Vessel-sparing prostate radiotherapy: dose limitation to critical erectile vascular structures (internal pudendal artery and corpus cavernosum) defined by MRI.
OBJECTIVE
Most evidence suggests that impotence after prostate radiation therapy has a vascular etiology. The corpus cavernosum (CC) and the internal pudendal artery (IPA) are the critical vascular structures related to erectile function. This study suggests that it is feasible to markedly decrease radiation dose to the CC and the IPA and directly determine the impact of dose limitation on potency.


METHODS
Twenty-five patients (10 external beam, 15 brachytherapy) underwent MRI/CT-based treatment planning for prostate cancer. In addition, 10 patients entered on the vessel-sparing protocol underwent a time-of-flight MRI angiography sequence to define the IPA. The distance from the MRI-defined prostate apex to the penile bulb (PB), CC, and IPA was measured and compared to the distance from the CT-defined apex. Doses (D5 and D50) to the PB, CC, and IPA were determined for an 80 Gy external beam course. In 5 patients, CT plans were generated and compared to MRI-based plans.


RESULTS
The combination of coronal, sagittal, and axial MRI data sets allowed superior definition of the prostate apex and its relationship to critical vascular structures. The apex to PB distance averaged 1.45 cm (0.36 standard deviation) with a range of 0.7 cm to 2.1 cm. Peak dose (D5) to the proximal CC in the MRI-planned 80 Gy course was 26 (9) Gy (0.36 of CT-planned dose), and peak dose to the IPA was 39 (13) Gy (0.61 of CT-planned dose).


CONCLUSIONS
The distance between the prostate apex and critical vascular structures is highly variable. Current empiric rules for CT contouring (apex 1.5 cm above PB) overestimate or underestimate the distance between the prostate apex and critical vascular structures. When defined by MRI T2 and MRI angiogram with CT registration, limitation of dose to critical erectile structures is possible, with a more significant gain than has been previously reported using dose limitation by commonly applied intensity modulated radiation therapy studies based on CT imaging. These techniques make "vessel-sparing" prostate radiotherapy feasible.
McLaughlin
Patrick W
PW
Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48109, USA. mclaughb@umich.edu
Narayana
Vrinda
V
Meirovitz
Amichay
A
Meriowitz
Amichay
A
Troyer
Sara
S
Roberson
Peter L
PL
Gonda
Roger
R
Sandler
Howard
H
Marsh
Lon
L
Lawrence
Theodore
T
Kessler
Marc
M
D000792:Angiography / Q000379:methods; D001158:Arteries / Q000033:anatomy & histology; D006801:Humans; D018783:Impotence, Vasculogenic / Q000517:prevention & control*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010413:Penis / Q000033:anatomy & histology* / Q000098:blood supply / Q000000981:diagnostic imaging; D011467:Prostate / Q000000981:diagnostic imaging / Q000473:pathology; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011832:Radiation Injuries / Q000517:prevention & control; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D014057:Tomography, X-Ray Computed