Patient-specific adaptive planning margin for whole bladder radiation therapy.

Area of Science:

• Radiation Oncology
• Medical Physics
• Oncologic Imaging

Background:

• Whole bladder irradiation is a key organ-preservation strategy for muscle-invasive bladder cancer (MIBC).
• Conventional planning margins (15-20 mm) for whole bladder irradiation lead to increased normal tissue toxicity and limit dose escalation.
• Minimizing margins is crucial for improving treatment outcomes and reducing side effects.

Purpose of the Study:

• To develop and validate a patient-specific anisotropic adaptive margin recipe for whole bladder irradiation.
• The primary goal was to minimize the planning target volume (PTV) while ensuring adequate dose coverage.
• To account for inter-fractional bladder volume changes during treatment.

Main Methods:

• Retrospective analysis of 16 MIBC patients undergoing whole bladder irradiation.
• A patient-specific anisotropic adaptive margin recipe was derived from the first five fractions of kV-CBCTs.
• The recipe was validated using kV-CBCTs from subsequent fractions, aiming for <5% residual volume (daily bladder volume outside PTV).
• Dosimetric comparison between adaptive and conventional planning margins was performed.

Main Results:

• Patient-specific adaptive margins averaged 6 mm, achieving <5% residual volume.
• The adaptive approach reduced PTV volume by an average of 135.3 cc compared to conventional planning.
• Adaptive plans demonstrated superior sparing of organs-at-risk (OARs) and maintained high coverage of the daily bladder volume.

Conclusions:

• The proposed patient-specific adaptive margin recipe is effective for whole bladder irradiation in MIBC.
• This adaptive strategy allows for smaller margins than conventional methods.
• It optimizes normal tissue sparing and maintains adequate PTV coverage, potentially improving treatment efficacy and patient safety.