Shattered pellet injection (SPI) has been selected as the baseline concept for Disruption Mitigation System (DMS) in ITER and its design will be finalized by 2022. DMS Task Force Theory & Modelling Group is focusing on the physics validation of optimized SPI performance. Because a robust suppression of runaway electron (RE) avalanche requires increasing the electron density by a factor of the order of 20 to 40, it is important to assess the key engineering parameters (pellet composition, injection velocity, shattering angles, pellet size, etc.) for achieving such a large factor by SPI as well as the flexibility and the margin to apply SPI to different target plasmas including both the pre-fusion and fusion operation scenario. Here we will report the status of SPI modelling for ITER reference scenarios using a 1.5D disruption transport code INDEX and we would like to discuss open physics issues for successful operation of ITER DMS in the direction of runaway electron avoidance by pre-TQ and post-TQ material injection.