Turning the Tide on Hepatitis C Virus-Related Liver Transplantation: The Return on Investment in Hepatitis C Virus Treatment in Australia and New Zealand.

Introduction of universal access to direct-acting antiviral (DAA) therapy for hepatitis C virus (HCV) in Australia and New Zealand on March 1st , 2016, has had a major impact on the number of people with chronic HCV infection, but the impact on liver transplantation rates is unknown. We conducted a retrospective registry study including all adult liver transplantations from the Australia and New Zealand Liver and Intestinal Liver Transplant Registry (ANZLITR) data set. Interrupted time series analysis determined the impact of DAAs in 2016 on the number of HCV liver transplantations per year. Cox regression analysis was used to determine the impact of DAAs on post-liver transplantation survival. Between January 1, 1990, and December 31, 2019 5318 adult liver transplantations were performed, and 29% (1531) were for HCV infection. Prior to the introduction of DAAs, there was a mean increase of 3.5 adult liver transplantations performed for HCV per annum, but between 2016 and 2019 there was a mean decrease of 7.9 adult liver transplantations per annum (P < 0.001). Similarly, the proportion of liver transplantations performed for HCV increased from 9% (1990) to 33% in 2016 and then fell to 23% in 2019 (P < 0.001). The number and proportion of patients with HCV added to the liver transplantation waiting list also fell in 2016 (P < 0.001) when compared with other indications. The introduction of DAAs was associated with a 31% reduction in death after liver transplantation, adjusted for age at transplant and hepatocellular carcinoma (HCC; hazard ratio [HR], 0.69; 95% confidence interval [CI], 0.48-0.99; P = 0.047). The number of adult liver transplantations performed for HCV-related liver cirrhosis and HCC has reduced since the introduction of universal access to DAAs in 2016 in Australia and New Zealand.

The MAAPE score in intermediate and advanced hepatocellular carcinoma treated with Yttrium-90 resin microsphere radioembolization.

BACKGROUND AND AIM: Yttrium-90 resin microsphere radioembolization (RE) is not recommended for routine use in intermediate or advanced hepatocellular carcinoma (HCC) by recent guidelines. This study aims to establish pre-treatment variables which predict survival in HCC patients treated with RE to identify those who will benefit most from it, and to inform patient selection for future trials. METHODS: Single center, retrospective study of consecutive patients with HCC treated with RE from 2007 to 2018. Patients included if undergoing their first RE treatment for intermediate or advanced HCC; a Child-Pugh score of B7 or less; and a performance status of 1 or less. Multivariable Cox regression identified variables that were significantly associated with survival. A predictive score was developed based upon coefficients from the fitted Cox regression model, and cubic spline regression was used to identify prognostic groups. RESULTS: One hundred thirteen patients with intermediate (53.1%) and advanced HCC (45.1%) followed for a median of 13.2 months were included. Variables associated with superior survival used to derive the MAAPE score were lower Model for End-Stage Liver Disease score (≤ 7), lower Alpha-fetoprotein (≤ 150 IU/L), higher serum Albumin (> 37 g/L), absence of Portal vein tumor thrombus, and better performance status (Eastern Cooperative Oncology Group = 0). Three survival prognostic groups were identified: good (median overall survival 25.0 months), average (15.3 months), and poor (6.3 months) (overall log-rank test, P < 0.001). CONCLUSION: The MAAPE score accurately identifies HCC patients in whom RE is safe and effective. This will allow for optimal patient selection for future trials of RE versus systemic therapy.

Baseline and Post-treatment 18F-Fluorocholine PET/CT Predicts Outcomes in Hepatocellular Carcinoma Following Locoregional Therapy.

BACKGROUND AND AIMS: 18F-fluorocholine positron emission tomography/computed tomography (18F-FCH PET/CT) is an emerging functional imaging technique in the diagnosis and management of hepatocellular carcinoma (HCC). The aim of this study was to assess the ability of a pre- and post-treatment 18F-FCH PET/CT to predict prognosis and treatment response in early-stage HCC. METHODS: Patients with early- or intermediate-stage HCC planned for locoregional therapy were prospectively enrolled. Baseline demographic and tumor information was collected and baseline and post-treatment 18F-FCH PET/CT performed. Maximum standardized uptake values (SUVmax) were determined for each HCC lesion, and the difference between baseline and post-treatment SUVmax values were compared with progression-free survival outcomes. RESULTS: A total of 29 patients with 39 confirmed HCC lesions were enrolled from a single clinical center. Patients were mostly men (89.7%) with hepatitis C or alcohol-related cirrhosis (65.5%) and early-stage disease (89.7%). Per-patient and per-lesion sensitivity of 18F-FCH PET/CT was 72.4% and 59.0%, respectively. A baseline SUVmax < 13 was associated with a superior median progression-free survival compared with an SUVmax of > 13 (17.7 vs. 5.1 months; p = 0.006). A > 45% decrease in SUVmax between baseline and post-treatment 18F-FCH PET/CT ("responders") was associated with a superior mean progression-free survival than a percentage decrease of < 45% ("non-responders," 36.1 vs. 11.6 months; p = 0.034). CONCLUSIONS: Baseline and post-treatment 18F-FCH PET/CT predicts outcomes in early-stage HCC undergoing locoregional therapy. This technique may identify patients with an objective response post-locoregional therapy who would benefit from further therapy.

Hepatocellular carcinoma in Australia 1982-2014: Increasing incidence and improving survival.

BACKGROUND & AIMS: There is a paucity of accurate and current data on hepatocellular carcinoma (HCC) trends in incidence and survival in developed countries. We performed an Australia-wide assessment of HCC epidemiology across a 33-year time span aiming to accurately describe changes in incidence and survival. METHODS: Cases of HCC from 1982 to 2014 were identified via the Australian Cancer Database (ACD). Trends in incidence rates were explored using piecewise linear regression. Survival was compared by Kaplan-Meier survival curves and 1-, 3- and 5-year survival probabilities by year of diagnosis. RESULTS: Age-adjusted HCC incidence rate increased from 1.38 per 100 000 (95% CI: 1.34-1.43) in 1982 to 4.96 per 100 000 (95% CI: 4.89-5.03, P < 0.001) in 2014 with an average annual percentage increase of 4.46% (95% CI: 4.24%-4.69%). The highest incidence rate in 2014 was in those aged 75-79 (24.31 per 100 000; 95% CI: 19.50-29.12). Almost 80% of cases across the period were men who had significantly higher age-adjusted incidence rates in 2014 than women (8.55 per 100 000 [95% CI: 8.42-8.68] vs 1.65 per 100 000 [95% CI: 1.60-1.70]; P < 0.001). A hepatitis C (HCV) birth cohort effect was identified and associated with rapid increases in HCC incidence when members of the cohort aged and entered into age groups 45-49, 50-54 and 55-59. Median survival increased from 2.10 months (95% CI: 1.57-2.62 months) in those diagnosed between 1982 and 1984 to 12.07 months (95% CI: 11.17-12.97 months) when diagnosed between 2010 and 2014 (P < 0.001). CONCLUSIONS: An Australia-wide analysis of HCC epidemiological trends across three decades shows significant and consistent increases in both incidence and survival. LAY SUMMARY: There has been a significant increase in hepatocellular cancer (HCC) reported in Australia over the last three decades without evidence of slowing. Across the same time period, a significant improvement in survival has been identified with the average life expectancy after diagnosis now one year. This research lays the foundation for important public health service delivery.

Rate of Nonsurveillance and Advanced Hepatocellular Carcinoma at Diagnosis in Chronic Liver Disease.

AIMS: Ultrasound surveillance for hepatocellular carcinoma (HCC) is recommended in cirrhotic patients to allow early diagnosis. This study investigated risk factors for nonsurveillance and advanced HCC at diagnosis and their effect on survival. MATERIALS AND METHODS: Two hundred seventy HCC patients were included. Clinical data were collected from hospital databases. RESULTS: One hundred twenty-eight (47.1%) patients had 6-monthly ultrasound surveillance before HCC diagnosis. Ninety-two (34.1%) patients had advanced HCC (multifocal or total diameter ≥6 cm) at diagnosis. The nonsurveillance rate was significantly higher in nonalcoholic fatty liver disease (NAFLD) (79%) compared with other causes of chronic liver disease (31.6% to 58.1%, P<0.001). Nonrecognition of NAFLD was significantly higher (68.4%) compared with other causes of chronic liver disease (0% to 23.2%, P<0.001). In NAFLD HCC patients, 23.7% were noncirrhotic and smoking was significantly associated HCC in this noncirrhotic group (P=0.041). No-surveillance for HCC was significantly associated with advanced HCC at diagnosis with an odds ratio (OR) of 8.1. Compared with nondrinkers, heavy alcohol consumption was significantly associated with advanced HCC (OR=7.6). In the surveillance group, diagnosis using computed tomography rather than magnetic resonance imaging was significantly associated with advanced HCC (OR=3.36). Patients without HCC surveillance had a significantly shorter median survival compared with those who had HCC surveillance (27.4 vs. 52.0 mo, P=0.0006). CONCLUSIONS: The lack of HCC surveillance is associated with advanced HCC at diagnosis and decreased survival. NAFLD patients with HCC have a significantly lower rate of diagnosis of chronic liver disease and HCC surveillance compared with the other causes of chronic liver disease.

The Prognostic Ability of Major Hepatocellular Carcinoma Staging Systems Is Improved by Including a Treatment Variable.

BACKGROUND AND AIMS: There has been significant debate regarding which hepatocellular carcinoma (HCC) staging system is best able to predict survival. We hypothesized that the prognostic ability of the Barcelona Clinic Liver Cancer (BCLC) and Hong Kong Liver Cancer (HKLC) systems would be improved with the addition of an explicit treatment variable. METHODS: We performed an analysis of a prospectively enrolled cohort of 292 patients undergoing 532 treatment episodes for HCC from 2006 to 2014. BCLC, standard nine-stage HKLC (HKLC9), and modified five-stage HKLC (HKLC5) for each treatment episode were assessed. Overall survival and time to disease progression were calculated for the initial treatment, re-treatment, and overall treatment cohorts. We compared the performance of various prognostic models including staging system alone, treatment alone, and staging system plus treatment using the corrected Akaike information criterion and Harrell's C statistic. RESULTS: The BCLC, HKLC5, and HKLC9 systems were significant predictors of survival and time to progression for all treatment cohorts (log rank test, p < 0.001). The addition of a treatment variable significantly improved (p < 0.01) the prognostic ability of the survival and time to progression models compared with those containing only the BCLC or HKLC stage across all treatment cohorts other than survival in re-treatment for BCLC (p = 0.094). CONCLUSIONS: Adding a treatment variable to major HCC staging systems improves their ability to predict survival and time to progression in initial treatment, re-treatment, and overall.

HKLC Triages More Hepatocellular Carcinoma Patients to Curative Therapies Compared to BCLC and Is Associated with Better Survival.

BACKGROUND AND AIMS: The Hong Kong Liver Cancer (HKLC) system proposes to triage hepatocellular carcinoma (HCC) patients to more aggressive treatment and may be associated with superior survival compared with the Barcelona Clinic Liver Cancer (BCLC) system. We aimed to identify the influence of adherence to HKLC or BCLC treatment recommendations on survival and time to progression. METHODS: We examined a prospectively enrolled cohort of 292 patients undergoing 532 treatment episodes from a single clinical center. RESULTS: The BCLC and HKLC systems accurately predicted overall survival and time to progression after each treatment episode (BCLC: p < 0.001; HKLC: p < 0.001). Adherence to treatment recommendations was higher for HKLC than for BCLC (55.6 vs. 47.9%, p = 0.01). Survival was superior with adherence to HKLC recommendations compared to non-adherence (45.3 vs. 27.1 months, p < 0.001). There was no difference in survival in BCLC with adherence compared to non-adherence (34.6 vs. 32.3 months, p = 0.96). The survival benefit was limited to early- and very early stage disease for both HKLC (p < 0.001) and BCLC (p = 0.007). More patients were triaged to curative therapies by HKLC than BCLC (p = 0.004). The use of transarterial chemoembolization instead of ablation or resection in early- and very early stage disease for technical reasons was the major cause for non-recommended treatment and was associated with worse survival (p < 0.001). CONCLUSIONS: These data support the use of HKLC in early- and very early stage HCC. Efforts should be made to overcome technical reasons for not performing ablation in early- and very early stage disease.

Pathobiology of liver fibrosis: a translational success story.

Reversibility of hepatic fibrosis and cirrhosis following antiviral therapy for hepatitis B or C has advanced the prospect of developing antifibrotic therapies for patients with chronic liver diseases, especially non-alcoholic steatohepatitis. Mechanisms of fibrosis have focused on hepatic stellate cells, which become fibrogenic myofibroblasts during injury through 'activation', and are at the nexus of efforts to define novel drug targets. Recent studies have clarified pathways of stellate cell gene regulation and epigenetics, emerging pathways of fibrosis regression through the recruitment and amplification of fibrolytic macrophages, nuanced responses of discrete inflammatory cell subsets and the identification of the 'ductular reaction' as a marker of severe injury and repair. Based on our expanded knowledge of fibrosis pathogenesis, attention is now directed towards strategies for antifibrotic therapies and regulatory challenges for conducting clinical trials with these agents. New therapies are attempting to: 1) Control or cure the primary disease or reduce tissue injury; 2) Target receptor-ligand interactions and intracellular signaling; 3) Inhibit fibrogenesis; and 4) Promote resolution of fibrosis. Progress is urgently needed in validating non-invasive markers of fibrosis progression and regression that can supplant biopsy and shorten the duration of clinical trials. Both scientific and clinical challenges remain, however the past three decades of steady progress in understanding liver fibrosis have contributed to an emerging translational success story, with realistic hopes for antifibrotic therapies to treat patients with chronic liver disease in the near future.

Emerging and disease-specific mechanisms of hepatic stellate cell activation.

The last decade has seen a rapid expansion in our understanding of the mechanisms leading to hepatic stellate cell activation. The classic activation pathway of initiation, perpetuation and regression remains as a useful model; however, the emergence of several new pathways and mediators has revealed a deeper complexity than previously appreciated. Although core fibrogenic pathways exist across organs and disease types, there is accumulating evidence for disease- and context-specific mechanisms that may modulate or drive hepatic fibrogenesis. Hence, a "one size fits all" approach to antifibrotic therapy may not be appropriate for all disease settings. The authors present a focused and concise update of the most recent advances in our understanding of hepatic stellate cell activation pathways, while highlighting several challenges that may be constraining progress. This summary provides a foundation to further expand our knowledge of this unique cell type and its contributions to human liver disease.

Hepatic fibrosis and the microenvironment: fertile soil for hepatocellular carcinoma development.

Hepatocellular carcinoma is an emerging worldwide health threat that has few curative treatment options and poor overall survival. Progressive hepatic fibrosis is a common pathway for all forms of chronic liver disease and is closely linked epidemiologically to hepatocellular carcinoma risk. However, the molecular events that predispose a fibrotic liver to cancer development remain elusive. Nonetheless, a permissive hepatic microenvironment provides fertile soil for transition of damaged hepatocytes into hepatocellular carcinoma. Key predisposing features include alterations in the extracellular matrix, bidirectional signaling pathways between parenchymal and nonparenchymal cells, and immune dysfunction. Emerging research into the contributions of autophagy, tumor-associated fibroblasts, and hepatocellular carcinoma progenitor cells to this dangerous milieu also provides new mechanistic underpinnings to explain the contribution of fibrosis to cancer. As effective antifibrotic therapies are developed, these approaches could attenuate the rising surge of hepatocellular carcinoma associated with chronic liver disease.

Predictors of survival in autoimmune liver disease overlap syndromes.

BACKGROUND: Survival in patients with autoimmune liver disease overlap syndromes (AILDOS) compared to those with single autoimmune liver disease is unclear. AIM: To investigate the survival of patients with AILDOS and assess the accuracy of non-invasive serum models for predicting liver-related death. METHODS: Patients with AILDOS were defined as either autoimmune hepatitis and primary biliary cholangitis overlap (AIH-PBC) or autoimmune hepatitis and primary sclerosing cholangitis overlap (AIH-PSC) and were identified from three tertiary centres for this cohort study. Liver-related death or transplantation (liver-related mortality) was determined using a population-based data linkage system. Prognostic scores for liver-related death were compared for accuracy [including liver outcome score (LOS), Hepascore, Mayo Score, model for end-stage liver disease (MELD) score and MELD incorporated with serum sodium (MELD-Na) score]. RESULTS: Twenty-two AILDOS patients were followed for a median of 3.1 years (range, 0.35-7.7). Fourteen were female, the median age was 46.7 years (range, 17.8 to 82.1) and median Hepascore was 1 (range, 0.07-1). At five years post enrolment, 57% of patients remained free from liver-related mortality (74% AIH-PBC, 27% AIH-PSC). There was no significant difference in survival between AIH-PBC and AIH-PSC. LOS was a significant predictor of liver-related mortality (P < 0.05) in patients with AIH-PBC (n = 14) but not AIH-PSC (n = 8). A LOS cut-point of 6 discriminated liver-related mortality in AIH-PBC patients (P = 0.012, log-rank test, 100% sensitivity, 77.8% specificity) (Harrell's C-statistic 0.867). The MELD score, MELD-Na score and Mayo Score were not predictive of liver-related mortality in any group. CONCLUSION: Survival in the rare, AILDOS is unclear. The current study supports the LOS as a predictor of liver-related mortality in AIH-PBC patients. Further trials investigating predictors of survival in AILDOS are required.

The Hidden Epidemic: The Prevalence and Impact of Concurrent Liver Diseases in Patients Undergoing Liver Transplantation in Australia and New Zealand.

Prevalence of concurrent liver diseases among liver transplant recipients and impact on posttransplant outcomes are unknown. Methods: This retrospective study included adult liver transplants between January 1' 1985' and December 31' 2019' from the Australian and New Zealand Liver and Intestinal Transplant Registry. Up to 4 liver disease causes were recorded for each transplant; concurrent liver diseases were defined as >1 liver disease indication for transplantation, excluding hepatocellular carcinoma. Impact on posttransplant survival was determined using Cox regression. Results: A total of 840 (15%) of 5101 adult liver transplant recipients had concurrent liver diseases. Recipients with concurrent liver diseases were more likely male (78% versus 64%) and older (mean age 52 versus 50 y). A higher proportion of liver transplants for hepatitis B (12% versus 6%), hepatitis C (33% versus 20%), alcohol liver disease (23% versus 13%), and metabolic-associated fatty liver disease (11% versus 8%, all P < 0.001) were identified when all indications were included than with primary diagnosis only. The number and proportion of liver transplants performed for concurrent liver diseases have increased from 8 (6%) during Era 1 (1985-1989) to 302 (20%) during Era 7 (2015-2019; P < 0.001). Concurrent liver diseases were not associated with increased posttransplant mortality (adjusted hazard ratio, 0.98, 95% confidence interval, 0.84-1.14). Conclusions: Concurrent liver diseases are increasing among adult liver transplant recipients in Australia and New Zealand; however, they do not appear to impact posttransplant survival. Reporting all liver disease causes in the transplant registry reports provides more accurate estimates of liver disease burden.

Monitoring quality of care in hepatocellular carcinoma: A modified Delphi consensus.

Although there are several established international guidelines on the management of hepatocellular carcinoma (HCC), there is limited information detailing specific indicators of good quality care. The aim of this study was to develop a core set of quality indicators (QIs) to underpin the management of HCC. We undertook a modified, two-round, Delphi consensus study comprising a working group and experts involved in the management of HCC as well as consumer representatives. QIs were derived from an extensive review of the literature. The role of the participants was to identify the most important and measurable QIs for inclusion in an HCC clinical quality registry. From an initial 94 QIs, 40 were proposed to the participants. Of these, 23 QIs ultimately met the inclusion criteria and were included in the final set. This included (a) nine related to the initial diagnosis and staging, including timing to diagnosis, required baseline clinical and laboratory assessments, prior surveillance for HCC, diagnostic imaging and pathology, tumor staging, and multidisciplinary care; (b) thirteen related to treatment and management, including role of antiviral therapy, timing to treatment, localized ablation and locoregional therapy, surgery, transplantation, systemic therapy, method of response assessment, and supportive care; and (c) one outcome assessment related to surgical mortality. Conclusion: We identified a core set of nationally agreed measurable QIs for the diagnosis, staging, and management of HCC. The adherence to these best practice QIs may lead to system-level improvement in quality of care and, ultimately, improvement in patient outcomes, including survival.

Transcriptome-based repurposing of apigenin as a potential anti-fibrotic agent targeting hepatic stellate cells.

We have used a computational approach to identify anti-fibrotic therapies by querying a transcriptome. A transcriptome signature of activated hepatic stellate cells (HSCs), the primary collagen-secreting cell in liver, and queried against a transcriptomic database that quantifies changes in gene expression in response to 1,309 FDA-approved drugs and bioactives (CMap). The flavonoid apigenin was among 9 top-ranked compounds predicted to have anti-fibrotic activity; indeed, apigenin dose-dependently reduced collagen I in the human HSC line, TWNT-4. To identify proteins mediating apigenin's effect, we next overlapped a 122-gene signature unique to HSCs with a list of 160 genes encoding proteins that are known to interact with apigenin, which identified C1QTNF2, encoding for Complement C1q tumor necrosis factor-related protein 2, a secreted adipocytokine with metabolic effects in liver. To validate its disease relevance, C1QTNF2 expression is reduced during hepatic stellate cell activation in culture and in a mouse model of alcoholic liver injury in vivo, and its expression correlates with better clinical outcomes in patients with hepatitis C cirrhosis (n = 216), suggesting it may have a protective role in cirrhosis progression.These findings reinforce the value of computational approaches to drug discovery for hepatic fibrosis, and identify C1QTNF2 as a potential mediator of apigenin's anti-fibrotic activity.

Transmastoid partial labyrinthectomy for brainstem vascular lesions: clinical outcomes and assessment of postoperative cochleovestibular function.

OBJECTIVES: To discuss the transmastoid partial labyrinthectomy approach for brainstem vascular lesions, with respect to hearing and balance preservation. DESIGN: Retrospective case series. SETTING: Tertiary referral center (University Health Network, Toronto). PARTICIPANTS: Nine consecutive surgical patients between 1999 and 2004. MAIN OUTCOME MEASURES: Clinical, audiometric, and electrophysiological vestibular data. RESULTS: Nine transmastoid partial labyrinthectomy procedures (all females) were performed. In seven patients the underlying pathology was an intra-axial brainstem cavernous malformation. Two patients were treated for a basilar artery aneurysm. All patients had progressive neurological signs. Serviceable hearing (pure tone average (PTA): < 50 dB; speech discrimination score (SDS): > 50%) was preserved in seven patients. Partial vestibular function (clinical and electrophysiological) was maintained in most patients. CONCLUSIONS: The partial labyrinthectomy approach to the skull base provides excellent exposure while preserving cochleovestibular function in most patients.

The evolving epidemiology of hepatocellular carcinoma: a global perspective.

Primary liver cancer, the majority of which are hepatocellular carcinomas, is now the second leading cause of cancer death worldwide. Hepatocellular carcinoma is a unique cancer that typically arises in the setting of chronic liver disease at a rate dependent upon the complex interplay between the host, disease and environmental factors. Infection with chronic hepatitis B or C virus is currently the dominant risk factor worldwide. However, changing lifestyle and environmental factors in western countries plus rising neonatal hepatitis B vaccination rates and decreasing exposure to dietary aflatoxins in developing countries are driving an evolution of the epidemiology of this cancer. An understanding of this change is crucial in combating the rising incidence currently being seen in western regions and will underpin the efforts to reduce the mortality rates associated with this cancer.

Acute gastrointestinal bleeding after percutaneous coronary intervention.

Bleeding from the GI tract is a commonly encountered clinical problem after percutaneous coronary intervention. The GI tract is likely to become the most commonly encountered site of bleeding as cardiologists adopt smaller access sheath sizes, percutaneous closure devices and a radial artery approach, further reducing access-site bleeding. To appropriately manage gastrointestinal bleeding in this setting, the clinician must strike a balance between arresting hemorrhage and preventing ischemic coronary complications. To do so, an appreciation of both cardiovascular and gastrointestinal issues is required. This review aims to provide the required knowledge, as well as a series of recommendations from our practice, to assist in the management of this potentially fatal complication.

Resolution of severe hepato-pulmonary syndrome following transjugular portosystemic shunt procedure.

The hepato-pulmonary syndrome (HPS) is a relatively common complication of hepatic disease that leads to hypoxaemia and dyspnoea secondary to pulmonary shunting. A number of pharmacological therapies have been trialled, yet liver transplantation remains the only definitive treatment. The use of a transjugular intrahepatic portosystemic shunt (TIPS) to reduce portal hypertension and improve oxygenation remains controversial in HPS due to the lack of large clinical series or randomised controlled trials. We present a case of HPS successfully treated with TIPS and review the relevant literature.