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Impact of enhanced leaf model on dose calculation accuracy in single-isocenter multitarget stereotactic radiosurgery
Hem Moktan1,2, Hongyu Jiang1, H Harold Li1
1Department of Radiation Oncology, University of Kansas Cancer Center, Kansas City, Kansas, USA.
Purpose:
Single-isocenter multitarget (SIMT) radiosurgery has become increasingly popular as advancement in planning and delivery systems have made this approach clinically viable. With targets varying in size and distance from isocenter, SIMT plans are highly complex with dynamic multileaf collimator (MLC) motion. Our department recently commissioned Eclipse Treatment Planning System v18.0, which included a novel enhanced leaf model (ELM) for photon dose calculation. ELM represents the biggest update in MLC modeling on a commercial treatment planning system over the past decade, yielding improvements in leaf modeling and ray tracing. Considering its dependence on dynamic MLC movements, we set out to assess the potential clinical impact of ELM on SIMT.
Methods:
Dynamic zebra crosswalk (DZC) plans were delivered on a Varian Edge to investigate ELM. DZCs consisted of sweeping MLC gaps (ranging 1-15 mm) across a 3 cm width at isocenter, 4 cm, 8 cm, and 12 cm along the x-axis. Phantom dose calculations were performed using AAA v15.6 and v18.0 (with ELM) for 6 MV flattening filter free DZC plans and compared to measurements using stereotactic radiosurgery MapCHECK (Sun Nuclear Corporation) and Gafchromic EBT4 films (Ashland). To assess potential impact on SIMT, ten patients were retrospectively planned with RapidArc and HyperArc. Both versions of AAA were used for dose calculation.
Results:
DZC measurements showed improved agreement with ELM; differences between measured and calculated doses were reduced by as much as 19% for the smallest sweeping gaps at off-axis distances. Differences in central profile dose for DZCs increased with reduced gap size and increased off-axis position. SIMT plans showed up to 4.0% increase in planning target volume (PTV) maximum dose when switching from AAA v15.6 to v18.0.
Conclusion:
Dose calculations with ELM mirrored diode and film measurements for highly modulated SIMT plans. ELM represents a major improvement in MLC modeling that more accurately reflects current treatment delivery practice.
