Deriving patient-specific planning target volume for partial bladder image guided radiation therapy

Image guided radiation therapy (IGRT) using bony anatomy for bladder cancer requires the use of large population-based planning target volume (PTV) margins to compensate for geometric uncertainties. This may result in a large volume of normal tissue being irradiated unnecessarily. Identification of the clinical target volume (CTV) is also a challenge during target delineation and treatment position verification. This study describes the use of lipiodol (Guerbet, US) and cone beam computed tomography (CBCT) in deriving patient-specific PTV (PS-PTV) for partial bladder IGRT.

Twelve patients underwent lipiodol injection into the bladder wall prior to radiation treatment. A PS-PTV was generated by the following: (1) Delineating partial bladder CTV (CTVpb) on 15 CBCTs; (2) registering the CBCTs with the planning CT image using lipiodol; (3) combining the 15 CTVpb to create an occupancy volume (OV); and (4) expanding the OV by 3 mm. Its efficacy in reducing irradiated volume and in providing coverage was assessed by comparing it with a 20-mm population-based PTV (popPTV) and using phase 2 CBCTs.

The median PS-PTV and popPTV (cm(3)) were 102 (range, 37-336) and 325 (range, 211-631), respectively. Median distance between the CTVpb and the PS-PTV edge (mm) were 6 superior, 6 right, 7 left, 7 anterior, 8 posterior, and 11 inferior. The absolute median reduction in the overlapping volume of rectum, small bowel, and large bowel were 0.3 cm(3), 5.3 cm(3), and 13.0 cm(3), respectively. Despite large reductions in volume and margin compared with popPTV, PS-PTV achieved 100% target coverage.

Using lipiodol and CBCT to derive PS-PTV facilitated large reductions in the irradiated normal tissue volume without compromising target coverage.