Preliminary Clinical Evaluation of Intrafraction Prostate Displacements for Two Immobilization Systems

Area of Science:

• Radiation Oncology
• Medical Physics
• Prostate Cancer Treatment

Background:

• Accurate clinical target volume to planning target volume (CTV-PTV) margins are crucial for hypofractionated prostate stereotactic body radiotherapy (SBRT).
• Immobilization systems and setup errors can affect these margins through prostate displacement.

Purpose of the Study:

• To evaluate and compare intrafraction prostate displacement between two different immobilization systems (A and B).
• To determine the impact of immobilization systems on CTV-PTV margins for prostate SBRT.

Main Methods:

• A preliminary study involving six patients with localized prostate cancer and implanted marker seeds.
• Planar X-ray imaging pre- and post-treatment to measure intrafraction prostate displacement.
• Calculation of average absolute and 3D displacements for each system and subsequent CTV-PTV margin computation.

Main Results:

• System A showed average 3D displacement of 2.6 ± 0.2 mm, resulting in margins of 2.5 mm (lateral), 2.5 mm (longitudinal), and 3.6 mm (vertical).
• System B exhibited lower average 3D displacement of 1.3 ± 0.2 mm, with computed margins of 1.4 mm (lateral), 1.6 mm (longitudinal), and 2.4 mm (vertical).
• Both systems yielded margins comparable to those typically used in hypofractionated prostate SBRT.

Conclusions:

• Immobilization system choice significantly influences intrafraction prostate displacement.
• System B offers potentially tighter margins due to reduced intrafraction motion compared to System A.
• The findings support the importance of selecting appropriate immobilization for precise prostate SBRT delivery.