Quantitative characterization of tomotherapy MVCT dosimetry.

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Dosimetry

Background:

• Megavoltage computed tomography (MVCT) is integral for image guidance in tomotherapy treatments.
• Increasing use of MVCT raises concerns regarding cumulative ionizing radiation dose to patients.
• Accurate dosimetry is essential for understanding and managing imaging dose in radiotherapy.

Purpose of the Study:

• To quantitatively characterize and analyze the dosimetry of tomotherapy MVCT.
• To investigate the influence of scanning parameters and phantom size on MVCT dose distribution.
• To validate dose calculation algorithms against experimental measurements.

Main Methods:

• Dose calculations performed using a commissioned dose engine.
• Film measurements used for experimental validation of dose calculations.
• Homogeneous cylindrical water phantoms of varying diameters used to simulate patient anatomy.
• Systematic variation of scanning parameters: jaw openings (J4, J1, J0.1) and couch speeds (fine, normal, coarse).
• Analysis of dose profiles, center dose, maximal dose, surface dose, and mean dose.

Main Results:

• Excellent agreement observed between calculated and film-measured doses, with a maximum difference of ~6% for specific parameters.
• Center dose demonstrated a near-linear decrease with increasing phantom diameter.
• Dose varied significantly with couch speed: fine mode received twice the dose of normal mode, and three times that of coarse mode.
• Maximal dose ranged from 100% to ~200% of center dose, influenced by phantom size, jaw settings, and couch speed.
• Mean and average surface doses ranged from 95% to 125% of center dose, increasing with phantom size.

Conclusions:

• A comprehensive quantitative dosimetric characterization of tomotherapy MVCT was achieved.
• The study provides a valuable reference dataset for estimating CT dose index for MVCT.
• Understanding dose distribution based on scanning parameters and phantom size is crucial for optimizing imaging protocols and minimizing patient dose.