R Pidikiti

Deficits in Functional Connectivity of Hippocampal and Frontal Lobe Circuits after Traumatic Axonal Injury

OBJECTIVE To examine the functional connectivity of hippocampal and selected frontal lobe circuits in patients with traumatic axonal injury (TAI). DESIGN Observational study. SETTING An inpatient traumatic brain injury unit. Imaging and neurocognitive assessments were conducted in an outpatient research facility. PARTICIPANTS Twenty-five consecutive patients with brain injuries consistent with TAI and acute subcortical white matter abnormalities were studied as well as 16 healthy volunteers of similar age and sex. INTERVENTIONS Echo-planar and high-resolution T1-weighted images were acquired using 3-T scanners. Regions of interest (ROI) were drawn bilaterally for the hippocampus, anterior cingulate cortex (ACC), and dorsolateral prefrontal cortex and were used to extract time series data. Blood oxygenation level-dependent data from each ROI were used as reference functions for correlating with all other brain voxels. Interhemispheric functional connectivity was assessed for each participant by correlating homologous regions using a Pearson correlation coefficient. Patient functional and neurocognitive outcomes were assessed approximately 6 months after injury. MAIN OUTCOME MEASURES Interhemispheric functional connectivity, spatial patterns of functional connectivity, and associations of connectivity measures with functional and neurocognitive outcomes. RESULTS Patients showed significantly lower interhemispheric functional connectivity for the hippocampus and ACC. Controls demonstrated stronger and more focused functional connectivity for the hippocampi and ACC, and a more focused recruitment of the default mode network for the dorsolateral prefrontal cortex ROI. The interhemispheric functional connectivity for the hippocampus was correlated with delayed recall of verbal information. CONCLUSIONS Traumatic axonal injury may affect interhemispheric neural activity, as patients with TAI show disrupted interhemispheric functional connectivity. More careful investigation of interhemispheric connectivity is warranted, as it demonstrated a modest association with outcome in chronic TBI.

Correlations of noninvasive BOLD and TOLD MRI with pO2 and relevance to tumor radiation response

To examine the potential use of blood oxygenation level dependent (BOLD) and tissue oxygenation level dependent (TOLD) contrast MRI to assess tumor oxygenation and predict radiation response.

Dosimetric characterization of an image-guided stereotactic small animal irradiator.

Small animal irradiation provides an important tool used by preclinical studies to assess and optimize new treatment strategies such as stereotactic ablative radiotherapy. Characterization of radiation beams that are clinically and geometrically scaled for the small animal model is uniquely challenging for orthovoltage energies and minute field sizes. The irradiator employs a commercial x-ray device (XRAD 320, Precision x-ray, Inc.) with a custom collimation system to produce 1-10 mm diameter beams and a 50 mm reference beam. Absolute calibrations were performed using the AAPM TG-61 methodology. Beams half-value layer (HVL) and timer error were measured with an ionization chamber. Percent depth dose (PDD), output factors (OFs) and off-axis ratios were measured using radiochromic film, a diode and a pinpoint ionization chamber at 19.76 and 24.76 cm source-to-surface distance (SSD). PDD measurements were also compared with Monte Carlo (MC) simulations. In-air and in-water absolute calibrations for the reference 50 mm diameter collimator at 19.76 cm SSD were measured as 20.96 and 20.79 Gy min(-1), respectively, agreeing within 0.8%. The HVL at 250 kVp and 15 mAs was measured to be 0.45 mm Cu. The reference field PDD MC simulation results agree with measured data within 3.5%. PDD data demonstrate typical increased penetration with increasing field size and SSD. For collimators larger than 5 mm in diameter, OFs measured using film, an ion chamber and a diode were within 3% agreement.

An x-ray image guidance system for small animal stereotactic irradiation

An x-ray image-guided small animal stereotactic irradiator was developed and characterized to enable tumor visualization and accurate target localization for small field, high dose irradiation. The system utilizes a custom collimation system, a motorized positioning system (x, y, θ), a digital imaging panel and operating software, and is integrated with a commercial x-ray unit. The essential characteristics of the irradiator include small radiation fields (1-10 mm), high dose rate (>10 Gy min(-1)) and submillimeter target localization. The software enables computer-controlled image acquisition, stage motion and target localization providing simple and precise automated target localization. The imaging panel was characterized in terms of signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and spatial resolution. Overall localization accuracy and precision were assessed. SNR, CNR and spatial resolution are 24 dB, 21 dB and 2.8 lp mm(-1), respectively, and localization accuracy is approximately 65 µm with 6 µm precision. With the aid of image guidance, system performance was subsequently used to evaluate radiation response in a rat orthotopic lung tumor effectively sparing normal tissues and in a mouse normal lung. The capabilities of 3D treatment and cone-beam computed tomography are presented for 3D localization and delivery as a work in progress.

Partial Depletion of Regulatory T Cells Does Not Influence the Inflammation Caused by High Dose Hemi-Body Irradiation

There is clinical interest in the modulation of regulatory T cells for cancer therapy. The safety of these therapies in combination with conventional anti-cancer therapies, including radiation therapy, can be studied in animal models. The effects of partial depletion of regulatory T (Treg) cells with an anti-CD25 antibody in conjunction with ionizing radiation on inflammation and tissue injury were analyzed in C57BL/6 mice. An anti-CD25 antibody (PC61) was administered 3 days prior to 13 Gy lower-half hemi-body irradiation (HBI). The blood, spleen, mesenteric lymph nodes (mLNs) and inguinal lymph nodes (iLNs) were harvested at various times thereafter. Alterations in the proportion of leukocyte subsets including CD4+ T cells, CD8+ T cells, Treg cells, B cells, NK cells, NK1.1+ T cells, macrophages and granulocytes were analyzed by FACS. The lungs, liver, pancreas, stomach, jejunum, duodenum, ileum, colon and kidney were harvested and studied by H&E staining. Expression of inflammatory mediators in plasma and tissue were investigated by ELISA. HBI significantly decreased the leukocyte pool though the various leukocyte subsets had different sensitivities to HBI. The administration of PC61 significantly decreased the proportion of Treg cells in spleen, iLN, mLN and blood (reduction of approximately 60%). Irradiation significantly increased the proportion of Treg cells in the spleen, iLN and mLN. HBI induced a systemic inflammatory reaction as demonstrated by increased plasma levels of IL-6, KC/CXCL1 and circulating granulocytes in the blood. Neutrophils also infiltrated the small bowel. The same general patterns were observed whether or not Treg cells were partially depleted with PC61 prior to HBI. These data demonstrate that partial depletion of Treg cells in these mice does not influence HBI-induced inflammatory response and tissue injury, and that combining anti-CD25 therapy with radiation may be safe and well tolerated in a clinical setting.

A role for dynamic contrast-enhanced magnetic resonance imaging in predicting tumour radiation response

Background:Dynamic contrast-enhanced (DCE) MRI may provide prognostic insights into tumour radiation response. This study examined quantitative DCE MRI parameters in rat tumours, as potential biomarkers of tumour growth delay following single high-dose irradiation.Methods:Dunning R3327-AT1 prostate tumours were evaluated by DCE MRI following intravenous injection of Gd-DTPA. The next day tumours were irradiated (single dose of 30 Gy), while animals breathed air (n=4) or oxygen (n=4); two animals were non-irradiated controls. Growth was followed and tumour volume-quadrupling time (T4) was compared with pre-irradiation DCE assessments.Results:Irradiation caused significant tumour growth delay (T4 ranged from 28 to 48 days for air-breathing rats, and 40 to 75 days for oxygen-breathing rats) compared with the controls (T4=7 to 9 days). A strong correlation was observed between T4 and extravascular-extracellular volume fraction (ve) irrespective of the gas inhaled during irradiation. There was also a correlation between T4 and volume transfer constant (Ktrans) for the air-breathing group alone.Conclusions:The data provide rationale for expanded studies of other tumour sites, types and progressively patients, and are potentially significant, as many patients undergo contrast-enhanced MRI as part of treatment planning.

SU‐E‐T‐274: Monte Carlo Simulations of Output Factors for a Small Animal Irradiator

PURPOSE Measurement of dosimetric parameters of small photon beams, with field sizes as small 1 mm in diameter, is particularly challenging. This work utilizes Monte Carlo techniques to calculate percent depth dose (PDD) and output factors for small photon fields from a kV x-ray based small animal irradiator. METHODS Absolute dose calibration of a commercial small animal stereotactic irradiator (XRAD225, Precision X-ray) was performed in accordance with the recommendations of AAPM TG-61 protocol. Both in-air and in-water calibrations were performed at a 30.4 cm source-to-surface distance (SSD) for a reference collimator 50 mm in diameter. The BEAM/EGS was used to model 225 kV photon beams used for most therapeutic applications. The Monte Carlo model was provided good agreement with measured beam characteristics, e.g. PDD and off-axis ratios. Subsequently, output factors for various square and circular applicators were measured using an ionization chamber and radiochromic film, and compared with MC simulations. Directional Bremsstrahlung splitting (DBS) was utilized for variance reduction to improve efficiency of the output factor simulations. The statistical uncertainty on the MC- calculated results is between 0.5% and 1% for most points. RESULTS The absolute dose measured for reference collimator at 30.4 cm SSD in water and in air is 4.1 and 4.12 Gy/min. The agreement between simulated and measured output factors was excellent, ranging from 1% to 2.84%. The MC- simulated and measured depth dose data, normalized at the surface, show excellent agreement, with a maximum deviation is approximately 2.5 %. CONCLUSIONS Monte Carlo simulation provides an indispensible tool for validating measurements of the smallest field sizes used in preclinical small animal irradiation.

SU‐GG‐J‐06: Optical and X‐Ray Image Guided Stereotactic Body Irradiator Dedicated to Small Animals

Purpose: To develop a dual modality, optical and x‐ray, image guided stereotactic irradiator, and to improve tumor visualization and the accuracy of target localization in small animal irradiation. Method and Materials: A bioluminescence (BL) optical imaging system has been integrated into an existing x‐ray image‐guided stereotactic irradiator developed previously by our group. The new system provides precise target localization and accurate radiation delivery in three dimensions. All localization procedures are computer‐controlled. An air‐cooled CCDcamera with sufficiently reduced dark current for better signal‐to‐noise ratio is employed for bioluminescenceimaging. The CCDcamera is positioned off‐axis, in the same transverse plane as the x‐ray radiation axis. To acquire BL images, the animal is rotated through a 30 degree angle towards the camera, and then back to the original (treatment) position for the x‐ray imaging. The BL images are registered to the x‐ray images for image guidance. Results: The dual modality image guided irradiator was evaluated using an orthotopic lung tumor in a rat. The anterior and lateral BL images clearly showed the tumor with high contrast, while the same tumor was visible only in the anterior view in the x‐ray images. This indicated that the x‐ray image guidance alone is not sufficient to provide the precise localization. On the other hand, the registered BL and x‐ray images overcome this limitation by combining information of tumor position in the BL image with the anatomy in the x‐ray image.Conclusion: The accuracy of tumor localization in small animal SBRT is dramatically improved by integrating optical imaging with x‐ray imaging. The developed system also enhances the sparing of dose limiting normal tissues and organs‐at‐risk (OARs) and has facilitated accurate assessment of experiments in small animal SBRT investigations.

TH‐C‐204B‐08: Energy Dependency and Dosimetric Modeling in Small Animal Stereotactic Irradiaton

Purpose: Small animal irradiation provides an invaluable tool for preclinical studies to assess and optimize new treatment strategies such as SBRT. However dosimetric accuracy for beams that are clinically and geometrically scaled for the small animal model is uniquely challenged by low energies and minute field sizes. Recent experiences with GafChromic film have evinced energy dependencies that preclude standard methods of dosimetric calibration with MV beams. Characterization of such dependencies has led to comprehensive recommissioning of the small animal stereotactic beam model based on the AAPM TG‐61 protocol to maximize the accuracy required by preclinical foundational research. Method and Materials: The irradiator couples a commercial x‐ray device (XRAD 320 Precision X‐Ray Inc. N. Branford CT) with a custom collimation system to produce 1–10 mm diameter beams and a 50 mm reference beam. Absolute calibrations were performed using TG‐61 methodology. Beam HVLs and timer error were measured with an ionization chamber. PDD output factors (OF) and OAR were measured using EBT‐2 a radiochromic film from ISP diode and pinpoint ionization chamber at 19.76 and 24.76 cm SSD. Results were compared with Monte Carlo simulations.Results: Absolute calibration for irradiator reference conditions were measured as 19.95 and 19.79 Gy/min for in‐air and in‐water methods respectively agreeing within 0.8%. Beam quality at 250 kVp and 15 mAs was measured to be 0.45 mm Cu. PDD data demonstrate typical increased penetration with increasing field size. For collimators >5 mm output factors measured using film chamber and diode were within 3% agreement. For smaller collimators output factors measured with film agree well with Monte Carlo simulation results. Conclusion: Careful application of TG‐61 methodology is essential for both commissioning small animal treatment beams and mitigating Gafchromic film energy dependencies. This enables beam modeling and subsequent small animaltreatment planning with the accuracy required of preclinical studies.

Image-Guided Stereotactic Small Animal Irradiator

Stereotactic body radiation therapy (SBRT) is a promising modality in the treatment of cancer. Despite success in early clinical applications and in subsequent clinical trials, there remains much to learn to understand and optimize the effects in tumors and normal tissues. In this study, we develop and characterize an image-guided small animal stereotactic body irradiator capable of delivering a highly localized radiation beam accurately to a small target. The irradiator employs a commercial X-ray device (XRAD 320, Precision X-Ray, Inc.) and a modular collimation system, consisting of a brass collimator holder and variable tungsten alloy collimators with apertures ranging from 1 to 10 mm in diameter. The unit is typically operated at 30 kVp for image guidance and at 250 kVp for therapy. To characterize the radiation beam, we measured percent depth dose (PDD), off-axis ratios (OARs), and absolute dose rate for each collimator using radiochromic film (Gafchromic EBT, International Specialty Products, Wayne, NJ). For all collimators the penumbra, defined as distance between 80% and 20% isodoses, was measured at a source-to-surface distance (SSD) of approximately 20 cm. For a 5 mm collimator, in-plane and cross-plane measurements of penumbra were 0.7 and 1.1 mm, respectively. Absolute dose rates ranged from 6.7 to 11.6 Gy/min for the 1 to 10 mm collimators. The image guidance system provided accuracy commensurate with stereotactic localization in small animals. We have demonstrated the capability of an image-guided stereotactic radiotherapy by the animal studies. Subsequent studies in a variety of pre-clinical tumor and normal tissue models suggest that use of the irradiator can significantly contribute to the understanding of new clinical therapies such as SBRT.