TOPAS wraps and extends the Geant4 Simulation Toolkit. Geant4 is built directly into TOPAS (the user does not need to themselves install Geant4). The following table lists which Geant4 version is built into each TOPAS version.

TOPAS version Geant4 version
3.5.0 10.06.p01
3.4.0 10.05.p02
3.3.0 10.05.p01
3.2.0 10.05.p01
3.1.0 10.03.p01
3.0.0 10.02.p01
2.0.0 10.01.p02
1.0.0 9.06.4

A detailed list of changes introduced by each TOPAS version can be found below:

Prior to the release of version 1.0, TOPAS had a long public beta phase.