Intensifying supportive care is associated with improved survival in gastric cancer patients with malignant ascites.

BACKGROUND: Gastric cancer patients with malignant ascites often have poor functional status and malnutrition that preclude receipt of systemic therapies. Thus, these patients have a very poor prognosis. Beginning in 2019, our multidisciplinary gastric cancer disease-oriented team implemented a more aggressive supportive care plan for gastric cancer patients with malignant ascites. The initiative included measures such as supplemental enteral nutrition, ascites drainage, and initiation of chemotherapy on an inpatient basis. We compared outcomes for gastric cancer patients who presented with synchronous malignant ascites treated before and after the implementation of the care plan. METHODS: We performed a retrospective review of our institutional database to identify patients diagnosed with gastric adenocarcinoma and synchronous malignant ascites between 2010 and 2022. We compared overall survival (OS) between patients diagnosed from 2010 to 2018, which will be referred to as the historical control era and patients diagnosed from 2019 to 2022, which will be called the aggressive supportive care era. RESULTS: Fifty-four patients were included in our analysis; 31 patients were treated in the historical control time frame, and 23 patients were treated during the aggressive supportive care era. Demographic, clinical, and pathologic characteristics were similar between groups. 3% of historical controls received supplemental tube feeds at diagnosis as compared to 30% of the aggressive supportive care cohort (p < 0.01). 3% of historical controls received their first cycle of chemotherapy in the inpatient setting versus 39% of patients treated during the aggressive supportive care era (p < 0.01). The median number of chemotherapy cycles received was 5 among historical controls and 9.5 among aggressive supportive care era patients (p = 0.02). There was no difference in the number of days spent as an inpatient between the two groups. The median OS for historical control patients was 5.4 months as compared with 10.4 months for patients treated during aggressive supportive care era (p = 0.04). CONCLUSIONS: Gastric cancer patients with synchronous malignant ascites treated during a timeframe when our multidisciplinary team implemented more aggressive supportive care measures had improved OS as compared with historic controls. Our results suggest that aggressive supportive measures for these patients with highly challenging clinical issues and poor prognosis can prolong survival. Specifically, initiation of chemotherapy in the inpatient setting and supplemental nutrition should be considered for patients at high risk for treatment intolerance.

Characteristics and clinical outcomes in young-onset cholangiocarcinoma.

BACKGROUND: While the incidence of cholangiocarcinoma is rising, little is known about young-onset disease. We compared clinical characteristics and outcomes between patients with young-onset cholangiocarcinoma, diagnosed between the ages of 18 and <50 years, and patients with typical-onset cholangiocarcinoma, diagnosed at age 50 years or greater. METHODS: We used the National Cancer Database to identify patients with young-onset cholangiocarcinoma (n = 2520) and typical-onset cholangiocarcinoma (n = 23,826). We compared the frequency of demographic and clinical characteristics between the two groups. We compared overall survival between the two groups using multivariable Cox regression analysis after adjusting for age, gender, race/ethnicity, comorbidity, facility type, tumor location, tumor stage, surgical status, and receipt of radiotherapy, chemotherapy and surgery. RESULTS: When compared to patients with typical-onset disease (median age 68 years), patients with young-onset cholangiocarcinoma (median age 44 years) were more likely to be non-White (35.0% vs. 27.4%, p < 0.01), and had lower overall comorbidity burden. Patients with young-onset disease had a greater proportion of intrahepatic cholangiocarcinoma (56.0% vs. 45.5%, p < 0.001) and stage IV disease (50.5% vs. 43.5%, p < 0.001). Younger patients were more likely than typical-onset patients to receive definitive surgery (30.9% vs. 25.0%, p < 0.001), radiation (27.7% vs. 19.6%, p < 0.001) and chemotherapy (73.1% vs. 50.1%, p < 0.001). In adjusted analyses, patients with young-onset disease had a 15% decreased risk of death, compared with patients with typical-onset disease (HR 0.85 [95% CI 0.80-0.89], p < 0.001). CONCLUSIONS: Patients with young-onset cholangiocarcinoma may represent a demographically and clinically distinct group from those with more typical-onset disease.

Trends in Adjuvant Chemotherapy Use Among Stage III Colon Cancer in Non-Elderly and Low Comorbidity Patients.

BACKGROUND: Adjuvant chemotherapy for stage III colon cancer is underutilized in the United States. The aim of this study was to assess the use of adjuvant chemotherapy in younger and medically fit patients and analyze the socioeconomic factors associated with its utilization. METHODS: Using the National Cancer Database from 2004 to 2015, we selected stage III colon cancer patients between age 18 to 65, Charlson-Deyo Comorbidity Index (CDCI) of 0 or 1, and those that survived at least 12 months after surgery. We then compared patients that underwent surgery only with those that received adjuvant chemotherapy. Multivariable logistic regression analysis was performed to identify variables associated with adjuvant chemotherapy use in the population. Overall survival was estimated by Kaplan-Meier curves. RESULTS: Of the 48,336 patients that met inclusion criteria, 43,315 (90%) received adjuvant chemotherapy. The utilization of adjuvant chemotherapy increased from 87% in 2004 to 91% in 2015. On multivariable regression analysis, the use of adjuvant chemotherapy was lower among males, Non-Hispanic Blacks and Hispanics, low-grade cancer, left-sided tumors, CDCI 1, those who travel ≥ 50 miles, yearly income < $40,227, and uninsured patients. The most common reason for the omission of adjuvant chemotherapy was the patient or caregiver's choice (40% between 2013 and 2015). The 5-year and 10-year overall survival rates were 76.7% and 63.8% respectively, in those who received adjuvant chemotherapy as compared to 65.1% and 49.3% in those who underwent surgery only (P < .001). CONCLUSION: In young and medically fit stage III colon cancer patients, most patients received guideline-compliant care in the United States. However, socioeconomic disparities adversely impacted the use of adjuvant chemotherapy. The patient or caregiver's decision was the most common reason for non-adherence to adjuvant chemotherapy and lead to poor survival outcomes. Emphasis should be placed on developing patient-centered strategies to improve adherence to chemotherapy in all patients.

Implementation of a Quality Improvement Project to Increase Access to Dietitian Services for Patients With Gastrointestinal Cancer in a Safety-Net Hospital System.

PURPOSE: Rates of malnutrition are high in patients with GI cancer, leading to poor outcomes. The aim of our project was to increase the rate of documented dietitian assessment in patients with GI cancer at Parkland Health and Hospital System from 5% to 25%. METHODS: Three PDSA cycles were conducted after identifying barriers to dietitian services. A registered dietitian was assigned to the GI oncology clinic during the first cycle, an adapted Malnutrition Screening Tool was implemented through the electronic medical record during the second cycle, and clinical staff training was performed during the third cycle. New patients with GI cancer seen by the registered dietitian had weight, Eastern Cooperative Oncology Group performance status, and serum albumin recorded at initial visit and 3-month follow-up. Paired t tests were performed. Emergency department visits and hospital admissions were also recorded during this time. RESULTS: Through these interventions, the percentage of patients with GI cancer with documented assessment by the registered dietitian increased from 5.1% in October 2018 prior to our interventions to 21.8% in July 2019 and has sustained in the 15%-20% range thereafter. From May to July 2019, there were 63 new patients with GI cancer seen by a registered dietitian. No significant difference was observed in average difference in weight and serum albumin level at initial visit and 3-month follow-up. CONCLUSION: A nutrition-focused quality improvement project led to a more than three-fold increase in the rate of documented dietitian assessment in patients with GI cancer.

Disparities in Characteristics, Access to Care, and Oncologic Outcomes in Young-Onset Colorectal Cancer at a Safety-Net Hospital.

PURPOSE: Young-onset colorectal cancer is an emerging cause of significant morbidity and mortality globally. Despite this, limited data exist regarding clinical characteristics and outcomes, particularly in safety-net populations where access to care is limited. We aimed to study disparities in clinical characteristics and outcomes in patients with young-onset colorectal cancer in the safety-net setting. METHODS: We performed a retrospective review of patients < 50 years old diagnosed and/or treated for colorectal cancer between 2001 and 2017 at a safety-net hospital. Kaplan-Meier and Cox regression models were constructed to compare overall survival (OS), progression-free survival (PFS), and relapse-free survival (RFS) by race and ethnicity, stratifying for relevant clinical and pathologic factors. RESULTS: A total of 395 young-onset patients diagnosed at a safety-net hospital were identified and 270 were included in the analysis (49.6% Hispanic, 25.9% non-Hispanic Black, 20.0% non-Hispanic White, and 4.4% other). Non-Hispanic White race was independently associated with worse OS (hazzard ratio [HR], 0.53; 95% CI, 0.29 to 0.97), as were lack of insurance, higher clinical stage, and mismatch repair proficiency. There was no significant difference seen in PFS or RFS between racial and ethnic groups. CONCLUSION: Non-Hispanic White race or ethnicity was found to be independently associated with worse OS in a safety-net population of patients with young-onset colorectal cancer. Other independent predictors of worse OS include higher stage, lack of insurance, and mismatch repair proficiency.

Racial/Ethnic Disparities and Survival Characteristics in Non-Pancreatic Gastrointestinal Tract Neuroendocrine Tumors.

Background: We studied the effect of race and ethnicity on disease characteristics and survival in gastrointestinal neuroendocrine tumors. Methods: The Surveillance, Epidemiology, and End Results database was used to select patients with non-pancreatic gastrointestinal neuroendocrine tumors diagnosed between 2004 and 2015. Trends in survival were evaluated among three groups: Hispanic, non-Hispanic White, and non-Hispanic Black. Kaplan-Meier and Cox regression methods were performed to calculate overall survival and cause-specific survival after adjusting for patient and tumor characteristics. Results: A total of 26,399 patients were included in the study: 65.1% were non-Hispanic White, 19.9% were non-Hispanic Black, and 15% were Hispanic. Non-Hispanic White patients were more likely to be male (50.0%, p < 0.001), older than 60 years (48.0%, p < 0.001), and present with metastatic disease (17.7%, p < 0.001). Non-Hispanic White patients had small intestine neuroendocrine tumors, while Hispanic and non-Hispanic Black patients had rectum neuroendocrine tumors as the most common primary site. Hispanic patients had better overall survival, while non-Hispanic Black patients had better cause-specific survival versus non-Hispanic White patients. This finding was confirmed on multivariable analysis where Hispanic patients had improved overall survival compared to non-Hispanic White patients (Hazard ratio (HR): 0.89 (0.81-0.97)), whereas non-Hispanic Black patients had better cause-specific survival compared to non-Hispanic White patients (HR: 0.89 (0.80-0.98)). Conclusions: Race/ethnicity is an independent prognostic factor in patients with gastrointestinal neuroendocrine tumors.

Trends in Primary Surgical Resection and Chemotherapy for Metastatic Colorectal Cancer, 2000-2016.

BACKGROUND: When, whether, and in whom primary tumor resection (PTR) for patients with metastatic colorectal cancer (CRC) is indicated remains unknown. With advances in multiagent systemic chemotherapy, PTR may be undertaken less frequently. The aim of this study was to obtain estimates of changes in the utilization of PTR and chemotherapy for metastatic CRC. METHODS: Patients diagnosed with metastatic CRC between 2000 and 2016 were identified from Surveillance Epidemiology, and End Results (SEER) registry. Multivariable logistic regression defined odds of undergoing PTR. The analysis was also stratified by primary site (colon vs. rectum), age (younger than 50 vs. 50 y and older), and whether patients also underwent resection of metastatic sites (yes vs. no). The secondary endpoint of interest was the receipt of any chemotherapy, also assessed by multivariable logistic regression. RESULTS: Among 99,835 patients with metastatic CRC, 55,527 (55.7%) underwent PTR. The odds of undergoing PTR decreased with a later year of diagnosis, with patients diagnosed in 2016 being 61.1% less likely to undergo surgery than those diagnosed in 2000 (adjusted odds ratio=0.39, 95% confidence interval: 0.36-0.42, P<0.0001; absolute percentage: 62.3% to 43.8%). Similar trends by year for PTR were observed among each of the subgroups, although patients with colon primary, young adults (age younger than 50 y), and patients also undergoing metastasectomy were more likely to undergo PTR (P<0.001 for all). In contrast, the odds of receiving chemotherapy increased dramatically with a later year of diagnosis (adjusted odds ratio=2.21, 95% confidence interval: 2.04-2.40, P<0.0001). CONCLUSIONS: From 2000 to 2016, there was a sharp decline in the rate of PTR for patients with metastatic CRC, while the use of chemotherapy increased over the same period. Prospective studies are needed to define the optimal local treatment for patients with metastatic CRC.

Distinct Clinical Characteristics in Young-Onset Pancreatic Neuroendocrine Tumor.

BACKGROUND: We aimed to study the effect of socioeconomic differences and molecular characteristics on survival in patients with young-onset pancreatic neuroendocrine tumors (YOPNET) and typical-onset PNET (TOPNET). METHODS: We identified the patients with YOPNET (<50 years) and TOPNET (≥50 years) who underwent definitive surgery diagnosed between 2004 and 2016 using the National Cancer Database. We evaluated overall survival (OS) using the Kaplan-Meier and Cox regression methods before and after propensity score matching. A publicly available genomic dataset was used to compare mutation frequencies among the two groups. RESULTS: A total of 6259 patients with PNET were included, of which 27% were YOPNET. Patients with YOPNET were more likely to be Black, Hispanic, female, and have private insurance versus patients with TOPNET (all p < 0.001). Patients with YOPNET had a lower comorbidity score, but higher stage and tumor size (all p < 0.001). YOPNET was associated with a greater improved OS than TOPNET before and after propensity score matching (p < 0.001). On multivariable analysis, this survival difference persisted for YOPNET as an independent prognostic factor (unmatched p = 0.008; matched p = 0.01). For genomic analysis, patients with YOPNET had a lower rate of multiple endocrine neoplasia type-1 (MEN-1) mutation than patients with TOPNET (26% vs. 56%, p < 0.001). CONCLUSIONS: YOPNET represents a disease with distinct clinical features. Patients with YOPNET who underwent definitive surgery had better OS than patients with TOPNET despite having higher stage and tumor size. YOPNET also had lower rate of MEN-1 mutation.

Racial Disparities in Time to Treatment Initiation and Outcomes for Early Stage Anal Squamous Cell Carcinoma.

OBJECTIVES: Although cure rates for early stage anal squamous cell cancer (ASCC) are overall high, there may be racial disparities in receipt of treatment and outcome precluding favorable outcomes across all patient demographics. Therefore, the authors aimed to assess the time to treatment initiation and overall survival (OS) in Black and White patients receiving definitive chemoradiation for early stage ASCC. MATERIALS AND METHODS: The authors identified patients diagnosed with early stage (stage I-II) ASCC and treated with chemoradiation diagnosed between 2004 and 2016 in the National Cancer Database. Clinical and treatment variables were compared by race using the χ test, and OS assessed through Cox regression with 1:1 nearest neighbor propensity score matching. RESULTS: Among 9331 patients, 90.6% were White. Black patients had longer median time to treatment initiation as compared with White patients (47 vs. 36 d, P<0.001), and on multivariable analysis, the Black race was associated with higher odds of >6 weeks of time to treatment initiation (hazard ratio, 1.78; 95% confidence interval, 1.53-2.08; P<0.001). Furthermore, Black patients had worse OS (5-year survival 71% vs. 77%; P<0.001), which persisted after propensity score matching (P=0.007). CONCLUSIONS: Black patients had a longer time to treatment initiation and worse OS as compared with White patients with early stage ASCC treated with chemoradiation. Further research is needed to better elucidate the etiologies of these disparities.

Stage-specific Conditional Survival Among Young (Age Below 50 y) Versus Older (Age 50 y and Above) Adults With Colorectal Cancer in the United States.

BACKGROUND: Conditional survival (CS) is a relevant prognostic measure and may be particularly important for young adult patients with colorectal cancer (CRC), whose incidence is rising. We sought to compare CS among young versus older adults with CRC. METHODS: Patients diagnosed with CRC between 2004 and 2010 were identified from the Surveillance, Epidemiology, and End Results registry. Smoothed yearly hazards of death due to CRC, other causes and any cause were estimated, stratified by age at diagnosis (below 50 vs. 50 y and above) and stage (I-III vs. IV). Stage-specific conditional 5-year overall survival and cancer-specific survival given that patients had already survived 1 to 5 years after diagnosis was calculated. RESULTS: Among 161,859 patients with median follow-up of 54 months, 35,411 (21.9%) were aged below 50 years. For older adults with nonmetastatic CRC, hazards of death due to noncancer causes exceeded that of rectal and colon cancer ∼6.1 and 3.8 years after diagnosis, respectively. Patients experienced improved CS over time with greater improvement seen for more advanced stages. However, young adults had less CS improvement over time than older adults. For example, the 5-year cancer-specific survival for stage IV colon cancer improved from 15.6% to 77.2% (change=61.6%) 0 to 5 years after diagnosis for older adults but only 20.3% to 67.7% (change=47.4%) for young adults. CONCLUSIONS: Prognosis for CRC improves over time for all patients, although the increase in survival appears to be less for young than older adults. Up to 10 years after diagnosis, the primary cause of death in young adults with CRC remains their incident cancer.

Patterns of Care for Stage II-III Rectosigmoid Cancers in the United States, 2004-2015.

OBJECTIVES: Although current guidelines continue to recommend trimodality therapy for stage II to III rectal cancers, a lower incidence of local recurrence has been observed in patients with upper rectal tumors, including those in the rectosigmoid. In practice, patients with upper rectal tumors may not be receiving all 3 modalities of therapy. Patterns of care for patients with rectosigmoid cancers have not previously been described. METHODS: The National Cancer Database (NCDB) was used to identify patients diagnosed with stage II to III rectosigmoid cancer who underwent definitive surgery between 2004 and 2015. Multivariable logistic regression defined adjusted odds ratio and associated 95% confidence intervals of receipt of any pelvic radiotherapy and preoperative and postoperative pelvic radiotherapy. Multivariable logistic regression also assessed odds of treatment with any chemotherapy and multiagent chemotherapy. RESULTS: Among 8410 patients, 3566 (42.4%) received any pelvic radiotherapy, of which 2516 (70.6%) were treated with preoperative radiotherapy. Factors associated with receipt of radiotherapy included male sex, white race, younger age, positive clinical nodes and positive margins (P<0.001). Among patients with clinically positive nodes, 1980 (48.6%) received any radiotherapy and among those with pathologically positive nodes, 1532 (37.9%) received radiotherapy. A total of 5708 patients (67.9%) received any chemotherapy including 3020 (52.9%) with multiagent chemotherapy. A total of 2579 (30.7%) of the cohort was treated with surgery alone and among patients who were T3N0, this proportion rose to 42.5%. CONCLUSIONS: Less than half of patients with stage II to III rectosigmoid cancers are treated with radiation therapy and approximately one third do not receive chemotherapy. Ongoing and future studies may help to better tailor treatment for rectosigmoid tumors to optimize the therapeutic ratio. Our work may serve as a benchmark on which to compare future practice patterns.

Gastric Cancer with Radiographically Occult Metastatic Disease: Biology, Challenges, and Diagnostic Approaches.

Gastric adenocarcinoma is an aggressive cancer that demonstrates heterogeneous biology depending on patient ethnicity, tumor location, tumor type, and genetic profile. It remains the third leading cause of cancer deaths worldwide and was estimated to result in 782,000 deaths in 2018. Challenges exist in accurately assessing the disease burden, as available radiological staging often underestimates metastatic disease. This diagnostic handicap, along with the poor understanding of the heterogeneous biology of gastric cancer, has hindered the development of effective therapeutic solutions and thus halted improvement in patient outcomes over the last few decades. The management of occult peritoneal disease is complicated, as most patients are understaged by standard imaging studies and therefore thought to have local diseases. In this article, we systematically review recent literature on the limitations that are associated with standard radiographic staging, discuss recent molecular biology advances to better identify and diagnose occult peritoneal disease, and propose possible management strategies to approach this complicated clinical problem.

Liquid Biopsy for Cancer: Review and Implications for the Radiologist.

Imaging and image-guided procedures play an imperative role in the screening, diagnosis, and surveillance of cancer. Although emerging imaging techniques now enable more precise molecular characterization of tumors, multigenetic tumor profiling for targeted therapeutic selection remains limited to direct tissue acquisition. Even in the context of targeted therapy, tumors adapt to acquire resistance. This necessitates serial monitoring, traditionally through tissue acquisition, to identify the molecular mechanism of resistance and to guide second-line therapy. An alternative to tissue acquisition is the collection of circulating tumor markers such as cell-free nucleic acids and circulating tumor cells in the peripheral blood. This noninvasive diagnostic approach is referred to as the liquid biopsy. The liquid biopsy is currently used clinically for therapeutic guidance when tissue acquisition is impossible or when the specimen is inadequate. It is also being studied in the context of screening, diagnosis, and surveillance. As cancer treatment continues to move toward a focus on precision medicine, this developing technology may alter and/or augment the role of imaging in the management of cancer. This review aims to outline the use of liquid biopsy in cancer and its potential impact on diagnostic imaging and image-guided procedures.

Gastric Cancer: Recent Molecular Classification Advances, Racial Disparity, and Management Implications.

Gastric adenocarcinoma remains an aggressive and poorly understood malignancy with a heterogeneous presentation and tumor biology. The current histologic and anatomic classification has been ineffective in guiding therapy, with only marginal improvement in outcome over time. Furthermore, the variation in presentation and disease among racial and ethnic groups amplifies the complexity of this cancer. An understanding of the clinical and molecular variability is important for effective treatment. Recent advances in molecular biology have better defined gastric cancer subtypes. We systematically review recent literature on the molecular classification of gastric adenocarcinoma and the associated management implications, with an emphasis on Hispanic and Native American populations.

Approach and management of checkpoint inhibitor-related immune hepatitis.

Immune checkpoint inhibitors have promising clinical activity across multiple gastrointestinal cancers and immune-mediated hepatotoxicity is particularly relevant for this group of patients. In this article we will review the recognition, workup and management of suspected checkpoint inhibitor related immune-hepatitis.

Pro-inflammatory angiogenesis is mediated by p38 MAP kinase.

Chronic inflammation is tightly linked to diseases associated with endothelial dysfunction including aberrant angiogenesis. To better understand the endothelial role in pro-inflammatory angiogenesis, we analyzed signaling pathways in continuously activated endothelial cells, which were either chronically exposed to soluble TNF or the reactive oxygen species (ROS) generating H2O2, or express active transmembrane TNF. Testing in an in vitro capillary sprout formation assay, continuous endothelial activation increased angiogenesis dependent on activation of p38 MAP kinase, NADPH oxidase, and matrix metalloproteinases (MMP). p38 MAP kinase- and MMP-9-dependent angiogenesis in our assay system may be part of a positive feed forward autocrine loop because continuously activated endothelial cells displayed up-regulated ROS production and subsequent endothelial TNF expression. The pro-angiogenic role of the p38 MAP kinase in continuously activated endothelial cells was in stark contrast to the anti-angiogenic activity of the p38 MAP kinase in unstimulated control endothelial cells. In vivo, using an experimental prostate tumor, pharmacological inhibition of p38 MAP kinase demonstrated a significant reduction in tumor growth and in vessel density, suggesting a pro-angiogenic role of the p38 MAP kinase in pathological angiogenesis in vivo. In conclusion, our results suggest that continuous activation of endothelial cells can cause a switch of the p38 MAP kinase from anti-angiogenic to pro-angiogenic activities in conditions which link oxidative stress and autocrine TNF production.

Robust functional vascular network formation in vivo by cooperation of adipose progenitor and endothelial cells.

Rapid induction and maintenance of blood flow through new vascular networks is essential for successfully treating ischemic tissues and maintaining function of engineered neo-organs. We have previously shown that human endothelial progenitor cells (EPCs) form functioning vessels in mice, but these are limited in number and persistence; and also that human adipose stromal cells (ASCs) are multipotent cells with pericytic properties which can stabilize vascular assembly in vitro. In this study, we tested whether ASCs would cooperate with EPCs to coassemble vessels in in vivo implants. Collagen implants containing EPCs, ASCs, or a 4:1 mixture of both were placed subcutaneously into NOD/SCID mice. After a range of time periods, constructs were explanted and evaluated with regard to vascular network assembly and cell fate; and heterotypic cell interactions were explored by targeted molecular perturbations. The density and complexity of vascular networks formed by the synergistic dual-cell system was many-fold higher than found in implants containing either ASCs or EPCs alone. Coimplantation of ASCs and EPCs with either pancreatic islets or adipocytes produced neoorgans populated by these parenchymal cells, as well as by chimeric human vessels conducting flow. This study is the first to demonstrate prompt and consistent assembly of a vascular network by human ASCs and endothelial cells and vascularization by these cells of parenchymal cells in implants. Mixture of these 2 readily available, nontransformed human cell types provides a practical approach to tissue engineering, therapeutic revascularization, and in vivo studies of human vasculogenesis.