Localization of deformable tumors from short-arc projections using Bayesian estimation.

PURPOSE: The authors present a stochastic framework for radiotherapy patient positioning directly utilizing radiographic projections. This framework is developed to be robust against anatomical nonrigid deformations and to cope with challenging imaging scenarios, involving only a few cone beam CT projections from short arcs. METHODS: Specifically, a Bayesian estimator (BE) is explicitly derived for the given scanning geometry. This estimator is compared to reference methods such as chamfer matching (CM) and the minimization of the median absolute error adapted as tools of robust image processing and statistics. In order to show the performance of the stochastic short-arc patient positioning method, a CIRS IMRT thorax phantom study is presented with movable markers and the utilization of an Elekta Synergy(®) XVI system. Furthermore, a clinical prostate CBCT scan of a Varian(®) On-Board Imager(®) system is utilized to investigate the robustness of the method for large variations of image quality (anterior-posterior vs lateral views). RESULTS: The results show that the BE shifts reduce the initial setup error of up to 3 cm down to 3 mm at maximum for an imaging arc as short as 10° while CM achieves residual errors of 7 mm at maximum only for arcs longer than 40°. Furthermore, the BE can compensate robustly for low image qualities using several low quality projections simultaneously. CONCLUSIONS: In conclusion, an estimation method for marker-based patient positioning for short imaging arcs is presented and shown to be robust and accurate for deformable anatomies.

Stochastic formulation of patient positioning using linac-mounted cone beam imaging with prior knowledge.

PURPOSE: In this work, a novel stochastic framework for patient positioning based on linac-mounted CB projections is introduced. Based on this formulation, the most probable shifts and rotations of the patient are estimated, incorporating interfractional deformations of patient anatomy and other uncertainties associated with patient setup. METHODS: The target position is assumed to be defined by and is stochastically determined from positions of various features such as anatomical landmarks or markers in CB projections, i.e., radiographs acquired with a CB-CT system. The patient positioning problem of finding the target location from CB projections is posed as an inverse problem with prior knowledge and is solved using a Bayesian maximum a posteriori (MAP) approach. The prior knowledge is three-fold and includes the accuracy of an initial patient setup (such as in-room laser and skin marks), the plasticity of the body (relative shifts between target and features), and the feature detection error in CB projections (which may vary depending on specific detection algorithm and feature type). For this purpose, MAP estimators are derived and a procedure of using them in clinical practice is outlined. Furthermore, a rule of thumb is theoretically derived, relating basic parameters of the prior knowledge (initial setup accuracy, plasticity of the body, and number of features) and the parameters of CB data acquisition (number of projections and accuracy of feature detection) to the expected estimation accuracy. RESULTS: MAP estimation can be applied to arbitrary features and detection algorithms. However, to experimentally demonstrate its applicability and to perform the validation of the algorithm, a water-equivalent, deformable phantom with features represented by six 1 mm chrome balls were utilized. These features were detected in the cone beam projections (XVI, Elekta Synergy) by a local threshold method for demonstration purposes only. The accuracy of estimation (strongly varying for different plasticity parameters of the body) agreed with the rule of thumb formula. Moreover, based on this rule of thumb formula, about 20 projections for 6 detectable features seem to be sufficient for a target estimation accuracy of 0.2 cm, even for relatively large feature detection errors with standard deviation of 0.5 cm and spatial displacements of the features with standard deviation of 0.5 cm. CONCLUSIONS: The authors have introduced a general MAP-based patient setup algorithm accounting for different sources of uncertainties, which are utilized as the prior knowledge in a transparent way. This new framework can be further utilized for different clinical sites, as well as theoretical developments in the field of patient positioning for radiotherapy.

Dosimetric consequences of a translational isocenter correction based on image guidance for intensity modulated radiotherapy (IMRT) of the prostate.

Interfractional prostate motion during radiotherapy can have deleterious clinical consequences. It has become clinical practice to re-position the patient according to ultrasound or other imaging techniques. We investigated the dosimetric consequences of the linear translational position correction (isocenter correction) when a conformal IMRT technique with nine fields was used. Treatment plans of seven patients with empty and distended rectums were analyzed. The reference plans were calculated on the CT with an empty rectum. The treatment plans were transferred to a second CT with a distended rectum for an uncorrected setup of the patient referenced to bony anatomy and a corrected setup after translational position correction of the isocenter. The dosimetric consequences (with and without correction) were analyzed. For single treatment fractions, organ motion decreased the volume of the prostate encompassed by the 95% isodose (V95%) by up to -24%-p (percentage points). The mean rectum dose increased by up to 41%-p. Linear translational correction increased V95% of the prostate by up to 17%-p while the mean rectum dose was reduced by up to -23%-p compared to the uncorrected setup. Linear translational correction can improve radiation treatment accuracy for prostate cancer if geometrical changes are within certain limits.

IMRT for breast. a planning study.

BACKGROUND AND PURPOSE: To evaluate the performance of ten different treatment-planning systems when intensity modulated (IMRT) plans are designed for breast treatments that include the irradiation of the internal mammary chain. PATIENTS AND METHODS: A dataset of five patients (CT images and volumes of interest) was distributed to design IMRT plans on the ten systems. To minimise biases, the same geometry and clinical planning aims were imposed on the individual plans. Results were analysed in terms of dose distributions and dose volume histograms. RESULTS AND CONCLUSIONS: For target coverage, the volume receiving more than 95% of the prescribed dose ranged from 77% (OTP) to 91% (Eclipse and Pinnacle), the volume receiving more than 107% ranged from 3.3% (Hyperion) to 23.2% (OTP). The mean dose to ipsilateral lung ranged from 13 Gy (Eclipse) to 18 Gy (OTP). The volume of the contralateral breast receiving more than 10 Gy ranged from 3% (Pinnacle) to 26% (Precise). The volume of heart receiving more than 20 Gy ranged from 7% (Eclipse) to 47% (Precise), the maximum significant dose to heart ranged from approximately 27 Gy (XiO) to approximately 49 Gy (Precise). The maximum significant dose to healthy tissue ranged from approximately 51 Gy (Eclipse) to approximately 62 Gy (OTP). It was also possible to show that the treatment geometry proposed here enables to minimise contralateral breast irradiation while keeping minimal ipsilateral lung (or heart) involvement and satisfactory target coverage.

[Intensity modulated radiotherapy (IMRT) of head and neck tumors. Increased biological effectiveness in high-risk situations by "integrated boost" therapy]. / Intensitätsmodulierte Bestrahlung (IMRT) von Kopf-Hals-Tumoren. Höhere biologische Wirksamkeit in Risikosituationen durch "integrierten Boost"

Primary tumors of the paranasal sinuses are rare entities which, because of precarious localization and frequently diffuse propagation into neighbouring cavities and the skull base, pose a significant therapeutic problem. Even after complete surgical resection, local relapses are frequent. Postoperative radiotherapy is therefore usually indicated. Intensity modulated radiotherapy (IMRT) is a new technique that helps creating dose distributions that conform closely to the target volume while maximally sparing the organs at risk. This results in the possibility of applying escalated doses to the target while still keeping the incidence of side effects low. What is especially appealing is the possibility of shaping the dose distribution within the target in such a way that areas with a presumably high tumor cell load receive increased doses, a concept which is best described by the term "integrated boost". We present the case of a patient with a sinunasal carcinoma and describe the implications of the clinical implementation of this technique.

[Optimizing the use of radiotherapy with IMRT and image guided location of advanced prostate cancer]. / Optimierter Einsatz der Strahlentherapie durch IMRT und Präzisionslokalisationsverfahren bei der Behandlung des fortgeschrittenen Prostatakarzinoms.

Intensity modulated radiotherapy (IMRT) combined with recently developed noninvasive image-guided targeting techniques for tumor localization/repositioning provide a means to further improve on conformal radiotherapy of prostate cancer by optimally sparing the rectum. This refined approach may potentially improve treatment results for locally advanced prostate cancer while reducing side effects. This review summarizes the clinical requirements for effective prostate radiotherapy and describes the new technology that helps to better fulfil these requirements. These noninvasive developments, their potential benefit as well as their limitations, together with new data on fractionation sensitivity of prostate cancer that may lead to shortened overall treatment times may be of interest for all physicians treating patients with prostate cancer.

Refractive-index measurement of gradient-index microlenses by diffraction tomography.

Diffraction tomography is applied to reconstruction of the gradient-index distribution of planar gradient-index microlenses, fabricated by thermal ion exchange. Measurements of the single-phase projections are performed by phase-shifting interferometry For reconstruction, the Rytov approximation for smooth inhomogeneities is applied. Results are compared with measurement results from other methods and simulation results.