title: Biological effect of dose distortion by fiducial markers in spot-scanning proton therapy with a limited number of fields: a simulation study.
abstract: OBJECTIVE
  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. The markers were with two different diameters of 2 and 1.5 mm, both visible on fluoroscopy. Three beam arrangements of single-field uniform dose (SFUD) 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, α∕β 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 α∕β values when two or more fields are used. On the other hand, if the marker diameter 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.
authors.affiliation: Department of Medical Physics, Hokkaido University Graduate School of Medicine, Sapporo, Hokkaido, Japan. matsura@med.hokudai.ac.jp
authors.forename: Taeko
authors.initials: T
authors.lastname: Matsuura
authors.forename: Kenichiro
authors.initials: K
authors.lastname: Maeda
authors.forename: Kenneth
authors.initials: K
authors.lastname: Sutherland
authors.forename: Taisuke
authors.initials: T
authors.lastname: Takayanagi
authors.forename: Shinichi
authors.initials: S
authors.lastname: Shimizu
authors.forename: Seishin
authors.initials: S
authors.lastname: Takao
authors.forename: Naoki
authors.initials: N
authors.lastname: Miyamoto
authors.forename: Hideaki
authors.initials: H
authors.lastname: Nihongi
authors.forename: Chie
authors.initials: C
authors.lastname: Toramatsu
authors.forename: Yoshihiko
authors.initials: Y
authors.lastname: Nagamine
authors.forename: Rintaro
authors.initials: R
authors.lastname: Fujimoto
authors.forename: Ryusuke
authors.initials: R
authors.lastname: Suzuki
authors.forename: Masayori
authors.initials: M
authors.lastname: Ishikawa
authors.forename: Kikuo
authors.initials: K
authors.lastname: Umegaki
authors.forename: Hiroki
authors.initials: H
authors.lastname: Shirato
mesh_terms: D057918:Fiducial Markers; D006801:Humans; D008297:Male; D009010:Monte Carlo Method; D011336:Probability; D011471:Prostatic Neoplasms / Q000532:radiotherapy; D061766:Proton Therapy; D011829:Radiation Dosage; D011878:Radiotherapy / Q000592:standards*; D011879:Radiotherapy Dosage; D011522:Protons
chemical_list: D011522:Protons