Anisotropic Bladder Planning Target Volume in Bladder Radiation Therapy.

Area of Science:

• Radiation oncology
• Medical physics
• Urologic oncology

Background:

• Bladder preservation therapy for bladder cancer often involves intensity-modulated radiation therapy (IMRT).
• Accurate patient positioning using daily imaging, such as cone-beam computed tomography (CBCT), is crucial for effective treatment.
• Determining appropriate planning target volume (PTV) margins is essential to account for organ motion and ensure adequate coverage of the clinical target volume (CTV).

Purpose of the Study:

• To investigate three planning target volume (PTV) margin expansions for bladder cancer treatment.
• To determine the most suitable PTV margin for bladder preservation therapy utilizing daily CBCT.
• To assess the feasibility of smaller PTV expansions without compromising treatment accuracy and risking geographical miss.

Main Methods:

• Retrospective analysis of bladder cancer patients treated with hypofractionated IMRT.
• Definition of three PTV margins: 1.5 cm uniform, 1 cm uniform, and an anisotropic margin (1.5 cm superior/anterior, 1 cm other directions).
• Daily CBCT imaging was used to evaluate bladder volume variations and PTV coverage, recording instances of clinical target volume (CTV) miss.

Main Results:

• A uniform 1 cm PTV margin resulted in a 1% frequency of clinically significant CTV miss (≥5 cm³).
• Both the uniform 1.5 cm PTV margin and the anisotropic PTV margin demonstrated a reduced miss frequency of 0.5%.
• The anisotropic margin effectively accounted for the predominant superior and anterior movement of the bladder.

Conclusions:

• The anisotropic PTV margin of 1.5 cm superiorly/anteriorly and 1 cm in other directions is a safe and effective approach for bladder cancer treatment.
• This margin strategy, combined with daily CBCT for localization of an empty bladder, minimizes geographical miss.
• The findings support the use of tailored anisotropic margins to optimize PTV definition in image-guided radiation therapy for bladder preservation.