Dose deposited in the DICOM example

Hello dear experts, I have some questions regarding the DICOM example of GEANT4.
I import the DICOM file for a phantom and i need to get the deposited dose in specific organ. The DICOM example deals with voxels.
So how can I irradiate the entire target organ, given that it is made up of several CT slices?
Similarly, i’m interested only in accumulated dose in specific organ how can I measure the dose deposited in the whole organ, given that it is also made up of several voxels spread over several CT slices?

If you have any suggestion I would be great full.
Thank you for your help!