Hualin Zhang PhD, Presenter: Nothing to Disclose

Alexander Law BSC, Abstract Co-Author: Nothing to Disclose

Jian Z. Wang PhD, Abstract Co-Author: Nothing to Disclose

Kaile Li PhD, Abstract Co-Author: Nothing to Disclose

Nilendu Gupta PhD, Abstract Co-Author: Nothing to Disclose

Douglas D. Martin MD, Abstract Co-Author: Nothing to Disclose

Nina A. Mayr MD, Abstract Co-Author: Nothing to Disclose

et al, Abstract Co-Author: Nothing to Disclose

To verify the deviation of dose calculation based on commonly used TG43U1- algorithms from Monte Carlo simulations in eyeplaque implants with I-125 and recently developed Cs-131 seeds.

Monte Carlo technique was used to generate 3D dose distributions of single IsoAidTM I-125 and Isoray Cs-1TM Cs-131 brachytherapy sources. A novel computer code EyeMC was developed to utilize the single-source 3D dose distribution to generate dose distributions of multiple-source implants for COMS eye-plaques, through rotating, superposing, displacing and interpolating methods. In addition, Monte Carlo simulations were performed for the 8-seed implant to verify the accuracy of the EyeMC and treatment planning system, and to verify the magnitude seed-eyeplaque effects. Eight seeds were placed as two isocentric squares at the same plane, with the inner square width of 0.6 cm and outside square width of 1.1 cm. The treatment planning system BrachyvisionTM was also used to calculate the doses at points of interest with point- and line-source approximations.

The point-source approximation agreed with line-source approximation within 10%. The dosimetric characteristics of Cs-131 source in yeplaque implant are similar to the I-125. However, Compared to the Monte Carlo simulation, the treatment planning system using line- and point-source approximations significantly overestimated the dose values at the eyeplaque axis, by 15% at 2 mm, 9% at 1.0 cm, 4% at 2.0 cm, and 2% at 3 cm. The Seed-eyeplauqe inter-effect was small but noticeable. The presence of the eyeplaque could reduce dose up to 10% at the distance of interest up to 3 cm for both I-125 and Cs-131.

Our comparison of commonly used TG43U1-based algorithm with Monte Carlo simulation shows that neither the point-source nor line-source approximation provides accurate dose calculations for eyeplaque implants at short distances.  The overestimation of TG43U1 is fully understandable, because the TG 43U1 considerers the sources acting like points reside at the centers of the sources, though anisotropy functions may be used, rather than the sources having physical dimensions and emiting photons from the whole length. Our study suggests that a Monte Carlo-aided code, such as EyeMC, is needed to get accurate dosimetric calculation in eyeplaque brachytherapy.

Our study is directly related to the clinical applications of eye cancer radiotherapy.

Zhang, H, Law, A, Wang, J, Li, K, Gupta, N, Martin, D, Mayr, N, et al, , Dosimetric Evaluation in Eye-plaque Brachytherapy Implant with I-125 and Cs-131 Seeds.  Radiological Society of North America 2008 Scientific Assembly and Annual Meeting, February 18 - February 20, 2008 ,Chicago IL. http://archive.rsna.org/2008/6016687.html