Consideration of Dose Error in Dynamic MLC IMRT Using MLC Speed Control with Dose Rate Change.

In dynamic multi-leaf collimator (MLC) intensity-modulated radiotherapy (IMRT), the accuracy of delivered dose is influenced by the positional accuracy of the moving MLC. In order to assess the accuracy of the delivered dose during dynamic MLC IMRT, the delivered dose error in dynamic MLC IMRT using the MLC speed control with dose rate change was investigated. Sweeping gap sequence irradiation was performed with constant MLC leaf speed at 0.6 to 5 cm/s or changed MLC speed (4 steps). The positional accuracy of the moving MLC was evaluated from the trajectory log file. Absorbed dose measurements with sweeping field (Field size: 10 cm×10 cm, MLC leaf speed: 0.6 to 2.7 cm/s, MLC leaf gap width: 0.2 to 2.0 cm) were performed. The delivered dose error at each gap width was evaluated according to MLC leaf speed change. MLC positional errors and changes in delivered dose according to MLC leaf speed were within 0.07 mm and 0.6%, respectively, for all measurements. Beam hold-off did not occur under any conditions. We confirmed that TrueBeam can regulate MLC leaf speed below the maximum limit (2.5 cm/s) by changing the dose rate in real-time during irradiation in dynamic MLC IMRT. MLC gap error during irradiation was estimated within 0.2 mm at the maximum dose rate from the results of absolute dose measurements using dynamic MLC irradiation. In conclusion, TrueBeam can use the maximum dose rate for the treatment planning of dynamic MLC IMRT, which has an advantage of shorter treatment time.