Patient-specific PTV margins in radiotherapy for bladder cancer - a feasibility study using cone beam CT

To assess the feasibility of using cone beam computed tomography (CBCT) to generate patient-specific PTV margins for bladder cancer patients treated with radiation therapy (RT).

Eleven patients underwent CT simulation and daily RT (full bladder and empty rectum). CBCT was done prior to each fraction, and the whole bladder was contoured off-line. For the first 15 CBCTs of each patient, the bladder was aligned with CT-simulation bladder (pBladder) to create an occupancy volume (OV). A 5mm isotropic margin was added to OV (OV+5). A measurement-based PTV (mPTV) was generated by measuring maximal displacement between pBladder and OV in six directions. OV, OV+5, mPTV, and a standard PTV (2cm isotropic margin) were compared for absolute and relative volume differences. Using the final 10 CBCT of each patient, the ability of each study volume to encompass the entire CBCT bladder was determined.

161/165 CBCT images were of adequate quality for contouring. No daily trend in bladder volume variation was noted. The median absolute volumes (cm(3)) were: 221, 271, 426, 440, and 914 for pBladder, OV, OV+5, mPTV, and standard PTV, respectively. The median ratios of the study volumes/pBladder were: 1.4 OV, 2.1 OV+5, 2.4 mPTV, 4.1 standard PTV. OV+5 was smaller than mPTV in 9 patients. There was considerable inter-patient variability in study volumes and no apparent association of the magnitude of margin expansion and pBladder. The bladder was encompassed in 69%, 99%, 99%, and 100% of the final 10 fractions by OV, OV+5, mPTV, and standard PTV, respectively.

The use of daily CBCT to generate patient-specific PTV margins is feasible and results in a marked reduction in the irradiated volume compared to population-based margins. As daily bladder volume varied unpredictably with considerable differences between patients, these findings support the use of patient-specific PTV margin expansions for bladder radiotherapy.