Performance evaluation of the binary MLC system on a novel biology-guided radiotherapy machine using beamlet sequence...

To evaluate the clinical performance of a binary multi-leaf collimator (MLC) that makes high-frequency transitions on a novel ring-gantry medical linac.

Beamlet sequence data from treatment plans and daily treatments were retrieved for ten patients treated with image-guided radiotherapy (IGRT) on the novel machine, with five patients treated in the pelvic and thoracic regions respectively. The prescription dose ranged from 1.8 to 7 Gy per fraction with an average beam-on time of 484.1 ± 129.6 s (range: 299-666 s). All plans used a field size of 40 cm x 2 cm. A 64-leaf binary MLC modulates the beam at 50 gantry firing positions in each gantry rotation while the gantry rotates at 60 RPM. Gantry angle, couch position, delivered monitor units (MU), and MLC leaf opening status were recorded at each beam firing position. Daily beamlet sequence log data was compared with those in the treatment plan at each couch position.

The ten treatment plans had an average MU per fraction of 5269 ± 1462 MU (range: 3184-7259) with an average total gantry firing positions of 15 835 ± 4391 (range: 9566-21 817). In daily delivery, an average number of 29.9 ± 8.9 (range: 12.3-41.0) gantry firing positions were skipped, resulting in an average reduction of 9.9 ± 3.0 MU (range: 4.1-13.6 MU), or 0.19 ± 0.04% (range: 0.13-0.28%), relative to plan MUs. The MU reduction was proportional to the number of skipped gantry firing positions, while the number of skipped gantry firing positions had an approximate linear relationship with the total number of gantry firing positions in a plan.

In-house software tools have been developed to efficiently evaluate daily performance of the binary MLC on the novel machine. In delivering IGRT plans, the number of skipped gantry positions or MU reduction showed no clinically significant effect on delivered dose.