Assessment of Adaptive Margins Using a Single Planning Computed Tomography Scan for Bladder Radiotherapy

A "plan of the day" (PoD) adaptive radiotherapy approach is presented for bladder cancer. The potential reduction in volumes of normal tissue and bowel bag receiving high-dose radiation is evaluated.

Planning computed tomography (pCT) and daily cone beam CT (CBCT) data sets were analyzed for eight previously treated bladder cancer patients. For each patient, a whole bladder clinical target volume (CTV) was delineated on a pCT. Then, the clinical target volume was expanded using five sets of anisotropic or isotropic margins to create five planning target volumes (PTVs). A library of five corresponding treatment PoDs was then created using volumetric modulated arc therapy. Offline PoD selection was performed by three independent radiation therapists (RTs) using daily CBCTs. Dosimetric results were compared between PoD treatments and two conventional treatments using isotropic 1.5- and 2.0-cm margins.

The smallest PTV using 1.0-cm isotropic margins was selected most frequently (70%). Three RTs demonstrated good agreement for daily PTV selections, choosing identical PoDs for 51% of all CBCTs. In addition, acceptable dosimetric coverage of the whole bladder was achieved for all PoD selections, similar to standard treatments. The average volume of bowel bag receiving 45 and 50 Gy and normal tissue receiving 95% prescription dose was significantly (P < .01) lower for PoD treatments compared with both conventional treatments.

A PoD strategy using one pCT with isotropic and anisotropic margins can be used to treat bladder cancer and improve sparing of the bowel bag. Minimal dosimetric differences observed between three RTs suggests that PoD strategies are feasible for clinical implementation.