SU-E-J-54: Bone Detection in MR Images and Absorbed Dose in a Material Behind Bones in Radiotherapy

Area of Science:

• Medical Physics
• Radiotherapy
• Medical Imaging

Background:

• Magnetic Resonance (MR) imaging is increasingly used in radiotherapy treatment planning.
• Accurate bone localization is crucial for dose calculation and treatment efficacy.
• The impact of bone density assumptions in pseudo-CT images on dose calculation requires investigation.

Purpose of the Study:

• To assess the accuracy of bone localization using MR images alone for radiotherapy planning.
• To measure absorbed dose in materials situated behind bone structures.
• To evaluate dose calculation errors introduced by using a single electron density for bone in pseudo-CT images.

Main Methods:

• A phantom with deer bones and gelatine was used to evaluate MR image accuracy.
• Bone edge location and diameter were compared between MR images and actual measurements.
• Absorbed dose behind bone was measured using a matrix detector at 6 and 15 MV.
• Dose calculation errors were quantified by comparing pseudo-CT derived results with standard CT-based calculations using superposition and Monte Carlo algorithms.

Main Results:

• Bone localization accuracy within 1 mm was achieved using MR images.
• Measured absorbed dose behind a 2.5-cm thick femur was approximately 1% lower than at the bone edge.
• Bulk density pseudo-CT images introduced dose calculation errors up to nearly 2% behind the femur.

Conclusions:

• Bone localization using MR images alone is not a limiting factor for radiotherapy treatment planning.
• The reduction in absorbed dose behind bone is not exclusively dependent on bone diameter.
• Further research is recommended for generating complex pseudo-CT images and improving dose calculations.