Dose perturbation behind tantalum clips in ocular proton therapy

Abstract

Abstract Proton therapy of eye tumors requires precise positioning in the sub-millimeter range. For this reason small (2.5 mm in diameter and a thickness of 0.2 mm) tantalum markers (clips) are sutured onto the sclera around the tumor base and used for radiological verification of the position and orientation of the eye at each treatment session. In some cases during irradiation clips might be positioned between the tumor and radiation source that may lead to perturbation of the dose distribution and underdosing of some parts of a tumor. The aim of this work was to determine experimentally the dose distribution behind the tantalum clips after irradiation with the parallel proton beam. The dose measurements were performed using the two-dimensional thermoluminescent dosimetric system, newly developed at the IFJ PAN in Poland, and a clip-phantom consisting of a 100 mm × 100 mm × 5.3 mm PMMA plate with holes for clips placing at five irradiation angles (0, 30 45, 60 and 90°). The clip-phantom was irradiated with 68 MeV proton beam. The measurements were performed at different depths ranging from 6.3 to 24.8 mm water equivalent depth. The measurements of dose modification due to tantalum clips showed underdosing ranged from 4% to 32%. For those reasons ophthalmologist need to take this effect into account during clip surgery and medical physicist need to consider the position of the clips in treatment planning.