Skeletal dosimetry in cone beam computed tomography.

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Radiation Dosimetry

Background:

• Cone beam computed tomography (CBCT) is vital for image-guided radiotherapy (IGRT), aiding treatment verification and patient positioning.
• CBCT is known to increase bone dose, potentially up to 10% of the prescribed radiotherapy dose.
• Understanding dose distribution within bone, specifically to red bone marrow (RBM) and bone surface cells (BSCs), is crucial for risk assessment.

Purpose of the Study:

• To quantify the dose delivered to RBM and BSCs within human bone structures during simulated CBCT scans.
• To compare the dose received by BSCs and RBM to other organs at risk (e.g., eye lens, brain) in different anatomical regions.
• To evaluate the implications of elevated bone dose from CBCT in the context of radiotherapy.

Main Methods:

• Utilized voxelized human phantoms (FAX06/MAX06) with detailed trabecular bone and bone marrow microstructure.
• Employed phase-space data from a realistic CBCT imager model as the radiation source.
• Modified EGSnrc-based Monte Carlo code for accurate radiation transport simulation and voxel-by-voxel dose scoring in RBM and BSCs.

Main Results:

• CBCT scans result in significantly higher doses to bone surface cells (BSCs) compared to red bone marrow (RBM).
• In head and neck scans, average BSC dose was 25% higher than eye lens dose and 80% higher than brain dose.
• Average RBM dose was approximately 50% of the average BSC dose and lower than doses to other organs at risk.

Conclusions:

• The elevated dose to BSCs from CBCT is a significant finding.
• This increased BSC dose warrants careful consideration, especially when CBCT is used in conjunction with radiotherapy.
• Further research into mitigating BSC dose during IGRT may be necessary to ensure patient safety.