Variability of prostate brachytherapy pre-implant dosimetry: a multi-institutional analysis.
OBJECTIVE
To conduct a multi-institutional comparison of prostate brachytherapy pre-implant dosimetry of Pd-103 and I-125.


METHODS
Eight experienced brachytherapists submitted Pd-103 and I-125 monotherapeutic and boost pre-implant dosimetry plans for central review. All 32 plans were calculated using the same transrectal ultrasound volumetric study. Seeds of any strength were acceptable, but were restricted to Theraseed Model 200 (Theragenics Inc., Buford, GA) and Oncura Oncoseed Model 6711 (Oncura, Plymouth Meeting, PA). The dosimetric analysis included evaluation of target volume, target to prostate ratio, target length, number of needles, seed activity, number of seeds, total activity, total activity divided by treatment planning volume, the use of extracapsular seeds, and average treatment margins (defined as the perpendicular distance between the prostate capsule and the 100% isodose line). Prostate coverage was defined in terms of V(100)/V(150)/V(200)/V(300) and D(100)/D(90)/D(50), whereas urethral dosimetry consisted of UV(100)/UV(150)/UV(200) and UD(90)/UD(50).


RESULTS
The mean planning target volume to prostate volume ratio varied dramatically (mean 1.29, range 0.99-1.76) with the target length ranging from 3.5 to 4.5 cm. Although the prostate V(100) was >95% in all cases, the V(150) ranged from 29.9% to 92.1% and the V(200) from 6.72% to 52.5%. The urethral V(100) was 100% in all cases with six of the eight brachytherapists limiting the UV(150) to <3%. However, the median urethral dose varied by up to 50%. Treatment margins also varied significantly (average 3.98 mm, range 0.32-7.68 mm). All brachytherapists used extracapsular seeds with five implanting >25% of the seeds in extracapsular locations (range 6.4-58.2%). In addition, significant variability existed in the number of needles, number of seeds, and seed strength.


CONCLUSIONS
This study highlights the substantial variability that exists regarding target volume, seed strength, dose homogeneity, treatment margins, and extracapsular seed placement, although prostate brachytherapy prescription doses are uniform. The standardization of pre-implant dosimetry is essential for meaningful multi-institutional comparisons of biochemical outcomes and morbidity.
Merrick
Gregory S
GS
Schiffler Cancer Center and Wheeling Jesuit University, Wheeling, WV 26003-6300, USA. gmerrick@wheelinghospital.com
Butler
Wayne M
WM
Wallner
Kent E
KE
Blasko
John C
JC
Michalski
Jeff
J
Aronowitz
Jesse
J
Grimm
Peter
P
Moran
Brian J
BJ
McLaughlin
Patrick W
PW
Usher
Jacqueline
J
Lief
Jonathan H
JH
Allen
Zachariah A
ZA
D001918:Brachytherapy / Q000379:methods; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D007457:Iodine Radioisotopes / Q000008:administration & dosage; D008297:Male; D015588:Observer Variation; D010165:Palladium / Q000008:administration & dosage; D011471:Prostatic Neoplasms / Q000453:epidemiology / Q000473:pathology / Q000532:radiotherapy*; D011868:Radioisotopes / Q000008:administration & dosage; D011874:Radiometry; D011879:Radiotherapy Dosage; D016896:Treatment Outcome; D047368:Tumor Burden / Q000528:radiation effects; D036761:Ultrasound, High-Intensity Focused, Transrectal; D014523:Urethral Neoplasms / Q000453:epidemiology / Q000473:pathology / Q000532:radiotherapy
D007457:Iodine Radioisotopes; D011868:Radioisotopes; D010165:Palladium