using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        var contentType = @"Multipart/Related; boundary=""----=_Part_17500_382902305.1682622011945""; action=""urn:ihe:iti:2007:CrossGatewayRetrieve""; type=""application/xop+xml""; start-info=""application/soap+xml""";

        var manuallyFixed = @"multipart/related; boundary=""----=_Part_17500_382902305.1682622011945""; action=""urn:ihe:iti:2007:CrossGatewayRetrieve""; type=""application/xop+xml""; start-info=""application/soap+xml""";

        var replaced = Regex.Replace(contentType, "multipart/related", "multipart/related", RegexOptions.IgnoreCase);

        Console.WriteLine(replaced);

        Console.WriteLine(replaced == manuallyFixed);

    }

}