A treatment planning system IMRT beam model is usually validated using phantom-based measurement, however this will not detect errors related to patient anatomy and inhomogeneity. In this study a secondary treatment system (CMS XIO) was used as a 3D dosimeter to verify an IMRT beam model recently commissioned in a Philips Pinnacle treatment planning system. Data sets from three head-neck and two prostate patients previously treated were utilised. The IMRT plans for these patients were planned in Pinnacle and transferred to XIO. The dose at each voxel in the patient volume was calculated in both XIO and Pinnacle. The 2D dose gamma maps for three orthogonal planes passing through the isocenter were calculated with a criteria of 3%/3mm. The mean gamma pass rate for all patients was 96.86% with maximum and minimum values of 99.6% and 95%. One coronal dose plane at 5.5 cm depth in the phantom was also measured and compared with dose calculated by the Pinnacle IMRT beam model using same gamma criteria. The measured mean gamma pass rate for this coronal plane dose was 96.7% with maximum and minimum of 98.41% and 95.3%. This was comparable with the gamma map pass rates for the three orthogonal dose planes calculated by XIO for the patient data. A secondary treatment planning system was shown to provide a supplementary verification tool based on calculation-based 3D dosimetry using patient anatomy.