High beta and electron dose from 192Ir: implications for "gamma" intravascular brachytherapy.
OBJECTIVE
Trains of multiple 192Ir seeds are used in many clinical trials for intravascular brachytherapy. 192Ir source is commonly considered as a gamma emitter, despite the understanding that this radionuclide also emits a wide range of electron and beta energies, with a similar range of energy. The high dose from betas and electrons in the submillimeter range due to unsealed ends of seed sources should be precisely quantified to fully understand the backdrop for complications associated with 192Ir coronary artery brachytherapy.


METHODS
Monte Carlo simulations (MCNP4C code) were performed for a model 5-seed 192Ir train used in SCRIPPS, GAMMA, and the Washington Radiation for In-Stent Restenosis (WRIST) randomized clinical trials. A stack of radiochromic films was also used to measure the dose distributions for an actual 6-seed train.


RESULTS
In the submillimeter range very close to the source, Monte Carlo results show that betas and electrons deposit a higher dose than 192Ir photons (gamma and X-rays) over the interseed gap. A high luminal dose from the combined effects of betas, electrons, and photons emitted from 192Ir can be deposited, particularly between seeds. When prescribing 15 Gy at 2 mm, the combined dose can be as high as 160 Gy at 0.5 mm. Different peak doses near the interseed gaps were noted, which may be due to variability of seed-end surfaces and nonuniformity of seed activity within a real multiseed train. Dose-volume histograms (DVH) of lumen surfaces were evaluated for an eccentric seed train. The DVH parameters indicating the extent of hot spots in the lumen wall, DV(10), DV(5), DV(2), and DV(1) (dose received by 10, 5, 2, 1% respectively of the total lumen surface), can be as high as 55, 76, 81, and 155 Gy for a lumen with 3-mm diameter, and 75, 80, 110, and 158 Gy for a narrow 2-mm lumen.


CONCLUSIONS
192Ir multiple seed trains used in the SCRIPPS, GAMMA, and WRIST trials can deposit a very high dose to the luminal wall. A particularly high electron and beta dose can be delivered near the interseed gap if the source is not centered in the catheter and lumen. The dose from 192Ir betas and electrons may partially explain adverse outcomes reported from 192Ir multiseed clinical trials. Improvement of the encapsulation design to filter out the betas and electrons should be seriously considered.
Patel
Neil S
NS
Department of Radiation Oncology, Beth Israel Medical Center and St. Luke's-Roosevelt Hospital Center, New York, NY, USA.
Chiu-Tsao
Sou-Tung
ST
Ho
Yunsil
Y
Duckworth
Tamara
T
Shih
J Allen
JA
Tsao
Hung-Sheng
HS
Quon
Harry
H
Harrison
Louis B
LB
D001610:Beta Particles; D001918:Brachytherapy / Q000295:instrumentation* / Q000379:methods; D003251:Constriction, Pathologic / Q000517:prevention & control / Q000532:radiotherapy; D023903:Coronary Restenosis / Q000517:prevention & control / Q000532:radiotherapy; D004583:Electrons; D006801:Humans; D007496:Iridium Radioisotopes / Q000627:therapeutic use*; D009010:Monte Carlo Method; D017785:Photons; D011879:Radiotherapy Dosage; D016032:Randomized Controlled Trials as Topic; D014652:Vascular Diseases / Q000517:prevention & control / Q000532:radiotherapy*; D014962:X-Ray Film
D007496:Iridium Radioisotopes