Impact of image registration surrogates on the planning target volume geometry for bladder radiation therapy.

Purpose: The use of Lipiodol and cone beam computed tomography (CT) has facilitated the generation of patient-specific planning target volumes (PTV) to reduce irradiation of normal tissue. However, injecting Lipiodol to demarcate the target volume is an invasive procedure. Center of bladder (COB) and bladder wall surface (BWS) encompassed by the clinical target volume (CTV) are proposed to be the alternative noninvasive surrogates. This study examines the impact of using these 3 surrogates for image registration on the resultant geometry of patient-specific PTVs.

Methods And Materials: Twenty bladder cancer patients who had Lipiodol injection before planning CT were included. Lipiodol, CTV, and bladder were delineated on the planning CT. In addition, CTVs were delineated on 5 cone beam CT scans from each patient. Cone beam CT scans were registered to planning CT using Lipiodol, COB, and BWS to generate Lipiodol-PTV, COB-PTV, and BWS-PTV. Using Lipiodol as the reference, the difference in the 2-dimensional/3-dimensional displacement values and the geometry of the resultant PTVs were quantified.

Results: A total of 1200 displacement values and 60 volumes were included for analysis. The overall median and standard deviation (SD) of the 3-dimensional displacement (mm) measured by Lipiodol, BWS, and COB are 25 (SD, 15), 24 (SD, 14), and 21 (SD, 15), respectively. Lipiodol-PTV has the smallest mean volume, followed by BWS-PTV and COB-PTV. BWS-PTV was more geometrically similar to Lipiodol-PTV when compared with COB-PTV. Six of 20 COB-PTVs who had CTV located at either the dome or the base of the bladder were larger than the corresponding Lipiodol-PTV by more than 20%.

Conclusion: Lipiodol is the optimal image registration surrogate for partial bladder radiation therapy. However, for patients who are contraindicated for Lipiodol injection, BWS is the preferred noninvasive surrogate because the derived PTV has a smaller geometric variation from the Lipiodol-PTV when compared with COB, especially when the CTV was located at the dome or the base of the bladder.