Neuronal STING activation in amyotrophic lateral sclerosis and frontotemporal dementia

The stimulator of interferon genes (STING) pathway has been implicated in neurodegenerative diseases, including Parkinson's disease and amyotrophic lateral sclerosis (ALS). While prior studies have focused on STING within immune cells, little is known about STING within neurons. Here, we document neuronal activation of the STING pathway in human postmortem cortical and spinal motor neurons from individuals affected by familial or sporadic ALS. This process takes place selectively in the most vulnerable cortical and spinal motor neurons but not in neurons that are less affected by the disease. Concordant STING activation in layer V cortical motor neurons occurs in a mouse model of C9orf72 repeat-associated ALS and frontotemporal dementia (FTD). To establish that STING activation occurs in a neuron-autonomous manner, we demonstrate the integrity of the STING signaling pathway, including both upstream activators and downstream innate immune response effectors, in dissociated mouse cortical neurons and neurons derived from control human induced pluripotent stem cells (iPSCs). Human iPSC-derived neurons harboring different familial ALS-causing mutations exhibit increased STING signaling with DNA damage as a main driver. The elevated downstream inflammatory markers present in ALS iPSC-derived neurons can be suppressed with a STING inhibitor. Our results reveal an immunophenotype that consists of innate immune signaling driven by the STING pathway and occurs specifically within vulnerable neurons in ALS/FTD.

Optimal Planning Target Margin for Prostate Radiotherapy Based on Interfractional and Intrafractional Variability Assessment during 1.5T MR-Guided Radiotherapy

PURPOSE/OBJECTIVE(S)

MR-guided radiotherapy (MRgRT) provides superior soft-tissue contrast over CT-based image guidance. We collected and analyzed daily pre-treatment (PRE) and real-time motion-monitoring (MM) MR images of patients receiving prostate radiotherapy to assess interfractional and intrafractional variability of prostate using two localization methods: pelvic bony anatomy (bone) and prostate during online adaptive radiotherapy (ART).

MATERIALS/METHODS

PRE and MM MRIs for the first five fractions of twenty prostate cancer patients who received definitive MRgRT with 1.5T MRI were collected. Using MIM software, rigid registration between PRE MRI and planning CT images based on pelvic bony anatomy and prostate reproduced bone localization and online ART, respectively. To determine interfractional setup margin (SM), prostate was delineated on all PRE MRIs registered after bone and prostate localizations by a radiation oncologist, and centroid values of prostate contours between planning CT and PRE MRIs were compared. To determine interobserver variability, another radiation oncologist, a medical physicist, and a radiotherapist contoured prostate for both localization methods. For internal margin (IM) assessment, we used MM MRIs of the five patients who had all three sets of coronal, sagittal, and axial cine images and determined the maximum contour displacement using in-house MATLAB-based software converting binary image files to 2D cine images with a superimposed grid of 1 mm spacing.

RESULTS

A total of 100 PRE and 25 MM MRIs were analyzed. Four hundred prostate contours were delineated on MR images registered with planning CT based on both bony anatomy and prostate. After bone localization, SM was 0.57±0.42 mm in left-right (LR), 2.45±1.98 mm in anterior-posterior (AP), and 2.28±2.08 mm in superior-inferior (SI) directions, and IO was 1.06±0.58 mm in LR, 2.32±1.08 mm in AP, and 3.30±1.85 mm in SI directions. After prostate localization, SM was 0.76±0.57 mm in LR, 1.89±1.60 mm in AP, and 2.2±1.79 mm in SI directions, and IO was 1.11±0.55 mm in LR, 2.13±1.07 mm in AP, and 3.53±1.65 mm in SI directions. Average IM was 2.12±0.86 mm, 2.24±1.07 mm, and 2.84±0.88 mm in LR, AP, and SI directions, respectively.

CONCLUSION

Using daily MRIs from MRgRT, we showed that movements in the SI direction were the largest source of variability in prostate definitive RT. In addition, interobserver variability was a non-negligible source of margin. Optimal PTV margin should also consider internal margin, especially in the SI direction.

Randomized evaluation of an online single-session intervention for minority stress in LGBTQ+ adolescents

Background

LGBTQ+ youth face myriad adverse health outcomes due to minority stress, creating a need for accessible, mechanism-targeted interventions to mitigate these minority stress-related risk factors. We tested the effectiveness and acceptability of Project RISE, an online single-session intervention designed to ameliorate internalized stigma and improve other outcomes among LGBTQ+ youth. We hypothesized that youth assigned to RISE (versus a control) would report significantly reduced internalized stigma and increased identity pride at post-intervention and at two-week follow-up and would find RISE acceptable.

Methods

We recruited adolescents nationally through Instagram advertisements in May 2022 (N = 538; M age = 15.06, SD age = 0.97). Participants were randomly assigned to RISE or an information-only control and completed questionnaires pre-intervention, immediately post-intervention, and two weeks post-intervention. Inclusion criteria included endorsing: (1) LGBTQ+ identity, (2) age 13-16, (3) English fluency (4) Internet access, and (5) subjective negative impact of LGBTQ+ stigma.

Results

Relative to participants in the control condition, participants who completed RISE reported significant decreases in internalized stigma (d = -0.49) and increases in identity pride (d = 0.25) from pre- to immediately post-intervention, along with decreased internalized stigma (d = -0.26) from baseline to two-week follow-up. Participants rated both RISE and the information-only control as highly, equivalently acceptable.

Conclusions

RISE appears to be an acceptable and useful online SSI for LGBTQ+ adolescents, with potential to reduce internalized stigma in both the short- and longer-term. Future directions include evaluating effects of Project RISE over longer follow-ups and in conjunction with other mental health supports.

Understanding patient-level costs of weekly isoniazid-rifapentine (3HP) among people living with HIV in Uganda

BACKGROUND: Twelve weeks of weekly isoniazid and rifapentine (3HP) prevents TB disease among people with HIV (PWH), but the costs to people of taking TB preventive treatment is not well described.METHODS: We surveyed PWH who initiated 3HP at a large urban HIV/AIDS clinic in Kampala, Uganda, as part of a larger trial. We estimated the cost of one 3HP visit from the patient perspective, including both out-of-pocket costs and estimated lost wages. Costs were reported in 2021 Ugandan shillings (UGX) and US dollars (USD; USD1 = UGX3,587)RESULTS: The survey included 1,655 PWH. The median participant cost of one clinic visit was UGX19,200 (USD5.36), or 38.5% of the median weekly income. Per visit, the cost of transportation was the largest component (median: UGX10,000/USD2.79), followed by lost income (median: UGX4,200/USD1.16) and food (median: UGX2,000/USD0.56). Men reported greater income loss than women (median: UGX6,400/USD1.79 vs. UGX3,300/USD0.93), and participants who lived further than a 30-minute drive to the clinic had higher transportation costs than others (median: UGX14,000/USD3.90 vs. UGX8,000/USD2.23).CONCLUSION: Patient-level costs to receive 3HP accounted for over one-third of weekly income. Patient-centered approaches to averting or defraying these costs are needed.

Risk of stroke with antivenom usage after venomous snakebite in Taiwan: a population-based cohort study

BACKGROUND AND PURPOSE

Stroke is a rare complication of snakebites, but may lead to serious sequelae. We aimed to explore the relationship between venomous snakebite and the risk for acute stroke, in a nationwide population-based cohort study.

METHODS

This retrospective cohort study used claims data between 1 January 2000 and 31 December 2012, from the Taiwan National Health Insurance Research Database. The study included data of patients aged 18 years or older with venomous snakebite (n = 535), matched for propensity score with controls without venomous snakebite (n = 2140). The follow-up period was the duration from the initial diagnosis of venomous snakebite and administration of antivenom to the date of an acute stroke, or until 31 December 2013. The competing risk model was used to estimate the hazard ratio (HR) and 95% confidence intervals (CIs) of stroke, ischemic stroke and hemorrhagic stroke, after adjusting for demographic and other possible stroke risk factors.

RESULTS

The adjusted HR for the venomous snakebite group compared with the control group was 2.68 for hemorrhagic stroke (95% CI = 1.35-5.33). Stratified analysis showed that the older age group (>65 years old) had a higher risk of hemorrhagic stroke. A 2.72-fold significant increase in the risk for hemorrhagic stroke was observed following venomous snakebite with antivenom usage (95% CI = 1.41-5.26).

CONCLUSION

Venomous snakebite is associated with an increased risk of hemorrhagic stroke after the use of antivenom. Further study of the underlying mechanism is warranted.

The impact of nonoptimal lipids on the progression of coronary artery calcification in asymptomatic young adults: results from the KOICA registry

Funding Acknowledgements

Type of funding sources: None.

Background

Recently, as cumulative exposure of lipids for a lifetime has become important to prevent and predict atherosclerotic cardiovascular disease (ASCVD), it is recommended to obtain the optimal lipid levels from a young age. However, questions remain regarding the vascular changes in young adulthood by nonoptimal lipid levels.

Purpose

We aimed to investigate the progression of coronary artery calcification (CAC) according to lipid profiles in Korean young adults.

Methods

From the KOrea Initiative on Coronary Artery calcification registry, we collected 2,940 statin-naïve adults under 45-year-old, undergoing serial coronary artery calcium scans for the purpose of routine health check-ups between 2002 and 2017. CAC progression was assessed according to the optimality of lipid levels and each lipid variable.

Results

In this cohort (mean age, 41.3 years; man 82.4%), only 477 subjects (16.2%) had the optimal lipid profile, defined as triglycerides <150 mg/dl, LDL cholesterol <100 mg/dl, and HDL cholesterol ≥45 mg/dl. During follow-up (median 39.7 months), CAC progression was observed in 438 participants (14.8%) and more frequent in nonoptimal lipid group (16.5% vs 5.9%; p<0.001). Nonoptimal lipid levels during young adulthood increased the risk of CAC progression after adjusting for other cardiovascular risk factors (adjusted HR, 2.36; p=0.001), with a stepwise risk increase according to lipid levels. In particular, in the subjects with an initial calcium score of zero (adjusted HR, 2.13; p=0.014), those in their 20s or 30s (adjusted HR 2.70; p=0.029), and those without any other risk factors (adjusted HR 2.51; p=0.025), deemed as very low-risk groups, nonoptimal lipid levels more than doubled the risk of CAC progression, respectively. Among lipid variables, high triglycerides appeared to provide the greatest impact on CAC progression of young adults.

Conclusions

The proportion of young adults with optimal lipid levels was lower than expected. Nonoptimal lipid level was significantly associated with the risk of CAC progression in young adults, even with low-risk. Triglycerides had the strongest association with the risk of CAC progression. Screening and intervention for nonoptimal lipid levels, particularly triglycerides, from an early age might be of clinical value.

Association of blood pressure classification using the 2017 ACC/AHA guidelines with the progression of coronary artery calcification

Funding Acknowledgements

Type of funding sources: None.

Aims

The recent American College of Cardiology/American Heart Association guidelines reinforced the criteria for diagnosis of hypertension. We assessed the association of this reinforced criteria with coronary artery calcification (CAC) progression in asymptomatic adults according to different age groups.

Methods and results

We included 10,839 asymptomatic Korean adults (23.4% aged ≤45 years) who underwent at least two coronary artery calcium score (CACS) evaluations between 2012 and 2016. All participants were categorised into normal blood pressure (BP) (untreated systolic/diastolic <120/<80 mmHg), elevated BP (untreated systolic/diastolic 120–129/<80 mmHg), stage 1 hypertension (untreated systolic/diastolic BP 130–139/80–89 mmHg), or stage 2 hypertension (systolic/diastolic BP ≥140/≥90 mmHg or taking anti-hypertensive medication) groups. CAC progression was defined as a difference ≥2.5 between the square roots (√) of the baseline and follow-up CACSs (Δ√transformed CACS). During a mean 3.3-year follow-up, the incidence of CAC progression was 13.5% and 36.3% in groups with ≤45 and >45 years, respectively. After multivariable adjustment for sex, diabetes, dyslipidaemia, obesity, current smoking, and baseline CACS, hazard ratios (95% confidence interval) for CAC progression versus normal BP in elevated BP, stage 1 hypertension, and stage 2 hypertension were 1.39 (0.93–2.07)(P = 0.107), 1.59 (1.17–2.17)(P = 0.003), and 2.38 (1.82–3.12)(P <0.001) in the ≤45 years group and 1.13 (0.97–1.32)(P = 0.111), 1.20 (1.06–1.34)(P = 0.003), and 1.72 (1.57–1.87)(P <0.001) in the >45 years group.

Conclusion

Newly defined stage 1 hypertension is independently associated with CAC progression in asymptomatic Korean adults irrespective of different age groups.

Development and Validation of a Deep Learning-Based Automatic Brain Segmentation and Classification Algorithm for Alzheimer Disease Using 3D T1-Weighted Volumetric Images

BACKGROUND AND PURPOSE

Limited evidence has suggested that a deep learning automatic brain segmentation and classification method, based on T1-weighted brain MR images, can predict Alzheimer disease. Our aim was to develop and validate a deep learning-based automatic brain segmentation and classification algorithm for the diagnosis of Alzheimer disease using 3D T1-weighted brain MR images.

MATERIALS AND METHODS

A deep learning-based algorithm was developed using a dataset of T1-weighted brain MR images in consecutive patients with Alzheimer disease and mild cognitive impairment. We developed a 2-step algorithm using a convolutional neural network to perform brain parcellation followed by 3 classifier techniques including XGBoost for disease prediction. All classification experiments were performed using 5-fold cross-validation. The diagnostic performance of the XGBoost method was compared with logistic regression and a linear Support Vector Machine by calculating their areas under the curve for differentiating Alzheimer disease from mild cognitive impairment and mild cognitive impairment from healthy controls.

RESULTS

In a total of 4 datasets, 1099, 212, 711, and 705 eligible patients were included. Compared with the linear Support Vector Machine and logistic regression, XGBoost significantly improved the prediction of Alzheimer disease (P < .001). In terms of differentiating Alzheimer disease from mild cognitive impairment, the 3 algorithms resulted in areas under the curve of 0.758-0.825. XGBoost had a sensitivity of 68% and a specificity of 70%. In terms of differentiating mild cognitive impairment from the healthy control group, the 3 algorithms resulted in areas under the curve of 0.668-0.870. XGBoost had a sensitivity of 79% and a specificity of 80%.

CONCLUSIONS

The deep learning-based automatic brain segmentation and classification algorithm allowed an accurate diagnosis of Alzheimer disease using T1-weighted brain MR images. The widespread availability of T1-weighted brain MR imaging suggests that this algorithm is a promising and widely applicable method for predicting Alzheimer disease.

Medical infrared thermal imaging of canine appendicular bone neoplasia

Background

Medical infrared thermal imaging (MITI) is a noninvasive imaging modality used in veterinary medicine as a screening tool for musculoskeletal and neurological disease processes. An infrared camera measures the surface body heat and produces a color map that represents the heat distribution. Local trauma or disease can impair the autonomic nervous system, which leads to changes in the local dermal microcirculation and subsequent alteration of surface body heat. Disruption of autonomic flow to the cutaneous vasculature at deeper levels can also result in asymmetric thermographic results. The purpose of this study was to evaluate surface temperature differences between limbs affected by bone neoplasia and their normal contralateral limbs.

Results

A statistically significant difference in average temperature was noted between regions of interest of the two groups (paired difference: 0.53 C° ± 0.14; P = 0.0005). In addition, pattern recognition analysis yielded a 75–100% success rate in lesion identification.

Conclusions

Significant alterations noted with average temperature and thermographic patterns indicate that MITI can document discernible changes associated with the presence of canine appendicular bone tumors. While MITI cannot be used as the sole diagnostic tool for bone cancer, it can be used as a screening modality and may be applicable in early detection of cancer.

1423Adherence to triple component antihypertensive regimen is higher in single-pill combination than two-pill regimen: data from a randomized controlled trial using medication event monitoring system

Background

Simplicity of regimen is known to be an important determinant of medication adherence and using single-pill combination (SPC) in hypertension treatment resulted in better adherence and persistence than free-equivalent combination. However, this finding has been studied only in dual-component antihypertensive treatments and in observational studies using medication possession ratio as an index of adherence. Medication event monitoring system (MEMS) is considered to be the gold standard in estimating medication adherence.

Purpose

To investigate the superiority in adherence of triple-component SPC compared to equivalent two-pill regimen using MEMS

Methods

This is a multi-center open-label randomized controlled trial. Inclusion criteria were hypertensive patients whose clinic blood pressure is not adequately controlled (systolic >140 mmHg or diastolic >90 mmHg) with combination antihypertensive regimen comprising two of three classes (angiotensin receptor blocker, calcium channel blocker and thiazide diuretics) for at least 4 weeks. Eligible patients were randomized either to single-pill (triple-component SPC, olmesartan/amlodipine/ hydrochlorothiazide 20/5/12.5 mg) or two-pill (dual-component SPC + one free pill, olmesartan/hydrochlorothiazide 20/12.5 mg + amlodipine 5 mg) groups and maintained for 12 weeks. Medications were dispensed in MEMS. Primary outcomes were the difference of percentage of dose taken (PDT) and percentage of days with prescribed dose taken correctly (PDTc) between single- and two-pill therapy, calculated from MEMS data.

Results

From 8 hospitals, 146 hypertensive patients were randomized into single- and two-pill groups. Final analysis was done in 65 and 66 patients in each group from which adherence index could be obtained. Baseline clinical characteristics of the two groups were not different. The single-pill group had significantly higher PDT and PDTc compared to the two-pill group. (median (25–75 percentile) (%), PDT 95.1 (87.9 - 100.0) vs 91.2 (79.8 - 96.5); PDTc 93.1 (79.8 - 96.5) vs 91.3 (70.7 - 96.4), p = both 0.04, by Wilcoxon rank sum test)

Percent dose taken

Conclusion

Single-pill combination of triple-component antihypertensive regimen showed superior adherence compared to equivalent two-pill therapy. Reducing pill burden by using SPC is a relevant strategy to enhance the adherence to multi-drug antihypertensive therapy.

Acknowledgement/Funding

Daiichi-Sankyo

The recruitment of extra-intestinal cells to the injured mucosa promotes healing in radiation enteritis and chemical colitis in a mouse parabiosis model

Mucosal healing occurs through migration and proliferation of cells within injured epithelium, yet these processes may be inadequate for mucosal healing after significant injury where the mucosa is denuded. We hypothesize that extra-intestinal cells can contribute to mucosal healing after injury to the small and large intestine. We generated parabiotic pairs between wild-type and tdTomato mice, which were then subjected to radiation-induced enteritis and 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis. We now show that as compared with singleton mice, mice with a parabiotic partner were protected against intestinal damage as revealed by significantly reduced weight loss, reduced expression of pro-inflammatory cytokines, reduced enterocyte apoptosis, and improved crypt proliferation. Donor cells expressed CD45-, Sca-1+, c-kit+, and CXCR4+ and accumulated around the injured crypts but did not transdifferentiate into epithelia, suggesting that extra-intestinal cells play a paracrine role in the healing response, while parabiotic pairings with Rag1-/- mice showed improved healing, indicating that adaptive immune cells were dispensable for mucosal healing. Strikingly, ablation of the bone marrow of the donor parabionts removed the protective effects. These findings reveal that the recruitment of extra-intestinal, bone marrow-derived cells into the injured intestinal mucosa can promote mucosal healing, suggesting novel therapeutic approaches for severe intestinal disease.

Associations between birth size and later height from infancy through adulthood: An individual based pooled analysis of 28 twin cohorts participating in the CODATwins project

BACKGROUND

There is evidence that birth size is positively associated with height in later life, but it remains unclear whether this is explained by genetic factors or the intrauterine environment.

AIM

To analyze the associations of birth weight, length and ponderal index with height from infancy through adulthood within mono- and dizygotic twin pairs, which provides insights into the role of genetic and environmental individual-specific factors.

METHODS

This study is based on the data from 28 twin cohorts in 17 countries. The pooled data included 41,852 complete twin pairs (55% monozygotic and 45% same-sex dizygotic) with information on birth weight and a total of 112,409 paired height measurements at ages ranging from 1 to 69 years. Birth length was available for 19,881 complete twin pairs, with a total of 72,692 paired height measurements. The association between birth size and later height was analyzed at both the individual and within-pair level by linear regression analyses.

RESULTS

Within twin pairs, regression coefficients showed that a 1-kg increase in birth weight and a 1-cm increase in birth length were associated with 1.14-4.25 cm and 0.18-0.90 cm taller height, respectively. The magnitude of the associations was generally greater within dizygotic than within monozygotic twin pairs, and this difference between zygosities was more pronounced for birth length.

CONCLUSION

Both genetic and individual-specific environmental factors play a role in the association between birth size and later height from infancy to adulthood, with a larger role for genetics in the association with birth length than with birth weight.

Genetic influence on the association between bone mineral density and testosterone in Korean men

Low bone mineral density (BMD) leads to an increased risk of osteoporotic fracture. Total testosterone and free testosterone were positively associated with BMD, which was significantly influenced by the additive genetic effects.

INTRODUCTION

This cross-sectional study aimed to evaluate an association between testosterone and BMD and the influence of genetic factors on the association.

METHODS

Study subjects were 1070 Korean men including 144 pairs of monozygotic twins and their family members. Levels of serum total testosterone and sex hormone binding globulin (SHBG) were measured by chemiluminescence immunoassay. Calculated free testosterone (cFT) was then determined using Vermeulen's method. BMDs of the whole body and specific regions were measured using dual-energy X-ray absorptiometry.

RESULTS

Linear mixed regression analyses showed that total testosterone and cFT were positively associated with BMD at most regions, after considering intra-familial relationship and covariates including fat mass, lean mass, and SHBG. SHBG had an inverse association with BMD at the pelvis but not with the BMD at other regions after adjusting for all covariates and cFT. Co-twin control analysis in monozygotic twins found no association between pairwise difference of testosterone and pairwise difference of BMD. Bivariate variance component analysis showed that both total testosterone and cFT had a significant positive additive genetic correlation with BMD at rib, spine, and arm, whereas SHBG had no significant genetic correlation with BMD. Inverse environmental correlations were seen between total testosterone and BMDs at the lumbar spine and arm.

CONCLUSIONS

This Korean twin and family study showed that both total testosterone and free testosterone were positively associated with BMD and that genetic effects were significant on the association between testosterone and BMD.

Five-year decreased incidence of surgical site infections following gastrectomy and prosthetic joint replacement surgery through active surveillance by the Korean Nosocomial Infection Surveillance System

BACKGROUND

Surveillance of healthcare-associated infection has been associated with a reduction in surgical site infection (SSI).

AIM

To evaluate the Korean Nosocomial Infection Surveillance System (KONIS) in order to assess its effects on SSI since it was introduced.

METHODS

SSI data after gastrectomy, total hip arthroplasty (THA), and total knee arthroplasty (TKA) between 2008 and 2012 were analysed. The pooled incidence of SSI was calculated for each year; the same analyses were also conducted from hospitals that had participated in KONIS for at least three consecutive years. Standardized SSI rates for each year were calculated by adjusting for SSI risk factors. SSI trends were analysed using the Cochran-Armitage test.

FINDINGS

The SSI rate following gastrectomy was 3.12% (522/16,918). There was a significant trend of decreased crude SSI rates over five years. This trend was also evident in analysis of hospitals that had participated for more than three years. The SSI rate for THA was 2.05% (157/7656), which decreased significantly from 2008 to 2012. The risk factors for SSI after THA included the National Nosocomial Infections Surveillance risk index, trauma, reoperation, and age (60-69 years). The SSI rate for TKA was 1.90% (152/7648), which also decreased significantly during a period of five years. However, the risk-adjusted analysis of SSI did not show a significant decrease for all surgical procedures.

CONCLUSION

The SSI incidence of gastrectomy and prosthetic joint replacement declined over five years as a result of active surveillance by KONIS.

Fatigue-induced microdamage in cancellous bone occurs distant from resorption cavities and trabecular surfaces

Impaired bone toughness is increasingly recognized as a contributor to fragility fractures. At the tissue level, toughness is related to the ability of bone tissue to resist the development of microscopic cracks or other tissue damage. While most of our understanding of microdamage is derived from studies of cortical bone, the majority of fragility fractures occur in regions of the skeleton dominated by cancellous bone. The development of tissue microdamage in cancellous bone may differ from that in cortical bone due to differences in microstructure and tissue ultrastructure. To gain insight into how microdamage accumulates in cancellous bone we determined the changes in number, size and location of microdamage sites following different amounts of cyclic compressive loading. Human vertebral cancellous bone specimens (n=32, 10 male donors, 6 female donors, age 76 ± 8.8, mean ± SD) were subjected to sub-failure cyclic compressive loading and microdamage was evaluated in three-dimensions. Only a few large microdamage sites (the largest 10%) accounted for 70% of all microdamage caused by cyclic loading. The number of large microdamage sites was a better predictor of reductions in Young's modulus caused by cyclic loading than overall damage volume fraction (DV/BV). The majority of microdamage volume (69.12 ± 7.04%) was located more than 30 μm (the average erosion depth) from trabecular surfaces, suggesting that microdamage occurs primarily within interstitial regions of cancellous bone. Additionally, microdamage was less likely to be near resorption cavities than other bone surfaces (p<0.05), challenging the idea that stress risers caused by resorption cavities influence fatigue failure of cancellous bone. Together, these findings suggest that reductions in apparent level mechanical performance during fatigue loading are the result of only a few large microdamage sites and that microdamage accumulation in fatigue is likely dominated by heterogeneity in tissue material properties rather than stress concentrations caused by micro-scale geometry.