A new Monte Carlo model of a Cyberknife® system for the precise determination of out-of-field doses

In a previous work, a PENELOPE Monte Carlo model of a Cyberknife system equipped with fixed collimator was developed and validated for in-field dose evaluation. The aim of this work is to extend it to evaluate peripheral doses and to determine the precision of the treatment planning system (TPS) Multiplan in evaluating the off-axis doses. The Cyberknife^® head model was completed with surrounding components based on manufacturer drawings. The contribution of the different head parts on the out-of-field dose was studied. To model the attenuation and the modification of particle energy caused by components not modelled, the photon transport was modified in one of the added components. The model was iteratively adjusted to fit dose profiles measured with EBT3 films and an ionization chamber for several collimator sizes. Finally, dose profiles were calculated using the two Multiplan TPS algorithms and were compared to our simulations. The contributions to out-of-field dose were identified as scattered radiation from the phantom and head leakage and scatter originating at the secondary collimator level. Particle transport in the additional pieces was modified to model this radiation. The maximum differences between simulated and measured doses are of 20.4%. Regarding the detector responses away from axis, EBT3 films and the Farmer chamber give similar response (less than 20% difference). The TPS Monte Carlo algorithm underestimates the doses away from axis more importantly for the smaller field sizes (up to 98%). Besides, RayTracing simplifies peripheral dose to a constant value with no inclusion of particle transport. A Monte Carlo model of a Cyberknife system for the determination of out-of-field doses up to 14 cm off-axis was successfully developed and validated for different depths and field sizes in comparison with measurements. This study also confirms that TPS algorithms do not model peripheral dose properly.

Study of dental prostheses influence in radiation therapy

Dental prostheses made of high density material contribute to modify dose distribution in head and neck cancer treatment. Our objective is to quantify dose perturbation due to high density inhomogeneity with experimental measurements and Monte Carlo simulations. Firstly, measurements were carried in a phantom representing a human jaw with thermoluminescent detectors (GR200A) and EBT2 Gafchromic films in the vicinity of three samples: a healthy tooth, a tooth with amalgam and a Ni-Cr crown, irradiated in clinical configuration. Secondly, Monte Carlo simulations (BEAMnrc code) were assessed in an identical configuration. Experimental measurements and simulation results confirm the two well-known phenomena: firstly the passage from a low density medium to a high density medium induces backscattered electrons causing a dose increase at the interface, and secondly, the passage from a high density medium to a low density medium creates a dose decrease near the interface. So, the results show a 1.4% and 23.8% backscatter dose rise and attenuation after sample of 26.7% and 10.9% respectively for tooth with amalgam and crown compared to the healthy tooth. Although a tooth with amalgam has a density of about 12-13, the changes generated are not significant. However, the results for crown (density of 8) are very significant and the discordance observed may be due to calculation point size difference 0.8 mm and 0.25 mm respectively for TLD and Monte Carlo. The use of Monte Carlo simulations and experimental measurements provides objective evidence to evaluate treatment planning system results with metal dental prostheses.

[Simulation of lung motions using an artificial neural network]

PURPOSE

A way to improve the accuracy of lung radiotherapy for a patient is to get a better understanding of its lung motion. Indeed, thanks to this knowledge it becomes possible to follow the displacements of the clinical target volume (CTV) induced by the lung breathing. This paper presents a feasibility study of an original method to simulate the positions of points in patient's lung at all breathing phases.

PATIENTS AND METHODS

This method, based on an artificial neural network, allowed learning the lung motion on real cases and then to simulate it for new patients for which only the beginning and the end breathing data are known. The neural network learning set is made up of more than 600 points. These points, shared out on three patients and gathered on a specific lung area, were plotted by a MD.

RESULTS

The first results are promising: an average accuracy of 1mm is obtained for a spatial resolution of 1 × 1 × 2.5mm(3).

CONCLUSION

We have demonstrated that it is possible to simulate lung motion with accuracy using an artificial neural network. As future work we plan to improve the accuracy of our method with the addition of new patient data and a coverage of the whole lungs.

[Dosimetric influence of hip prosthesis during radiotherapeutic treatement]

As the population become aged, many patients with hip prosthesis are treated for a pelvic cancer. The recommended ballistic must avoid to pass in the prosthesis, but sometimes it is inevitable. So it is essential to quantify with accuracy the dose modifications linked to the presence of metallic implant. The aim of this study is to analyze by Monte Carlo method these modifications in simple and complex models (anthropomorphic phantom) which take into account the geometry and the composition of the prosthesis and its coatings. Then, this methodology was used to study the behaviour of a treatment planning system in theses extreme conditions.

Calculations of dose distributions using a neural network model

The main goal of external beam radiotherapy is the treatment of tumours, while sparing, as much as possible, surrounding healthy tissues. In order to master and optimize the dose distribution within the patient, dosimetric planning has to be carried out. Thus, for determining the most accurate dose distribution during treatment planning, a compromise must be found between the precision and the speed of calculation. Current techniques, using analytic methods, models and databases, are rapid but lack precision. Enhanced precision can be achieved by using calculation codes based, for example, on Monte Carlo methods. However, in spite of all efforts to optimize speed (methods and computer improvements), Monte Carlo based methods remain painfully slow. A newer way to handle all of these problems is to use a new approach in dosimetric calculation by employing neural networks. Neural networks (Wu and Zhu 2000 Phys. Med. Biol. 45 913-22) provide the advantages of those various approaches while avoiding their main inconveniences, i.e., time-consumption calculations. This permits us to obtain quick and accurate results during clinical treatment planning. Currently, results obtained for a single depth-dose calculation using a Monte Carlo based code (such as BEAM (Rogers et al 2003 NRCC Report PIRS-0509(A) rev G)) require hours of computing. By contrast, the practical use of neural networks (Mathieu et al 2003 Proceedings Journees Scientifiques Francophones, SFRP) provides almost instant results and quite low errors (less than 2%) for a two-dimensional dosimetric map.

Comparative genomic hybridization reveals frequent chromosome 13q and 4q losses in renal carcinomas with sarcomatoid transformation

Renal cell carcinomas (RCCs) with sarcomatoid transformation show the most malignant behaviour of all renal carcinoma types. In this study, comparative genomic hybridization was used to screen for losses and gains of DNA sequences along all chromosome arms in 12 sarcomatoid (S) RCCs. On average, there were 8.6 aberrations per tumour. DNA sequence losses (5.2 +/- 4.4) were slightly more frequent than gains (3.4 +/- 2.6). DNA gains most often involved chromosomes 17 (33 per cent), 7, and 8q (25 per cent each). High-level co-amplification involving 11q22-23 and 7p21-22 in one SRCC was not present in adjacent non-sarcomatous tumour areas, raising the possibility of oncogene involvement at these loci for sarcomatoid transformation. DNA losses were most prevalent at 13q (75 per cent) and 4q (50 per cent), suggesting that inactivation of tumour suppressor genes at chromosomes 13q and 4q may be linked to sarcomatoid growth of RCC. It is concluded that SRCCs are genetically highly complex. Chromosomes 13q, 4q, 7p21-22, and 11q22-23 may carry genes with relevance for sarcomatoid growth in RCC.

Solution structure of the antitermination protein NusB of Escherichia coli: a novel all-helical fold for an RNA-binding protein

The NusB protein of Escherichia coli is involved in the regulation of rRNA biosynthesis by transcriptional antitermination. In cooperation with several other proteins, it binds to a dodecamer motif designated rrn boxA on the nascent rRNA. The antitermination proteins of E.coli are recruited in the replication cycle of bacteriophage lambda, where they play an important role in switching from the lysogenic to the lytic cycle. Multidimensional heteronuclear NMR experiments were performed with recombinant NusB protein labelled with 13C, 15N and 2H. The three-dimensional structure of the protein was solved from 1926 NMR-derived distances and 80 torsion angle restraints. The protein folds into an alpha/alpha-helical topology consisting of six helices; the arginine-rich N-terminus appears to be disordered. Complexation of the protein with an RNA dodecamer equivalent to the rrn boxA site results in chemical shift changes of numerous amide signals. The overall packing of the protein appears to be conserved, but the flexible N-terminus adopts a more rigid structure upon RNA binding, indicating that the N-terminus functions as an arginine-rich RNA-binding motif (ARM).

Morphological, immunohistochemical, stereological and nuclear shape characteristics of proliferative Leydig cell alterations in rats

Proliferative Leydig cell (LC) alterations (hyperplasia, adenoma) of laboratory rats often pose diagnostic problems because the progression from normal to hyperplasia to neoplasia is continuous. The LC compartments of 130 Wistar rats (kfm: WIST strain) of approximately 2 years of age were examined. Ten typical cases conventionally classified as being normal or as showing diffuse or focal hyperplasia or small or large adenomata were investigated in more detail. In large adenomata, areas with large and small LC nuclei were identified. Immunohistochemical characterization, EM examination, as well as stereologic and planimetric investigations were performed. Hyperplastic and neoplastic LC essentially retained their normal appearance and immunohistochemical characteristics, but were found to contain more lipid droplets, fibroblast-like cells and patches of collagen than normal LC at the EM level. LC proliferation was accompanied by significant LC hypertrophy. LC nuclei of hyperplastic LC compartments were slightly larger while those of LC adenoma were markedly larger than nuclei of normal LC. The values for circle-related and ellipticity factors indicated that the nuclei of normal and hyperplastic LC were more markedly oval than nuclei of neoplastic LC. Concavity factor and bending energy measurements revealed that the small and oval nuclei of normal and hyperplastic LC had significantly more and deeper indentations than the larger and somewhat rounder nuclei of neoplastic LC. It is concluded that LC proliferations conventionally diagnosed as hyperplasia or adenoma on the basis of their size were composed of cytologically different LC populations.

Some fundamental aspects of morphometry in clinical pathology, demonstrated on a simple, multipurpose analysis system

The aim of this study was (1) to investigate the value of morphometry, (2) to fix a set of parameters suitable for analyzing diagnostic problems, and (3) to create a general strategy for data storage and for user-friendly data management. The intrinsic value of morphometry lies in the fact that in contrast to other morphologic methods, it permits the presentation of findings in the form of numbers. The following set of morphometric parameters, in the broad sense of the term morphometry, is standard in our laboratory: planimetric parameters (shape descriptors), parameters of the gray value histogram (descriptors of the general gray value distribution), texture parameters (descriptors of the correlation between various image segments), invariant moments (descriptors of the size and localization of textural image segments) and densitometric parameters. The introduction of morphometric procedures into the daily routine is facilitated if data registration and evaluation are performed separately. Original data generated by direct measurement are primary or raw data, which are stored as such. In a separate, second step these raw data are used to compare more or less complex morphometric parameters, which are called "secondary data". A system designed for separate data registration and evaluation can easily be adapted to new methodologic developments. For instance, primary data on objects (gray values, coordinates of the contour) measured one time in the past can be reused at any other time for computing new features from these data. This procedure is comparable to the possibilities in immunohistochemical staining: new immunohistochemical stains can be applied to newly prepared sections of old tissue blocks.

The significance of morphometric methods in cytologic diagnostics: differentiation between mesothelial cells, mesothelioma cells and metastatic adenocarcinoma cells in pleural effusions with special emphasis on chromatin texture

Opinions differ widely as to the value of morphometric techniques for differentiating between mesothelial cells, mesothelioma cells and metastatic adenocarcinoma cells in pleural effusions. The significance of the changes in the chromatin structure to differentiate between these three cell types was stressed by various scientific groups only qualitatively, and until now no group has used digital methods to quantify chromatin structures. The controversy about the value of morphometric methods was the reason for conducting the following study. We wanted to investigate (i) the importance of various planimetric and texture parameters as well as invariant moments as morphometric discriminators; and (ii) to set up a data structure to make possible the further diagnostic use of the multivariate morphometric analyses method for other applications. The present findings show that nuclear size and form are not the decisive factors in differentiating between reactive cells and mesothelioma cells, but pixel related texture parameters and invariant moments are. All parameter types used showed differences between reactive cells and cells of adenocarcinoma metastases. Invariant moments and shape descriptors, but not texture parameters are used to discriminate between mesothelioma cells and cells of adenocarcinoma metastases.

Digital image analysis in cytological diagnosis: a morphometric analysis on pleural mesotheliomas

In the course of the last years digital image analysis has become increasingly important in the field of quantitative pathology. This may be due to the technical development of computers which have now reached a high user-friendly standard. In the present paper we present: (1) the development of a simple system for the measurement of digital image analysis; (2) the development of a general data structure applicable to various measurements and classification strategies; and (3) the analysis of the value of invariant moments (IM) as regional descriptors to distinguish mesothelial cells from mesothelioma cells in cytologic smears of pleural fluid. The results show that the invariant moments contain information concerning the geometric distribution of the grey values of the pixels as well as the staining intensity. The invariant moments present combination parameters and are therefore ideal morphometric parameters. With the invariant moments 7 out of 7 patients with mesotheliomas and 8 out of 11 control subjects could be assigned correctly in a nonparametric discriminant analysis procedure.

Prognostic significance (relapse, non-relapse) of nuclear shape parameters in lymph node negative breast cancer

A series of 15 postmenopausal patients with stage I invasive breast cancer (pT1-2, pNO, cMO) of uniform histologic type (ductal, NOS) was selected from the Basel Cancer Registry for retrospective analysis. A group of seven patients with a minimal disease-free survival of 9 years was compared to a group of eight patients with clinical and/or histopathological relapse within 8 years of mastectomy. Computer-assisted morphometry of cancer nuclei in routine histological material was used to investigate possible correlations of clinical outcome with nuclear size and shape parameters. Significant differences were found for certain shape descriptors by nonparametric statistical analysis. Stepwise discriminant analysis was applied to find the best combination of parameters separating the two prognostic groups. Application of the 'leaving one out method' showed correct classification in 6 of 8 recurrent cases and in all non-recurrent cases. The number of patients is small, therefore the results should be carefully interpreted.

Evaluation of shape descriptors for the morphometric analysis of cell nuclei

A series of shape descriptors were developed for the morphometrical analysis of cell nuclei. These included five descriptors measuring ellipticity, two measuring concavity, and one measuring the bending energy of a contour. These different shape descriptors were compared using a test sample of 1800 contours of cell nuclei.

Fluctuations of light absorption of healthy papillae repeatedly photographed over a long period of time

In order to distinguish normal, age-related changes of the papilla from changes due to glaucoma, we investigated 7 normal papillae that had been photographed several times over a period of 11 years. By means of digital-image processing techniques, the red- and green-filtered, digitized pictures were compared directly using a subtraction method. The results seem to indicate that normal papillae do not blanch over a long period of time. The difficulties of comparison are discussed.