Index of content:
Volume 33, Issue 6, June 2006
- Joint Imaging/Therapy Scientific Session: Valencia B
- Imaging for Therapy Assessment
33(2006); http://dx.doi.org/10.1118/1.2241941View Description Hide Description
Purpose:Tumor hypoxia is an important resistant component that significantly affects response to treatment. Our aim was to concurrently monitor cell proliferation and tumor hypoxia distribution during radiation therapy.Materials and Methods: Several canine subjects with soft tissuesarcomas were repeatedly imaged with PET/CT before, during and after radiation treatment. The tumors were treated with 60Co in four 8 Gy weekly fractions. 3′‐Deoxy‐3′‐fluorothymidine ([F‐18]‐FLT) and Cu‐diacetyl‐bis(N4‐methylthiosemicarbazone) ([Cu‐61]‐ATSM) as surrogates of cell proliferation and tissue hypoxia, respectively, were used to follow the response. Approximately 200 MBq of FLT or Cu‐ATSM activity was administered per scan. The CT data between the imaging sessions was co‐registered and the corresponding PET data compared and analyzed.Results:Tumor response to therapy varied significantly between the subjects and even for different tumors in the same patient in case of multicentric disease. High heterogeneity of both cell proliferation and hypoxia (up to 50% in SUV) of the tumor was observed in several cases. Early proliferative response seems to be indicative of the overall tumor response. Both cell proliferation and hypoxia distributions changed during treatment, but their differential response remained rather constant. The distributions of cell proliferation and tissue hypoxia were often found to be complementary, but not exclusively. Conclusions: Concurrent monitoring of cell proliferation and tissue hypoxia represents a new dimension in tumor monitoring and provides basis for more efficient, biologically based treatment optimization. High heterogeneity of tumor kinetics and microenvironment together with spatially variable response calls for individualized approach to cancer management.
TH‐E‐ValB‐02: Image Registration‐Based Tool for Correlation Studies of Radiation‐Induced Fibrosis and Local Dose‐Related Parameters in Conformal Non‐Small Cell Lung Cancer Radiation Therapy33(2006); http://dx.doi.org/10.1118/1.2241942View Description Hide Description
Purpose: Despite the clinical importance of radiation therapy (RT) induced pulmonary injury, methods to accurately predict the degree of RT‐induced dysfunction are still lacking. Many investigators are trying to develop methods to relate dose‐volumetric parameters to the risk of RT‐induced lung injury, but no consensus has been reached about which of these parameters should be used. Other investigators are attempting to develop a dose‐response curve for regional RT‐induced damage and several local parameters like computed tomography(CT) density and single photon emission computed tomographylung perfusion and ventilation have been measured to allow an estimate of local injury. Method and Materials: A software tool was developed for the evaluation of the correlation between RT‐induced fibrosis and local dose‐related parameters for a group of non‐small cell lungcancer(NSCLC) patients. Local dose‐related parameters were determined using both conventional and Monte Carlo (MC)dose calculations algorithms. The relation between dose, calculated on the planning CT scans and RT‐induced fibrosis, identified on follow‐up CT scans, was established through linear registration. Subsequently, tissues densities were determined and automatic segmentation methods were developed for lung and fibrotic tissues.Results: One patient participating in a phase I/II NSCLC multi‐center clinical trial was chosen for illustration. Patients' response to treatment was quantified by evaluating the variation of lung and fibrotic tissue volumes over the follow‐up period. MC and conventional dose‐ and normalized total dose‐response curves were generated for the RT‐induced fibrosis. Fibrosis probability was shown to increase with increasing conventional and MCdose as well as with increasing conventional and MC normalized total dose. Moreover, fibrosis probability was also correlated with MC predicted hot spots in high dose regions.Conclusion: The presented tool allows a systematic numerical study of the relations between RT‐induced fibrosis and dose, normalized total dose and MC predicted hot‐spots in high dose regions.
TH‐E‐ValB‐03: Tumor Volume Regression During Radiation Therapy to Predict Treatment Outcome for Cervical Cancer33(2006); http://dx.doi.org/10.1118/1.2241943View Description Hide Description
Purpose: To investigate the outcome predictive power of tumor volume measured by serial MR imaging(MRI) of cervical cancer, including the sensitivity and specificity to identify patients at risk of local failure. Method and Materials: Seventy‐nine patients with cervical cancer stages IB2‐IVA, treated with radiation/chemotherapy (RT/CT), underwent serial MRI:MRI 1(pre‐RT), MRI 2(at 20–25 Gy/2 weeks), MRI 3(at 40–50 Gy/4 weeks), and MRI 4(at 1–2 months post‐RT). Mean follow up was 6.2 (0.2–9.4) years. Tumor volumes (V1,V2,V3,V4) and regression ratios (V2/V1,V3/V1,V4/V1) were measured by MRI 3D volumetry, and correlated with local tumor‐control and disease‐free survival using Mann‐Whitney rank‐sum test. Results: The volume data collected in this study were analyzed and the predictive power in terms of p‐value to discriminate local tumor‐control and disease‐free survival was computed. The absolute tumor volumes (V2,V3,V4) and the regression ratios (V2/V1,V3/V1,V4/V1) strongly correlated with local tumor‐control (p<0.001). These parameters also correlated with disease‐free survival, but only the last measurement (MRI 4) showed significant predictive value (p=0.02). Four methods had been developed to identify patients at risk for tumor recurrence (sensitivity 61%–100% and specificity 87%–100%). The most powerful method is based on the volume regression measured in MRI 3 and MRI 4 (V3/V1 >20% and V4/V1 >10%), which have a sensitivity of 89% and a specificity of 100%. Local failure can also be predicted as early as 2–3 weeks (MRI 2), the method of V1 >40 cc and V2/V1 >75% shows a sensitivity of 61% and a specificity of 93%. Conclusion: MRI‐based volumetric tumor measurement provides important predictive information about tumor response to the ongoing RT/CT. The methods developed in this study demonstrate a high specificity (87%–100%) for patients at risk of local failure based on long‐term follow‐up. These methods may classify patients who require more aggressive therapeutic intervention.
33(2006); http://dx.doi.org/10.1118/1.2241944View Description Hide Description
Purpose: The PETradiotracer [F‐18]FLT (3′‐deoxy‐3′‐[F‐18]fluorothymidine), used to measure cellularproliferation, has the potential to validate the efficacy of chemotherapy. We investigate the effect of chemotherapy on the biological distribution and radiationdosimetry of FLT in patients with acute myeloid leukemia (AML). Method and Materials:Cellularproliferation was measured in adult AML patients injected with 5 mCi of FLT. Dynamic and whole body PET/CT scans were acquired one day prior to chemotherapy and one week after the completion of chemotherapy using a GE Discovery PET/CT Scanner. Organs were manually contoured in the PETimages at multiple time points and time‐activity curves were generated for each contoured organ.Organ cumulative activities, organradiotracerdoses, and total body dose were determined using the standard adult male model and the RADAR method of dose calculation. Results: The biological distribution of FLT changed as a result of chemotherapy and this redistribution affected individual organ and total body radiationdoses. The toxic effect of the chemotherapeutic drugs on the leukemia cells resulted in a five‐fold reduction of FLT activity in the bone marrow post‐chemotherapy. This reduction in the bone marrow uptake was accompanied by a three‐fold increase in FLT activities and radiationdoses to the liver, kidneys, gallbladder, and adrenals while that of the spleen doubled. The total body radiationdose increased 30% post‐chemotherapy, given identical bladder voiding conditions.Conclusion: Systemic therapies such as chemotherapy can lead to significant changes in the biological distribution and dosimetry of radiotracers used in PETimaging for treatment assessment. Knowledge of these changes could impact the administered radiotracerdose to patients. Care should be taken in determining a suitable radiotracerdose for each specific case in order to avoid unnecessary dose yet maintain appropriate signal‐to‐noise ratios.
TH‐E‐ValB‐05: Analysis of Early Treatment Failure in Patients with Newly Diagnosed GBM Using Advanced MR Imaging33(2006); http://dx.doi.org/10.1118/1.2241946View Description Hide Description
Purpose: To seek imagingcharacteristics predictive of early treatment failure (EF) following concurrent radiation/chemotherapy (RT/CHT) in patients with newly diagnosed GBM s/p surgical resection using advanced MRI techniques (3D1H spectroscopy (MRSI),diffusion weighted (DWI) and perfusion weighted (PWI) imaging). Methods and Materials: 26 patients were imaged at 1.5T prior to RT/CHT (pre‐RT) and immediately after RT (post‐RT). Analyzedimaging parameters included peak heights of choline (Cho), creatine, N‐acetyl‐aspartate (NAA), lactate and lipid; Cho‐to‐NAA (CNI), Cho‐to‐Cr (CCrI) indices and excess‐choline (Ex(Cho)); parametric maps of percent‐recovery and apparent diffusion coefficient (ADC) were calculated. Mutually exclusive morphologic abnormalities were contoured as contrast‐enhancement (CE), T2‐hyperintensity (T2), resection‐cavity, necrosis, and a reference for normal appearing white matter. Patients were categorized as EF if any new CE appeared or if the CE volume increased by >25% at post‐RT. Imaging parameters were subjected to a Wilcoxon Rank Sum to test statistical significance between EF and non‐EF. Results: 9/26 patients were classified as EF. Both patient groups did not differ statistically in terms of age, volume of CE or T2 at pre‐RT. There were trends to higher Cho, CNI, and Ex(Cho) for the EF group at pre‐RT, however, these did not reach statistical significance. Statistically significant findings within CE at post‐RT were mainly associated with Cho and related indices and included lower ADC and %recovery values suggesting higher cellularity and increased leakiness of vessels in the EF vs non‐EF group. Conclusion: Even though our preliminary data on 26 patients could not identify imaging parameters significantly different between EF and non‐EF patients at pre‐RT, the demonstrated trend encourages further evaluation of additional 31 additional patient data sets acquired at 3T in order to increase statistical power. In addition, the reported significant changes at post‐RT suggest that the parameters may be valuable in assessing treatment effects.
TH‐E‐ValB‐06: Quantitative Characterization of Tumor Vascular Dysfunction in High‐Grade Gliomas Prior to and During Radiotherapy33(2006); http://dx.doi.org/10.1118/1.2241947View Description Hide Description
Introduction: Vascular properties within and adjacent to tumors may not be distinguishable by cerebral blood flow [CBF] or cerebral blood volume [CBV] alone, since the rates of CBV change may not be proportional in magnitude to CBF change. Hence, the empirical and physiological relationships between CBF and CBV were examined to estimate vasculature‐specific hemodynamiccharacteristics in high‐grade gliomas. Methods: Twenty patients with gliomas were studied with dynamic contrast‐enhanced T2* MRI [DCE‐MRI] before and during radiotherapy [RT]. CBF and CBV were calculated from DCE‐MRI and the relationships between the two were evaluated using two different metrics: The physiological measure of Mean Transit Time [MTT]=CBV/CBF; and, Empirical fitting of CBV and CBF using the power law, expressed as CBV=Constant*(CBF)β. Three tissue types were assessed, Gd‐enhanced tumor volume [GdTV], non‐enhancing abnormal tissue located beyond GdTV but within the abnormal hyperintense volume on FLAIR images [NEV], and normal tissue in hemisphere contralateral to tumor[CNT]. Effects of tissue types, CBV magnitudes (low[L], medium[M] and high[H] CBV), before and during RT, on MTT and β were analyzed by factorial ANOVA. Results: Both, MTT and β were significantly different (p<0.009) among the three tissue types. MTT increased from CNT(=1.60s) to NEV(=1.93s) to GdTV(=2.28s) (<0.0005). The power exponent β was significantly greater in GdTV(=1.079) and NEV(=1.070) than CNT(=1.025), but β in NEV and GdTV were not significantly different from each other. β increased with increasing CBV magnitude. There was a significant decrease in MTT and a significant increase in β in tumor (GdTV) and peritumoral (NEV) tissue during RT compared with pre‐RT values. Conclusions: β was strongly dependent on CBV magnitude and MTT on tissue type. Progressive abnormalities in functional characteristics of the vascular bed were noted, with significant disorder in tumor, but mild abnormality in peritumoral tissue. Early vascular response to radiation was first observed in functional rather than structural properties.
TH‐E‐ValB‐07: Blood‐Tumor‐Barrier Permeability Changes in High‐Grade Gliomas During Radiation Therapy Using DCE MRI33(2006); http://dx.doi.org/10.1118/1.2241948View Description Hide Description
Introduction: Radiation could affect vascular permeability in tumor and normal tissue. A previous study using high‐resolution MR images and a contrast uptake index demonstrated that an increase in contrast uptake in the tumor occurs after 30 Gy and persists up to one month after radiation therapy (RT). This finding suggested that an optimal time window exists to increase the efficiency of drug delivery to tumors. In this study, a quantitative method, dynamic contrast enhanced (DCE) MR imaging , was used to assess tumor vascular permeability changes during RT, in order to maximize potential therapeutic benefit. Methods: Twenty patients with high‐grade gliomas who underwent conformal RT participated in a MRI protocol. DCE T2* weighted images were acquired before RT, after ∼30 Gy, and one month after the completion of RT. A transfer constant (K) of Gd‐DTPA from blood to tissue was estimated voxel‐by‐voxel and used as a metric for assessment of vascular permeability. In the tumor volume (TV) defined on FLAIR MRI, statistically significant changes in K after ∼30 Gy and one month after RT were evaluated, compared to before treatment, using a students' t test. Results: An average fractional volume of 29.6% in tumor manifested substantial contrast leakage with K>0.005 min−1 pre RT. In the TV where there was no substantial leakage pre RT, the mean K increased significantly from K=0.0003 min−1 to 0.0073 min−1 after ∼30 Gy (p<0.0005) and to 0.0053 min−1 one month after RT (P<0.003). The fraction of TV that showed substantial contrast leakage significantly increased by 23% after ∼30 Gy (p<0.02), but not one month after RT (p>0.5). Conclusion: DCE MR imaging reveals vascular permeability increases after ∼30Gy in the portion of tumor where leakage is not substantial before RT. This finding suggests that the optimal time to administer chemotherapy is during the course of radiation therapy.
- Kilovolt and Megavolt Imaging for Therapy Guidance
33(2006); http://dx.doi.org/10.1118/1.2241902View Description Hide Description
Purpose: To evaluate artifacts caused by treatment couch attenuation on 3D image reconstruction for a new kV on‐board‐imager (OBI) and cone beam CT(CBCT) system and to develop an algorithm that filters couch effects from two‐dimensional radiographic projections prior to inputting to the 3D reconstruction algorithm. Material and methods: A standard quality assurance phantom was scanned in air and on couch top using both full and half fan cone‐beam scanning modes with and without bowtie filter combination. A spatial domain filter algorithm was developed to remove couch attenuation from each radiographic projection. This filter is based on a pixel‐by‐pixel subtraction technique of radiographic projections of cone‐beam scans of the couch from the corresponding radiographic projections of scans with phantom on top of the couch. The net couch‐filtered radiographic projections were used to reconstructCTimages.Results:CT numbers for scans of the phantom on couch top are less uniform than for scans of the phantom in air. The couch artifacts vary the linearity of the CT numbers by 5–15%, depending on the density of the material. Noise of the scans with phantom on couch top (3.5%) is higher than that with phantom in air (1.5%). The increased noise hinders the ability of the CBCT system to resolve low‐contrast regions when the couch is present. Pre‐reconstruction processing of the couch suppresses noise (< 1.5%) improves uniformity by a factor of 2 and removes ring and streak artifacts in the couch‐filtered reconstructedCBCTimages.Conclusion: The treatment couch produces streaking artifacts, enhances noise, and causes drifting of CT numbers in the reconstructed OBI CBCTimages. The developed couch pre‐processing algorithm suppresses noise, improves CT number uniformity by a factor of 2 and removes ring and streak artifacts in the couch‐filtered reconstructedCBCTimages.
Conflict of Interest: Supported by NCI Grant P01‐CA59017.
TH‐D‐VaIB‐02: Skin and Body Dose Measurements for Varian Cone‐Beam CT (CBCT) During IMRT for Prostate Cancer33(2006); http://dx.doi.org/10.1118/1.2241903View Description Hide Description
Purpose: With the increased use of CBCT for daily patient setup, kV dose delivered to patient should be investigated. This study is to measure skin and body dose from Varian daily CBCT for prostate patients. Methods and Materials: CBCT scans were acquired in half‐fan and pulsed‐fluoro mode with a half bow‐tie mounted. A technical setting of 125kV, 80mA and 25ms was used. Skin and body doses were first measured for a Rando pelvic and an IMRT QA phantom, set centrally, with TLD and a cylindrical chamber. Then skin dose for 7 prostate patients undergoing daily CBCT was measured. To avoid the ring artifacts centered at prostate, the treatment couch was dropped 3cm from patient's tattoo. TLD capsules were placed on patient's skin at 3 sites: AP, Lt Lat and Rt Lat. Phantom measurement was also made for this setup. The absorbed dose was determined by the air‐kerma‐based calibration method recommended by TG61. Results: For phantoms set centrally, measured skin dose was ∼6 cGy, ∼5.6 cGy, ∼3.7cGy at AP, Lt Lat, and Rt Lat, respectively. Body dose at the center was ∼3–4 cGy. With table dropping (TD), only AP skin dose was increased ∼12%. Patient AP skin dose varied with separation, ranging 4–6 cGy for thicker patients (AP 23 – 33 cm) and 6 – 8 cGy for thinner patients. Minimum changes were observed on lateral dose for patients with different size. Lt Lat skin (4cGy) and bone (9cGy) doses were higher than Rt Lat skin (3cGy) and bone dose (6cGy) Conclusions: Daily CBCT provides better patient setup but it increases skin and body dose. The dose can range from 120 – 330 cGy for skin and 120 – 380 cGy for body during the 42 daily fractions delivered for IMRT prostate patients.
TH‐D‐VaIB‐03: Unified Algorithm for KV and MV Scatter and Beam‐Hardening Correction Using the Convolution‐Superposition Method33(2006); http://dx.doi.org/10.1118/1.2241904View Description Hide Description
Purpose: Quantitative cone beam CT(CBCT) is essential for advanced radiation oncology (RO) applications such as portal image‐based 3D dose reconstruction. Quantitative CT requires accurate modeling of scatter, beam‐hardening and detector response. Scatter correction methods are typically semi‐empirical in nature and are designed to reduce visible artifacts while incurring low computational cost. In contrast, Monte Carlo(MC) methods are accurate but impractically slow. Convolution‐superposition (CS) scatter models offer a good balance between accuracy and computational complexity. We show how CS can be employed to implement a unified correction method that enables quantitative kV and MV imaging.Method and Materials: (1) We perform detailed MC modeling of the kV and MV cone beam imaging systems. (2) Using MC, we generate calibration data that map intensities recorded on the flat panel imagers to water‐equivalent thicknesses (WETs). (3) The MC models are used to generate pencil beam kernels for water cylinders of varying thickness. (4) Scattergrams are generated from acquired projection images via the CS method using these kernels indexed by the WET at each pixel. (5) Scattergrams are iteratively refined using a multiplicative correction formula that ensures that the estimated primary image remains non‐negative even when scatter‐to‐primary ratios are very high. (6) The FDK reconstruction algorithm is applied directly to the thickness maps corresponding to the estimated primary images.Results: The algorithm is able to reduce maximum non‐uniformity in the reconstruction of a 16cm cylindrical homogeneous tissue equivalent phantom from 11.7% to 1.5%. When applied to a challenging 35cm × 22.5cm oblong water phantom, a non‐uniformity reduction in from 36% to 2.5% is achieved. A dataset of 200 1024×1024 projections can be processed in 25 seconds. Conclusions: CS methods can be used at both kV and MV energies to enable reconstruction of quantitative CBCTimages.Conflict of Interest: Supported by Siemens.
33(2006); http://dx.doi.org/10.1118/1.2241905View Description Hide Description
Purpose: The growing use of conformal radiotherapy techniques has motivated the development of in‐room imaging systems capable of producing a patient 3D image that can be compared with the planning CT. We report on the Megavoltage Cone‐Beam CT (MVCBCT) positioning accuracy and its first clinical use for alignment of head and neck patients. Method and Materials: Using a standard treatment unit equipped with a flat panel detector, we compared a 2D setup technique using digitally reconstructedradiographs and portal images with a 3D setup technique using a diagnostic CT and MVCBCT. A gold seed placed at isocenter was imaged over time to measure the MVCBCT absolute positioning accuracy and stability. A Rando head phantom was imaged at 23 different locations in the treatment field to measure the capability of both setup techniques to determine shifts. A total of 18 MVCBCTs and corresponding pairs of orthogonal portal images were acquired on 8 patients undergoing treatment for head and neck cancers. Results: The absolute positioning accuracy of MVCBCT was better than 1.5 mm over several weeks. The mean and standard deviations of the differences between applied and measured shifts on Rando were (0.0±0.5) and (0.0±0.9) mm for MVCBCT and portal imaging respectively. For patient images, bony anatomy and soft‐tissue was visualized on MVCBCT while only bony structures could be used for alignment on portal images. The shift measurements made with the two methods were within 2 mm of each other in 68% of cases. However, differences as large as 4 mm were observed. Conclusion: The phantom measurements indicate that portal imaging and MVCBCT have the potential to verify patient shifts with sub‐millimeter precision. MVCBCT performed on patients showed translation shifts, rotations and anatomy deformations not always appreciated using portal imaging.Conflict of Interest: Research sponsored by Siemens OCS.
33(2006); http://dx.doi.org/10.1118/1.2241906View Description Hide Description
Purpose: We report on the characteristics of Megavoltage Cone Beam Digital Tomosynthesis (MVCB DTS) and its potential clinical application for imaging of pulmonary lesions. Method and Materials: MVCB CT refers to the reconstruction of a 3D image from a set of 2D projections, acquired using a medical linear accelerator equipped with an electronic portal imaging device(EPID). A typical MVCB CT scan is acquired over a 200 degrees arc. In the case of MVCB DTS, the angular range is limited to reduce the acquisition time. This limited angular range affects the image quality of the reconstructed tomograms. To study the image quality as a function of the angular range, phantom measurements were performed and data from a head and neck patient were analyzed. The image quality was analyzed in terms of effective slice thickness, shape distortion and contrast sensitivity. MVCB DTS of the lung was performed on patients, with localized and diffuse lesions. Results: The image quality and the capability to distinguish overlaid structures decreases with decreasing angular range: a 20 degrees arc DTS results in a slice thickness of 2.7cm (vs. 1mm), a ratio of the vertical to lateral diameter of a sphere of 0.15, and a reduced contrast sensitivity. The acquisition is faster than MVCB CT. It takes 5–10 seconds for arcs of 20–40 degrees, compared to 45 seconds for a 200 degrees arc. In lungimages, the faster acquisition results in a reduced blur due to the respiratory motion. Conclusion: This study indicates some potential advantages of DTS for imaginglung patients in the treatment position. Compared with EPID, DTS provides 3D information and better soft tissue contrast. Compared with CBCT, DTS allows shorter acquisition times, compatible with breath holding.
Conflict of Interest: Research sponsored by Siemens OCS.
33(2006); http://dx.doi.org/10.1118/1.2241907View Description Hide Description
Purpose: To reconstruct 4D‐CT images using temporal re‐binning of sinogram projections from a slow‐CT scan of an object simulating respiratory motion. Methods and Materials: A slow‐CT scan of a battery‐powered motion phantom was taken using a helical tomotherapy machine. The motion phantom consisted of an elliptical disk with maximum diameter of 8.5‐cm, mounted on a spindle offset from the center of the major axis of the disk. The rotating spindle created a wobble in the disk's motion that simulated respiration. Atop the motion phantom, a CT resolution plug rested on a platform that moved vertically 1.5‐cm with a rotational period of 7 seconds. The plug was made of water equivalent material with seven holes of with diameters ranging from 2.00‐mm to 0.5‐mm. The vertical motion of the resolution plug was measured using a real‐time respiratory gating system. The CTimagesinogram obtained from the machine detector was reconstructed using a program from the manufacturer of the helical tomotherapy machine. A program was written to shuffle sinogram projections, producing reconstructed slices over a range of movement (bin). The sorted slices were compared to the original CTimages of the resolution plug. Not every bin contained a full gantry rotation of projections, and missing projections were replaced. Three replacement methods were used to obtain the best possible slice reconstructions.Results: The reconstructedCT slices showed improvement in resolution over the CTimage of the plug in motion over the entire range of bins examined. Smaller bins with fewer missing projections had similar resolution to the static CTimage of the plug. It was shown that bin enlargement showed the finest resolution of the projection replacement methods examined. Conclusions: Temporal re‐binning of slow‐CT sinogram projections can reduce motion artifacts and improve image resolution, but methods must be devised to accommodate for missing data.
TH‐D‐ValB‐07: Development of a Novel High Quantum Efficiency Flat Panel Detector for Megavoltage Cone Beam CT/DT: Construction and Evaluation of a Prototype Single‐Row Detector33(2006); http://dx.doi.org/10.1118/1.2241908View Description Hide Description
Purpose: Most electronic portal imaging devices (EPIDs) developed so far have low x‐ray absorption, i.e., low quantum efficiency (QE) of 2–4% for megavoltage x rays. A significant increase of QE is desirable for applications such as megavoltage cone‐beam CT (MV‐CBCT) and digital tomosynthesis (MV‐CBDT). Our overall goal is to develop a new generation of area detectors for MV‐CBCT/DT, with a QE an order of magnitude higher than that of current EPIDs and yet an equivalent spatial resolution. To this end, a novel direct‐conversion design of such a high QE detector was introduced recently [Pang and Rowlands, Med. Phys. 31, 3004 (2004)]. The purpose of this work is to construct and evaluate a prototype single‐row detector.Method and Materials: A prototype single‐row detector was constructed based on the novel design. It consists of a single custom‐made printed circuit board (with microsize cavities, charge collection electrodes and microstrip spacers) sandwiched between two identical tungsten plates. The detector array has 128 pixels each with dimensions of 0.45mm (width)×0.6mm (length)×22mm (height). The detector array was placed inside a sealed vessel filled with Xe gas (ionization medium) and then connected to a data acquisition board (XDAS, Electron Tubes Ltd.) for readout. Some fundamental imaging properties including QE, noise power spectrum (NPS) and detective quantum efficiency (DQE) were measured with a 6MV beam. Results: Phantom images were obtained using a dose as low as one Linac pulse. The QE of the prototype is ∼ 66% at 6MV. The DQE at zero frequency is more than an order of magnitude higher than that of current EPIDs. Conclusion: This work demonstrated the feasibility of our novel design for a high QE MV detector. Construction and evaluation of a prototype flat‐panel area detector is in progress.
Conflict of Interest: Supported by the DOD Prostate Cancer Research Program (DAMD17‐04‐1‐0276).
33(2006); http://dx.doi.org/10.1118/1.2241909View Description Hide Description
Purpose: Linac‐based kilo‐voltage x‐ray systems for image‐guidance have been widely adopted in radiation therapy. Over recent months, six cone‐beam CT enabled linacs have been installed and commissioned in our clinic. In the era of image‐guidance, a reference image of the patient anatomy and the treatment plan must be delivered to the unit for matching. This requires coordination of the simulation, planning and delivery systems. This work describes the commissioning and integration process for kilo‐voltage x‐ray image‐guidance systems in the modern radiotherapy clinic. Method and Materials: Accuracy and reproducibility of the kV source was measured using the RTI Barracuda diagnostic x‐ray meter. Image quality of the cone‐beam CT acquisition system was assessed using the CatPhan 500 multi‐slice CTimage quality phantom. Metrics here include spatial resolution, uniformity, CT# accuracy and linearity as well as scale, orientation and slice thickness. Geometric coincidence of the imaging and treatment iso‐centers was verified using a steel BB localized to the MV radiation iso‐center using a Winston‐Lutz technique. A custom‐made anthropomorphic torso phantom was CT simulated and planned in four orientations (head first supine, head first prone, feet first supine and feet first prone) and exported to each linac for image‐guided setup correction and “treatment”. This tested the DICOM connectivity, orientation, scale, structure sets and isocenter of the imported reference images as well as magnitude and direction of couch corrections. Results: The average absolute error after correction across all orientations and all platforms was (0.8±0.7, 0.6±0.7, 1.0±0.7) [mm±1S.D.] (L/R, S/I, A/P). Conclusion: A commissioning process for linacs with kilo‐voltage imaging was described. Image‐guidedradiotherapy increases precision and accuracy of the delivered treatment but it also increases the demand for integration and coordination of other systems in the modern clinic. Conflict of Interest: This work is sponsored by the Elekta Synergy Research Consortium.
- Margin Assessment and Modeling of Inter‐Fraction Motion
TU‐C‐ValB‐01: Evaluation of Clinical Margins Via Simulation of Patient Setup Errors in 27 Prostate IMRT Plans33(2006); http://dx.doi.org/10.1118/1.2241513View Description Hide Description
Purpose: To evaluate: (i) the size of random and systematic setup errors that can be absorbed by 5mm CTV‐to‐PTV margins in prostate IMRTtreatment plans; (ii) whether findings are consistent with published margin recipes; (iii) if shifting contours with respect to a static dose distribution accurately predicts dose coverage due to setup errors.
Method and Materials: 27 IMRTtreatment plans with 5mm CTV‐to‐PTV margins were utilized. Random setup errors with standard deviations (SDs) of 1.5, 3 and 5mm were simulated by fluence convolution. Systematic errors with the same SDs were simulated using two methods: (a) shifting the isocenter and recomputing dose (isocenter shift), and (b) shifting patient contours with respect to the static dose distribution (contour shift). Maximum tolerated errors were evaluated such that 90% of plans had target coverage greater than a specified minimum. Results: For coverage criteria consistent with margin formulas, plans generated with a 5mm margin were able to absorb SDs >3mm. Most structures, including the prostate CTV, showed close agreement between isocenter and contour shift methods. Exceptions were the nodal CTV and small bowel. For 3mm SDs, contour vs isocenter shift estimates for the percent of plans with acceptable dose differed by >2% for the nodal CTV, and >7% for the small bowel. Contour shift small bowel D30 values differed from isocenter shift values by >120% for some simulated shifts. Conclusion: Published recipes require margins of 8–10mm for 3mm SDs. For the IMRT cases presented here, a 5mm margin would suffice. Approximating structure doses by shifting contours with respect to a static dose distribution was acceptable for most structures, but resulted in significant errors for the nodal CTV and small bowel doses for some shifts due to proximity to high dose gradients. (Work supported by NIH R01CA98524).
TU‐C‐ValB‐02: Patient Specific Differences in Setup Error Variability and Its Effect On Treatment Margins in Fractioned Radiotherapy33(2006); http://dx.doi.org/10.1118/1.2241514View Description Hide Description
Purpose: It is often assumed that geometric error distributions in radiotherapy differ from patient to patient. It is however, problematic to substantiate this assumption because, generally, limited measurements are available per patient, giving a high uncertainty in the estimate of the standard deviation (SD). Our aim is to develop a simple method to estimate the true distribution variability based on statistical data analysis of a large patient population and to investigate the effect of detected variability on population based PTV‐margins. Method and Materials: Setup error data of 470 prostate cancer patients (11 portalimaging measurements per patient, used for off‐line corrections), were analyzed for random errors. The SD of the setup error was computed for each patient. The RMS‐values of these numbers estimate the random uncertainty in the patient population. Next, the SD of the SD per patient is computed, containing the real distribution variability diluted by “measurement error in the SD” due to the limited number of samples. To estimate the true distribution variability, a correction is applied for this “measurement error”. Finally, that margin was calculated that encloses the CTV with the 95% isodose for 90% of the population. Results: The true inter‐patient variability is 26% of the SD, found after correcting for a “measurement error” of 18% (11 samples). Inter‐patient distribution variability leads to larger PTV‐margins, partly because the range of dose blurring becomes patient dependent. Assuming normality and the same SD variability in random and systematic errors, the margin for systematic errors increases from 2.5SD to 2.8SD, maintaining the same margin for random errors. Conclusion: Inter‐patient distribution variability exists but only slightly exceeds its measurement error and it is therefore difficult to detect for individual patients. By grouping many patients, it can be detected. A variable distribution requires slightly larger margins than a homogenous one.
33(2006); http://dx.doi.org/10.1118/1.2241515View Description Hide Description
Purpose: To quantify the dosimetric impact of intrafractional motion on reduced‐margin IMRT treatments of prostate cancer.Methods and Materials:CTimages were acquired immediately before and after a daily treatment for 46 prostate cancer patients. These CT sets were registered to the bony anatomy of the patient using an in‐house 3D image registration software. To test the hypothesis that a 3‐mm isotropic target margin would adequately cover the target over the duration of the treatment, an 8‐field IMRT plan was designed on the pre‐treatment CT and subsequently copied and re‐calculated on the post‐treatment CT. For convenience of comparison, dose plans were designed to receive a full course of treatment (75.6Gy). Dosimetric impact was assessed with comparisons of prostate, seminal vesicle (SV), rectum, and bladder volumes receiving several dose levels as well as the minimum and maximum doses to 0.1cc of the prostate and SV. Anatomic variations were also quantified. Results: Over the duration of one treatment fraction (21.4+/−5.5 minutes), there were systematic reductions in the volumes of the prostate and SV receiving the prescription dose (1.8 and 7.2 % respectively, P<0.001) as well as the minimum dose to 0.1cc of their volumes (2.1 and 6.4Gy, P<0.001). Of the 46 patients, 4 patients' prostates (91%) and 8 patients' SVs (83%) did not maintain dose coverage above 70Gy. Rectal dose increased and dose to the percentage‐volume of the bladder decreased at all dose levels. Rectal volume filling was correlated with a decrease in percentage‐volume of the SV receiving 75.6, 70, and 60Gy (P<0.001, P<0.001, P=0.02). Conclusion: With a 3‐mm intrafractional margin, a considerable percent of patients will not receive full dose coverage. The rectal volume increase during a treatment fraction has significant dosimetric impact on SV dose coverage and rectal sparing. Proactive immobilization of the rectum during treatment may be warranted.
TU‐C‐ValB‐04: Margin‐Less Prostate IMRT Plans, Directly Optimized for TCP and NTCP Including Geometric Uncertainties33(2006); http://dx.doi.org/10.1118/1.2241516View Description Hide Description
Purpose: To account for geometric uncertainties without the use of margins during IMRT planning such that optimal values are obtained for the population averaged TCP and NTCP functions. Methods and materials: A new method of computing cost functions was implemented within the IMRT planning tool Hyperion. Population‐averaged values of biologic score functions (TCPpop and NTCPpop) are optimized, simulating random errors by blurring the dose, and systematic errors by displacing target and OARs relative to the dose distribution.
For 19 prostate (and seminal vesicle) patients, treatment plans for a five beam setup were created, optimising TCPpop while constraining rectum NTCPpop and the maximum dose to the target. Gaussian distributions were used for the systematic and random errors (translations only, no attempt was made to model rotations or deformations). Since geometric uncertainties were accounted for within the cost functions, no CTV to PTV margin was used. For comparison, conventional plans were created using a CTV‐to‐PTV margin (M=2.5Σ+0.7σ) and a Simultaneous Integrated Boost (SIB) technique (68Gy to the above PTV, 78Gy to PTVboost with 5mm margin, 0mm towards rectum). The resulting plans were evaluated using an independent tool that simulates the effects of geometric uncertainties. Results: Compared to conventional plans, our new technique reduced the planned dose to the rectum, while increasing the volume receiving 78Gy. We ensured that TCPpop of the new technique was not smaller than for conventional techniques. The average rectum NTCPpop values were 14% (margin recipe), 8% (SIB), and 4% (new technique), for average TCPpop values of 69%, 70%, and 71%. Conclusions: The computation of TCP and NTCP including knowledge of geometric uncertainties within the inverse IMRT optimization loop is feasible (less than 1 hour optimization time), and results in robust prostate treatment plans with an improved balance between local control and rectum toxicity.
33(2006); http://dx.doi.org/10.1118/1.2241517View Description Hide Description
Purpose: The purpose of the present work is to assess the changes in size and respiration‐induced motion of lungtumors resulting from radiation treatment.Methods and Materials: Six to ten four‐dimensional computed tomography (4‐DCT) image datasets were acquired for each of 5 stage‐III non‐small‐cell lungcancer patients who received chemoradiotherapy treatment over six weeks. Serial 4‐D datasets were obtained each week. Gross tumor volumes (GTV) were outlined on each data set. Software tools in the radiation treatment planning system were used to calculate the volumes and centroids of the GTVs on the 0% (end‐inspiration) and 50% (end‐expiration) phase for each dataset. Interfractional changes in GTV location was assessed by registering corresponding phases of the datasets based on vertebral body landmarks and determining variations in the position of the GTV centroids relative to the landmarks. Forty‐six scans including six primary tumors (involved nodal stations were not included) were analyzed. Results: The initial mean tumor volume was 53 cm3 (range: 1 to 137cm3). The interfractional changes in GTV position were predominantly in the superior‐inferior direction with a mean magnitude of 3.4mm (range: 0.1 to 9.3mm). Overall tumor regression ranged from 20–71% (0% phase) and 15–70% (50% phase). As tumors shrunk, the magnitude of intrafractional GTV motion increased in the anterior‐posterior and superior‐inferior directions while remaining constant in the right‐left direction. Reproducibility of the GTV‐centroid position at the 50% phase, based on same‐day repeat CT scans, was within 2 mm in each direction. Conclusions: Because of changes in tumor size and intrafractional tumor motion, care must be taken when reducing treatment portals based on explicit determination of the internal target volume (ITV). Repeat 4‐DCT scans might be warranted during treatment.