Dosimetric impact of tantalum markers used in the treatment of uveal melanoma with proton beam therapy

Abstract

Metallic fiducial markers are frequently implanted in patients prior to external-beam radiation therapy to facilitate tumor localization. There is little information in the literature, however, about the perturbations in proton absorbed-dose distribution these objects cause. The aim of this study was to assess the dosimetric impact of perturbations caused by 2.5 mm diameter by 0.2 mm thick tantalum fiducial markers when used in proton therapy for treating uveal melanoma. Absorbed dose perturbations were measured using radiochromic film and confirmed by Monte Carlo simulations of the experiment. Additional Monte Carlo simulations were performed to study the effects of range modulation and fiducial placement location on the magnitude of the dose shadow for a representative uveal melanoma treatment. The simulations revealed that the fiducials caused perturbations in the absorbed-dose distribution, including absorbed-dose shadows of 22% to 82% in a typical proton beam for treating uveal melanoma, depending on the marker depth and orientation. The clinical implication of this study is that implanted fiducials may, in certain circumstances, cause dose shadows that could lower the tumor dose and theoretically compromise local tumor control. To avoid this situation, fiducials should be positioned laterally or distally with respect to the target volume.

Monte Carlo calculations and measurements of absorbed dose per monitor unit for the treatment of uveal melanoma with proton therapy

“…In a proof-of-concept analysis, Newhauser et al (2005) found that accuracy could be improved by using a Monte Carlo simulation model. More recently, this model was used to study the relative dose perturbations caused by small implanted fiducial markers in the eye (Newhauser et al 2007b) Another limitation of presently available treatment planning systems is that they do not predict the dose per monitor unit (D/MU) value. Accurate D/MU predictions to within 3% were reported by Kooy et al (2003) using an analytical algorithm developed for proton fields with a wide cross-sectional area and deep penetration range.…”

Titanium Screws as Fiducial Markers for Image-Guided Stereotactic Proton Irradiation in Pediatric Central Nervous System Tumors: A Technical Report

“…Previous fiducials used in radiotherapy include steel beads, titanium cylinders, tantalum disks and gold beads [1,11,13,15]. These implants are not widely available and were used exclusively in the reporting centers.…”

“…Instead, various fiducial systems are frequently utilized to deliver daily image-guided stereotactic proton therapy. These include a rigid head frame attached to the skull, steel beads implanted into the outer table of the skull, tantalum markers attached to the sclera for ocular tumors, and stainless steel markers implanted into the prostate for prostate cancer [1,11,12,13]. …”

Noninvasive referencing of intraocular tumors for external beam radiation therapy using optical coherence tomography: A proof of concept

“…Furthermore, Newhauser et al indicated that implanted tantalum clips may, in certain circumstances, cause dose shadows that could lower the tumor dose and theoretically compromise local tumor control. 15 To overcome these disadvantages, several noninvasive tumorreferencing approaches have been proposed. Most of them rely on a 2D (two-dimensional) approach for the localization of the 2D eye position on the image captured by a single video camera.…”