Update the cache when dataset is updated

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C#

ASP.NET

cache

I have a data set storing all continents with their respective countries. I am caching the data table:

DataSet dset = new DataSet();
string cacheKey = "CoverageDataTable";
object cacheItem = Cache[cacheKey] as DataTable;
if (cacheItem == null)
{
    dset = (DataSet)_obj.GetAllContinent();
    cacheItem = dset.Tables[0];
    Cache.Insert(cacheKey, cacheItem, null, System.Web.Caching.Cache.NoAbsoluteExpiration, TimeSpan.FromHours(5), CacheItemPriority.High, null);
 }

Now I want that if there is any change in the data table, fresh data should be fetched from the database. How can I do this?