Four-dimensional dose calculations for dynamic tumour tracking with a gimbal-mounted linear accelerator

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Dynamic Tumor Tracking (DTT) radiotherapy requires accurate dose calculation across respiratory phases.
• Standard treatment planning often uses a single breath-hold phase, potentially underestimating dose to organs at risk (OARs).
• Accurate modeling of beam motion during DTT is crucial for precise dosimetry.

Purpose of the Study:

• To introduce and validate a novel
• rotation method
• for re-calculating DTT treatment plans on multiple respiratory phases.
• To assess the dosimetry accuracy by comparing dose distributions calculated on different numbers of breathing phases.
• To determine the most efficient and accurate method for verifying DTT plan safety.

Main Methods:

• sIMRT plans were optimized on exhale breath-hold CT images for 10 liver cancer patients.
• The rotation method recalculated dose distributions on inhale and exhale phases, and subsequently on all 10 phases of 4DCT datasets.
• Dose distributions were deformed to exhale CT and accumulated with phase-weighted sums; maximum OAR doses were compared.

Main Results:

• When calculating dose on only inhale and exhale phases, six OARs exceeded their dose limits.
• Calculating dose on all 10 breathing phases resulted in five OARs exceeding their dose limits.
• The rotation method accurately modeled beam geometry across respiratory phases.

Conclusions:

• DTT plans optimized for a single phase risk OAR dose constraint violations.
• Accumulating dose from all 10 breathing phases using deformable image registration offers the highest accuracy but is time-consuming.
• A weighted accumulation of dose from two extreme phases (inhale and exhale) provides accurate results efficiently, recommended for DTT plan safety confirmation.