¿Cómo cancelar tarea asíncrona en Asp.Net Mvc?

Escribi este codigo para cancelar una tarea asincrona pero no la esta cancelando:

    private CancellationTokenSource _cts1 = new CancellationTokenSource();
        private CancellationToken _ct1;
        private CancellationTokenSource _cts2 = new CancellationTokenSource();
        private CancellationToken _ct2;
        private CancellationTokenSource _cts3 = new CancellationTokenSource();
        private CancellationToken _ct3;
        private CancellationTokenSource _cts4 = new CancellationTokenSource();
        private CancellationToken _ct4;

   public ActionResult Report(ReportFilterModel filterModel, int report)
        {
            if (Session["GeneratingReport"] == null || !(bool)Session["GeneratingReport"])
            {
                _cts1 = new CancellationTokenSource();
                _ct1 = _cts1.Token;
                _cts2 = new CancellationTokenSource();
                _ct2 = _cts2.Token;
                _cts3 = new CancellationTokenSource();
                _ct3 = _cts3.Token;
                _cts4 = new CancellationTokenSource();
                _ct4 = _cts4.Token;

                Session["GeneratingReport"] = true; //Se registra en sesión
                string basePath = ConfigurationManager.AppSettings["BaseFilePath"] + "/ReportesExcel/";
                string fileName = null;
                try {
                    switch (report)
                    {
                        case 1:
                            fileName = "report-users" + "-" + DateTime.Now.ToString("yyyyMMddhmmss") + ".xlsx";
                            Task.Factory.StartNew(() => ExportReport(filterModel, basePath + fileName,_ct1),_ct1);
                            break;
                        case 2:
                            fileName = "report-plays" + "-" + DateTime.Now.ToString("yyyyMMddhmmss") + ".xlsx";
                           Task T = Task.Factory.StartNew(() => ExportReportPlays(filterModel, basePath + fileName,_ct2), _ct2);
                            break;
                        case 3:
                            fileName = "reporte-compras" + "-" + DateTime.Now.ToString("yyyyMMddhmmss") + ".xlsx";
                            Task.Factory.StartNew(() => ExportReportPurchases(filterModel, basePath + fileName,_ct3), _ct3);
                            break;
                        case 4:
                            fileName = "reporte-ventas-distribuidor" + "-" + DateTime.Now.ToString("yyyyMMddhmmss") + ".xlsx";
                            Task.Factory.StartNew(() => ExportReportDistributorPurchases(filterModel, basePath + fileName,_ct4), _ct4);
                            break;
                    }
                    Session["ReportPath"] = basePath + fileName;
                }
                catch(Exception ex) 
                { 
                    string error =  ex.ToString();
                }
                
            }
            return Content("Generando Reporte");
        }

Aqui un ejemplo del metodo que invoco:

public async Task ExportReport(ReportFilterModel filterModel, string path, CancellationToken _ct)
    {
        try
        {

            while (!_ct.IsCancellationRequested)
            {
                using (var taskDb = new CmsLiteData()) //Se define contexto de ejecución de la conexión para la tarea Asyncrona, forzando la llamada de dispose cuando finalice
                {
                    this.GenerateExcel(filterModel, "Informe compra y recompra (cantidad de usuarios únicos)", true, path, taskDb);
                }
            }
        }
        catch (Exception ex)
        {
            _cts1.Cancel();
            CmsHelper.SetLog(ex.ToString(), 1, "PurchaseReportController", "ExportReport", "TareaAsincrona", "N/A", "N/A");
        }
    }

Aqui el metodo desde donde trato de cancela no devuelve error pero no cancela la tarea:

  public ActionResult CancelTaskGenerateReport (int idReport)
        {
            string nameReport = string.Empty;
            switch (idReport)
            {
                case 1:
                    nameReport = "Reporte de compra/recompra";
                    _cts1.Cancel();
                    break;
                case 2:
                    nameReport = "Reporte de reproducciones";
                    _cts2.Cancel();
                    break;
                case 3:
                     nameReport = "Reporte de compra";
                    _cts3.Cancel();
                    break;
                case 4:
                    nameReport = "Reporte de ventas";
                    _cts4.Cancel();
                    break;

            }
            return Content("Generacion de Reporte Cancelada");
        }