modulated radiation therapy, and helical tomotherapy plans for rectal cancer

PurposeThis study evaluated the dosimetric effect of small bowel oral contrast on conventional radiation therapy, linear accelerator--based intensity modulated radiation therapy (IMRT), and helical tomotherapy (HT) treatment plans. Methods and materialsThirteen patients with rectal cancer underwent computed tomography (CT) simulation with oral contrast (CCT) in preparation for chemoradiation therapy. The contrast in the small bowel was contoured, and a noncontrast CT scan (NCCT) was simulated by reassigning a CT number of 0 Hounsfield units to the contrast volume. Conventional, IMRT, and HT plans were generated with the CCT. The plan generated on the CCT was then recalculated on the NCCT, maintaining the same number of monitor units for each field, and the plans were not renormalized. Dosimetric parameters representing coverage of the planning target volume with 45 Gy (D98%, D95%, D50%, and D2%) and sparing of the bladder and peritoneal cavity (D50%, D30%, and D10%) were recorded. The ratio of dose calculated in the presence of contrast to dose with contrast edited out was recorded for each parameter. A paired Student t test was used for comparison of plans. ResultsFor conventional plans, there was <0.1% variance in the dose ratio for all volumes of interest. For IMRT plans, there was a 1% decrease in the mean dose ratio, and the range of dose ratios for all volumes was greater than that for HT or conventional plans. For HT plans, for all volumes of interest, the mean dose ratio was <0.2%, and the range for all patients was <1%. For all IMRT dosimetric parameters, the difference was in the order of 1% of the mean dose (P < .05). The dose difference was not statistically significant for the conventional or HT plans. ConclusionsThe use of CCT during CT simulation has no clinically significant effect on dose calculations for conventional, IMRT, and HT treatment plans and may not require replacement of the contrast with a CT number of 0 Hounsfield units.

P13.25 * A RADIOBIOLOGICAL COMPARISON BETWEEN THE INVERSE INTENSITY MODULATED RADIATION THERAPY AND THE FIELD IN FIELD INTENSITY MODULATED RADIATION THERAPY FOR THE TREATMENT OF GLIOBLASTOMA

For critical structures the use of maximum or mean dose limits in the intensity modulated radiation therapy (IMRT) optimization is not very meaningful. The dose-volume limits and equivalent uniform dose (EUD) which is defined as the uniform dose that would lead to the same effect as the given non-uniform dose in as particular organ are combined to analyze the volume effect. The normal tissue complication probability (NTCP) is a predictor of radiobiological effect for organs at risk (OAR). The calculation of the NTCP is based on the DVH which is generated by the treatment planning system after calculation of the 3D dose distribution. Uniform EUD irradiation of an OAR results in the same NTCP as the original non-homogeneous distribution. The NTCP equation is therefore represented as a function of EUD. The inverse equation expresses EUD as a function of NTCP. Expressing NTCP in terms of EUD represents a step toward simplifying the conceptual framework for modeling probability of expected complications. The aim of this study is to compare radiobiological effects between the fif IMRT and inverse IMRT for treatment of Glioblastoma (GBM) using the equivalent uniform dose (EUD) and normal tissue complication probability (NTCP). Five representative patients treated with definitive radiation using IMRT at our clinic in the last two year were selected for treatment planning study. Criteria for inclusion were glioblastoma . The normal tissues (Organ at Risk, OaR) contoured included brain-stem, optic chiasm, optic nerves, right-left cohclea and whole brain. Brain was defined as total brain tissue minus the PTV. Two different radiation therapy techniques; inverse IMRT and FiF IMRT treatment plans were optimised with the prescription dose 60 Gy/30 fractions. All those calculations and IMRT optimizations were performed using the Prowess Panther DAQ TPS with 6 MV,Siemens Artiste, MLC with 160 leaves and evaluated by using EUD and NTCP models. In the present study, target dose coverage was improved with inverse IMRT planning as compared with fif IMRT planning. With respect to NTCP there is no significant differences between fif IMRT planning and inverse IMRT planning. For the optic chiasm, brainstem, right-left eyes and right-left lens the NTCP values were calculated to be smaller than %1 for both IMRT plannings, except for GBM3 patient as a result of eye(L) concurrent with PTV. So the maximum NTCP was 1 for lens(L) of GBM3 patient for both plannings. Eye(L) of NTCP values were 0.15479 and 0.10896 respectively for this patient with a frontosfenoidal tumor and a biggest size of tumor, 8.82 cm. After obtaining these results we decided to compare also the total monitor units (MU) of optimizations. Then, we find that total MU value of the fif IMRT plannings are smaller than inverse IMRT plannings. So, the fif IMRT planning can be chosen for short treatment time and comfort of the patients for GBM patients treatment.

Automation and Intensity Modulated Radiation Therapy for Individualized High-Quality Tangent Breast Treatment Plans

To demonstrate the large-scale clinical implementation and performance of an automated treatment planning methodology for tangential breast intensity modulated radiation therapy (IMRT). Automated planning was used to prospectively plan tangential breast IMRT treatment for 1661 patients between June 2009 and November 2012. The automated planning method emulates the manual steps performed by the user during treatment planning, including anatomical segmentation, beam placement, optimization, dose calculation, and plan documentation. The user specifies clinical requirements of the plan to be generated through a user interface embedded in the planning system. The automated method uses heuristic algorithms to define and simplify the technical aspects of the treatment planning process. Automated planning was used in 1661 of 1708 patients receiving tangential breast IMRT during the time interval studied. Therefore, automated planning was applicable in greater than 97% of cases. The time for treatment planning using the automated process is routinely 5 to 6 minutes on standard commercially available planning hardware. We have shown a consistent reduction in plan rejections from plan reviews through the standard quality control process or weekly quality review multidisciplinary breast rounds as we have automated the planning process for tangential breast IMRT. Clinical plan acceptance increased from 97.3% using our previous semiautomated inverse method to 98.9% using the fully automated method. Automation has become the routine standard method for treatment planning of tangential breast IMRT at our institution and is clinically feasible on a large scale. The method has wide clinical applicability and can add tremendous efficiency, standardization, and quality to the current treatment planning process. The use of automated methods can allow centers to more rapidly adopt IMRT and enhance access to the documented improvements in care for breast cancer patients, using technologies that are widely available and already in clinical use.

Evaluation of DVH-based treatment plan verification in addition to gamma passing rates for head and neck IMRT

Background and purposeTreatment plan verification of intensity modulated radiotherapy (IMRT) is generally performed with the gamma index (GI) evaluation method, which is difficult to extrapolate to clinical implications. Incorporating Dose Volume Histogram (DVH) information can compensate for this. The aim of this study was to evaluate DVH-based treatment plan verification in addition to the GI evaluation method for head and neck IMRT. Materials and methodsDose verifications of 700 subsequent head and neck cancer IMRT treatment plans were categorised according to gamma and DVH-based action levels. Fractionation dependent absolute dose limits were chosen. The results of the gamma- and DVH-based evaluations were compared to the decision of the medical physicist and/or radiation oncologist for plan acceptance. ResultsNearly all treatment plans (99.7%) were accepted for treatment according to the GI evaluation combined with DVH-based verification. Two treatment plans were re-planned according to DVH-based verification, which would have been accepted using the evaluation alone. DVH-based verification increased insight into dose delivery to patient specific structures increasing confidence that the treatment plans were clinically acceptable. Moreover, DVH-based action levels clearly distinguished the role of the medical physicist and radiation oncologist within the Quality Assurance (QA) procedure. ConclusionsDVH-based treatment plan verification complements the GI evaluation method improving head and neck IMRT-QA.

Novel, full 3D scintillation dosimetry using a static plenoptic camera.

Patient-specific quality assurance (QA) of dynamic radiotherapy delivery would gain from being performed using a 3D dosimeter. However, 3D dosimeters, such as gels, have many disadvantages limiting to quality assurance, such as tedious read-out procedures and poor reproducibility. The purpose of this work is to develop and validate a novel type of high resolution 3D dosimeter based on the real-time light acquisition of a plastic scintillator volume using a plenoptic camera. This dosimeter would allow for the QA of dynamic radiation therapy techniques such as intensity-modulated radiation therapy (IMRT) or volumetric-modulated arc therapy (VMAT). A Raytrix R5 plenoptic camera was used to image a 10 × 10 × 10 cm(3) EJ-260 plastic scintillator embedded inside an acrylic phantom at a rate of one acquisition per second. The scintillator volume was irradiated with both an IMRT and VMAT treatment plan on a Clinac iX linear accelerator. The 3D light distribution emitted by the scintillator volume was reconstructed at a 2 mm resolution in all dimensions by back-projecting the light collected by each pixel of the light-field camera using an iterative reconstruction algorithm. The latter was constrained by a beam's eye view projection of the incident dose acquired using the portal imager integrated with the linac and by physical consideration of the dose behavior as a function of depth in the phantom. The absolute dose difference between the reconstructed 3D dose and the expected dose calculated using the treatment planning software Pinnacle(3) was on average below 1.5% of the maximum dose for both integrated IMRT and VMAT deliveries, and below 3% for each individual IMRT incidences. Dose agreement between the reconstructed 3D dose and a radiochromic film acquisition in the same experimental phantom was on average within 2.1% and 1.2% of the maximum recorded dose for the IMRT and VMAT delivery, respectively. Using plenoptic camera technology, the authors were able to perform millimeter resolution, water-equivalent dosimetry of an IMRT and VMAT plan over a whole 3D volume. Since no moving parts are required in the dosimeter, the incident dose distribution can be acquired as a function of time, thus enabling the validation of static and dynamic radiation delivery with photons, electrons, and heavier ions.

Effects of spatial resolution and noise on gamma analysis for IMRT QA.

We investigated the sensitivity of the gamma index to two factors: the spatial resolution and the noise level in the measured dose distribution. We also examined how the choice of reference distribution and analysis software affect the sensitivity of gamma analysis to these two factors for quality assurance (QA) of intensity‐modulated radiation therapy (IMRT) treatment plans. For ten clinical IMRT plans, the dose delivered to a transverse dose plane was measured with EDR2 radiographic film. To evaluate the effects of spatial resolution, each irradiated film was digitized using three different resolutions (71, 142, and 285 dpi). To evaluate the effects of image noise, 1% and 2% local Gaussian noise was added to the film images. Gamma analysis was performed using 2%/2 mm and 3%/3 mm acceptance criteria and two commercial software packages, OmniPro I'mRT and DoseLab Pro. Dose comparisons were performed with the treatment planning system (TPS)‐calculated dose as the reference, and then repeated with the film as the reference to evaluate how the choice of reference distribution affects the results of dose comparisons. When the TPS‐calculated dose was designated as the reference distribution, the percentage of pixels with passing gamma values increased with both increasing resolution and noise. For 3%/3 mm acceptance criteria, increasing the film image resolution by a factor of two and by a factor of four caused a median increase of 0.9% and 2.6%, respectively, in the percentage of pixels passing. Increasing the noise level in the film image resulted in a median increase in percentage of pixels passing of 5.5% for 1% added local Gaussian noise and 5.8% for 2% added noise. In contrast, when the film was designated as the reference distribution, the percentage of pixels passing decreased with increased film noise, while increased resolution had no significant effect on passing rates. Furthermore, the sensitivity of gamma analysis to noise and resolution differed between OmniPro I'mRT and DoseLab Pro, with DoseLab Pro being less sensitive to the effects of noise and resolution. Noise and high scanning resolution can artificially increase the percentage of pixels with passing gamma values in IMRT QA. Thus, these factors, if not properly taken into account, can potentially affect the results of IMRT QA by causing a plan that should be classified as failing to be falsely classified as passing. In designing IMRT QA protocols, it is important to be aware that gamma analysis is sensitive to these parameters.PACS number: 87.55.Qr, 87.55.km, 87.56.Fc

TH-C-17A-03: Dynamic Visualization and Dosimetry of IMRT and VMAT Treatment Plans by Video-Rate Imaging of Cherenkov Radiation in Pure Water

Purpose: A novel optical dosimetry technique for the QA and verification of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) radiotherapy plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. Methods: An intensified CCD camera (ICCD) was used to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank. The ICCD acquisition was gated to the Linac, operated for single pulse imaging, and binned to a resolution of 512×512 pixels. The resulting videos were analyzed temporally for regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR) and summed to obtain an overall light distribution, which was compared to the expected dose distribution from the TPS using a gammaindex analysis. Results: The chosen camera settings resulted in data at 23.5 frames per second. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.2% and 95.6% agreement between the light distribution and expected TPS dose distribution based upon a 3% / 3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans respectively. Conclusion: The results from this initial study demonstrate the first documented use of Cherenkov radiation for optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications. NIH R01CA109558 and R21EB017559.

SU-E-T-592: Relationship Between Dose of Distribution and Area of Segment Fields Among Different Intensity-Modulated Radiotherapy Planning in Cervix Cancer

Purpose: In premise of uninfluenced to dose distribution of tumor target and organ at risk(OAR) in cervical cancer,area of segment fields was changed to increase efficacy and optimize treatment method by designing different plan of intensity modulated radiotherapy(IMRT). Methods: 12 cases of cervical cancer were confirmed in pathology and treated with step and shoot IMRT. Dose of PTV was 50Gy/25fractions. Every patient was designed 9 treatment plans of IMRT by Pinnacle 8.0m planning system,each plan was used with 9 beams of uniform distribution and fixing incidence direction(200°,240°,280°,320°,0°,40°,80°,120°and 160°respectively),and designed for delivery on Elekta Synergy linear accelerator. All plans were optimized with the direct machine parameter optimization(DMPO) algorithm using the same set of optimization objectives. Number of maximum segment field was defined at 80 and minimum MU in each segment was 5MU,and minimal segment area was 2*1cm2,2*2cm2,3*3cm2,4*4cm2,5*5cm2,6*6cm2,7*7cm2,8*8cm2and 9*9cm2,respectively.Coverage,homogeneity and conformity of PTV,sparing of OAR, MU and number of segment were compared. Results: In this group, mean volume of PTV was 916.8±228.7 cm3. Compared with the area of minimal segment field increased from 2*1cm2 to 9*9 cm2,the number of mean MU was decreased from 1405±170 to 490±47 and the number of segment field was reduced from 76±4 to 39±7 respectively(p 0.05),but when the minimal segment area was 8*8 cm2 and 9*9 cm2,they were changed compared with 7*7 cm2 and below(p<0.05). Conclusion: The minimal segment field of IMRT plan designed by Pinnacle 8.0m planning system in cervical carcinoma should be enlarge reasonably and minimal segment area of 7*7 cm2 was recommend.

SU‐E‐T‐436: Accelerated Gated IMRT: A Feasibility Study for Lung Cancer Patients

Purpose:To evaluate the feasibility of delivering a gated Intensity Modulated Radiotherapy (IMRT) treatment using multiple respiratory phases in order to account for all anatomic changes during free breathing and accelerate the gated treatment without increasing the dose per fraction.Methods:For 7 patients with lung cancer, IMRT treatment plans were generated on a full inspiration (FI) Computed Tomography (CT) and a Mid Intensity Position (MIP) CT. Moreover, in order to achieve an accelerated gated IMRT, multiple respiratory phase plans were calculated: 2‐phase plans including the FI and the full expiration phases, and 3‐phase plans by adding the mid‐inspiration phase. In order to assess the tolerance limits, plans' doses were registered and summed to the FI‐based plan. Mean dose received by Organs at Risk (OARs) and target volumes were used to compare obtained plans.Results:The mean dose differences between the FI plans and the multi‐phase plans never exceeded 0.4 Gy (Fig. 1). Concerning the clinical target volume these differences were even smaller: less than 0.1 Gy for both the 2‐phase and 3‐phase plans. Regarding the MIP treatment plan, higher doses in different healthy structures were observed, with a relative mean increase of 0.4 to 1.5 Gy. Finally, compared to the prescribed dose, the FI as well as the multi‐phase plans were associated with a mean difference of 0.4 Gy, whereas in the case of MIP a higher mean difference of 0.6 Gy was observed.Conclusion:The doses obtained while planning a multi‐phase gated IMRT treatment were within the tolerance limits. Compared to MIP, a better healthy tissue sparing was observed in the case of treatment planning based on one or multiple phases. Future work will consist in testing the multi‐phase treatment delivery while accounting for the multileaf collimator speed constraints.

SU-E-T-379: Concave Approximations of Target Volume Dose Metrics for Intensity- Modulated Radiotherapy Treatment Planning

Purpose: The widely used treatment plan metric Dx (mimimum dose to the hottest x% by volume of the target volume) is simple to interpret and use, but is computationally poorly behaved (non-convex), this impedes its use in computationally efficient intensity-modulated radiotherapy (IMRT) treatment planning algorithms. We therefore searched for surrogate metrics that are concave, computationally efficient, and accurately correlated to Dx values in IMRT treatment plans. Methods: To find concave surrogates of D95—and more generally, Dx values with variable x values—we tested equations containing one or two generalized equivalent uniform dose (gEUD) functions. Fits were obtained by varying gEUD ‘a’ parameter values, as well as the linear equation coefficients. Fitting was performed using a dataset of dose-volume histograms from 498 de-identified head and neck IMRT treatment plans. Fit characteristics were tested using a crossvalidation process. Reported root-mean-square error values were averaged over the cross-validation shuffles. Results: As expected, the two-gEUD formula provided a superior fit, compared to the single-gEUD formula. The best approximation uses two gEUD terms: 16.25 x gEUD[a=0.45] – 15.30 x gEUD[a=1.75] – 0.69. The average root-mean-square error on repeated (70/30) cross validation was 0.94 Gy. In addition, a formula was found that reasonably approximates Dx for xmore » between 80% and 96%. Conclusion: A simple concave function using two gEUD terms was found that correlates well with PTV D95s for these head and neck treatment plans. More generally, a formula was found that represents well the Dx for x values from 80% to 96%, thus providing a computationally efficient formula for use in treatment planning optimization. The formula may need to be adjusted for other institutions with different treatment planning protocols. We conclude that the strategy of replacing Dx values with gEUD-based formulas is promising.« less

Measurement-based Monte Carlo simulation of high definition dose evaluation for nasopharyngeal cancer patients treated by using intensity modulated radiation therapy

Measurement-based Monte Carlo (MBMC) simulation using a high definition (HD) phantom was used to evaluate the dose distribution in nasopharyngeal cancer (NPC) patients treated with intensity modulated radiation therapy (IMRT). Around nasopharyngeal cavity, there exists many small volume organs-at-risk (OARs) such as the optic nerves, auditory nerves, cochlea, and semicircular canal which necessitate the use of a high definition phantom for accurate and correct dose evaluation. The aim of this research was to study the advantages of using an HD phantom for MBMC simulation in NPC patients treated with IMRT. The MBMC simulation in this study was based on the IMRT treatment plan of three NPC patients generated by the anisotropic analytical algorithm (AAA) of the Eclipse treatment planning system (Varian Medical Systems, Palo Alto, CA, USA) using a calculation grid of 2 mm2. The NPC tumor was treated to a cumulative dose of 7000 cGy in 35 fractions using the shrinking-field sequential IMRT (SIMRT) method. The BEAMnrc MC Code was used to simulate a Varian EX21 linear accelerator treatment head. The HD phantom contained 0.5 × 0.5 × 1 mm3 voxels for the nasopharyngeal area and 0.5 × 0.5 × 3 mm3 for the rest of the head area. An efficiency map was obtained for the amorphous silicon aS1000 electronic portal imaging device (EPID) to adjust the weighting of each particle in the phase-space file for each IMRT beam. Our analysis revealed that small volume organs such as the eighth cranial nerve, semicircular canal, cochlea and external auditory canal showed an absolute dose difference of ≥200 cGy, while the dose difference for larger organs such as the parotid glands and tumor was negligible for the MBMC simulation using the HD phantom. The HD phantom was found to be suitable for Monte Carlo dose volume analysis of small volume organs.

Video-rate optical dosimetry and dynamic visualization of IMRT and VMAT treatment plans in water using Cherenkov radiation.

A novel technique for optical dosimetry of dynamic intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans was investigated for the first time by capturing images of the induced Cherenkov radiation in water. A high-sensitivity, intensified CCD camera (ICCD) was configured to acquire a two-dimensional (2D) projection image of the Cherenkov radiation induced by IMRT and VMAT plans, based on the Task Group 119 (TG-119) C-Shape geometry. Plans were generated using the Varian Eclipse treatment planning system (TPS) and delivered using 6 MV x-rays from a Varian TrueBeam Linear Accelerator (Linac) incident on a water tank doped with the fluorophore quinine sulfate. The ICCD acquisition was gated to the Linac target trigger pulse to reduce background light artifacts, read out for a single radiation pulse, and binned to a resolution of 512 × 512 pixels. The resulting videos were analyzed temporally for various regions of interest (ROI) covering the planning target volume (PTV) and organ at risk (OAR), and summed to obtain an overall light intensity distribution, which was compared to the expected dose distribution from the TPS using a gamma-index analysis. The chosen camera settings resulted in 23.5 frames per second dosimetry videos. Temporal intensity plots of the PTV and OAR ROIs confirmed the preferential delivery of dose to the PTV versus the OAR, and the gamma analysis yielded 95.9% and 96.2% agreement between the experimentally captured Cherenkov light distribution and expected TPS dose distribution based upon a 3%/3 mm dose difference and distance-to-agreement criterion for the IMRT and VMAT plans, respectively. The results from this initial study demonstrate the first documented use of Cherenkov radiation for video-rate optical dosimetry of dynamic IMRT and VMAT treatment plans. The proposed modality has several potential advantages over alternative methods including the real-time nature of the acquisition, and upon future refinement may prove to be a robust and novel dosimetry method with both research and clinical applications.

Evaluation of hippocampus dose for patients undergoing intensity-modulated radiotherapy for nasopharyngeal carcinoma

To evaluate the dose received by the hippocampus among patients undergoing intensity-modulated radiotherapy (IMRT) for nasopharyngeal cancer. 10 patients with biopsy-proven, locally advanced nasopharyngeal cancer constituted the study population. The total prescribed dose to the planning target volume (PTV) was 70 Gy (D95%) delivered in 2.12-Gy daily fractions using IMRT. Using established anatomical guidelines, MRI co-registration and the assistance of a board-certified neuroradiologist, the right and left hippocampi were delineated on axial imaging from the CT scan obtained at simulation for each patient beginning at the most anterior portion of the lateral ventricle. IMRT treatment plans were generated without dose-volume constraints to the hippocampus. A range of dose-volume statistics was calculated. The mean hippocampus volume was 6.01 ± 2.61 cm(3). The mean V20 was 72.2%; V40 was 22.0%; V50 was 10.2%; and V60 was 5.5%. The average mean, minimum and maximum hippocampus doses were 30.27 Gy (range, 19.08-47.99 Gy); 17.54 Gy (range, 11.66-33.17 Gy); and 54.95 Gy (range, 35.59-75.57 Gy), respectively. The hippocampus received a maximum dose exceeding 70 Gy in 30% of cases. Our dosimetric analysis suggests that, for patients undergoing IMRT for nasopharyngeal cancer, the hippocampus routinely receives significantly high doses. The hippocampus receives a fair amount of incidental radiation during treatment for nasopharyngeal cancer. Given the importance of this structure with respect to memory and neurocognitive function, consideration should be given to identifying the hippocampus as a critical organ at risk in the IMRT optimization process.

GPU-Monte Carlo based fast IMRT plan optimization

Purpose: Intensity-modulated radiation treatment (IMRT) plan optimization needs pre-calculated beamlet dose distribution. Pencil-beam or superposition/convolution type algorithms are typically used because of high computation speed. However, inaccurate beamlet dose distributions, particularly in cases with high levels of inhomogeneity, may mislead optimization, hindering the resulting plan quality. It is desire to use Monte Carlo (MC) methods for beamlet dose calculations. Yet, the long computational time from repeated dose calculations for a number of beamlets prevents this application. It is our objective to integrate a GPU-based MC dose engine in lung IMRT optimization using a novel two-steps workflow. Methods : A GPU-based MC code gDPM is used. Each particle is tagged with an index of a beamlet where the source particle is from. Deposit dose are stored separately for beamlets based on the index. Due to limited GPU memory size, a pyramid space is allocated for each beamlet, and dose outside the space is neglected. A two-steps optimization workflow is proposed for fast MC-based optimization. At first step, a rough dose calculation is conducted with only a few number of particle per beamlet. Plan optimization is followed to get an approximated fluence map. In the second step, more accurate beamlet doses are calculated, where sampled number of particles for a beamlet is proportional to the intensity determined previously. A second-round optimization is conducted, yielding the final result. Results: For a lung case with 5317 beamlets, 105 particles per beamlet in the first round, and 108 particles per beam in the second round are enough to get a good plan quality. The total simulation time is 96.4 sec. Conclusion : A fast GPU-based MC dose calculation method along with a novel two-step optimization workflow are developed. The high efficiency allows the use of MC for IMRT optimizations. -------------------------------- Cite this article as: Li Y, Tian Z, Shi F, Jiang S, Jia X. GPU-Monte Carlo based fast IMRT plan optimization. Int J Cancer Ther Oncol 2014; 2 (2):020244. DOI: 10.14319/ijcto.0202.44

A Novel Reduced-Order Prioritized Optimization Method for Radiation Therapy Treatment Planning

In this study, a novel reduced order prioritized algorithm is presented for optimization in radiation therapy treatment planning. The proposed method consists of three stages. In the first stage, the intensity space was sampled by solving a series of unconstrained optimization problems. The objective function of the first stage is expressed as a scalarized weighted sum of partial objectives for the target and organ at risk. Latin hypercube sampling was utilized to define the weights for each run of the unconstrained optimizations. In the second stage, principal component analysis is applied to the solutions determined in the first stage to identify the major eigen modes in the intensities space, significantly reducing the number of independent variables. In the third stage, treatment planning goals/objectives are prioritized, and the problem is solved in the reduced order space. After each objective is optimized, that objective function is converted into a constraint for the lower-priority objectives. In the current formulation, a slip factor is used to relax the hard constraints for planning target volume (PTV) coverage. The applicability of the proposed method is demonstrated for one prostate and one lung intensity-modulated radiation therapy treatment plan. Upon completion of the sequential prioritized optimization, the mean dose at the rectum and bladder was reduced by 21.3% and 22.4%, respectively. Additionally, we investigated the effect of the slip factor 's' on PTV coverage and we found minimal degradation of the tumor dose (∼4%). Finally, the speed up factors upon the dimensionality reduction were as high as 49.9 without compromising the quality of the results.

Interactive Dose Shaping - efficient strategies for CPU-based real-time treatment planning

Conventional intensity modulated radiation therapy (IMRT) treatment planning is based on the traditional concept of iterative optimization using an objective function specified by dose volume histogram constraints for pre-segmented VOIs. This indirect approach suffers from unavoidable shortcomings: i) The control of local dose features is limited to segmented VOIs. ii) Any objective function is a mathematical measure of the plan quality, i.e., is not able to define the clinically optimal treatment plan. iii) Adapting an existing plan to changed patient anatomy as detected by IGRT procedures is difficult.To overcome these shortcomings, we introduce the method of Interactive Dose Shaping (IDS) as a new paradigm for IMRT treatment planning. IDS allows for a direct and interactive manipulation of local dose features in real-time. The key element driving the IDS process is a two-step Dose Modification and Recovery (DMR) strategy: A local dose modification is initiated by the user which translates into modified fluence patterns. This also affects existing desired dose features elsewhere which is compensated by a heuristic recovery process.The IDS paradigm was implemented together with a CPU-based ultra-fast dose calculation and a 3D GUI for dose manipulation and visualization. A local dose feature can be implemented via the DMR strategy within 1-2 seconds. By imposing a series of local dose features, equal plan qualities could be achieved compared to conventional planning for prostate and head and neck cases within 1-2 minutes.The idea of Interactive Dose Shaping for treatment planning has been introduced and first applications of this concept have been realized.

Evaluation of the Dosimetric Feasibility of Hippocampal Sparing Intensity-Modulated Radiotherapy in Patients with Locally Advanced Nasopharyngeal Carcinoma

PurposeThe objective of this study was to evaluate the dosimetric feasibility of using hippocampus (HPC) sparing intensity-modulated radiotherapy (IMRT) in patients with locally advanced nasopharyngeal carcinoma (NPC).Materials/MethodsEight cases of either T3 or T4 NPC were selected for this study. Standard IMRT treatment plans were constructed using the volume and dose constraints for the targets and organs at risk (OAR) per Radiation Therapy Oncology Group (RTOG) 0615 protocol. Experimental plans were constructed using the same criteria, with the addition of the HPC as an OAR. The two dose-volume histograms for each case were compared for the targets and OARs.ResultsAll plans achieved the protocol dose criteria. The homogeneity index, conformity index, and coverage index for the planning target volumes (PTVs) were not significantly compromised by the avoidance of the HPC. The doses to all OARs, excluding the HPC, were similar. Both the dose (Dmax, D2%, D40%, Dmean, Dmedian, D98% and Dmin) and volume (V5, V10, V15, V20, V30, V40 and V50) parameters for the HPC were significantly lower in the HPC sparing plans (p<0.05), except for Dmin (P = 0.06) and V5 (P = 0.12).ConclusionsIMRT for patients with locally advanced NPC exposes the HPC to a significant radiation dose. HPC sparing IMRT planning significantly decreases this dose, with minimal impact on the therapeutic targets and other OARs.

Prospective study evaluating the use of IV contrast on IMRT treatment planning for lung cancer

To investigate the impact of exclusively using intravenous (IV) contrast x-ray computed tomography (CT) scans on lung cancer intensity-modulated radiation therapy (IMRT) treatment planning. Eight patients with lung cancer (one small cell, seven nonsmall cell) scheduled to receive IMRT consented to acquisition of simulation CT scans with and without IV contrast. Clinical treatment plans optimized on the noncontrast scans were recomputed on contrast scans and dose coverage was compared, along with the γ passing rates. IV contrast enhanced scans provided better target and critical structure conspicuity than the noncontrast scans. Using noncontrast scan as a reference, the median absolute/relative differences in mean, maximum, and minimum doses to the planning target volume (PTV) were -4.5 cGy/-0.09%, 41.1 cGy/0.62%, and -19.7 cGy/-0.50%, respectively. Regarding organs-at-risk (OARs), the median absolute/relative differences of maximum dose to heart was -13.3 cGy/-0.32%, to esophagus was -63.4 cGy/-0.89%, and to spinal cord was -16.3 cGy/-0.46%. The median heart region of interest CT Hounsfield Unit (HU) number difference between noncontrast and contrast scans was 136.4 HU (range, 94.2-161.8 HU). Subjectively, the regions with absolute dose differences greater than 3% of the prescription dose were small and typically located at the patient periphery and/or at the beam edges. The median γ passing rate was 0.9981 (range, 0.9654-0.9999) using 3% absolute dose difference/3 mm distance-to-agreement criteria. Overall, all evaluated cases were found to be clinically equivalent. PTV and OARs dose differences between noncontrast and contrast scans appear to be minimal for lung cancer patients undergoing IMRT. Using IV contrast scans as the primary simulation dataset could increase treatment planning efficiency and accuracy by avoiding unnecessary scans, manually region overriding, and planning errors caused by nonperfect image registrations.

Comparison of IMRT and VMAT plans with different energy levels using Monte-Carlo algorithm for prostate cancer

To make dosimetric comparisons of volumetric-modulated arc therapy (VMAT) and 7-field intensity-modulated radiotherapy (IMRT) with dynamic MLCs using the Monaco treatment planning system with Monte Carlo algorithm. Single-arc VMAT and 7-field IMRT treatment plans were compared for 12 intermediate risk prostate cancer patients treated with prostate and seminal vesicle radiotherapy. For all patients, the prescribed dose was 78 Gy delivered in 39 fractions. The dosimetric data of IMRT and VMAT plans with 6, 10 and 15 MV energies were compared. The comparison was made for target volume, organs at risk (OAR) doses, and for monitor units (MU). The normal tissue surrounding the target were lower in VMAT plans compared to IMRT plans. VMAT plans achieved lower doses to all OARs for nearly all dosimetric endpoints. VMAT plans achieved 9.4, 9.0 and 7.0 % relative decrease in MUs required for RT delivery, for 6, 10 and 15 MV energy levels, respectively. The target volume and OAR dosimetric values did not differ significantly between 6, 10 and 15 MV photon energies. VMAT plans were found to be dosimetrically equivalent to IMRT plans for prostate cancer patients, with better rectum and bladder sparing and fewer MUs required.

Dose calculation and treatment plan optimization including imaging dose from kilovoltage cone beam computed tomography

Background. With the increasing use of cone beam computed tomography (CBCT) for patient position verification and radiotherapy treatment adaptation, there is an increasing need to develop techniques that can take into account concomitant dose using a personalized approach.Material and methods. A total of 20 patients (10 pelvis and 10 head and neck) who had undergone radiation therapy using intensity modulated radiation therapy (IMRT) were selected and the dose from kV CBCT was retrospectively calculated using a treatment planning system previously commissioned for this purpose. The imaging dose was added to the CT images used for treatment planning and the difference in its addition prior to and after the planning was assessed.Results. The additional isocenter dose as a result of daily CBCT is in the order of 3–4 cGy for 35-fraction head and neck and 23–47 cGy for 25-fraction pelvis cases using the standard head and neck and pelvis image acquisition protocols. The pelvic dose is especially dependent on patient size and body mass index (BMI), being higher for patients with lower BMI. Due to the low energy of the kV CBCT beam, the maximum energy deposition is at or near the surface with the highest dose being on the patient's left side for the head and neck (∼ 7 cGy) and on the posterior for the pelvic cases (∼ 80 cGy). Addition of imaging dose prior to plan optimization resulted in an average reduction of 4% in the plan monitor units and 5% in the number of control points.Conclusion. Dose from daily kV CBCT has been added to patient treatment plans using previously commissioned kV CBCT beams in a treatment planning system. Addition of imaging dose can be included in IMRT treatment plan optimization and would facilitate customization of imaging protocol based on patient anatomy and location of isocenter.