Regional patterns of incidental pelvic nodal irradiation during bladder-only radiotherapy: A paired dosimetric analysis of VMAT and IMRT

Purpose Incidental pelvic nodal irradiation is an unavoidable consequence of bladder-only radiotherapy (RT) and may contribute to the irradiation of occult microscopic disease. We characterized incidental pelvic nodal dose and compared regional dose distributions between volumetric modulated arc therapy (VMAT) and step-and-shoot intensity-modulated RT (IMRT). Materials and Methods Planning computed tomography datasets from 30 patients receiving bladder-only RT were used to generate paired VMAT and step-and-shoot IMRT plans. Pelvic nodal regions, including the common iliac, external iliac, internal iliac, obturator, and presacral basins, were contoured according to consensus guidelines. Composite and region-specific dose–volume histogram parameters were compared between techniques using paired analyses. Treatment delivery efficiency was evaluated by monitor units (MUs). Results Both techniques achieved equivalent target coverage and comparable incidental pelvic nodal dose distributions. A consistent regional hierarchy of incidental nodal irradiation was observed, with the obturator, external iliac, internal iliac, and common iliac regions receiving approximately 75%, 45%, 25%, and < 5% of the prescribed dose, respectively. These patterns remained remarkably consistent regardless of delivery technique. Although small statistically significant differences were identified for selected regional dose–volume parameters, the magnitude of these differences was generally less than 1 Gy and was considered unlikely to be clinically meaningful. VMAT required significantly fewer MUs than step-and-shoot IMRT (median 914 vs. 1123, p < 0.001). Conclusions Bladder-only RT produces a reproducible regional pattern of incidental pelvic nodal irradiation that is largely independent of delivery technique. The observed consistency across techniques suggests that incidental nodal exposure may be influenced more by target geometry than by the delivery technique.