Six dimensional analysis with daily stereoscopic x-ray imaging of intrafraction patient motion in head and neck treatments using five points fixation masks

The safety margins used to define the Planning Target Volume (PTV) should reflect the accuracy of the target localization during treatment that comprises both the reproducibility of the patient positioning and the positional uncertainty of the target, so both the inter- and intrafraction motion of the target. Our first aim in this study was to determine the intrafraction motion of patients immobilized with a five-point thermoplastic mask for head and neck treatments. The five-point masks have the advantage that the patient's shoulders as well as the cranial part of the patient's head is covered with the thermoplastic material that improves the overall immobilization of the head and neck region of the patient. Thirteen patients were consecutively assigned to use a five-point thermoplastic mask. The patients were positioned by tracking of infrared markers (IR) fixed to the immobilization device and stereoscopic x-ray images were used for daily on-line setup verification. Repositioning was carried out prior to treatment as needed; rotations were not corrected. Movements during treatment were monitored by real-time IR tracking. Intrafraction motion and rotation was supplementary assessed by a six-degree-of-freedom (6-D) fusion of x-ray images, taken before and after all 385 treatments, with DRR images generated from the planning CT data. The latter evaluates the movement of the patient within the thermoplastic mask independent from the mask movement, where IR tracking evaluates the movement of the mask caused by patient movement in the mask. These two movements are not necessarily equal to each other. The maximum intrafraction movement detected by IR tracking showed a shift [mean (SD; range)] of -0.1(0.7; 6.0), 0.1(0.6; 3.6), -0.2(0.8;5.5) mm in the vertical, longitudinal, and lateral direction, respectively, and rotations of 0.0(0.2; 1.6), 0.0(0.2; 1.7) and 0.2(0.2; 2.4) degrees about the vertical, longitudinal, and lateral axis, respectively. The standard deviations and ranges found with the 6-D fusion demonstrate intrafraction patient displacements of -0.5(1.2; 7.4), 0.3(0.7; 5.3), 0.0(0.7; 5.7) mm in the vertical, longitudinal, and lateral direction, respectively, and rotations of -0.1(0.6; 4.1), 0.1(0.7; 8.3) and -0.2(0.8; 8.2) degrees about the vertical, longitudinal, and lateral axis, respectively. The 6-D fusions are considerably larger (p < 0.05) than detected by IR tracking. This indicates that the external marker tracking underestimates the magnitude of the actual intrafraction motion and rotation of the patient. The intrafraction motion detected for the patients immobilized with a conventional thermoplastic mask was relatively large. The feasibility to reduce this intrafraction movement by the application of alternative five-point thermoplastic mask types was evaluated as a second aim of this study. The preliminary results showed a clear reduction in the range, being an indication for the random movements, of both the intrafraction shift and rotation for both alternative mask types. The 6-D fusion is found a useful tool for a fast evaluation of the actual patient's intrafraction shift and rotation and shows the latter is not negligible and needs to be taken into account additional to the initial setup accuracy when determining the PTV margin. © 2006 American Association of Physicists in Medicine.SCOPUS: ar.jFLWNAinfo:eu-repo/semantics/publishe

Dosimetric evaluation of partially overlapping intensity modulated beams using dynamic mini-multileaf collimation

The dose distribution resulting from partially overlapping intensity modulated beams (IMBs) assigned to different isocenters for the treatment of the same planning target volume (PTV) was evaluated. These partially overlapping IMBs are used in static intensity modulated radiation therapy (IMRT) treatments with the Novalis® system using the mini-MultiLeaf Collimator (mini-MLC) in Dynamic MultiLeaf Collimation (DMLC) mode. The resultant dose distribution was verified dosimetrically for a cylindrical target defined in a homogeneous cubic phantom. The phantom positioning can introduce dose nonuniformities in the resultant dose distribution by nonperfect positioning of the isocenters in accordance with each other. The dose inhomogeneities are quantified mathematically by summation of the dose profiles of the used IMBs and experimentally by measurement of the resulting dose profiles with radiographic film and thermoluminescent detectors (TLD). The mathematical estimation of the resulting dose profile of the treatment with a perfect positioning of the isocenters showed a good agreement with the planned dose profile. The magnitude of the maximum dose inhomogeneities introduced by the simulated supplementary shifts between the isocenters decreases by - 8.54% mm-1 as the shift changes from -0.30±0.10 cm to +0.30±0.10 cm. The TLD measurements showed a similar variation of the magnitude of the maximum dose inhomogeneities: - 8.77% mm-1. The amount of dose variation was underestimated with the radiographic film measurements, which showed a variation of - 7.17% mm-1. The film measurements demonstrated that the magnitude of the introduced maximum dose inhomogeneities did not alter significantly throughout the PTV. The approach of using partially overlapping IMBs assigned to different isocenters to enlarge the treatment region introduces smaller dose inhomogeneities in the resultant dose distribution than when abutting treatment fields are used. The resultant dose distribution of this treatment technique is less sensitive to positioning errors of the used treatment isocenters. © 2003 American Association of Physicists in Medicine.SCOPUS: ar.jFLWNAinfo:eu-repo/semantics/publishe

An assessment of the use of skin flashes in helical tomotherapy using phantom and in-vivo dosimetry

Background and purpose: In helical tomotherapy the nature of the optimizing and planning systems allows the delivery of dose on the skin using a build-up compensating technique (skin flash). However, positioning errors or changes in the patient's contour can influence the correct dosage in these regions. This work studies the behavior of skin-flash regions using phantom and in-vivo dosimetry. Materials and methods: The dosimetric accuracy of the tomotherapy planning system in skin-flash regions is checked using film and TLD on phantom. Positioning errors are induced and the effect on the skin dose is investigated. Further a volume decrease is simulated using bolus material and the results are compared. Results: Results show that the tomotherapy planning system calculates dose on skin regions within 2 SD using TLD measurements. Film measurements show drops of dose of 2.8% and 26% for, respectively, a 5 mm and 10 mm mispositioning of the phantom towards air and a dose increase of 9% for a 5 mm shift towards tissue. These measurements are confirmed by TLD measurements. A simulated volume reduction shows a similar behavior with a 2.6% and 19.4% drop in dose, measured with TLDs. Conclusion: The tomotherapy system allows adequate planning and delivery of dose using skin flashes. However, exact positioning is crucial to deliver the dose at the exact location. © 2007 Elsevier Ireland Ltd. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Characteristics and clinical application of a treatment simulator with Ct-option

Background and purpose: The integration of a scanner for computed tomography (CT) and a treatment simulator (Sim-CT, Elekta Oncology Systems, Crawley, UK) has been studied in a clinical situation. Image quality, hounsfield units (HU) and linearity have been evaluated as well as the implications for treatment planning. The additional dose to the patient has also been highlighted. Material and methods: Image data is acquired using an array of solid state X-ray detectors attached externally to the simulator's image intensifier. Three different fields of view (FOV: 25.0 cm, 35.0 cm and 50.0 cm) with 0.2 cm, 0.5 cm and 1.0 cm slice thickness can be selected and the system allows for an aperture diameter of 92.0 cm at standard isocentric height. The CT performance has been characterized with several criteria: spatial resolution, contrast sensitivity, geometric accuracy, reliability of hounsfield units and the radiation output level. The spatial resolution gauge of the nuclear associates quality phantom (NAQP) as well as modulation transfer functions (MTF) have been applied to evaluate the spatial resolution. Contrast sensitivity and HU measurements have been performed by means of the NAQP and a HU conversion phantom that allows inserts with different electron densities. The computed tomography dose index (CTDI) of the CT-option has been monitored with a pencil shaped ionization chamber. Treatment planning and dose calculations for heterogeneity correction based on the Sim-CT images generated from an anthropomorphic phantom as well as from ten patients have been compared with similar treatment plans based on identical, yet diagnostic CT (DCT) images. Results: The last row of holes that are resolved in the spatial resolution gauge of the NAQP are either 0.150 cm or 0.175 cm depending on the FOV and the applied reconstruction filter. These are consistent with the MTF curves showing cut-off frequencies ranging from 5.3 lp/cm to 7.1 lp/cm. Linear regression analysis of HU versus electron densities revealed a correlation coefficient of 0.99. Contrast, pixel size and geometric accuracy are within specifications. Computed tomography dose index values of 0.204 Gy/As and 0.069 Gy/As have been observed with dose measurements in the center of a 16 cm diameter and 32 cm diameter phantom, respectively for large FOV. Small FOV yields CTDI values of 0.925 Gy/As and 0.358 Gy/As which is a factor ten higher than the results obtained from a DCT under similar acquisition conditions. The phantom studies showed excellent agreement between dose distributions generated with the Sim- CT and DCT HU. The deviations between the calculated settings of monitor units as well as the maximum dose in three dimensions were less than 1% for the treatment plans based on either of these HU both for pelvic as well as thoracic simulations. The patient studies confirmed these results. Conclusions: The CT-option can be considered as an added value to the simulation process and the images acquired on the Sim-CT system are adequate for dose calculation with tissue heterogeneity correction. The good image quality, however, is compromised by the relative high dose values to the patient. The considerable load to the conventional X-ray tube currently limits the Sim-CT to seven image acquisitions per patient and therefore the system is limited in its capability to perform full three-dimensional reconstruction.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Assessment of the uncertainties in dose delivery of a commercial system for linac-based stereotactic radiosurgery

Purpose: Linac-based stereotactic radiosurgery (SRS) was introduced in our department in 1992, and since then more than 200 patients have been treated with this method. An in-house-developed algorithm for target localization and dose calculation has recently been replaced with a commercially available system. In this study both systems have been compared, and positional accuracy, as well as dose calculation, have been verified experimentally. Methods and Materials: The in-house-developed software for target localization and dose calculation is an extension to George Sherouse's GRATIS® software for radiotherapy treatment planning, and has been replaced by a commercial (BrainSCAN version 3.1; BrainLAB, Germany) treatment planning system (TPS) for SRS. The positional accuracy for the entire SRS procedure (from image acquisition to treatment) has been investigated by treatment of simulated targets in the form of 0.2-cm lead beads inserted into an anthropomorphic phantom. Both dose calculation algorithms have been verified against manual calculations (based on basic beam data and CT data from phantom and patients), and measurements with the anthropomorphic phantom applying ionization chamber, thermoluminescent detectors, and radiographic film. This analysis has been performed on a variety experimental situations, starting with static beams and simple one-arc treatments, to more complex and clinical relevant applications. Finally, 11 patients have been evaluated with both TPS in parallel for comparison an continuity of clinical experience. Results: Phantom studies evaluating the entire SRS procedure have shown that a target, localized by CT, can be irradiated with a positional accuracy of 0.08 cm in any direction with 95% confidence. Neglecting the influencing of dose perturbation when the beam passes through bone tissue or air cavities, the calculated dose values obtained from both TPSs agreed within 1% (SD 1%) for phantom and patient studies. The application of a one dimensional path length correction for tissue heterogeneity influences the treatment prescription 4% on average (SD 1%), which is in compliance with theoretical predictions. The phantom measurements confirmed the predicted dose at isocenter within uncertainty for the different treatment schedules in this study. Conclusion: The full SRS procedure applied to an anthropomorphic phantom has been used as a comprehensive method to assess the uncertainties involved in dose delivery and target positioning. The results obtained with both TPSs are in agreement with AAPM Report 54, TG 42 and clinical continuity is assured. However, the use of one-dimensional path length correction will result in an increase of 4% in dose prescription, which is slightly more than that predicted in the literature.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Definition of gross tumor volume in lung cancer: inter-observer variability

BACKGROUND AND PURPOSE: To determine the inter-observer variation in gross tumor volume (GTV) definition in lung cancer, and its clinical relevance. MATERIALS AND METHODS: Five clinicians involved in lung cancer were asked to define GTV on the planning CT scan of eight patients. Resulting GTVs were compared on the base of geometric volume, dimensions and extensions. Judgement of invasion of lymph node (LN) regions was evaluated using the ATS/LCSG classification of LN. Clinical relevance of the variation was studied through 3D-dosimetry of standard conformal plans: volume of critical organs (heart, lungs, esophagus, spinal cord) irradiated at toxic doses, 95% isodose volumes of GTVs, normal tissue complication probabilities (NTCP) and tumor control probabilities (TCP) were compared for evaluation of observer variability. RESULTS: Before evaluation of observer variability, critical review of planning CT scan led to up- (two cases) and downstaging (one case) of patients as compared to the respective diagnostic scans. The defined GTVs showed an inter-observer variation with a ratio up to more than 7 between maximum and minimum geometric content. The dimensions of the primary tumor had inter-observer ranges of 4.2 (transversal), 7.9 (cranio-caudal) and 5.4 (antero-posterior) cm. Extreme extensions of the GTVs (left, right, cranial, caudal, anterior and posterior) varied with ranges of 2.8-7.3 cm due to inter-observer variation. After common review, only 63% of involved lymph node regions were delineated by the clinicians (i.e. 37% are false negative). Twenty-two percent of drawn in lymph node regions were accepted to be false positive after review. In the conformal plans, inter-observer ranges of irradiated normal tissue volume were on average 12%, with a maximum of 66%. The probability (in the population of all conformal plans) of irradiating at least 95% of the GTV with at least 95% of the nominal treatment dose decreased from 96 to 88% when swapping the matched GTV with an unmatched one. The average (over all patients) inter-observer range in NTCP varied from 5% (spinal cord) to 20% (ipsilateral lung), whereas the maximal ranges amounted 16% (spinal cord) to 45% (heart). The average TCP amounted 51% with an average range of 2% (maximally 5%) in case of matched GTVs. These values shifted to 42% (average TCP) with an average range of 14% (maximally 31%) when defining unmatched GTVs. Four groups of causes are suggested for the large inter-observer variation: (1) problems of methodology; (2) impossible differentiation between pathologic structures and tumor; or (3) between normal structures and tumor, and (4); lack of knowledge. Only the minority of these can be resolved objectively. For most of the causal factors agreements have to be made between clinicians, intra- and inter-departmentally. Some of the factors will never be unequivocally solved. CONCLUSIONS: GTV definition in lung cancer is one of the cornerstones in quality assurance of radiotherapy. The large inter-observer variation in GTV definition jeopardizes comparison between clinicians, institutes and treatment

Use of a simulator with CT option in radiotherapy of macular degeneration

Purpose: To assess the accuracy of a conventional simulation procedure in radiotherapy of age-related macular degeneration. Methods and Materials: A computed tomographic (CT) extension attached to the treatment simulator was used to acquire CT images immediately after conventional simulation in 18 patients referred for treatment of age-related macular degeneration. Analysis was performed on 16 one-sided treatment cases for whom images were obtained. Error was estimated by the displacement between the observed treatment isocenter and the intended isocenter based on reconstructed eye geometry. Results: Based on single slice measurements, the mean error amplitude was 2.3 mm (range 0.2-5.6). Based on three-dimensional eye globe reconstruction, the mean error amplitude was 2.8 mm (range 0.8-5.3). An incidental finding previously unreported was the lower image quality at the center of the simulator-CT image acquisition field. Conclusions: Small but significant errors from conventional simulation were noted. The integrated simulation-CT procedure may help correct the errors to improve the accuracy of simulation setup. The lower image quality at the center of image acquisition field requires adaptation of the simulation-CT procedure.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Urethra-sparing stereotactic body radiotherapy for prostate cancer: how much can the rectal wall dose be reduced with or without an endorectal balloon?

Abstract Background This is a dosimetric comparative study intended to establish appropriate low-to-intermediate dose-constraints for the rectal wall (Rwall) in the context of a randomized phase-II trial on urethra-sparing stereotactic body radiotherapy (SBRT) for prostate cancer. The effect of plan optimization on low-to-intermediate Rwall dose and the potential benefit of an endorectal balloon (ERB) are investigated. Methods Ten prostate cancer patients, simulated with and without an ERB, were planned to receive 36.25Gy (7.25Gyx5) to the planning treatment volume (PTV) and 32.5Gy to the urethral planning risk volume (uPRV). Reference plans with and without the ERB, optimized with respect to PTV and uPRV coverage objectives and the organs at risk dose constraints, were further optimized using a standardized stepwise approach to push down dose constraints to the Rwall in the low to intermediate range in five sequential steps to obtain paired plans with and without ERB (Vm1 to Vm5). Homogeneity index for the PTV and the uPRV, and the Dice similarity coefficient (DSC) for the PTV were analyzed. Dosimetric parameters for Rwall including the median dose and the dose received by 10 to 60% of the Rwall, bladder wall (Bwall) and femoral heads (FHeads) were compared. The monitor units (MU) per plan were recorded. Results Vm4 reduced by half D30%, D40%, D50%, and Dmed for Rwall and decreased by a third D60% while HIPTV, HIuPRV and DSC remained stable with and without ERB compared to Vmref. HIPTV worsened at Vm5 both with and without ERB. No statistical differences were observed between paired plans on Rwall, Bwall except a higher D2% for Fheads with and without an ERB. Conclusions Further optimization to the Rwall in the context of urethra sparing prostate SBRT is feasible without compromising the dose homogeneity to the target. Independent of the use or not of an ERB, low-to-intermediate doses to the Rwall can be significantly reduced using a four-step sequential optimization approach

Accurate assessment of LV function using the first automated 2D-border detection algorithm for small animals - evaluation and application to models of LV dysfunction

Abstract Echocardiography is the most commonly applied technique for non-invasive assessment of cardiac function in small animals. Manual tracing of endocardial borders is time consuming and varies with operator experience. Therefore, we aimed to evaluate a novel automated two-dimensional software algorithm (Auto2DE) for small animals and compare it to the standard use of manual 2D-echocardiographic assessment (2DE). We hypothesized that novel Auto2DE will provide rapid and robust data sets, which are in agreement with manually assessed data of animals. 2DE and Auto2DE were carried out using a high-resolution imaging-system for small animals. First, validation cohorts of mouse and rat cine loops were used to compare Auto2DE against 2DE. These data were stratified for image quality by a blinded expert in small animal imaging. Second, we evaluated 2DE and Auto2DE in four mouse models and four rat models with different cardiac pathologies. Automated assessment of LV function by 2DE was faster than conventional 2DE analysis and independent of operator experience levels. The accuracy of Auto2DE-assessed data in healthy mice was dependent on cine loop quality, with excellent agreement between Auto2DE and 2DE in cine loops with adequate quality. Auto2DE allowed for valid detection of impaired cardiac function in animal models with pronounced cardiac phenotypes, but yielded poor performance in diabetic animal models independent of image quality. Auto2DE represents a novel automated analysis tool for rapid assessment of LV function, which is suitable for data acquisition in studies with good and very good echocardiographic image quality, but presents systematic problems in specific pathologies