PURPOSE: To determine whether image guidance with rigid registration (RR) to intraprostatic markers (IPMs) yields acceptable coverage of the pelvic lymph nodes in the context of a stereotactic body radiation therapy (SBRT) regimen.
METHODS AND MATERIALS: Four to seven kilovoltage cone-beam CTs (CBCTs) from 12 patients with high-risk prostate cancer were analyzed, allowing approximation of an SBRT regimen. The nodal clinical target volume (CTV(N) and bladder were contoured on all kilovoltage CBCTs. The V100 CTVN, expressed as a ratio to the same parameter on the initial plan, and the magnitude of translational shift between RR to the IPMs versus RR to the pelvic bones, were computed. The ability of a multimodality bladder filling protocol to minimize bladder height variation was assessed in a separate cohort of 4 patients.
RESULTS: Sixty-five CBCTs were assessed. The average V100 CTVN was 92.6%, but for a subset of 3 patients the average was 80.0%, compared with 97.8% for the others (P< .0001). The average overall and superior-inferior axis magnitudes of the bony-to-fiducial translations were significantly larger in the subgroup with suboptimal nodal coverage (8.1 vs 3.9 mm and 5.8 vs 2.4 mm, respectively; P< .0001). Relative bladder height changes were also significantly larger in the subgroup with suboptimal nodal coverage (42.9% vs 18.5%; P< .05). Use of a multimodality bladder-filling protocol minimized bladder height variation (P< .001).
CONCLUSION: A majority of patients had acceptable nodal coverage after RR to IPMs, even when approximating SBRT. However, a subset of patients had suboptimal nodal coverage. These patients had large bony-to-fiducial translations and large variations in bladder height. Nodal coverage should be excellent if the superior-inferior axis bony-to-fiducial translation and the relative bladder height change (both easily measured on CBCT) are kept to a minimum. Implementation of a strict bladder filling protocol may achieve this goal.
Kishan AU, Lamb JM, Jani SS, Kang JJ, Steinberg ML, King CR. Are you the author?
Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California.
Reference: Int J Radiat Oncol Biol Phys. 2015 Mar 15;91(4):832-9.