gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

Techniques for Using Patient Positioning System Digital X-ray Images for Alignment QA in Proton Treatment Rooms

Meeting Abstract

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  • Chr. Allgower - Medical Physics, Midwest Proton Radiotherapy Institute, Bloomington, IN, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog006

DOI: 10.3205/09ptcog006, URN: urn:nbn:de:0183-09ptcog0063

Published: September 24, 2009

© 2009 Allgower.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.en). You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.


Outline

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Background: Proton therapy requires a high standard of patient positioning to deliver precision doses. For such precision, proper quality assurance (QA) is required. The high precision inherent in the digital x-ray images from a patient positioning system can be used to detect minute problems in x-ray source alignment, snout alignment, gantry isocentricity, crosshair alignment, and sag in extending x-ray panel arms.

Material and methods: The Midwest Proton Radiotherapy Institute (MPRI) uses the digital image positioning system (DIPS) originally developed at MGH in Boston specifically for proton therapy patient positioning. As implemented at MPRI, the DIPS system in each treatment room has three orthogonal x-ray sources pointing at flat panel x-ray detectors. In the gantry rooms, the Gaxis source points outward along the gantry rotation axis at a flat panel that does not rotate with the gantry. The other two sources (beamline and G90) point at digital panels mounted on extending arms which do rotate with the gantry. The robotic patient positioners used at MPRI provide a convenient means of putting fixed mechanical pointers at isocenter to a high degree of precision(<0.2 mm). Positions with the pointer pointed upward at the ceiling or into the gantry along the gantry rotation axis are both possible. This establishes a fixed isocenter reference point independent of gantry motion against which to compare apparent motions with gantry angle of crosshairs used for patient positioning, as well as edges of round apertures mounted in the snout. Images at 0, 90, 180, and 270 degrees in gantry angle are taken on a quarterly basis. At each gantry angle, double exposures at full extension and retraction of a snout with a mounted 10 cm round aperture are taken to test snout alignment as a function of radial extension. Images are analyzed by recording the nearest pixel values of pointer tip and the crosshair center. The beamline round aperture images are analyzed by averaging the up/down and left/right pixel numbers on the edges of the aperture shadow against the pixel values of the pointer tip.

Results: The precision of the image data allows one to quantify very small alignment errors down to a submillimeter level on X-ray source positioning, crosshair alignment/sag/reproducibility, and snout alignment.

Conclusion: The x-ray imaging systems in proton treatment rooms can be readily used as a precise QA tool to detect mechanical misalignments of patient positioning and beam delivery systems. The ability of the robotic patient positioner to serve as a precise platform for indicating the room isocenter makes the process quick and easy. The end result is a versatile tool that can give one confidence in mechanical alignments to better than 1 mm accuracy.