RapidArc patient specific mechanical delivery accuracy under extreme mechanical limits using linac log files

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Technology

Background:

• RapidArc (RA) radiotherapy enables faster treatment delivery by utilizing volumetric modulated arc therapy.
• Accurate dose delivery is crucial, especially when operating treatment machines near their mechanical limits.

Purpose of the Study:

• To evaluate the accuracy of RapidArc delivery under dynamic multileaf collimator (DMLC) leaf velocities, gantry speeds, and dose rates approaching machine limits.
• To assess dosimetric differences between planned and delivered dose distributions.

Main Methods:

• Thirty RA patient plans for various cancers were created and delivered on a Trilogy unit.
• Three delivery scenarios were tested: medium MLC velocities with high gantry/dose rates, maximal MLC velocities, and lowest gantry speeds.
• Machine log files were analyzed using custom software to reconstruct delivered dose distributions, which were then compared to planned doses using gamma analysis and dose difference metrics.

Main Results:

• MLC leaf position discrepancies averaged 0.8-1.2 mm, and gantry position discrepancies were up to 0.9°.
• Maximum dose differences reached 6.7% with 1 cm MLC leaves at high velocities, while 3D gamma acceptance rates exceeded 99% across all tested conditions.
• The majority of dose voxels showed minimal differences (<3%) from the planned dose.

Conclusions:

• RapidArc delivery accuracy is maintained across a wide range of operational parameters for 0.5 cm MLC leaves.
• Care should be taken when employing larger MLC leaves (1 cm) at high velocities due to potential increases in dosimetric uncertainty.
• A novel method for determining delivered 3D dose distributions from machine log files was successfully developed and applied.