Technical Note: Efficient and accurate MRI-only based treatment planning of the prostate using bulk density...

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Image-guided Radiation Therapy

Background:

• Magnetic resonance imaging (MRI) offers superior soft-tissue contrast compared to computed tomography (CT).
• MRI-only workflows eliminate the need for CT, reducing patient dose and streamlining treatment planning.
• Accurate electron density assignment is crucial for dose calculation in MRI-only radiotherapy.

Purpose of the Study:

• To evaluate the dosimetric accuracy and robustness of an MRI-only prostate radiotherapy planning approach.
• To assess a bulk density assignment method using automatically segmented bone and soft tissue regions.
• To compare MRI-only dose calculations with CT-based calculations for volumetric modulated arc therapy (VMAT).

Main Methods:

• Twenty prostate cancer patients underwent planning CT and MRI scans for VMAT.
• Atlas-based segmentation (ABS) was used to identify bone and soft tissue regions for bulk density assignment.
• Various soft tissue (0.95-1.03 g/cm³) and bone (1.15-1.65 g/cm³) density pairs were tested to minimize dose difference compared to CT.

Main Results:

• An optimal density pair of 0.98 g/cm³ for soft tissue and 1.20 g/cm³ for bone yielded dose differences <1% compared to CT.
• Atlas-based segmentation demonstrated high accuracy for bone contouring (Dice similarity coefficient 0.94, Mean Distance to Agreement <1 mm).
• Clinically acceptable dose differences were achieved with both automatic and manually adjusted bone contours.

Conclusions:

• Bulk density assignment on MRI images provides dosimetrically accurate and robust results for prostate VMAT planning.
• An integrated MRI-only workflow using two bulk densities is a feasible and reliable approach for prostate cancer radiotherapy.
• This method supports the transition to advanced MRI-only treatment planning systems.