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Spatial Variation of Dosimetric Leaf Gap and the Impact in Dose Delivery

L Kumaraswamy

L Kumaraswamy*, J Schmitt, M Podgorsak, Roswell Park Cancer Institute, Buffalo, NY

SU-C-BRCD-1 Sunday 1:30:00 PM - 2:15:00 PM Room: Ballroom CD

Purpose: To evaluate the variation of dosimetric leaf-gap (DLG) along the travel path of each MLC leaf pairs. Furthermore, dosimetric implication of this variation is studied for several dynamic MLC fields.

Methods: During dose calculation, the treatment planning systems (TPS) retracts the MLC leaf positions by half of the DLG value (measured at CA) for all leaf positions in a Dynamic MLC plan to accurately model the rounded leaf ends. The 6 MV DLG values were measured, for all leaf pairs, in increment of 0.5 cm at Dmax, in the direction of leaf motion. The measurements were performed on three Varian Linacs, employing the Millennium 120-leaf MLCs. Measurements were performed using both the Sun Nuclear MapCheck device and the PTW pinpoint ion chamber. Measurements with an ion chamber were also performed with varying leaf gaps to determine % change in dose with leaf gap discrepancy.

Results: The measured DLGs for all leaf pairs were very similar along the in-plane direction; varying maximally only by 0.3 mm. But in the in the leaf travel direction, the DLG increases gradually from CA to 6.0 cm off-axis, reaching to a maximum variation of 1.6 mm for a particular leaf pair. Varying gap measurements indicate that DLG deviation of about 1 mm can cause a dose difference of about 5.0 % for a fluence with 1.5 cm leaf width sweeps and significantly increases to about 12.5 % difference for 0.5 cm sweeps.

Conclusions: The spatial variation in DLG is caused by the variation in beam spectrum as well as MLC motion across the field. Fields with dynamic MLC centered on the CAX would not be affected since DLG does not vary as much; but any field residing significantly off-axis with narrow sweeping leafs can exhibit significant dose differences due to large DLG differences.

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