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

Background: Whole bladder irradiation is an organ preservation treatment approach for muscle-invasive bladder cancer (MIBC). Conventional planning margins, typically 15-20 mm, increase normal tissue toxicity and limit possible dose escalation.

Purpose: The study aimed to develop a patient-specific adaptive margin recipe for whole bladder irradiation to minimize the planning target volume (PTV) while preserving adequate dose coverage.

Methods: Sixteen patients who received whole-bladder irradiation were retrospectively selected for this study. We proposed a patient-specific anisotropic adaptive margin recipe, derived from the first five fractions of kV-CBCTs, to account for inter-fractional bladder changes. This recipe was validated using kV-CBCTs from fractions six to ten and the final five fractions. The goal was to achieve a residual volume, defined as the percentage of daily bladder volume (V_daily) outside the PTV, of less than 5%. Adaptive and conventional plans were created using proposed and conventional margins, respectively. A dosimetric comparison of targets and organs-at-risk (OARs) was performed between the two approaches.

Results: (V_daily) decreased throughout the treatment course. The most notable inter-fractional bladder variations were in the superior and anterior directions. The patient-specific anisotropic adaptive margins, averaging 6 mm (± 2.9 mm), achieved a residual volume of less than 5%. Compared to conventional planning, the adaptive approach reduced PTV volume by an average of 135.3 cc (± 46.6 cc). A significant correlation (p < 0.05) was identified between residual volume and adaptive margins in the anterior, superior, left, and right directions. Using the proposed adaptive margins, the median residual volume was 0.71% (interquartile range 0.09%-3.55%), and the median (V_daily) receiving the prescribed dose was 99.1% (interquartile range 95.3%-99.9%). Adaptive plans demonstrated superior OAR sparing compared to conventional plans.

Conclusions: The proposed patient-specific adaptive margin recipe for whole bladder irradiation resulted in margins smaller than conventional ones, optimized normal tissue sparing, and maintained adequate PTV coverage.