Bladder surface-based analysis proves the advantages of online adaptive radiotherapy over plan of the day

Background Radiotherapy delivery for bladder cancer is challenging due to inter- and intra-fraction anatomical variations. The literature shows that surface-based methods, such as cumulative Dose-Surface Histograms (cDSHs), are more appropriate than dose-volume histograms for evaluating dose distribution in hollow organs. Purpose To compare two adaptive radiotherapy approaches – Plan of the Day (POD) and online-Adaptive Radiotherapy (oART) – by focusing on target coverage, dose conformity, and healthy tissue sparing through surface-based methods. Methods Five patients with urothelial bladder cancer were selected. POD and oART treatment (9-field-IMRT plans, 55 Gy/20 fractions) were simulated using the Ethos™ Emulator. The cDSHs were compared using the D98% and D50% metrics, describing the dose covering 98% and 50% of the PTV/CTV surface, respectively. The Surface Distance Histogram (SDH) was introduced and analysed through Full Width at Half Maximum and mode value. The Healthy Tissue Overdose volume Factor (HTOF) at 95% and 50% isodoses was calculated. Statistical comparisons included the Friedman test and the Canberra dissimilarity index. Results oART achieved superior PTV coverage, with a mean D98% consistently above 98% of the prescribed dose, whereas for POD, the mean D98% dropped below 90% for two patients. oART SDHs showed narrower bell shape with respect to POD, indicating higher dose conformity. oART improved healthy tissue sparing, reducing HTOF95% by over 50% compared to POD. Inter- and intra-patient variability were lower for oART. Conclusions Surface analysis strengthens the notion that oART is advantageous in bladder cancer treatment, ensuring superior target coverage, enhanced dose conformity, and better healthy tissue sparing. The SDH findings suggest potential for reducing CTV-PTV margins.