Radiomics as a measure superior to common similarity metrics for tumor segmentation performance evaluation.

PURPOSE: To propose radiomics features as a superior measure for evaluating the segmentation ability of physicians and auto-segmentation tools and to compare its performance with the most commonly used metrics: Dice similarity coefficient (DSC), surface Dice similarity coefficient (sDSC), and Hausdorff distance (HD). MATERIALS/METHODS: The data of 10 lung cancer patients' CT images with nine tumor segmentations per tumor were downloaded from the RIDER (Reference Database to Evaluate Response) database. Radiomics features of 90 segmented tumors were extracted using the PyRadiomics program. The intraclass correlation coefficient (ICC) of radiomics features were used to evaluate the segmentation similarity and compare their performance with DSC, sDSC, and HD. We calculated one ICC per radiomics feature and per tumor for nine segmentations and 36 ICCs per radiomics feature for 36 pairs of nine segmentations. Meanwhile, there were 360 DSC, sDSC, and HD values calculated for 36 pairs for 10 tumors. RESULTS: The ICC of radiomics features exhibited greater sensitivity to segmentation changes than DSC and sDSC. The ICCs of the wavelet-LLL first order Maximum, wavelet-LLL glcm MCC, wavelet-LLL glcm Cluster Shade features ranged from 0.130 to 0.997, 0.033 to 0.978, and 0.160 to 0.998, respectively. On the other hand, all DSC and sDSC were larger than 0.778 and 0.700, respectively, while HD varied from 0 to 1.9 mm. The results indicated that the radiomics features could capture subtle variations in tumor segmentation characteristics, which could not be easily detected by DSC and sDSC. CONCLUSIONS: This study demonstrates the superiority of radiomics features with ICC as a measure for evaluating a physician's tumor segmentation ability and the performance of auto-segmentation tools. Radiomics features offer a more sensitive and comprehensive evaluation, providing valuable insights into tumor characteristics. Therefore, the new metrics can be used to evaluate new auto-segmentation methods and enhance trainees' segmentation skills in medical training and education.

Bactericidal Ability of Well-Controlled Cationic Polymer Brush Surfaces and the Interaction Analysis by Quartz Crystal Microbalance with Dissipation.

In this study, cationic poly(2-(methacryloyloxy)ethyl) trimethylammonium chloride) (PMTAC) brush surfaces were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP), and their properties were systematically investigated to discuss the factors affecting their bactericidal properties and interactions with proteins. Model equations for the analysis of electrophoretic behaviors were considered for accurate parameter estimation to indicate the charge density at the interface. The zeta potential dependency of the PMTAC brushes was successfully analyzed using Smolchowski's equation and the Gouy-Chapman model, which describes the diffusive electric double layer. The analysis of the quartz crystal microbalance with dissipation (QCM-D) indicated that the electrostatic interaction promoted protein adsorption, with a large quantity of a negatively charged protein, bovine serum albumin (BSA), being adsorbed. The bactericidal efficiency of the high-graft-density polymer brush (0.45 chains nm-2) was higher than that of the low-graft-density polymer brush (0.06 chains nm-2). To investigate the mechanism of this phenomenon, we applied the dissipation change (ΔD) of QCM-D analysis. The BSA was likewise adsorbed when the brush structure was changed; however, the negative ΔD indicated that the BSA-adsorbed, high-graft-density PMTAC brush became a rigid state. In the bacteria culture media, the behaviors were the same as BSA adsorption, and the high-graft-density polymer brush was also estimated to be more rigid than the low-graft-density polymer brush. Moreover, for S. aureus adhesion after incubating in TSB, a small slope of ΔD/ΔF plots considered initial adsorption of bacteria on the high-graft-density polymer brush strongly interacted compared to that of the low-graft-density polymer brush. The scattered value of the slope of ΔD/ΔF on the high-graft-density polymer brush was considered to be due to the dead bacteria between the bacteria and the polymer brush interface. These investigations for a well-defined cationic polymer brush will contribute to the design of antibacterial surfaces.

Use of a Commercial 7-T MRI Scanner for Clinical Brain Imaging: Indications, Protocols, Challenges, and Solutions-A Single-Center Experience.

The first commercially available 7-T MRI scanner (Magnetom Terra) was approved by the FDA in 2017 for clinical imaging of the brain and knee. After initial protocol development and sequence optimization efforts in volunteers, the 7-T system, in combination with an FDA-approved 1-channel transmit/32-channel receive array head coil, can now be routinely used for clinical brain MRI examinations. The ultrahigh field strength of 7-T MRI has the advantages of improved spatial resolution, increased SNR, and increased CNR but also introduces an array of new technical challenges. The purpose of this article is to describe an institutional experience with the use of the commercially available 7-T MRI scanner for routine clinical brain imaging. Specific clinical indications for which 7-T MRI may be useful for brain imaging include brain tumor evaluation with possible perfusion imaging and/or spectroscopy, radiotherapy planning; evaluation of multiple sclerosis and other demyelinating diseases, evaluation of Parkinson disease and guidance of deep brain stimulator placement, high-detail intracranial MRA and vessel wall imaging, evaluation of pituitary pathology, and evaluation of epilepsy. Detailed protocols, including sequence parameters, for these various indications are presented, and implementation challenges (including artifacts, safety, and side effects) and potential solutions are explored.

Utilization of CBCT to improve the delivery accuracy of Gamma Knife radiosurgery with G-frame.

PURPOSE: To quantify the G-frame based stereotactic coordinate definition accuracy of Leksell coordinate G-frame-based Gamma Knife radiosurgery (GKRS) by the on-board cone-beam CT (CBCT) and establish remedial action rules to minimize the delivery errors. METHODS: We analyzed the data of 108 patients (a total of 201 tumors) treated by GKRS with G-frame for head fixation. After co-registering the CBCT images and plan reference images, the Leksell GammaPlan (LGP) treatment planning system provided the amount of geometric translation and rotation required to minimize the position difference between the plan and treatment. The software also calculated maximum displacement, which characterizes the position shift more clearly. We studied how much these predicted dosimetric quantities changed if the treatment was delivered without correcting the patient's position. RESULTS: The maximum displacement of the patient position obtained from the co-registration of CBCT and plan reference images was 0.81 ± 0.38 mm (0.24-2.03 mm). The target coverage decreased by 3.3 ± 7.0% on average (-48.5% to +35.7%). The decrease of the target coverage, however, became smaller as the target volume increased. In particular, if the volume was greater than 2 cm3 , the %change in target coverage was always less than -5%. CONCLUSIONS: The position differences reported by the registration module of LGP were within the accuracy limit of image registration for most clinical cases, but the errors could be larger in some cases. Therefore, we propose the following decision process. We do not advise position adjustment for G-frame based GKRS if the maximum displacement is less than 1 mm. When this limit is exceeded, however, another criterion should be applied to the decision making by considering the tumor size (or the treatment volume) together with the acceptable change of the tumor coverage.

[Corticospinal Tract Mapping by Calculating the Ratio of Subcortical to Cortical Stimulation Intensity:A Technical Note].

OBJECTIVE: Motor evoked potentials(MEPs)have been developed and utilized as safe surgical procedures. A correlation between the threshold intensity of direct stimulation MEPs and the distance of the corticospinal tract(CST)has been already established. However, MEPs are affected by anesthesia and patient-related conditions. Here, we describe a unique technique to avoid these effects. METHOD: When tumors developed in proximity to the CST, the transcortical MEP monitoring was done by placing grid electrodes on the primary motor cortex continuously while direct subcortical MEP mapping was conducted with a monopolar probe. The ratios of the subcortical to the transcortical stimulation intensity were calculated. The point at which the ratios reached 50% was defined as the surgical excision limit. DISCUSSION: MEPs are affected by anesthesia, paralysis, body temperature, and other factors. By measuring the ratio of the cortical stimulation intensity instead of the absolute value of the white matter stimulation intensity, various affecting factors can be avoided, and more accurate monitoring can become possible. CONCLUSION: By calculating the ratio of subcortical to cortical stimulation intensity, the corticospinal tract mapping is less likely to be influenced by the stimulation condition or facility setup, and this warrants further investigation.

A mathematical model of tumor growth and its response to single irradiation.

BACKGROUND: Mathematical modeling of biological processes is widely used to enhance quantitative understanding of bio-medical phenomena. This quantitative knowledge can be applied in both clinical and experimental settings. Recently, many investigators began studying mathematical models of tumor response to radiation therapy. We developed a simple mathematical model to simulate the growth of tumor volume and its response to a single fraction of high dose irradiation. The modelling study may provide clinicians important insights on radiation therapy strategies through identification of biological factors significantly influencing the treatment effectiveness. METHODS: We made several key assumptions of the model. Tumor volume is composed of proliferating (or dividing) cancer cells and non-dividing (or dead) cells. Tumor growth rate (or tumor volume doubling time) is proportional to the ratio of the volumes of tumor vasculature and the tumor. The vascular volume grows slower than the tumor by introducing the vascular growth retardation factor, θ. Upon irradiation, the proliferating cells gradually die over a fixed time period after irradiation. Dead cells are cleared away with cell clearance time. The model was applied to simulate pre-treatment growth and post-treatment radiation response of rat rhabdomyosarcoma tumors and metastatic brain tumors of five patients who were treated with Gamma Knife stereotactic radiosurgery (GKSRS). RESULTS: By selecting appropriate model parameters, we showed the temporal variation of the tumors for both the rat experiment and the clinical GKSRS cases could be easily replicated by the simple model. Additionally, the application of our model to the GKSRS cases showed that the α-value, which is an indicator of radiation sensitivity in the LQ model, and the value of θ could be predictors of the post-treatment volume change. CONCLUSIONS: The proposed model was successful in representing both the animal experimental data and the clinically observed tumor volume changes. We showed that the model can be used to find the potential biological parameters, which may be able to predict the treatment outcome. However, there is a large statistical uncertainty of the result due to the small sample size. Therefore, a future clinical study with a larger number of patients is needed to confirm the finding.

One-year analysis of Elekta CBCT image quality using NPS and MTF.

The image quality (IQ) of imaging systems must be sufficiently high for image-guided radiation therapy (IGRT). Hence, users should implement a quality assurance program to maintain IQ. In our routine IQ tests of the kV cone-beam CT system (Elekta XVI), image noise was quantified by noise standard deviation (NSD), which was the standard deviation of CT numbers measured in a small area in an image of an IQ test phantom (Catphan), and the high spatial resolution (HSR) was evaluated by the number of line-pairs (LPN) visually recognizable in the image. We also measured the image uniformity, the low contrast resolution, and the distances of two points for geometrical accuracy. For this study, we did an additional evaluation of the XVI data for 12 monthly IQ tests by using noise power spectrum (NPS) for noise, modulation transfer function (MTF) for HSR, and CT number-to-density relationship. NPS was obtained by applying Fourier analysis in a small area on the uniformity test section of Catphan. The MTF analysis was performed by applying the Droege-Morin (D-M) method to the line-pair bar regions in the phantom. The CT number-to-density relationship was obtained for insert materials in the low-contrast test section of the phantom. All the quantities showed a noticeable change over the one-year period. Especially the noise level improved significantly after a repair of the imager. NPS was more sensitive to the IQ change than NSD. MTF could provide more quantitative and objective evaluation of HSR. The CT number was very different from the expected CT number, but the CT number-to-density curves were constant within 5% except for two months. Since the D-M method is easy to implement, we recommend using MTF instead of LPN even for routine QA. The IQ of the imaging systems was constantly changing; hence, IQ tests should be periodically performed. Additionally, we found the importance of IQ tests after every service work, including detector calibration as well as preventive maintenance.

[Development of a Novel Body Phantom with Bone Equivalent Density for Evaluation of Bone SPECT].

We developed a custom-designed phantom for bone single photon emission computed tomography (SPECT)-specific radioactivity distribution and linear attenuation coefficient. The aim of this study was to evaluate the accuracy of the phantom. The lumbar phantom consisted of the trunk of a body phantom (background) containing a cylinder (vertebral body), a sphere (tumor), and a T-shaped container (processus). The vertebral body, tumor, and processus phantoms contained a K(2)HPO(4) solution of bone equivalent density and 50, 300 and 50 kBq/mL of (99m)Tc, respectively. The body phantom contained 8 kBq/mL of (99m)Tc solution. SPECT images were acquired using low-energy high-resolution collimation, a 128 × 128 matrix and 120 projections over 360° with a dwell time of 15 sec/view × 4 times. Thereafter, CT images were acquired at 130 kV and 70 ref mAs using adaptive dose modulation. The SPECT data were reconstructed with ordered subset expectation maximization with three-dimensional, scatter, and CT-based attenuation correction. Count ratio, linear attenuation coefficient (LAC), and full-width at half-maximum (FWHM) were measured. Count ratios between the background, the vertebral body, and the tumor in SPECT images were 463.8: 2888.0: 15150.3 (1: 6.23: 32.7). The LAC of the background and vertebral body in the CT-derived attenuation map were 0.155 cm⁻¹ and 0.284 cm⁻¹, respectively, and the FWHM measured from the processus was 15.27 mm. The precise counts and LAC indicated that the phantom was accurate and could serve as a tool for evaluating acquisition, reconstruction parameters, and quantitation in bone SPECT images.

A simulation study on the radiation-induced immune response of tumors after single fraction high-dose irradiation.

PURPOSE: We investigated radiation-induced antitumor immunity and its suppression by hypoxia-inducible factor (HIF-1α) for radiosurgery (SRS) using an improved cellular automata (CA) model. METHOD: A two-dimensional Cellular Automata (CA) model was employed to simulate the impact of radiation on cancer cell death and subsequent immune responses. Cancer cells died from direct cell death from radiation and indirect cell death due to radiation-induced vascular damage. The model also incorporated radiation-induced immunity and immuno-suppression. It was incorporated into the model assuming that the death of cancer cells generates effector cells, forming complexes with cancer cells, and high radiation doses lead to vascular damage, inducing tumor hypoxia and increasing HIF-1α expression. The model was validated and subjected to sensitivity analysis by evaluating tumor volume changes post-irradiation and exploring the effects and sensitivity of radiation-induced immune responses. RESULTS: The ratios of the tumor volume at 360 days post-irradiation and the SRS day (rTV) decreased with a higher PME, a higher Pcomp, and a lower ThHIF. The rTVs were 4.6 and 2.0 for PME = 0.1 and 0.9, 12.0 and 2.2 for Pcomp = 0.1 and 0.9, and 1.5 and 15.3 for ThHIF = 0.1 and 10.0, respectively. CONCLUSIONS: By modeling the activation and deactivation of the effectors, the improved CA model showed that the radiation-induced immunogenic cell death in the tumor caused a decrease in the post-irradiation volume by a factor of four for the therapeutic doses relative to non-immune reaction cases. Furthermore, the suppressive effects of HIF-1α induced by hypoxia decreased radiation-induced immune effects by more than 50.

Comparison of Gamma Knife (GK) and Linear Accelerator (LINAC) radiosurgery of brain metastasis resection cavity: a systematic review and proportional meta-analysis.

PURPOSE: Stereotactic radiosurgery (SRS) to the resection cavity is essential in the treatment of brain metastasis (BM) amenable to surgical resection. The two most common platforms for SRS delivery include Gamma Knife (GK) and LINAC. Here we collated the available peer-reviewed literature and performed a meta-analysis on clinical outcomes after GK or LINAC resection cavity SRS. METHODS: Following PRISMA Guidelines, a search on PUBMED and MEDLINE was performed to include all studies evaluating each post-operative SRS modality. Local control, overall survival, radiation necrosis, and leptomeningeal disease were evaluated from the available data. A proportional meta-analysis was performed via R using the metafor package to pool the outcomes of studies and a moderator effect to assess the significance between groups. RESULTS: We identified 21 GK studies (n = 2009) and 28 LINAC studies (n = 2219). The radiosurgery doses employed were comparable between GK and LINAC studies. The pooled estimate of 1-year local control, 1-year overall survival, and risk of leptomeningeal disease were statistically comparable between GK and LINAC (81.7 v 85.8%; 61.4 v 62.7%; 10.6 v 12.5%, respectively). However, the risk of radiation necrosis (RN) was higher for LINAC resection cavity SRS (5.4% vs. 10%, p = 0.036). The volume of the resection cavity was a significant modifying factor for RN in both modalities (p = 0.007) with a 0.5% and 0.7% increase in RN risk with every 1 cm3 increase in tumor volume for GK and LINAC, respectively. CONCLUSIONS: Our meta-analysis suggests that GK and LINAC SRS of resection cavity achieve comparable 1-year local control and survival. However, resection cavity treated with GK SRS was associated with lowered RN risk relative to those treated with LINAC SRS.

Prediction of Obliteration After the Gamma Knife Radiosurgery of Arteriovenous Malformations Using Hand-Crafted Radiomics and Deep-Learning Methods.

INTRODUCTION: Brain arteriovenous malformations (bAVMs) are vascular abnormalities that can be treated with embolization or radiotherapy to prevent the risk of future rupture. In this study, we use hand-crafted radiomics and deep learning techniques to predict favorable vs. unfavorable outcomes following Gamma Knife radiosurgery (GKRS) of bAVMs and compare their prediction performances. METHODS: One hundred twenty-six patients seen at one academic medical center for GKRS obliteration of bAVMs over 15 years were retrospectively reviewed. Forty-two patients met the inclusion criteria. Favorable outcomes were defined as complete nidus obliteration demonstrated on cerebral angiogram and asymptomatic recovery. Unfavorable outcomes were defined as incomplete obliteration or complications relating to the AVM that developed after GKRS. Outcome predictions were made using a random forest model with hand-crafted radiomic features and a fine-tuned ResNet-34 convolutional neural network (CNN) model. The performance was evaluated by using a ten-fold cross-validation technique. RESULTS: The average accuracy and area-under-curve (AUC) values of the Random Forest Classifier (RFC) with radiomics features were 68.5 ±9.80% and 0.705 ±0.086, whereas those of the ResNet-34 model were 60.0 ±11.9% and 0.694 ±0.124. Four radiomics features used with RFC discriminated unfavorable response cases from favorable response cases with statistical significance. When cropped images were used with ResNet-34, the accuracy and AUC decreased to 59.3 ± 14.2% and 55.4 ±10.4%, respectively. CONCLUSIONS: A hand-crafted radiomics model and a pre-trained CNN model can be fine-tuned on pre-treatment MRI scans to predict clinical outcomes of AVM patients undergoing GKRS with equivalent prediction performance. The outcome predictions are promising but require further external validation on more patients.

Septic pulmonary embolism induced by dental infection.

Dental infection can be an important source for septic pulmonary embolism (SPE), but only a few cases of SPE accompanying dental infection have been reported. The aim of this study was to characterize the clinical features of SPE induced by dental infection. Patients who fulfilled the diagnostic criteria described in the text were recruited in a retrospective fashion. All 9 patients were men, with a median age of 59 years (range:47 to 74 years). Eight patients had chest pain (88.9%), 5 had a preceding toothache (55.6%) and 3 had preceding gingival swelling (33.3%). Blood cultures obtained from 7 patients were negative. Periodontitis was found in all of the cases, periapical periodontitis in 5 cases, and gingival abscess in 3 cases. The median duration of hospitalization was 15 days, and symptoms were mild in some cases. In addition to antimicrobial therapy, tooth extraction was performed in 3 cases, tooth scaling in 6. SPE induced by dental infection has prominent clinical characteristics such as male preponderance, chest pain, preceding toothache, and mild clinical course.

Errors introduced by dose scaling for relative dosimetry.

Some dosimeters require a relationship between detector signal and delivered dose. The relationship (characteristic curve or calibration equation) usually depends on the environment under which the dosimeters are manufactured or stored. To compensate for the difference in radiation response among different batches of dosimeters, the measured dose can be scaled by normalizing the measured dose to a specific dose. Such a procedure, often called "relative dosimetry", allows us to skip the time-consuming production of a calibration curve for each irradiation. In this study, the magnitudes of errors due to the dose scaling procedure were evaluated by using the characteristic curves of BANG3 polymer gel dosimeter, radiographic EDR2 films, and GAFCHROMIC EBT2 films. Several sets of calibration data were obtained for each type of dosimeters, and a calibration equation of one set of data was used to estimate doses of the other dosimeters from different batches. The scaled doses were then compared with expected doses, which were obtained by using the true calibration equation specific to each batch. In general, the magnitude of errors increased with increasing deviation of the dose scaling factor from unity. Also, the errors strongly depended on the difference in the shape of the true and reference calibration curves. For example, for the BANG3 polymer gel, of which the characteristic curve can be approximated with a linear equation, the error for a batch requiring a dose scaling factor of 0.87 was larger than the errors for other batches requiring smaller magnitudes of dose scaling, or scaling factors of 0.93 or 1.02. The characteristic curves of EDR2 and EBT2 films required nonlinear equations. With those dosimeters, errors larger than 5% were commonly observed in the dose ranges of below 50% and above 150% of the normalization dose. In conclusion, the dose scaling for relative dosimetry introduces large errors in the measured doses when a large dose scaling is applied, and this procedure should be applied with special care.

A practical strategy for incorporating the convolution algorithm in Leksell GammaPlan for routine treatment planning†.

Purpose: This study aims to establish criteria for convolution dose calculations and an efficient procedure to include the heterogeneity effects in GammaKnife radiosurgery (GKRS) treatment plans. Methods and materials: We analyzed 114 GKRS cases of various disease types, tumor locations, sizes, the number of fractions, and prescription doses. There was a total of 205 tumors. CT scans were performed in addition to routine MRI scans for all treatments. All treatment plans were created using the TMR10 algorithm (TMR10). We repeated the dose calculations for this study with the convolution algorithm (Conv). We calculated the ratios between Conv and TMR10 of the treatment volume (TxtVol), the volume covered by half of the prescription dose (TxtVol2), the minimum, maximum, and mean doses in the tumor (minDose, maxDose, and meanDose), and the volume of tumor covered by the prescription isodose (covVol). We then categorized those quantities for locations of tumors represented by the shortest distance of the skull surface from the tumor center (distC) and the tumor edge (distE). [Table: see text]. Results: All six ratios increased with increasing distC and distE. For example, the median minDose ratio increased from 0.885 to 0.933 as distE increased. There was a statistically significant difference in the minDose ratio between tumors of distE < 2 cm and distE ≥ 2 cm. On the other hand, the median maxDose ratio was about 0.933 [0.928-0.939], being almost independent of distE. This suggested a 6.1% overestimation of the delivered dose with TMR10. Conclusion: The heterogeneity effects must be considered for the volume dose calculations by applying the convolution algorithm when the distance of the skull surface from the closest point of the tumor is less than 2 cm to achieve less than 3% accuracy.

ACR-ABS-ASTRO Practice Parameter for the Performance of Low-Dose-Rate Brachytherapy.

AIM/OBJECTIVES/BACKGROUND: The American College of Radiology (ACR), the American Brachytherapy Society (ABS), and the American Society for Radiation Oncology (ASTRO) have jointly developed the following practice parameter for the performance of low-dose-rate (LDR) brachytherapy. LDR brachytherapy is the application of radioactive sources in or on tumors in a clinical setting with therapeutic intent. The advantages of LDR brachytherapy include improving therapeutic ratios with lower doses to nontarget organs-at-risk and higher doses to a specific target. METHODS: This practice parameter was developed according to the process described under the heading. The Process for Developing ACR Practice Parameters and Technical Standards on the ACR website (https://www.acr.org/Clinical-Resources/Practice-Parameters-and-Technical-Standards) by the Committee on Practice Parameters-Radiation Oncology of the Commission on Radiation Oncology, in collaboration with ABS and ASTRO. RESULTS: This practice parameter was developed to serve as a tool in the appropriate application of this evolving technology in the care of cancer patients or other patients with conditions where radiation therapy is indicated. It addresses clinical implementation of LDR brachytherapy including personnel qualifications, quality assurance standards, indications, and suggested documentation. This includes a contemporary literature search. CONCLUSIONS: This practice parameter is a tool to guide the use of LDR brachytherapy and does not assess relative clinical indication for LDR brachytherapy when compared with other forms of brachytherapy or external beam therapy, but to focus on the best practices required to deliver LDR brachytherapy safely and effectively, when clinically indicated. Comparative costs of versus other modalities therapy may also need to be considered.

Sexual differences in physiological integration in the dioecious shrub Lindera triloba: a field experiment using girdling manipulation.

BACKGROUND AND AIMS: It is important to consider the modular level when verifying sexual dimorphism in dioecious plants. Nevertheless, between-sex differences in resource translocation among modules (i.e. physiological integration) have not been tested at the whole-plant level. In this study, sexual differences in physiological integration were examined among ramets, within a genet in the dioecious sprouting shrub Lindera triloba, by a field experiment with girdling manipulation. METHODS: Female and male genets were randomly assigned to girdled or intact groups. Girdling of the main ramets was conducted in May 2009 by removing a ring of bark and cambium approx. 1 cm wide at a height of 80-100 cm. The effects of treatment and sex on ramet dynamics (mortality, recruitment and diameter growth) and inflorescence production during 1 year after girdling were examined. KEY RESULTS: The diameter growth rate of main ramets of both sexes was lower at ground level (D(0)) but higher at breast height (dbh) in girdled than in intact groups. In sprouted ramets with a dbh of 0-2 cm, males in girdled groups had lower growth rates at D(0) than those of intact groups, whereas no girdling effect was found for females. The main ramets in girdled groups produced more inflorescences than intact groups, irrespective of sex, but male ramets showed a greater response to the treatment than females. CONCLUSIONS: In L. triloba, physiological integration exists at the whole-plant level, and sprouted ramets are dependent on assimilates translocated from main ramets, but this dependence weakens as sprouted ramets get larger. Female sprouted ramets can grow in a physiologically independent manner from the main ramet earlier than those of males. This study highlights the importance of considering modular structures and physiological integration when evaluating sexual differences in demographic patterns of clonal plants.

A variable echo-number method for estimating R2 in MRI-based polymer gel dosimetry.

PURPOSE: Spin-spin relaxation rate R2 is commonly used to quantify absorbed dose for magnetic resonance imaging (MRI)-based polymer gel dosimetry. R2 is estimated by applying a parameter fitting algorithm to a train of spin-echo signals. However, a careless application of a large number of echoes can result in anomalous R2 values because the echo signal intensity decreases to the background signal offset level for a long echo time. In this article, the authors proposed and evaluated a variable echo-number (VAREC) method to remedy the problem. METHODS: The VAREC algorithm uses only echo signals, whose intensities are greater than a preset threshold. Here, the threshold is defined as the standard deviation of Gaussian noise times a multiplier alpha. The authors implemented three R2 estimation methods in an in-house program: The nonlinear least-squares algorithm (NLLS), the VAREC method, and the maximum likelihood estimator with the Rician signal intensity distribution (MLE_R). Those methods were used to estimate the R2 values of test phantoms with known R2 values and BANG3-type polymer gels, which were irradiated to 12 different doses ranging from 0 to 50 Gy. The R2 values were measured by using a 32-echo CPMG pulse sequence on 3 T MRI scanners. The R2 values of the VAREC method were compared with those of NLLS and MLE_R. RESULTS: The R2 values of the NLLS method incorrectly decreased to the zero-dose level for doses greater than 10 Gy. The R2 values of the VAREC method with alpha=2 agreed with those of MLE_R within the measurement uncertainty. The uncertainties of the R2 values were the smallest for alpha=2 or 3 among various alpha values. CONCLUSIONS: The VAREC algorithm is simple, fast, and robust for the R2 estimation. The authors recommend this method with alpha=2 or 3 for R2 estimation using multispin echo MRI protocols.

MRI-based polymer gel dosimetry for validating plans with multiple matrices in Gamma Knife stereotactic radiosurgery.

One of treatment planning techniques with Leksell GammaPlan (LGP) for Gamma Knife stereotactic radiosurgery (GKSRS) uses multiple matrices with multiple dose prescriptions. Computational complexity increases when shots are placed in multiple matrices with different grid sizes. Hence, the experimental validation of LGP calculated dose distributions is needed for those cases. For the current study, we used BANG3 polymer gel contained in a head-sized glass bottle to simulate the entire treatment process of GKSRS. A treatment plan with three 18 mm shots and one 8 mm shot in separate matrices was created with LGP. The prescribed maximum dose was 8 Gy to three shots and 16 Gy to one of the 18 mm shots. The 3D dose distribution recorded in the gel dosimeter was read using a Siemens 3T MRI scanner. The scanning parameters of a CPMG pulse sequence with 32 equidistant echoes were as follows: TR = 7 s, echo step = 13.6 ms, field-of-view = 256 mm × 256 mm, and pixel size = 1 mm × 1 mm. Interleaved acquisition mode was used to obtain 15 to 45 2-mm-thick slices. Using a calibration relationship between absorbed dose and the spin-spin relaxation rate (R2), we converted R2 images to dose images. MATLAB-based in-house programs were used for R2 estimation and dose comparison. Gamma-index analysis for the 3D data showed gamma values less than unity for 86% of the voxels. Through this study we accomplished the first application of polymer gel dosimetry for a true comparison between measured 3D dose distributions and LGP calculations for plans using multiple matrices for multiple targets.

Improved cellular automata model shows that indirect apoptotic cell death due to vascular damage enhances the local control of tumors by single fraction high-dose irradiation.

We investigated the effects of indirect apoptotic cell death due to vascular damage on tumor response to a single large dose with an improved two-dimensional cellular automata model. The tumor growth was simulated by considering the oxygen and nutrients supplied to the tumor through the blood vessels. The cell damage processes were modeled by taking account of the direct cell death and the indirect death due to the radiation-induced vascular damages. The radiation increased the permeation of oxygen and nutrients through the blood vessel or caused the breakdown of the vasculature. The amount of oxygen in cancer cells affected the response of cancer cells to radiation and the tumor growth rate after irradiation. The lack of oxygen led to the apoptotic death of cancer cells. We calculated the tumor control probability (TCP) at different radiation doses, the probability of apoptotic death, the threshold of the oxygen level for indirect apoptotic death, the average oxygen level in cancer cells and the vessel survival probability after radiation damage. Due to the vessel damage, indirect cell death led to a 4% increase in TCP for the dose ranging from 15 Gy to 20 Gy. TCP increased with increasing the probability of apoptotic death and the threshold of the oxygen level for indirect apoptotic death due to increased apoptotic death. The variation of TCP as a function of the average oxygen level exhibited the minimum at the average oxygen level of 2.7%. The apoptosis increased as the average oxygen level decreased, leading to an increasing TCP. On the other hand, the direct radiation damage increased, and the apoptosis decreased for higher average oxygen level, resulting in a higher TCP. We showed by modeling the radiation damage of blood vessels in a 2D CA simulation that the indirect apoptotic death of cancer cells, caused by the reduction of the oxygen level due to vascular damage after high dose irradiation, increased TCP.

Evaluation of two calibration methods for MRI-based polymer gel dosimetry.

Polymer gel dosimetry (PGD) can provide three-dimensional (3D) dose data for evaluation of the dose calculation algorithms used by treatment planning systems (TPS). Although the PGD technique, particularly with MRI, is now ready for clinical applications, an accurate calibration method is vital for treatment validation in 3D. This study evaluated the single-phantom electron beam (SPE) method that used the depth-dose data of a 9 MeV electron beam. This technique was compared with the multi-vial x-ray (MVX) method that used nine small vials irradiated with various doses. We tested two regression equations, i.e., third-order polynomial and tangent functions, and two dose-normalization methods, i.e., one-point and two-point methods. These methods were evaluated using a dose distribution generated by a 3 cm × 3 cm open arc beam. We used MAGAT polymer gel manufactured in-house. We found that the SPE method required a smaller dose scaling for the dose comparison. The tangent function showed better data fitting than the polynomial function with smaller uncertainty of the estimated coefficients. We did not observe a distinct advantage of the SPE method over the MVX method for the 3D dose comparison with the test case. From this study, we infer that the SPE method with the tangent function as the regression equation and one-point dose normalization is a good calibration option for the MRI-based polymer gel dosimetry.