Dose perturbation behind tantalum clips in ocular proton therapy

The physics of proton therapy

“…Specifically, implanted fiducial markers for image-guided patient alignment have been used in proton therapy for many years (Gall et al , 1993; Welsh et al , 2004; Newhauser et al , 2007a; Newhauser et al , 2007c; Ptaszkiewicz et al , 2010; Matsuura et al , 2012). Substantial recent improvements in on-board imaging systems, patient positioning, and patient immobilization have led to increased use of radiopaque implanted fiducial markers in proton therapy to many disease sites, with the goal of improving target coverage and/or normal tissue sparing.…”

Investigation of dose perturbations and the radiographic visibility of potential fiducials for proton radiation therapy of the prostate

“…In fiducial marker-based IGRT, daily radiographic images reveal the position of the markers, enabling precise alignment of the target with the treatment beam (Hsu et al ., 2007). However, a disadvantage of this technique is that implanted fiducial markers perturb the radiation beam and cause downstream dose shadows in both a photon beam (Chow and Grigorov, 2005) and a proton beam (Newhauser et al ., 2007a; Newhauser et al ., 2007b; Ptaszkiewicz et al ., 2010; Giebeler et al ., 2009). Furthermore, fiducial markers made of high-Z (atomic number) materials, such as gold, can also cause significant streak artifacts on computed tomography (CT) images that are used for treatment planning.…”

Quantification of dose perturbations induced by external and internal accessories in ocular proton therapy and evaluation of their dosimetric impact

“…If the clip is placed with an angle or in perpendicular orientation, these effects would be worsened. Indeed, in perpendicular orientation Ptaszkiewicz et al 13 measured 35% underdose and Newhauser et al 12 up to 82%. If the potential effects of tantalum fiducial markers are not taken into account, an overor underirradiation of the tumor can take place, or even situations with the tumor not totally covered by irradiation if it is located downstream the central part of the clip which is affected by range reduction.…”

“…In our case, a dose shadow ranging from 0.4% to −20%, with a range reduction of 1.2 mm (calculated equivalently) was determined. Ptaszkiewicz et al 13 reported maximum dose reduction obtained in parallel orientation less than −10% measured with TLD foils in perpendicular orientation for a real tantalum clip of 2.5 mm diameter and 0.2 mm thick. On the other hand, the dose perturbation generated by lateral effects on the edges of the clips should also be taken into consideration.…”

“…[9][10][11] Maximum underdose on the range of 30%-40% is generally reported. For ocular proton therapy, works are more scarce although important perturbations are also shown: Newhauser et al 12 reported dose variations ranging from 22% to 82%, depending on the clip depth and orientation and, Ptaszkiewicz et al 13 measured up to 35% dose reduction in the most unfavorable studied case. Besides fiducial markers, the occasional presence of other intraocular implants or substitutes may also greatly perturb range and dose.…”