Pelvic nodal dosing with registration to the prostate: implications for high-risk prostate cancer patients receiving stereotactic body radiation therapy.
OBJECTIVE
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
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 CTV(N), 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 CTV(N) 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).


CONCLUSIONS
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.
Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California. Electronic address: aukishan@mednet.ucla.edu.
Amar U
AU
Kishan
Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California.
James M
JM
Lamb
Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California.
Shyam S
SS
Jani
Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California.
Jung J
JJ
Kang
Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California.
Michael L
ML
Steinberg
Department of Radiation Oncology, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California.
Christopher R
CR
King
D059925:Anatomic Landmarks / Q000000981:diagnostic imaging; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D057918:Fiducial Markers; D006801:Humans; D008198:Lymph Nodes / Q000000981:diagnostic imaging; D015182:Lymphatic Irradiation; D008297:Male; D009068:Movement; D009929:Organ Size; D010384:Pelvic Bones / Q000000981:diagnostic imaging; D010388:Pelvis; D011467:Prostate / Q000000981:diagnostic imaging; D011471:Prostatic Neoplasms / Q000000981:diagnostic imaging / Q000601:surgery*; D016634:Radiosurgery; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D012007:Rectum / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D012669:Seminal Vesicles; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging