Accumulated Dose Distributions Research Articles

It is not appropriate to “deform” dose along with deformable image registration in adaptive radiotherapy

It is not appropriate to “deform” dose along with deformable image registration in adaptive radiotherapy

Combining advanced radiotherapy technologies to maximize safety and tumor control probability in stage III non-small cell lung cancer

The goal of the current study was to investigate the tumor control probability (TCP) of advanced radiotherapy technologies for stage III non-small cell lung cancer (NSCLC) and to evaluate potential interplay effects between their applications. Three-dimensional conformal radiotherapy (3D-CRT) with conventionally fractionated doses of 66 Gy served as reference for 13 patients with stage III NSCLC. Isotoxic dose escalation relative to the corresponding 3D-CRT plans was performed for three technologies and their combinations: intensity-modulated radiotherapy (IMRT), IMRT with a simultaneous integrated boost (IMRT-SIB) of 10% to the gross tumor volume (GTV), and adaptive re-planning twice during the treatment course (ART). All analyses were based on accumulated dose distributions using deformable image registration of CT images, which were acquired weekly during the treatment course. IMRT reduced the mean lung dose (MLD) by 5.6% ± 3.8% compared to 3D-CRT. ART resulted in lung sparing of 7.9% ± 4.8% and 9.2% ± 3.9% in 3D-CRT and IMRT planning, respectively. IMRT and ART escalated the irradiation dose by 6.6% ± 3.2% and 8.8% ± 6.3%, respectively, which was not statistically different. For the 7 patients with the largest GTVs, IMRT-SIB was superior to IMRT and ART with dose escalation of 11.9% ± 3.7%. The combination of ART, IMRT, and SIB achieved maximum dose escalation in all 13 patients by 17.1% ± 5.4% on average, which increased TCP from 19.9% ± 7.0 to 37.1% ± 10.1%. Adaptive re-planning was required to continuously conform the escalated and hypofractionated SIB doses to the shrinking tumor. Combining advanced radiotherapy technologies is considered as a safe and effective strategy to maximize local tumor control probability in stage III NSCLC.

SU-E-J-117: Computer-Assisted Verification of Accumulated Dose Distribution during the Treatment Time Based on Estimation of Four-Dimensional Dose Distribution Using an Electronic Portal Imaging Device.

The accumulated dose distributions during the course of radiation treatment are substantially important for verifying whether treatment dose distributions are produced according to planned dose distributions. The purpose of this study was to develop a computer-assisted verification method of accumulated dose distribution during the irradiation of a tumor based on estimation of four-dimensional (4D) dose distribution using an electronic portal imaging device (EPID). The 4D 'treatment' computed tomography (CT) images during the irradiation were estimated based on affine transformations including respiratory motions, which were derived by registration between a planning portal dose image and treatment portal dose dynamic image. Planning portal dose images were calculated from planning CT images and an algorithm for calculation of dose spatial distribution. Treatment portal dose images were estimated from EPID dynamic images obtained during a treatment time. The planning portal dose images were registered to the treatment portal dose images to obtain the affine transformation, which could include respiratory motion in a patient body. The CT images at a treatment time were determined by deforming the planning CT images using the affine transformation matrix. 4D dose distributions during a treatment delivery were obtained by applying a dose calculation algorithm to the 4D treatment CT images. Finally, accumulated dose distributions during the course of radiation treatment were verified with planned dose distributions. We applied the proposed method to EPID dynamic images of 2 lung cancer patients, and evaluated the difference in accumulated dose distribution between the plan and treatment using a gamma evaluation (3mm/3%). The average pass rate for 2 cases was 78.2%. The proposed method can be used for adaptively modifying the plan based on the dose discrepancy between the plan and treatment. This work was partially supported by Grant-in-Aid for Scientific Research (C) (22611011) and Okawa Foundation for Information and Telecommunications.

SU‐E‐T‐340: Estimation of the Delivered Fractional and Accumulative Patient Dose in IMRT and VMAT

Purpose: To estimate the delivered dose after each fraction of treatment as well as the delivered accumulative dose distribution. The goal of this project is to develop an automatic verification system using daily images and machine log files. Methods: First, we perform deformable registration from CT to CBCT and correct CBCT artifacts and CT numbers. Due to the limitation of the current scanning protocol, CBCT may not cover the whole treatment region needed for dose reconstruction. In that case, we use deformed CT for dose calculation which has correct CT number as well as daily patient geometry within whole treatment region. With machine log files, we generate actually delivered fluence maps at all delivery angles for that fraction. Then a GPU‐based Monte Carlo dose calculation package, gDPM, is employed to get delivered fractional dose on deformed CT or corrected CBCT. To compare the planned dose with the delivered dose, the delivered dose on new daily image is transferred back to the original CT geometry using the deformation vector fields obtained in the first step. This procedure is employed for each fraction to get the accumulative dose on original CT. Results: We demonstrate this treatment verification system on VMAT patients. We plot the DVHs for delivered and planned dose distributions and summarize the absolute errors in dose matrix report. Conclusions: We have proposed a procedure to estimate the delivered fractional and accumulative patient doses in IMRT and VMAT. If big errors are observed in the dose matrix report, then a warning should be sent out to responsible personnel, so the error can be indentified and corrected promptly.

A novel technique to enable experimental validation of deformable dose accumulation

To propose a novel technique to experimentally validate deformable dose algorithms by measuring 3D dose distributions under the condition of deformation using deformable gel dosimeters produced by a novel gel fabrication method. Five gel dosimeters, two rigid control gels and three deformable gels, were manufactured and treated with the same conformal plan that prescribed 400 cGy to the isocenter. The control gels were treated statically; the deformable gels were treated while being compressed by an actuation device to simulate breathing motion (amplitude of compression = 1, 1.5, and 2 cm, respectively; frequency = 16 rpm). Comparison between the dose measured by the control gels and the corresponding static dose distribution calculated in the treatment planning system (TPS) has determined the intrinsic dose measurement uncertainty of the gel dosimeters. Doses accumulated using MORFEUS, a biomechanical model-based deformable registration and dose accumulation algorithm, were compared with the doses measured by the deformable gel dosimeters to verify the accuracy of MORFEUS using dose differences at each voxel as well as the gamma index test. Flexible plastic wraps were used to contain and protect the deformable gels from oxygen infiltration, which inhibits the gels' dose sensitizing ability. Since the wraps were imperfect oxygen barrier, dose comparison between MORFEUS and the deformable gels was performed only in the central region with a received dose of 200 cGy or above to exclude the peripheral region where oxygen penetration had likely affected dose measurements. Dose measured with the control gels showed that the intrinsic dose measurement uncertainty of the gel dosimeters was 11.8 cGy or 4.7% compared to the TPS. The absolute mean voxel-by-voxel dose difference between the accumulated dose and the dose measured with the deformable gels was 4.7 cGy (SD = 36.0 cGy) or 1.5% (SD = 13.4%) for the three deformable gels. The absolute mean vector distance between the 250, 300, 350, and 400 cGy isodose surfaces on the accumulated and measured distributions was 1.2 mm (SD < 1.5 mm). The gamma index test that used the dose measurement precision of the control gels as the dose difference criterion and 2 mm as the distance criterion was performed, and the average pass rate of the accumulated dose distributions for all three deformable gels was 92.7%. When the distance criterion was relaxed to 3 mm, the average pass rate increased to 96.9%. This study has proposed a novel technique to manufacture deformable volumetric gel dosimeters. By comparing the doses accumulated in MORFEUS and the doses measured with the dosimeters under the condition of deformation, the study has also demonstrated the potential of using deformable gel dosimetry to experimentally validate algorithms that include deformations into dose computation. Since dose less than 200 cGy was not evaluated in this study, future investigations will focus more on low dose regions by either using bigger gel dosimeters or prescribing a lower dose to provide a more complete experimental validation of MORFEUS across a wider dose range.

Four-Dimensional Lung Treatment Planning in Layer-Stacking Carbon Ion Beam Treatment: Comparison of Layer-Stacking and Conventional Ungated/Gated Irradiation

We compared four-dimensional (4D) layer-stacking and conventional carbon ion beam distribution in the treatment of lung cancer between ungated and gated respiratory strategies using 4DCT data sets. Twenty lung patients underwent 4DCT imaging under free-breathing conditions. Using planning target volumes (PTVs) at respective respiratory phases, two types of compensating bolus were designed, a full single respiratory cycle for the ungated strategy and an approximately 30% duty cycle for the exhalation-gated strategy. Beams were delivered to the PTVs for the ungated and gated strategies, PTV(ungated) and PTV(gated), respectively, which were calculated by combining the respective PTV(Tn)s by layer-stacking and conventional irradiation. Carbon ion beam dose distribution was calculated as a function of respiratory phase by applying a compensating bolus to 4DCT. Accumulated dose distributions were calculated by applying deformable registration. With the ungated strategy, accumulated dose distributions were satisfactorily provided to the PTV, with D95 values for layer-stacking and conventional irradiation of 94.0% and 96.2%, respectively. V20 for the lung and Dmax for the spinal cord were lower with layer-stacking than with conventional irradiation, whereas Dmax for the skin (14.1 GyE) was significantly lower (21.9 GyE). In addition, dose conformation to the GTV/PTV with layer-stacking irradiation was better with the gated than with the ungated strategy. Gated layer-stacking irradiation allows the delivery of a carbon ion beam to a moving target without significant degradation of dose conformity or the development of hot spots.

Inverse planning for four‐dimensional (4D) volumetric modulated arc therapya)

To develop a 4D volumetric modulated arc therapy (VMAT) inverse planning framework. 4D VMAT inverse planning aims to derive an aperture and weight modulated arc therapy treatment plan that optimizes the accumulated dose distribution from all gantry angles and breathing phases. Under an assumption that the gantry rotation and patient breathing are synchronized (i.e., there is a functional relationship between the phase of the patient breathing cycle and the beam angle), the authors compute the contribution from different respiration phases through the registration of the phased CT images. The accumulative dose distribution is optimized by iteratively adjusting the aperture shape and weight of each beam through the minimization of the planning objective function. For comparison, traditional 3D VMAT plans are also performed for the two cases and the performance of the proposed technique is demonstrated. A framework for 4D VMAT inverse planning has been proposed. With the consideration of the extra dimension of time in VMAT, a tighter target margin can be achieved with a full duty cycle, which is otherwise not achievable simultaneously by either 3D VMAT optimization or gated VMAT. The 4D VMAT planning formulism proposed here provides useful insight on how the "time" dimension can be exploited in rotational arc therapy to maximally compensate for the intrafraction organ motion.

Evaluation of adaptive radiotherapy of bladder cancer by image-based tumour control probability modelling

Clinical implementation of adaptive radiotherapy strategies could benefit from extended tools for plan evaluation and selection. For this purpose we investigated the feasibility of image-based tumour control probability (TCP) modelling using the bladder as example of a tumour site with potential benefit from adaptive strategies. Material and methods. Two bladder cancer patients that underwent planning CT and daily cone beam CT (CBCT) imaging during the treatment course were included. The bladder was outlined in every image series. Following a previously published procedure, various adaptive planning target volumes (PTVs) were generated from the inter-fractional bladder variation observed during the first four CBCT sessions. Intensity modulated treatment plans delivering 60 Gy to a given PTV were generated. In addition, simultaneous integrated boost (SIB) plans giving a 10 Gy boost to the tumour were created. Using the daily CBCT images and polynomial warping, the dose in each bladder volume element was tracked fraction by fraction. TCP calculations employing the tracked accumulated dose distributions, together with radiosensitivity parameters estimated from published data on local control of bladder cancer were performed. The dependence of TCP on the simulated clonogenic cell distribution was also explored. Results. For a uniform clonogenic cell density in the whole bladder, TCP varied between 53% and 58% for the 60 Gy plans, while it was between 51% and 64% for the SIB plans. The lowest values were found when using the smallest PTVs, as they did not geometrically enclose the clinical target volume in all fractions. When increasing the clonogenic cell density in the tumour relative to that in the remaining bladder, the TCP saturated at approximately 75% for the SIB plans. Conclusion. Dose tracking and TCP calculation provided additional information to standard criteria such as geometrical coverage for the selected cases. TCP modelling may be a useful tool in plan evaluation and for selection between multiple plans.

SU-GG-T-28: Patient-Specific Rotational Tolerances and Margins Based on Prostate Shape

Purpose: To determine the impact of prostate shape and daily intra‐fraction translations and rotations on CTV coverage for PTV margins of 2, 3 and 5 mm. Materials/Methods: Twenty‐six patients with adenocarcinoma of the prostate were treated using the Calypso System on an IRB approved protocol. IMRT treatment plans meeting RTOG0126 dosimetric criteria (Dmin=79.2 Gy to 100% of CTV) were created with 2, 3, and 5 mm CTV‐to‐PTV expansions, where the CTV was the contoured prostate gland on the planning CT. Daily average translations and rotations were determined from daily real‐time electromagnetic tracking data. These rotations and translations were then applied to the planned dose distribution to obtain the daily and accumulated dose to structures for all fractions of each patient with 2, 3 and 5 mm PTV expansions. Dose volume histograms (DVHs) are computed on the planned and accumulated dose distributions to evaluate the PTV margin under real target motion. D95 and V79 were used to assess the CTV coverage of the accumulated dose distributions for each PTV margin. A parameter combining prostate average rotation and prostate elongation was deduced from patient data to assess the impact of non‐spherical geometry on target coverage. Results: When daily translations and rotations are included, V7995% was only achieved in 41%, 69% and 81% of cases with 2,3, and 5 mm margins, respectively. A strong correlation was found between elongated CTVs with large rotations, about the left‐right axis, and poor coverage. Conclusions: Rotations can cause large underdosing of the CTV depending on how elongated the prostate is along the sagittal axis. A patient‐specific geometrical parameter could be extracted from the 3D prostate structure and used to determine a PTV margin for adequate coverage within an allowed angular deviation limit. This work supported by supported by NIH R21 CA110485‐02

SU‐GG‐J‐104: Inclusion of Effects of Image Registration Errors in Accumulated Dose Distributions

Purpose: To consider the effects of image‐registration error in dose accumulation across different geometry instances and to display the resultant errors in a meaningful and useful way. Method and Materials: 4DCT phases acquired of a lung cancer patient were non‐rigidly registered by a B‐spline interpolation method (www.plastimatch.org) to the first phase chosen as a reference geometry. Dose was calculated on each phase and warped back to the reference phase by linear interpolation. The error in the deformation values at each voxel were assumed to be independent of that of neighboring voxels or of adjacent phases. This allows an error instance to be simulated by randomly perturbing the deformation used in dose accumulation by a value sampled from a Gaussian distribution for each voxel. The dose was accumulated for each instance and thirty instances were simulated to sample the total error distribution. Three methods of assigning the standard deviation of the error distribution to each voxel were considered: (i) fixed value of 3mm; (ii) intensity‐matching method: the local value is scaled by a distance to agreement metric calculated by comparing the registered phase image with the reference image; and (iii) vector‐discrepancy method: the local value is scaled by evaluating the invertability of the deformation maps. The effects of the simulated error were visualized by a spread of the accumulated isodose contours. Results: The vector‐discrepancy method resulted in the representation of reduced errors in the posterior region of the target where the extra image detail provided by the ribs and vertebrae increased the accuracy of the registration. Differences between the intensity‐matching method and fixed‐error method were less noticeable. Conclusion: A method of including image registration error in dose accumulation has been developed. Further work is required to compare results to a gold standard on a number of image datasets.

TU-E-BRB-05: Real-Time Dose Reconstruction for Treatment Monitoring

Purpose: To reconstruct in real‐time the dose distribution of intensity‐modulated radiation therapy(IMRT)delivered to a patient during treatment.Method and Materials: First, we use a GPU‐based FSPB algorithm to calculate the dose deposition coefficients Dij for all IMRT field. Dij is the dose contribution of the ith beamlets to the jth voxel on the patient image, and it is pre‐computed and stored in the memory. During the treatmentdelivery,MLC leaf positions will be read in real‐time from either the MLC controller or the cine EPIDimages. Then the delivereddose distribution is generated using the Dij matrix and the leaf positions. In this work, we simulate this process using the MLC DynaLog files in retrospective. We extract the leaf positions and fractional MU every 50 ms from the MLC DynaLog files and converted them to a beam mask Bi using an in‐house software based on MATLAB. The accumulated dose distribution delivered up to a time point is then computed by convolving Dij with Bi. Results: It takes on average less than 1 s to pre‐calculate Dij for each beam. It takes on average 0.2 s to update the accumulative dose distribution on patient's geometry. It is expected that this time can be further reduced by 10 to 20 times when we implement it on GPUs to meet the real‐time requirement. Conclusion: We have developed a dose reconstruction algorithm with pre‐calculated dose deposition coefficients using GPU‐based FSPB algorithm. The results indicate that real‐time dose reconstruction during treatmentdelivery is feasible, enabling dose‐based real‐time treatment monitoring.

Comparison of Respiratory-Gated and Respiratory-Ungated Planning in Scattered Carbon Ion Beam Treatment of the Pancreas Using Four-Dimensional Computed Tomography

We compared respiratory-gated and respiratory-ungated treatment strategies using four-dimensional (4D) scattered carbon ion beam distribution in pancreatic 4D computed tomography (CT) datasets. Seven inpatients with pancreatic tumors underwent 4DCT scanning under free-breathing conditions using a rapidly rotating cone-beam CT, which was integrated with a 256-slice detector, in cine mode. Two types of bolus for gated and ungated treatment were designed to cover the planning target volume (PTV) using 4DCT datasets in a 30% duty cycle around exhalation and a single respiratory cycle, respectively. Carbon ion beam distribution for each strategy was calculated as a function of respiratory phase by applying the compensating bolus to 4DCT at the respective phases. Smearing was not applied to the bolus, but consideration was given to drill diameter. The accumulated dose distributions were calculated by applying deformable registration and calculating the dose-volume histogram. Doses to normal tissues in gated treatment were minimized mainly on the inferior aspect, which thereby minimized excessive doses to normal tissues. Over 95% of the dose, however, was delivered to the clinical target volume at all phases for both treatment strategies. Maximum doses to the duodenum and pancreas averaged across all patients were 43.1/43.1 GyE (ungated/gated) and 43.2/43.2 GyE (ungated/gated), respectively. Although gated treatment minimized excessive dosing to normal tissue, the difference between treatment strategies was small. Respiratory gating may not always be required in pancreatic treatment as long as dose distribution is assessed. Any application of our results to clinical use should be undertaken only after discussion with oncologists, particularly with regard to radiotherapy combined with chemotherapy.

SU-FF-J-122: Dosimetric Impact of the Choice of a Deformation Algorithm for 4D Treatment Plannings with and Without Density Correction

Purpose: To compare the performance of two image registration algorithms used to obtain deformation vector fields (DVF) in the context of 4D radiation therapy, where DVFs are used to remap the dose on a reference breathing phase. Method and Materials: Two image registration methods were used in this study: 1) a multiresolution B‐splines using mutual information as a similarity measure 2) a Demons algorithm using a sum of squared differences as a similarity measure. The algorithms were implemented with the ITK toolkit and tested on a lungtumor case for which 4D data were available. The DVFs obtained were used in a simple anterior/posterior treatment plan with and without heterogeneity corrections. The doses were warped on the reference phase using a trilinear remapping with octant subdivision. Results: Large dose differences (20 Gy) were locally observed in the penumbra between static and motion‐corrected dose distributions but those differences had no effect on the CTV coverage. Indeed, Accounting for the tissue heterogeneity was shown to have more impact on the CTV mean dose than accounting for breath‐induced deformation (4 Gy compared to less than 1 Gy). Conclusion: Even though the accumulated dose distributions in the treatment of lungtumors is sensitive to the properties of the deformation vector fields (DVFs) used, no significant dosimetric difference was observed between the B‐splines corrected‐motion and the Demons corrected‐motion map doses. This situation is due to properly set structure margins, if these margins were to be tightened the differences would most probably increase.

Dosimetric variation due to CT inter-slice spacing in four-dimensional carbon beam lung therapy

When CT data with thick slice thickness are used in treatment planning, geometrical uncertainty may induce dosimetric errors. We evaluated carbon ion dose variations due to different CT slice thicknesses using a four-dimensional (4D) carbon ion beam dose calculation, and compared results between ungated and gated respiratory strategies. Seven lung patients were scanned in 4D mode with a 0.5 mm slice thickness using a 256-multi-slice CT scanner. CT images were averaged with various numbers of images to simulate reconstructed images with various slice thicknesses (0.5–5.0 mm). Two scenarios were studied (respiratory-ungated and -gated strategies). Range compensators were designed for each of the CT volumes with coarse inter-slice spacing to cover the internal target volume (ITV), as defined from 4DCT. Carbon ion dose distribution was computed for each resulting ITV on the 0.5 mm slice 4DCT data. The accumulated dose distribution was then calculated using deformable registration for 4D dose assessment. The magnitude of over- and under-dosage was found to be larger with the use of range compensators designed with a coarser inter-slice spacing than those obtained with a 0.5 mm slice thickness. Although no under-dosage was observed within the clinical target volume (CTV) region, D95 remained at over 97% of the prescribed dose for the ungated strategy and 95% for the gated strategy for all slice thicknesses. An inter-slice spacing of less than 3 mm may be able to minimize dose variation between the ungated and gated strategies. Although volumes with increased inter-slice spacing may reduce geometrical accuracy at a certain respiratory phase, this does not significantly affect delivery of the accumulated dose to the target during the treatment course.

血管造影撮影装置搭載コーンビームＣＴの特徴

Angiographic cone-beam CT, called DynaCT by SIEMENS, is a 3D imaging tool reconstructed from projection data by a rotational C-arm with a flat panel detector. It can visualize low-contrast objects such as soft tissue or small vessels as well as high-contrast structures such as enhanced vessels or bone. We need to understand its image characteristics and dose distribution during 200 degree rotation around a patient. In this research, we evaluated fundamental characteristics and dose effectiveness for optimized clinical images. DynaCT, including soft tissue information and isochronal voxel data along the z-axis, could provide enough CT-like image quality for IVR use. In addition, evaluation of accumulated dose distribution helped us to predict and avoid the occurrence of radiodermatitis. Thus, DynaCT is useful as a support and navigation tool for IVR.

SU-GG-T-109: Four Dimensional Inverse Planning for Intensity Modulated Radiation Therapy

Purpose: This work develops 4D inverse planning methods and demonstrates the potential benefit of 4D IMRT. Method and Materials: Two 4D planning strategies are proposed and compared. The first one treats all respiration phases as a system and optimizes the dose delivery collectively in space and phase. The method is referred to as collective optimization of all phases (COAP). In this approach, a deformable model is employed to establish a voxel-to-voxel correspondence and the goal is to maximize the accumulative dose to the tumor target while minimizing the dose to the organ-at-risk (OARs). The second one treats each phase as an independent 3D inverse planning problem and optimizes them separately. The final dose distribution is obtained by summing the dose of each phase after a deformable image registration. This method is called separate optimization of each phase (SOEP). In both approaches, the dose is optimized with a linear programming technique. Results: The resultant dose distribution of COAP is markedly better than that of SOEP in both target dose coverage and organ-at-risk sparing. The improvement of COAP is resulted from reallocation of dose among the phases to cater for anatomical changes during the breathing process. It is found that, for a phase with favorable geometry for dose delivery, more doses are allocated by COAP, and vise versa. COAP optimally assigns dose for all the involved phases. Because of the lack of this degree of freedom, SOEP yields almost identical intensity maps and dose distributions for all the phases. Conclusion: Simultaneous spatio-temporal dose optimization in 4D inverse planning allows one to take consideration of the spatial variation of the patient anatomy caused by respiration and yields the optimal accumulative dose distribution.

What CTV-to-PTV Margins Should Be Applied for Prostate Irradiation? Four-Dimensional Quantitative Assessment Using Model-Based Deformable Image Registration Techniques

To quantify adequate anisotropic clinical target volume (CTV)-to-planning target volume (PTV) margins for three different setup strategies used during prostate irradiation: (1) no setup corrections, (2) on-line corrections determined from bony anatomy, and (3) on-line corrections determined from gold markers. Three radiation oncologists independently delineated the CTV on computed tomography images of 30 prostate cancer patients. Eight repeat scans were acquired to allow simulation of the delivered dose distributions in changing geometry. Different registration approaches were taken to mimic the different setup strategies. A surface model-based deformable image registration system was used to warp the delivered dose distributions back to the dose in the planning computed tomography scan. On the basis of the geometric extent of the underdosed areas, a set of anisotropic margins was derived to ensure a minimal dose to the CTV of 95% for 90% of the patients. Without setup correction, margins of approximately 11 mm for the corpus of the prostate and 15 mm for the seminal vesicles were required. These margins could be reduced to 8 and 13 mm when aligning the patient to the bony anatomy and to 3 and 8 mm aligning the patient to implanted gold markers. A larger margin at the apex was required to account for the significant observer variability and steep dose gradients at this location (11 mm using skin marker registration, 9 mm using bony anatomy registration, and 6 mm using gold marker registration). Novel voxel tracking techniques have enabled us to calculate accumulated dose distributions and design accurate three-dimensional CTV-to-PTV margins for prostate irradiation.

SU-FF-J-124: The Effects of Anatomy Motion On Dose Distribution

Purpose: To evaluate the effects of respiration‐induced anatomy motion on dose distribution. Method and Materials: We have implemented a 4D Monte Carlo dose calculation system. It produces dose distribution on each of the respiratory phases. Deformable registration based on B‐spline optimization is used to register lung 4DCT data from other phases to the end‐exhalation (EEH) phase. The dose maps are then all warped to the EEH phase and time‐weighted accumulated dose distribution is computed. Dose distribution based on free‐breathing CT is also calculated to simulate the case when organ motion was not taken into account. This study presents the preliminary analysis of the DVH differences among the accumulated doses with and without registration as well as different time‐weights on one patient IMRT plan. Results: For the patient case, treatment was planned with 3 ITVs. We implemented two time‐weighting functions: 1. Dividing the respiratory cycle equally into 5 phases and summing up the dose with equal weights; 2. Accumulating the dose with 70% weight on EEH and 30% on end‐inhalation (EIH). The registration and time‐weighting functions did not make significant difference of the DVH in lungs. While the discrepancies between the two time‐weighting functions in the ITVs are small, the disparities to the free‐breathing are more prominent. For one of the 3 ITVs, 95% volume received 2.8Gy less dose than the static plan, which is more than one fraction. Conclusion: This preliminary study suggests that using 2 phases (EIH and EEH) to calculate the accumulated dose may be sufficient for estimating the effect of motion on a DVH. Respiration‐introduced organ motion may cause significant dose discrepancy, especially for tumors that have a large magnitude of motion. Hence, it is desired to incorporate the motion into dose calculation.

TH-E-M100F-02: CTV-To-PTV Margins for Prostate Irradiations, a Quantitative Assessment Using Novel Voxel Tracking Techniques

Purpose: To quantify adequate anisotropic CTV-to-PTV margins for three different setup strategies used during prostate irradiations: (1) no setup corrections, (2) on-line corrections based on bony anatomy and (3) on-line corrections based on gold markers. Method and Materials: Three radiation oncologists independently delineated the CTV, the bladder and the rectum in CT images of 30 prostate patients with implanted gold markers using a 3D model-based segmentation technique. IMRT plans with zero CTV-to-PTV margins were generated for each of the 3 times 30 contoured image datasets. Eight repeat scans were acquired to allow simulation of the delivered dose distributions in changing geometry. Different registration approaches were taken to mimic the different setup strategies. A surface-model based deformable image registration system was used to warp the delivered dose distributions back to the dose in the planning CT. Based on the geometric extent of underdosed areas in this patient population, a set of anisotropic margins was derived. A second simulation was carried out to assess the target coverage of the derived margins. Results: Without setup correction, margins of 10 mm for the corpus of the prostate and 15 mm for the seminal vesicles were required. These margins could be reduced to 7 mm and 12 mm respectively with on-line correction using bony anatomy registration methods and to 3 mm and 7 mm using on-line repositioning based on gold markers. A larger margin at the apex was required to account for the significant observer variability and steep dose gradients at this location (11 mm — skin marker registration, 9 mm — bony anatomy registration, 7 mm — gold marker registration). Conclusion: Novel voxel tracking techniques enable us to calculate accumulated dose distributions and design accurate 3-D CTV-to-PTV margins for prostate irradiations.

SU‐FF‐T‐402: Synchronized Dynamic Dose Reconstruction

Purpose: To present a dose accumulation method that explicitly incorporates real‐time target volume motion, and real‐time machine configuration, MLC motion and fluence state. Method and Materials: Effects of inter‐ and intra‐fraction motion on dose distributions delivered to a target volume are typically estimated by statistical methods that rely on idealized assumptions such as constant random and systematic errors, and constant offsets, periods and amplitudes of motion over the entire course of therapy. In addition, implementation and delivery related issues such as leaf sequencing limitations, spatial interplay between MLC leaf and target volume motion, and temporal interplay of the fluence state in IMRT, are assumed to average out during treatment. To include these effects, a dose accumulation technique is proposed which explicitly incorporates real‐time target volume motion data, and real‐time treatment machine data. Several technologies are becoming available to continuously monitor (10–30 Hz) the patient position, organs‐at‐risk, and the target volume during therapy. Likewise, real‐time (20 Hz) machine configuration, leaf position and fluence state data, is currently available in the Varian DynaLog files. These datasets are synchronized at the beginning of each beam; and the Monte Carlo method, which inherently accounts for time dependence, was used for dose calculations. Results: By synchronizing target volume position, machine configuration, leaf positions and fluence state, with sub‐second resolution, the dose delivered by each beam, may be accumulated. Such accumulated dose distributions reflect the interplay between target volume motion, MLC leaf motion and other machine‐related delivery effects based on real‐time, patient specific, measured data. The delivered dose distribution to date may then be compared to the planned dose distribution to provide input for dynamic refinement of treatment planning and delivery. Conclusion: Delivered target volume dose may be reconstructed using real‐time motion and machine data, and serve as a basis for dynamic refinement of treatments.