Verification procedure for isocentric alignment of proton beams

George Ciangaru,Shin Nakamura,Martin Bues,Patrick J Oliver,Mosumi Umezawa,Mengping Zhu,James N Yang,Hirofumi Yoshino,Alfred R Smith,Hitoshi Chiba,Fumio Nakagawa

doi:10.1120/jacmp.v8i4.2671

Abstract

We present a technique—based on the Lutz, Winston, and Maleki test used in stereotactic linear accelerator radiosurgery—for verifying whether proton beams are being delivered within the required spatial coincidence with the gantry mechanical isocenter. Our procedure uses a proton beam that is collimated by a circular aperture at its central axis and is then intercepted by a small steel sphere rigidly supported by the patient couch. A laser tracker measurement system and a correction algorithm for couch position assures precise positioning of the steel sphere at the mechanical isocenter of the gantry. A film‐based radiation dosimetry technique, chosen for the good spatial resolution it achieves, records the proton dose distribution for optical image analysis. The optical image obtained presents a circular high‐dose region surrounding a lower‐dose area corresponding to the proton beam absorption by the steel sphere, thereby providing a measure of the beam alignment with the mechanical isocenter. We found the self‐developing Gafchromic EBT film (International Specialty Products, Wayne, NJ) and commercial Epson 10000 XL flatbed scanner (Epson America, Long Beach, CA) to be accurate and efficient tools. The positions of the gantry mechanical and proton beam isocenters, as recorded on film, were clearly identifiable within the scanning resolution used for routine alignment testing (0.17 mm per pixel). The mean displacement of the collimated proton beam from the gantry mechanical isocenter was 0.22±0.1 mm for the gantry positions tested, which was well within the maximum deviation of 0.50 mm accepted at the Proton Therapy Center in Houston.PACS numbers: 87.53.Xd, 87.53.Oc, 87.56.‐v, 87.66.‐a, 87.56.Fc

Highlights

A major attraction of proton therapy is that the physics of the proton dose deposition, as reflected by the Bragg depth–dose distribution, offers the theoretical possibility to achieve millimetric accuracy in cancer treatment planning
The hardware jig for the isocenter alignment testing was attached to the snout on the gantry nozzle so that the beam was passing through the aperture, but for these measurements, the steel sphere was not mounted on the couch
During acceptance tests conducted at Proton Therapy Center in Houston (PTC-H), we developed a testing system to verify the isocentric alignment of proton beam delivery

Summary

Introduction

A major attraction of proton therapy is that the physics of the proton dose deposition, as reflected by the Bragg depth–dose distribution, offers the theoretical possibility to achieve millimetric accuracy in cancer treatment planning. Such performance requires accurate delivery of the proton beam to the target volume, a task of utmost clinical importance for conformal radiation therapy in cancer. Gantry and misalignments in the beam delivery system can, if not corrected, result in nonisocentric relative motion between the delivered beam and the nominal isocenter This problem is perhaps not very significant for passive wide scattered proton beams, but it is a matter of importance for small-field treatments and for scanned proton pencil beams. Our proton isocentric gantries are very large (three stories high) and massive (about 190 tons)

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