you can look at papers/proposals such as these to get an idea about the algorithm:

https://innsida.ntnu.no/documents/portlet_file_entry/10157/Goa-Monte+Carlo+simulations+of+diffusion-weighted+MRI+in+breast+cancer-2016.pdf/ac6faa34-706e-4db7-9afc-362f9b3df88d?status=0

Thank you very much sir.

EGSnrc or GATE for CT simulation.

Qingyang Wei Sir, Could you please detail instructions. I downloaded EGSnrc and GATE. But, could not figure out how to begin???

Vikas Ramachandra Sir, there is no name of software in the document attached. It just mentions that they are working on that particular subject.

You can follow the UserGuide and the example provided by the software. There is an "examples" folder in GATE.

Yes. It costs 750€. 

Expensive for us.

If you want calculate organ equivalent and effective dose from CT examinations you can use ImpactDose  software which developed by ImPACT group. The computation is done based on tables pre-computed for different scanners and selected anthropomorphic phantoms.

Another dose estimation software which is easy to use and not expensive is CT-expo developed by Dr. Georg Stamm. its user friendly and inexpensive. CT-expo in based on computational methods which were used to evaluate the data collected in the German surveys on CT practice. 

VirtualDose in another GPU-based Monte Carlo dose estimator which enables users to assess organ doses, in addition to CTDI and DLP data provided by the CT scanner.

for more information see below links: 

http://www.ct-imaging.de/en/ct-software-e/impactdose-e.html

http://www.virtualphantoms.com/virtualdose/

http://www.mh-hannover.de/fileadmin/kliniken/ diagnostische_radiologie/download/ct-expo-e.zip

Dear Maharjan; I guess you are new in MC simulations. GATE is the part of Geant4 project. You have to get experience on Geant4 as first step. In this case, i can offer you to follow User Forums and User Guides. Sometime, to be in user forums of a MC program is giving fastest response to your questions but somehow, do not stop to read about User Guides. Another point is, program of impact dose need a big cost since it is trade company. There is many free sources. You have to discover them especially by reading the literature. For any question please do not hestitate contact me directly.. Good Luck !

@Tekin Sir. Thank you very much.

NCI dose is free tools for CT, NM, and radiography/fluoroscopy. http://ncidose.cancer.gov

GATE would be the one. You can try it.