How to read data from check box in PDF using iTextsharp?

We are trying to read data from PDF file using pixel coordinates (by using iTextsharp). below is the sample code and working to retrieve data from text boxes

string file = @"E:\CA Dan Truman Services.pdf";
PdfReader reader = new PdfReader(file);
Rectangle rect = new Rectangle(18, 558, 32, 570);
RenderFilter[] renderFilter = { new RegionTextRenderFilter(rect) };

but its not working while retrieving data from check box. please refer attachment for checkbox control.

Sample pdf with check box control. when check box selected X symbol appears

Please let us know that how can we read data from check box using above mentioned code .

Any suggestions is highly appreciated.

Regards
Nanda Kishore.CH