How to get mass density from DICOM CT?

Hi all,

I have found that the CT value (in Hounsfield units) provided in Slicer (shown in the data probe when moving the mouse pointer to the point of interest) was different from the Pinnacle treatment planning system (used in radiation oncology). And I want to know how to correlate the two values. Besides, how to get mass density from the CT number provided in Slicer? Is there any information provided in DICOM tag could help?

Hope for some advice. Your help is highly appreciated.