Mapping motion from 4D-MRI to 3D-CT for use in 4D dose calculations: a technical feasibility study

Area of Science:

• Medical Physics
• Radiotherapy
• Medical Imaging

Background:

• Accurate dose calculation in radiation therapy requires time-resolved (4D) data to account for organ motion.
• Conventional 4D computed tomography (4D-CT) provides a snapshot of motion and assumes regular breathing, with added patient dose burden.
• Repeated 4D-CT acquisitions are undesirable due to significant radiation exposure.

Purpose of the Study:

• To test the feasibility of an alternative method for generating patient-specific 4D-CT datasets using 4D-MRI.
• To evaluate the accuracy of simulated 4D-CT datasets derived from 4D-MRI for dose calculations.
• To assess the impact of irregular breathing patterns on dose distribution in proton therapy.

Main Methods:

• Motion information was extracted from 4D-MRI sequences.
• Simulated 4D-CT datasets (4D-CT(MRI)) were created by deforming a static 3D-CT using motion fields from 4D-MRI.
• 4D proton dose calculations were performed using a validated algorithm that accounts for displacements and deformations based on individual time stamps.

Main Results:

• 4D dose distributions calculated using 4D-CT(MRI) were nearly identical to those from conventional 4D-CT, despite proton beam sensitivity to geometric differences.
• Simulations using 4D-CT(MRI) with variable breathing patterns demonstrated significantly different proton dose distributions, highlighting the effect of irregular breathing.
• The 4D dose algorithm accurately adjusted dose grid points based on time-stamped displacements and density variations.

Conclusions:

• Motion information from 4D-MRI can successfully generate realistic 4D-CT datasets from a single static 3D-CT.
• 4D-CT(MRI) offers a robust method for testing treatment plan resilience against organ motion.
• This approach facilitates the study of breathing irregularities and their impact on radiation dose delivery.