Patient dose considerations for routine megavoltage cone-beam CT imaging

Area of Science:

• Medical Physics
• Radiation Oncology
• Imaging Science

Background:

• Megavoltage cone-beam CT (MVCBCT) is an in-room imaging system for image-guided radiation therapy (IGRT).
• It utilizes a conventional treatment unit with a flat panel detector for 3D patient imaging in treatment position.
• MVCBCT is used for anatomy verification and patient alignment before radiation delivery.

Purpose of the Study:

• To evaluate the image acquisition dose delivered to patients during MVCBCT procedures.
• To develop a simple method for reducing the additional dose from routine MVCBCT imaging.
• To integrate MVCBCT dose calculation into treatment planning systems (TPS) for compensation.

Main Methods:

• Simulated MVCBCT acquisition using a treatment planning system (Phillips Pinnacle) with conventional CT scans.
• Dose measurement validation using a water-equivalent phantom with MOSFETs and an ion chamber.
• Development and application of a compensation factor equation for composite treatment and imaging dose plans.
• Testing the compensation method on clinical plans for prostate, head and neck, and pelvis patients.

Main Results:

• TPS calculations of MVCBCT dose showed excellent agreement with phantom measurements (within 3%).
• A small anterior-posterior dose gradient (0.6–1.2 cGy/MU) was observed for typical MVCBCT acquisitions.
• Daily MVCBCT for prostate patients added less than 4% to the total treatment dose.
• Compensated plans demonstrated no additional dose to the target and minimal increases at low doses.

Conclusions:

• The radiation dose from MVCBCT imaging can be precisely calculated within the TPS.
• A simple compensation factor allows for the minimization of total dose to critical structures.
• This approach reduces the overall treatment burden for patients undergoing daily MVCBCT imaging.
• Reducing the imaging volume can further decrease the delivered dose, enhancing safety and focus.