Comprehensive measurement for clinical dosimetry of tertiary multileaf collimator

To initiate the use of a tertiary multileaf collimator (MLC) in the clinic, a set of dosimetry data for clinical use of the MLC, the secondary field size jaw and the MLC tracked by the jaw were measured. The dose calculation technique from the commissioned jaw field data was established. The dosimetry characteristics included absolute output (Dw), collimator scatter factor (Sc), total scatter factor (Scp), phantom scatter factor (Sp), percentage depth dose (PDD), tissue-maximum ratio (TMR), and peak scatter factor (PSF). The absolute output of the MLC field was +5% to +2% greater than that of the same jaw size field from 4 x 4 to 24 x 24 cm2 fields. The variation of Sc and Scp ranged from 4 x 4 to 24 x 24 cm2 fields and were less than that of the jaw fields, while the Sp, PDD and TMR values remained the same. Importantly, when the MLC-only field was performed without the collimator jaws tracking close to the field segments, greater output was delivered, and PSFs should be used to calculate the MLC field output.