Albert Zacarias

Comparative measurements of multicomponent phospholipid mixtures by electrospray mass spectroscopy: relating ion intensity to concentration.

Electrospray mass spectrometry allows direct identification and sensitive detection of multiple phospholipids in non-derivatized cell extracts. However, quantitative analyses are not straightforward, and are confounded by analyte and mass discrimination effects, and non-linear dependence of the ion intensity on concentration. This non-linearity is particularly severe in the negative mode and precludes even comparative measurements of anion concentrations. Herein, we report a general method for relating negative electrospray ion intensity to concentration when analyzing multicomponent phospholipid samples. In this method, the intensity of individual ions is measured at several different concentrations of the total mixture and the slope (n(E)) of the double log plot of sample concentration vs. intensity for each analyte is determined. The n(E) is then used to map intensity data to a quantity proportional to concentration for each analyte. The method allows facile and accurate comparison of negative spectra of complex mixtures containing structurally different anions.

Binding of Pyridine Nucleotide Coenzymes to the β-Subunit of the Voltage-sensitive K+ Channel

The β-subunit of the voltage-sensitive K+ (Kv) channels belongs to the aldo-keto reductase superfamily, and the crystal structure of Kvβ2 shows NADP bound in its active site. Here we report that Kvβ2 displays a high affinity for NADPH (K d = 0.1 μm) and NADP+(K d = 0.3 μm), as determined by fluorometric titrations of the recombinant protein. The Kvβ2 also bound NAD(H) but with 10-fold lower affinity. The site-directed mutants R264E and N333W did not bind NADPH, whereas, the K d NADPH of Q214R was 10-fold greater than the wild-type protein. TheK d NADPH was unaffected by the R189M, W243Y, W243A, or Y255F mutation. The tetrameric structure of the wild-type protein was retained by the R264E mutant, indicating that NADPH binding is not a prerequisite for multimer formation. A C248S mutation caused a 5-fold decrease inK d NADPH, shifted the pK a of K d NADPH from 6.9 to 7.4, and decreased the ionic strength dependence of NADPH binding. These results indicate that Arg-264 and Asn-333 are critical for coenzyme binding, which is regulated in part by Cys-248. The binding of both NADP(H) and NAD(H) to the protein suggests that several types of Kvβ2-nucleotide complexes may be formed in vivo.

VOLUMETRIC MODULATED ARC THERAPY (VMAT) TREATMENT PLANNING FOR SUPERFICIAL TUMORS

The physicians planning objective is often a uniform dose distribution throughout the planning target volume (PTV), including superficial PTVs on or near the surface of a patients body. Varians Eclipse treatment planning system uses a progressive resolution optimizer (PRO), version 8.2.23, for RapidArc dynamic multileaf collimator volumetric modulated arc therapy planning. Because the PRO is a fast optimizer, optimization convergence errors (OCEs) produce dose nonuniformity in the superficial area of the PTV. We present a postsurgical cranial case demonstrating the recursive method our clinic uses to produce RapidArc treatment plans. The initial RapidArc treatment plan generated using one 360 degrees arc resulted in substantial dose nonuniformity in the superficial section of the PTV. We demonstrate the use of multiple arcs to produce improved dose uniformity in this region. We also compare the results of this superficial dose compensation method to the results of a recursive method of dose correction that we developed in-house to correct optimization convergence errors in static intensity-modulated radiation therapy treatment plans. The results show that up to 4 arcs may be necessary to provide uniform dose to the surface of the PTV with the current version of the PRO.

Tumor volume change with stereotactic body radiotherapy (SBRT) for early-stage lung cancer: evaluating the potential for adaptive SBRT.

Objectives:To quantify gross tumor volume (GTV) change during stereotactic body radiotherapy (SBRT) and on first follow-up, as well as to evaluate for any predictive prognostic risk factors related to GTV decrease. An attempt was also made to identify the potential timing for adaptive SBRT. Methods:Twenty-five tumors in 24 consecutive patients were treated with SBRT to total dose of 50 Gy in 5 fractions. Median age was 72.5 years. Tumor stage was T1, 68%; T2, 20%; and other, 12%. The GTVs of on the 5 cone-beam computed tomographies (CBCT1-5) obtained before each fraction and the first follow-up CT (CTPOST) were analyzed. Results:Median time from diagnosis to initiation of radiotherapy was 64 days. GTV on CBCT1 was the baseline for comparison. GTV decreased by a mean of 7% on CBCT2 (P=0.148), 11% on CBCT3 (P=0.364), 19% on CBCT4 (P=0.0021), and 32% on CBCT5 (P=0.0004). Univariate analyses of GTV shrinkage was significantly associated with “time from CBCT5 to CTPOST” (P=0.027) and “T-stage” (P=0.002). In multivariate analyses, “T-stage” remained significant with T1 tumors showing greater GTV shrinkage than T2 tumors. Conclusions:Significant decrease in GTV volume based on daily CBCT was demonstrated during SBRT treatment. Adaptive SBRT has the potential to minimize integral dose to the surrounding normal tissues without compromising GTV coverage.

Algorithm for correcting optimization convergence errors in Eclipse

IMRT plans generated in Eclipse use a fast algorithm to evaluate dose for optimization and a more accurate algorithm for a final dose calculation, the Analytical Anisotropic Algorithm. The use of a fast optimization algorithm introduces optimization convergence errors into an IMRT plan. Eclipse has a feature where optimization may be performed on top of an existing base plan. This feature allows for the possibility of arriving at a recursive solution to optimization that relies on the accuracy of the final dose calculation algorithm and not the optimizer algorithm. When an IMRT plan is used as a base plan for a second optimization, the second optimization can compensate for heterogeneity and modulator errors in the original base plan. Plans with the same field arrangement as the initial base plan may be added together by adding the initial plan optimal fluence to the dose correcting plan optimal fluence. A simple procedure to correct for optimization errors is presented that may be implemented in the Eclipse treatment planning system, along with an Excel spreadsheet to add optimized fluence maps together. PACS number: 87.53.Bn, 87.56.By

Radiation shielding design of a new tomotherapy facility

It is expected that intensity modulated radiation therapy (IMRT) and image guided radiation therapy (IGRT) will replace a large portion of radiation therapy treatments currently performed with conventional MLC-based 3D conformal techniques. IGRT may become the standard of treatment in the future for prostate and head and neck cancer. Many established facilities may convert existing vaults to perform this treatment method using new or upgraded equipment. In the future, more facilities undoubtedly will be considering de novo designs for their treatment vaults. A reevaluation of the design principles used in conventional vault design is of benefit to those considering this approach with a new tomotherapy facility. This is made more imperative as the design of the TomoTherapy system is unique in several aspects and does not fit well into the formalism of NCRP 49 for a conventional linear accelerator.

An automated landmark-based elastic registration technique for large deformation recovery from 4-D CT lung images

The treatment plan evaluation for lung cancer patients involves pre-treatment and post-treatment volume CT imaging of the lung. However, treatment of the tumor volume lung results in structural changes to the lung during the course of treatment. In order to register the pre-treatment volume to post-treatment volume, there is a need to find robust and homologous features which are not affected by the radiation treatment along with a smooth deformation field. Since airways are well-distributed in the entire lung, in this paper, we propose use of airway tree bifurcations for registration of the pre-treatment volume to the post-treatment volume. A dedicated and automated algorithm has been developed that finds corresponding airway bifurcations in both images. To derive the 3-D deformation field, a B-spline transformation model guided by mutual information similarity metric was used to guarantee the smoothness of the transformation while combining global information from bifurcation points. Therefore, the approach combines both global statistical intensity information with local image feature information. Since during normal breathing, the lung undergoes large nonlinear deformations, it is expected that the proposed method would also be applicable to large deformation registration between maximum inhale and maximum exhale images in the same subject. The method has been evaluated by registering 3-D CT volumes at maximum exhale data to all the other temporal volumes in the POPI-model data.

Strain as a novel index of regional pulmonary function from thoracic 4D CT images: in-vivo validation with tomographic SPECT ventilation and perfusion

Since many diseases or injuries can cause biomechanical or structural property changes that can alter lung function, there is great interest in measuring regional lung function by measurement of regional mechanical changes. To date, the most prevalent approach for assessing regional lung function from 4-D X-ray CT data has been a measure of Jacobian of deformation. However, although the Jacobian describes regional volume changes of the lung during deformation, it lacks any consideration of directional changes of local compressions and expansions during respiration. Herein, we propose the use of strain as a measure of regional lung function from 4-D thoracic CT and we perform correlation of principal strains of calculated deformation by s recently proposed 3-D optical flow technique (MOFID) computed from radiotherapy treatment planning 4-D X-ray CT data sets collected in seven subjects suffering from non-small cell primary lung cancer. In addition to 4-D CT data, both SPECT ventilation (VSPECT), and SPECT perfusion (QSPECT) data were acquired in all subjects. For each subject, we performed voxel-wise statistical correlation of the Jacobian as well as principal strains of deformation (CT-derived pulmonary function images) with both ventilation and perfusion SPECT. For all subjects, the maximum principal strain resulted in a higher correlation with both SPECT ventilation and SPECT perfusion than other indices including the previously established Jacobian metric.

Comparison of indices of regional lung function from 4-D X-ray CT: Jacobian vs. strain of deformation

In the literature, a widely adopted approach for assessing the regional lung function in patients undergoing radiation therapy for lung cancer from 4-D X-ray CT data is the Jacobian of deformation. The Jacobian which describes regional lung volume changes however lacks consideration of directional length changes during respiration. Previously, we proposed the use of strain for measurement of regional deformation in the Lung [1]. In this paper, we perform physiologic validation of lung strain and compare the results with those from the Jacobian measure. Lung deformation fields were computed through application of a recently proposed 3-D optical flow technique (MOFID) [2] to 4-D X-ray CT data sets collected in seven subjects diagnosed with non-small cell primary lung cancer. In addition to 4-D CT data, both SPECT ventilation (VSPECT), and SPECT perfusion (QSPECT) data were acquired in all subjects. For each case, voxel-wise statistical correlation of the Jacobian as well as principal strains of deformation (CT-derived pulmonary function images) with both ventilation and perfusion SPECT was performed. The results indicate that the maximum principal strain has a higher correlation with both SPECT ventilation and SPECT perfusion than other indices including the Jacobian.

A Method for Correcting IMRT Optimizer Heterogeneity Dose Calculations

Radiation therapy treatment planning for volumes close to the patients surface, in lung tissue and in the head and neck region, can be challenging for the planning system optimizer because of the complexity of the treatment and protected volumes, as well as striking heterogeneity corrections. Because it is often the goal of the planner to produce an isodose plan with uniform dose throughout the planning target volume (PTV), there is a need for improved planning optimization procedures for PTVs located in these anatomical regions. To illustrate such an improved procedure, we present a treatment planning case of a patient with a lung lesion located in the posterior right lung. The intensity-modulated radiation therapy (IMRT) plan generated using standard optimization procedures produced substantial dose nonuniformity across the tumor caused by the effect of lung tissue surrounding the tumor. We demonstrate a novel iterative method of dose correction performed on the initial IMRT plan to produce a more uniform dose distribution within the PTV. This optimization method corrected for the dose missing on the periphery of the PTV and reduced the maximum dose on the PTV to 106% from 120% on the representative IMRT plan.