Biological effect of dose distortion by fiducial markers in spot‐scanning proton therapy with a limited number of fields: A simulation study

Abstract

Purpose: In accurate proton spot-scanning therapy, continuous target tracking by fluoroscopic X-ray during irradiation is beneficial not only for respiratory moving tumors of lung and liver but also for relatively stationary tumors of prostate. Implanted gold markers have been used with great effect for positioning the target volume by a fluoroscopy, especially for the cases of liver and prostate with the targets surrounded by water-equivalent tissues.However, recent studies have revealed that gold markers can cause a significant underdose in proton therapy. This paper focuses on prostate cancer and explores the possibility that multiple-field irradiation improves the underdose effect by markers on Tumor Control Probability (TCP). Methods:A Monte Carlo simulation was performed to evaluate the dose distortion effect. A spherical gold marker was placed at several characteristic points in a water phantom. Markers were with two different diameters of 2 mm and 1.5 mm, both visible on fluoroscopy. Three beam arrangements of SFUD (single-field uniform dose)were examined: one lateral field, two opposite lateral fields, and three fields (two opposite lateral fields + anterior field). The Relative Biological Effectiveness (RBE) was set to 1.1 and a dose of 74 Gy (RBE) was delivered to the target of a typical prostate size in 37 fractions. The ratios of TCP to that without the marker (TCP r ) were compared with the parameters of the marker sizes, number of fields, and marker positions. To take into account the dependence of biological parameters in TCP model,  values of 1.5, 3, and 10 Gy (RBE) were considered.Results: It was found that the marker of 1.5 mm diameter does not affect the TCPs with all  values when two or more fields are used. On the other hand, if the marker size is 2 mm, more than two irradiation fields are required to suppress the decrease in TCP from TCP r by less than 3%. This is especially true when multiple (two or three) markers are used for alignment of a patient. Conclusions:It is recommended that 1.5 mm markers be used to avoid the reduction of TCP as well as to spare the surrounding critical organs, as long as the markers are visible on X-ray fluoroscopy. When 2 mm markers are implanted, more than two fields should be used and the markers should not be placed close to the distal edge of any of the beams.

Preliminary analysis for integration of spot-scanning proton beam therapy and real-time imaging and gating

“…Also, the time structure of the synchrotron is modeled based on specification given by the manufacturer, which may be too simplified as compared to the actual system. The other shortcoming is that the simulation studies have been performed using rigid model, influenced by the small fiducial to the dose distribution [7], minute change around the tumor and fiducial, phase control of the log data and beam delivery in each simulation. The above are the main concerns that we need to address in the future.…”

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.…”

“…Several studies reported that implanted metallic fiducials can cause dose shadows that may compromise local tumor control for certain diseases, e.g. , prostate carcinoma and uveal melanoma (Newhauser et al , 2007a; Newhauser et al , 2007c; Giebeler et al , 2009; Cheung et al , 2010; Huang et al , 2011; Matsuura et al , 2012; Zhang et al , 2012; Carnicer et al , 2013). …”

A Proton Beam Therapy System Dedicated to Spot-Scanning Increases Accuracy with Moving Tumors by Real-Time Imaging and Gating and Reduces Equipment Size

“…Since SSPT does not require compensators and collimators, SSPT is suitable for multiple beam treatment. In a previous study, the effect of gold markers was found to be biologically negligible if multiple beams were used to irradiate the target [18].…”