What CTV-to-PTV margins should be applied for prostate irradiation? Four-dimensional quantitative assessment using...

Area of Science:

• Radiation Oncology
• Medical Physics
• Radiotherapy Planning

Background:

• Prostate irradiation requires precise targeting to maximize tumor dose while minimizing toxicity.
• Accurate clinical target volume (CTV)-to-planning target volume (PTV) margins are crucial for effective prostate cancer treatment.
• Setup uncertainties significantly impact dose delivery accuracy in prostate radiotherapy.

Purpose of the Study:

• To quantify adequate anisotropic CTV-to-PTV margins for prostate irradiation.
• To evaluate the impact of three setup strategies on margin requirements: no correction, bony anatomy alignment, and gold marker alignment.
• To determine optimal margins ensuring 95% CTV coverage for 90% of patients.

Main Methods:

• Three radiation oncologists delineated CTVs on CT images from 30 prostate cancer patients.
• Repeat scans simulated delivered dose distributions under different registration approaches.
• A deformable image registration system calculated accumulated dose distributions to derive anisotropic margins.

Main Results:

• Without correction, margins of 11 mm (prostate) and 15 mm (seminal vesicles) were needed.
• Bony anatomy alignment reduced margins to 8 mm (prostate) and 13 mm (seminal vesicles).
• Gold marker alignment further reduced margins to 3 mm (prostate) and 8 mm (seminal vesicles), with smaller margins at the apex.

Conclusions:

• Voxel tracking techniques enable accurate calculation of accumulated dose distributions.
• Three-dimensional CTV-to-PTV margins can be accurately designed for prostate irradiation.
• Gold marker-based image guidance offers the most significant margin reduction, enhancing treatment precision.