Purpose Cone-beam computed tomographic images (CBCTs) are increasingly used for set up correction, soft tissue targeting and image-guided adaptive radiotherapy (IGART). However, CBCT image quality is limited by low contrast and imaging artifacts. This analysis investigates the detectability of soft tissue boundaries in CBCT by performing a multiple-observer segmentation study. Material and Methods In 4 prostate cancer patients prostate, bladder and rectum were repeatedly delineated by 5 observers on CBCTs and fan beam CTs (FBCTs). A volumetric analysis of contouring variations was performed by calculating coefficients of variation (COV: standard deviation/average volume). The topographical distribution of contouring variations was analyzed using an average surface mesh-based method. Results Observer- and patient-averaged COVs for FBCT/CBCT were 0.09/0.19 for prostate, 0.05/0.08 for bladder and 0.09/0.08 for rectum. Contouring variations on FBCT were significantly smaller than on CBCT for prostate (p<0.03) and bladder (p<0.04), but not for rectum (p<0.37) (intermodality differences). Intraobserver variations from repeated contouring of the same image set were not significant for either FBCT or CBCT (p<0.05). Average standard deviations of individual observers’ contour differences from average surface meshes on FBCT versus CBCT were 1.5 versus 2.1 mm for prostate, 0.7 versus 1.4 mm for bladder, and 1.3 versus 1.5 mm for rectum. The topographical distribution of contouring variations was similar for FBCT and CBCT. Conclusion Contouring variations were larger on CBCT than FBCT, except for rectum. Given the well-documented uncertainty in soft tissue contouring in the pelvis, improvement of CBCT image quality and establishment of well-defined soft tissue identification rules are desirable for image-guided radiotherapy.

This article presents simulation studies of field shaping in rotational radiation therapy by means of two categories of beam modifying devices: protectors and shapers. The protectors used are diminished copies of the organs at risk (OARs) and stay parallel to them during gantry rotation. Thus, each protector always keeps the corresponding OAR in its shadow, significantly reducing the irradiation. The shapers are used in order to obtain a more uniform dose distribution in the planning target volume (PTV) while preserving their initial orientation during gantry rotation. Thus, the use of beam modifying devices allows modulation of the beam intensity, to better fit irradiation requirements, at every gantry position. A software tool for calculations of geometrical position and dimensions of the beam modifying devices, using information about the shape, size, and position of the protected organ or area at risk as input, was developed. This tool was integrated into the in-house-developed Monte Carlo radiation therapy simulator (MCRTS), used to simulate the particle transport through the designed system. The verification of the software tool showed good agreement between experimental and simulation data, with discrepancies of less than 3%. Dose distributions in solid-geometry and voxel-based neck models were evaluated. Furthermore, the effectiveness of the shapers to modify the dose distribution outside the protected area was studied. Results demonstrated that the use of the shapers effectively improves dose uniformity. Studies using shapers of different materials were also carried out and resulted in similar dose distributions. The results of the simulation studies with a voxel-based model showed that rotational therapy with beam modifying devices offers adequate protection of the OAR and a uniform dose distribution outside the protected region.

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Purpose: This is a proof of concept study with the objective of reconstructing the position of an HDR source in 3D in real‐time using a flat panel detector (FPD). It can potentially become the new standard in Quality Assurance (QA) for treatment delivery. Method and Materials: A matrix of markers (Ball Bearings 4mm in diameter) with precisely known locations was mounted on the cover of a flat panel detector (Acuity, Varian Inc) at variable height. Images acquired with the x‐ray source were used to calibrate the system. A plan with three dwell positions, well defined in 3D was created and delivered. Images were acquired with the FPD during the delivery of ‘treatment’. In house software was created to automatically segment and label the markers' images. A mathematical solution for the ‘near‐intersection’ of two 3D lines was implemented and used to determine the ‘true’ 3D source position. Each line was defined by the 3D positions of each marker and its projection on the FPD. A matrix with N markers will produce N*(N‐1)/2 points of intersection and their mean will result in a more accurate source position. The HDR source was placed on a 5cm solid water to mimic the patient and the FPD was placed at distances varying from 50 to 70cm. Results: The best imaging geometry was determined and images of markers obtained with the HDR source (strength of 6.2Ci) were properly segmented at all distances. During delivery, the source was located at [0,0,50], [0.5,0,50] and [2.0, 0, 50]. The reconstructed positions were [0,0,50.130], [0.497,−0.008,50.106] and [1.984, −0.005,50.053] with a standard deviation of [0.027,0.019,0.115]cm. When intersecting lines in 3D, the mean shortest distance between any two lines was 0.025cm with standard deviation 0.016cm. Conclusion: We proved that the accuracy of source position detection in 3D using a FPD is sub‐millimeter.

We present calculations based on the R-matrix method for electron impact excitation of Ne-like selenium. In these elaborate calculations, the 27 fine-structure levels arising from the four lowest configurations 2s'2p6, 2s22p53s, 2s'2p53p and 2s'2p53d are included. Terms of the Breit-Pauli Hamiltonian are explicitly included in determining both the target and the scattering wavefunction. All target levels included in the calculations are represented by configuration interaction wavefunctions using the Is, 2s, 2p, 3s, 3p and 3d orbitals. The collision strengths are compared with previous theoretical results. For many transitions we found complicated resonance structures in the excitation cross sections. The excitation cross sections are integrated over a Maxwellian distribution of electron energies to give electron excitation rate coefficients; the rates for transitions from the ground state to each fine-structure level are tabulated and are compared with results from distortedwave calculations. At low temperatures ( T , = 500 eV) some of our rates are six times larger than earlier calculations. We also present estimates of the relative populations of the (2p53p)'D2 and IS, levels of Se xxv at T, = 1000 eV and at various electron densities. R-matrix calculationof the functions: K PMl( r ) = Cirpt exp( -&) i = l electron impact excitation 3291