open access

https://hdl.handle.net/2115/50429

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

英語

tumor-control probability

2012-09

American Association of Physicists in Medicine

Medical Physics

39(9)

5584-5591

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 (TCPr) 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 φ2 mm, more than two irradiation fields are required to suppress the decrease in TCP from TCPr 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 φ1.5 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 φ2 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.

学術雑誌論文

Accepted Manuscript

0094-2405