Normal tissue doses from MV image-guided radiation therapy (IGRT) using orthogonal MV and MV-CBCT

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Imaging

Background:

• Accurate dose calculation is crucial for radiotherapy planning.
• Mega-voltage (MV) imaging is increasingly used for patient positioning and verification.
• The Halcyon linear accelerator offers various MV imaging techniques.

Purpose of the Study:

• To measure and compare MV imaging doses from the Halcyon linac.
• To validate the accuracy of the Eclipse treatment planning system in calculating these doses.

Main Methods:

• An anthropomorphic thorax phantom was used for dose measurements.
• MV cone-beam computed tomography (MV-CBCT) and MV-MV imaging were performed in high-quality and low-dose modes.
• Ion chambers measured doses at 11 points, compared against Eclipse calculations.

Main Results:

• In-target doses ranged from 0.59 to 9.75 cGy; extra-target doses from MV-MV were 0-2.54 cGy.
• MV-CBCT doses were less sensitive to organ location than MV-MV.
• Low-dose modes reduced imaging dose by approximately 50% compared to high-quality modes.

Conclusions:

• The Eclipse system accurately calculates MV imaging doses from the Halcyon linac, with differences under 0.5% of a typical treatment dose.
• This accuracy supports reliable integration of imaging and treatment dose distributions.
• Understanding these doses is vital for optimizing radiotherapy planning and minimizing patient exposure.