Independent absorbed-dose calculation using the Monte Carlo algorithm in volumetric modulated arc therapy.

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Volumetric Modulated Arc Therapy (VMAT) is a sophisticated radiation therapy technique.
• Accurate dose calculation is critical for effective VMAT treatment planning.
• Independent verification of dose calculations ensures patient safety and treatment efficacy.

Purpose of the Study:

• To compare absorbed dose calculations using a Monte Carlo (MC) algorithm against a superposition/convolution (SC) algorithm for VMAT.
• To evaluate the accuracy of MC-based dose recalculations for various cancer treatment sites.
• To assess the utility of independent MC dose calculations in VMAT quality assurance.

Main Methods:

• Treatment plans generated with the SC algorithm (SmartArc, Pinnacle) were recalculated using the X-ray Voxel Monte Carlo (XVMC) algorithm (Monaco).
• Validation was performed using phantom irradiations. Independent dose calculations were conducted for 15 patients across prostate, lung, head and neck, rectal, and esophageal cancer sites.
• Dose comparisons focused on the planning target volume (PTV) and organs at risk (OAR), with analysis of discrepancies against measurement data.

Main Results:

• Excellent agreement (<1.5% difference at isocenter) was observed between SC and XVMC algorithms for VMAT phantom calculations.
• Independent absorbed dose calculations showed strong agreement, with mean PTV dose differences within 2.0% for all patients.
• Significant differences were noted in mean OAR doses, particularly a smaller calculated rectal dose in Monaco for prostate cancer patients.

Conclusions:

• MC and SC algorithms demonstrate comparable accuracy in high-dose regions for VMAT.
• Discrepancies in low-dose regions may be attributed to intrinsic MC variations, modeling, or CT-to-density table differences.
• Independent MC dose calculations are valuable for commissioning and quality assurance in intensity-modulated radiation therapy.