Radiotherapy patients will from time to time be treated on another linac than originally planned due to service or logistical challenges. For patients treated with dynamic intensity modulated radiotherapy (IMRT), extra care should be taken to make sure the delivered dose remains as planned. Four linacs with the same type of dynamic multileaf collimator (MLC) were compared to find a general prediction of the potential dosimetric error caused by treating IMRT patients on another linac without recalculating the treatment plan. The MLC parameters, transmission and dosimetric leaf separation (DLS) were measured for all four linacs. The dynamic fields that were measured to find the DLS value were imported into the treatment planning system to compare the calculated and measured doses. Measured values of transmission and DLS were used directly in the calculations to obtain dose differences of less than 1% between the calculated and measured doses at the reference setup. The dosimetric discrepancy between the linacs was acceptable for all but one linac. Recalculation of the treatment plan therefore remains as standard procedure for this linac when a planned patient must switch linac during the course of treatment. The depth and field size dependences of the MLC parameters were also tested, finding dose differences of up to 4%.