Deformable image registration for dose mapping between external beam radiotherapy and brachytherapy images of cervical cancer.

For locally advanced cervical cancer (LACC), anatomy correspondence with and without BT applicator needs to be quantified to merge the delivered doses of external beam radiation therapy (EBRT) and brachytherapy (BT). This study proposed and evaluated different deformable image registration (DIR) methods for this application. Twenty patients who underwent EBRT and BT for LACC were retrospectively analyzed. Each patient had a pre-BT CT at EBRT boost (without applicator) and a CT and MRI at BT (with applicator). The evaluated DIR methods were the diffeomorphic Demons, commercial intensity and hybrid methods, and three different biomechanical models. The biomechanical models considered different boundary conditions (BCs). The impact of the BT devices insertion on the anatomy was quantified. DIR method performances were quantified using geometric criteria between the original and deformed contours. The BT dose was deformed toward the pre-CT BT by each DIR method. The impact of boundary conditions to drive the biomechanical model was evaluated based on the deformation vector field and dose differences. The GEC-ESTRO guideline dose indices were reported. Large organ displacements, deformations, and volume variations were observed between the pre-BT and BT anatomies. Rigid registration and intensity-based DIR resulted in poor geometric accuracy with mean Dice similarity coefficient (DSC) inferior to 0.57, 0.63, 0.42, 0.32, and 0.43 for the rectum, bladder, vagina, cervix and uterus, respectively. Biomechanical models provided a mean DSC of 0.96 for all the organs. By considering the cervix-uterus as one single structure, biomechanical models provided a mean DSC of 0.88 and 0.94 for the cervix and uterus, respectively. The deformed doses were represented for each DIR method. Caution should be used when performing DIR for this application as standard techniques may have unacceptable results. The biomechanical model with the cervix-uterus as one structure provided the most realistic deformations to propagate the BT dose toward the EBRT boost anatomy.

Adaptive radiotherapy for head and neck cancer.

INTRODUCTION: Large anatomical variations can be observed during the treatment course intensity-modulated radiotherapy (IMRT) for head and neck cancer (HNC), leading to potential dose variations. Adaptive radiotherapy (ART) uses one or several replanning sessions to correct these variations and thus optimize the delivered dose distribution to the daily anatomy of the patient. This review, which is focused on ART in the HNC, aims to identify the various strategies of ART and to estimate the dosimetric and clinical benefits of these strategies. MATERIAL AND METHODS: We performed an electronic search of articles published in PubMed/MEDLINE and Science Direct from January 2005 to December 2016. Among a total of 134 articles assessed for eligibility, 29 articles were ultimately retained for the review. Eighteen studies evaluated dosimetric variations without ART, and 11 studies reported the benefits of ART. RESULTS: Eight in silico studies tested a number of replanning sessions, ranging from 1 to 6, aiming primarily to reduce the dose to the parotid glands. The optimal timing for replanning appears to be early during the first two weeks of treatment. Compared to standard IMRT, ART decreases the mean dose to the parotid gland from 0.6 to 6 Gy and the maximum dose to the spinal cord from 0.1 to 4 Gy while improving target coverage and homogeneity in most studies. Only five studies reported the clinical results of ART, and three of those studies included a non-randomized comparison with standard IMRT. These studies suggest a benefit of ART in regard to decreasing xerostomia, increasing quality of life, and increasing local control. Patients with the largest early anatomical and dose variations are the best candidates for ART. CONCLUSION: ART may decrease toxicity and improve local control for locally advanced HNC. However, randomized trials are necessary to demonstrate the benefit of ART before using the technique in routine practice.

A Nomogram to predict parotid gland overdose in head and neck IMRT.

PURPOSES: To generate a nomogram to predict parotid gland (PG) overdose and to quantify the dosimetric benefit of weekly replanning based on its findings, in the context of intensity-modulated radiotherapy (IMRT) for locally-advanced head and neck carcinoma (LAHNC). MATERIAL AND METHODS: Twenty LAHNC patients treated with radical IMRT underwent weekly computed tomography (CT) scans during IMRT. The cumulated PG dose was estimated by elastic registration. Early predictors of PG overdose (cumulated minus planned doses) were identified, enabling a nomogram to be generated from a linear regression model. Its performance was evaluated using a leave-one-out method. The benefit of weekly replanning was then estimated for the nomogram-identified PG overdose patients. RESULTS: Clinical target volume 70 (CTV70) and the mean PG dose calculated from the planning and first weekly CTs were early predictors of PG overdose, enabling a nomogram to be generated. A mean PG overdose of 2.5Gy was calculated for 16 patients, 14 identified by the nomogram. All patients with PG overdoses >1.5Gy were identified. Compared to the cumulated delivered dose, weekly replanning of these 14 targeted patients enabled a 3.3Gy decrease in the mean PG dose. CONCLUSION: Based on the planning and first week CTs, our nomogram allowed the identification of all patients with PG overdoses >2.5Gy to be identified, who then benefitted from a final 4Gy decrease in mean PG overdose by means of weekly replanning.

[Benefit of a pretreatment planning library-based adaptive radiotherapy for cervix carcinoma?]. / Bénéfice de la radiothérapie adaptative par bibliothèque de plans de traitement pour les cancers du col utérin ?

PURPOSE: In case of intensity-modulated radiotherapy (IMRT) for locally advanced cervix carcinoma, the objectives were to quantify the difference between the planned and the delivered doses by a standard irradiation, and to estimate the dosimetric benefit of a pretreatment planning library-based adaptive radiotherapy. MATERIAL AND METHODS: Ten patients with locally advanced cervix carcinoma had three planning CTs corresponding to three bladder volumes: empty, intermediate (vi) and full. On each CT, two IMRT plans were generated to deliver 45 Gy to the planning target volume (PTV), with two different margins: clinical target volume (CTV)+10mm and CTV+15 mm. Using bi-weekly CBCTs, three scenarios of treatment have been simulated and compared: standard IMRT (one vi planning) with 10 and 15 mm margins and adaptive radiotherapy with 10mm margin. The cumulated dose in the organs at risk was estimated by elastic registration. RESULT: In case of standard IMRT, the cumulated dose was significantly different than the planning dose, with an under-dose of the CTV and the bladder, and an over-dose of the rectum and the peritoneal cavity. For 54% of the fractions, the adaptive radiotherapy planning was not based on vi. Considering the cumulated dose and compared to IMRT with 10-mm margin, adaptive radiotherapy increased the dose to the CTV (1.4 Gy for D98%) and decreased slightly the dose to the rectum and the peritoneal cavity. Compared to a standard IMRT with 15 mm margin, adaptive radiotherapy decreased significantly the dose to the rectum (20% for V40), the bladder (13% for V40) and the peritoneal cavity (2% for V35). CONCLUSION: A pretreatment planning library-based adaptive radiotherapy in cervix carcinoma decreases the dose to the organs at risk and increases the dose to the CTV.

Resistor mesh model of a spherical head: part 1: applications to scalp potential interpolation.

A resistor mesh model (RMM) has been implemented to describe the electrical properties of the head and the configuration of the intracerebral current sources by simulation of forward and inverse problems in electroencephalogram/event related potential (EEG/ERP) studies. For this study, the RMM representing the three basic tissues of the human head (brain, skull and scalp) was superimposed on a spherical volume mimicking the head volume: it included 43 102 resistances and 14 123 nodes. The validation was performed with reference to the analytical model by consideration of a set of four dipoles close to the cortex. Using the RMM and the chosen dipoles, four distinct families of interpolation technique (nearest neighbour, polynomial, splines and lead fields) were tested and compared so that the scalp potentials could be recovered from the electrode potentials. The 3D spline interpolation and the inverse forward technique (IFT) gave the best results. The IFT is very easy to use when the lead-field matrix between scalp electrodes and cortex nodes has been calculated. By simple application of the Moore-Penrose pseudo inverse matrix to the electrode cap potentials, a set of current sources on the cortex is obtained. Then, the forward problem using these cortex sources renders all the scalp potentials.

Resistor mesh model of a spherical head: part 2: a review of applications to cortical mapping.

A resistor mesh model (RMM) has been validated with reference to the analytical model by consideration of a set of four dipoles close to the cortex. The application of the RMM to scalp potential interpolation was detailed in Part 1. Using the RMM and the same four dipoles, the different methods of cortical mapping were compared and have shown the potentiality of this RMM for obtaining current and potential cortical distributions. The lead-field matrices are well-adapted tools, but the use of a square matrix of high dimension does not permit the inverse solution to be improved in the presence of noise, as a regularisation technique is necessary with noisy data. With the RMM, the transfer matrix and the cortical imaging technique proved to be easy to implement. Further development of the RMM will include application to more realistic head models with more accurate conductivities.

Ex vivo discrimination between normal and pathological tissues in human breast surgical biopsies using bioimpedance spectroscopy.

Ex vivo bioimpedance data measured on normal and cancerous female breast tissues are reported. They clearly show that the electrical properties of normal tissues, surrounding tissues, and carcinoma are different. These differences lie in the conductivity, in the characteristic frequency (frequency of the maximum of the imaginary part of the bioimpedance), and also in the shape of the Bode plots. Modeling using an R-S-Zcpe model is reported as well as indexes extracted from the real and imaginary parts of the bioimpedance. Even if a classification of the different types of tissues remains a difficult task and leads to much less precise diagnosis than microscopic examination, the electrical behavior of mammary tissue could be used to develop a noninvasive technique for early breast cancer detection.

A multifrequency serial EIT system.

A multifrequency (1 kHz-1 MHz) serial electrical impedance tomography (EIT) system has been developed. It is based on 16 active electrodes and can be extended up to 32. Each active electrode can be programmed for current driving and for measuring either the injected current or the voltage difference between adjacent electrodes, and includes calibration facilities. Real and imaginary parts of the impedance are obtained by applying a parametric identification method (extended Prony), but other techniques are easily adaptable. Image reconstruction is carried out using the Sheffield filtered back-projection algorithm. Characteristic frequency images are under development and should be of great interest to distinguish between normal and tumorous tissues.

Parametric modelling for electrical impedance spectroscopy system.

Three parametric modelling approaches based on the Cole-Cole model are introduced. Comparison between modelling only the real part and modelling both the real and imaginary parts is carried out by simulations, in which random and systematic noise are considered, respectively. The results of modelling the in vitro data collected from sheep are given to reach the conclusions.

Bioelectrical impedance techniques in medicine. Part I: Bioimpedance measurement. Second section: impedance spectrometry.

Electrical impedance spectrometry is an important application field of bioimpedance measurements. After introducing the electrical properties of biological tissues, this part presents instrumental aspects and applications of electrical impedance spectrometry. The main instrumental constraints encountered in spectrometric electrical impedance measurements are reviewed, focusing on low-frequency applications. Examples of impedance cells and probes are presented and several instrumental setups operating in the frequency and time domain are described. Some examples of applications are presented, including in vitro characterization and modeling of normal tissues, in vitro and in vivo characterization of cancerous tissues, and assessment of tissue perfusion/ischemia levels.

Bioelectrical impedance techniques in medicine. Part III: Impedance imaging. First section: general concepts and hardware.

Measurement accuracy is a key point in impedance imaging and is mainly limited by factors that take place in the acquisition system. This part is a review of hardware solutions developed in acquisition systems for electrical impedance tomography (EIT). The general principles of EIT along with the changes that have taken place in the last decade, in terms of measurement strategy, and a certain number of definitions are introduced. The major hardware error sources that occur in the front end of EIT systems are presented. A review of the various alternatives published in the literature that are used to drive current, including current and voltage approaches, and the main solutions recommended in the literature to overcome the key point drawbacks of voltage measurement systems, including voltage buffers, instrumentation amplifiers, and demodulators, are provided. Some calibration procedures and approaches for the evaluation of the performance of EIT systems are also presented.

Bioelectrical impedance techniques in medicine. Part III: Impedance imaging. Third section: medical applications.

In several areas of clinical medicine, electrical impedance tomography could offer significant advantages over existing methods. These advantages have been supported by preliminary studies or by validation studies, which are described. The suggested applications are reviewed in this section. They mainly concern developments in impedance variations on brain, lung (neonatal, edema, emphysema), and heart; changes in blood volume, gastrointestinal system (gastric emptying, gastroesophageal reflux, pharyngeal transit time); pelvis (pelvis congestion); and thermal mapping in hyperthermia and breast (tissue characterization). The conductivity information at one frequency in a pixel is insufficient to take into account the very complex physiological mechanisms that underlie the observed impedance changes. To gain a better understanding of these mechanisms, research is currently being carried out on imaging of the imaginary part, parametric imaging, spectroscopic imaging, and 3D imaging, which are developed at the end of this section.

In vitro tissue characterization and modelling using electrical impedance measurements in the 100 Hz-10 MHz frequency range.

In vitro electrical impedance spectrometry was performed on tissue samples excised from sheep. Measured data have been processed to reduce dispersion in measurements and to provide criteria useful for tissue comparison. Two electrical models are proposed for tissues exhibiting a one-circle impedance locus and a two-circle impedance locus. Measurement results and electrical parameters of tissues and models fitted to experimental data are presented. Model sensitivity to parameter variations is discussed.

Tissue characterization by impedance: a multifrequency approach.

Two experimental set-ups for in vitro characterization of electrical bio-impedance are described. The first one, based on a commercially available instrument, operates in the frequency range 1 Hz-10 MHz. The second one uses an identification process and operates in the frequency range 1 Hz-1 MHz. Some results are presented and discussed in the context of multifrequency electrical impedance tomography.

Experimental acquisition system for impedance tomography with active electrode approach.

An experimental system for impedance tomography has been constructed. The acquisition system uses 16 multifunctional active electrodes, each including a current source and a voltage buffer. Images of active and reactive parts of different target impedances in a phantom filled with liquid have been obtained. The system performance has been compared with those of other systems using either a mesh phantom or rods as point sources used for the determination of the modulation transfer function.

The modulation transfer function in impedance imaging.

The concepts of the point spread function (PSF) and its Fourier transform, the modulation transfer function (MTF) are introduced to evaluate an impedance imaging system. The effects of some practical factors which should be taken into account in calculating the MTF are analysed. Experimental results from the groups in Sheffield, Barcelona and Toulouse are presented.

Electrical impedance tomography. Technical and experimental problems encountered in impedance spectroscopy in the alpha and beta dispersion regions.

The main experimental problems (temperature, electrode, polarisation, non-uniformity of electric field, non-homogeneity of biological material) of impedance measurements of biological tissue are discussed. Minimisation of the inherent sources of experimental error is proposed.