Patient‐specific adaptive planning margin for whole bladder radiation therapy

Abstract

BackgroundWhole 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.PurposeThe 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.MethodsSixteen 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 (Vdaily) 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(Vdaily) 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 (Vdaily) receiving the prescribed dose was 99.1% (interquartile range 95.3%−99.9%). Adaptive plans demonstrated superior OAR sparing compared to conventional plans.ConclusionsThe 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.