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

Design and Performance of a Multileaf Collimator for Proton Therapy

Meeting Abstract

  • Chr. Ainsley - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • S. Avery - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • E. Diffenderfer - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • D. Dolney - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • L. Lin - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • J. McDonough - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • J. Metz - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • R. Scheuermann - Radiation Oncology, University of Pennsylvania, Philadelphia, USA
  • Z. A. Tochner - Radiation Oncology, University of Pennsylvania, Philadelphia, 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. Doc09ptcog002

DOI: 10.3205/09ptcog002, URN: urn:nbn:de:0183-09ptcog0029

Veröffentlicht: 24. September 2009

© 2009 Ainsley et al.
Dieser Artikel ist ein Open Access-Artikel und steht unter den Creative Commons Lizenzbedingungen (http://creativecommons.org/licenses/by-nc-nd/3.0/deed.de). Er darf vervielfältigt, verbreitet und öffentlich zugänglich gemacht werden, vorausgesetzt dass Autor und Quelle genannt werden.


Gliederung

Text

Background: The proton facility under construction at the University of Pennsylvania (Penn) has installed a multileaf collimator (MLC) in the first gantry room. The purpose of the MLC is to shape the dose transverse to the beam direction thus replacing custom-machined apertures and the need to change snouts. The MLC is the result of a development agreement between Penn, Varian Medical Systems, Inc. (VMS), and Ion Beam Applications S.A. (IBA). Penn defined the clinical specifications, performed Monte Carlo simulations to aid the design, and made the measurements to validate the system. VMS engineers were responsible for the design and manufacture of the MLC including the electronics and software control. IBA engineers interfaced their nozzle and control system to the MLC. The work described here focuses on the Penn contribution, which was supported by DOD grant W81XWH-04-2-0022.

Material and methods: Using the GEANT4. (version 9.1) Monte Carlo toolkit we constructed a model of the IBA beam delivery nozzle, including multiple variations of the material in the final 50 cm of the nozzle (shielding structures and the MLC itself). The design goals were to minimize the leakage of extraneous radiation (protons, neutrons, and gammas) while retaining a clinically useful system and adhering to the constraint on total weight. One challenge came from the need that, unlike snouts, MLCs must rotate ±90° to be effective. Measurements were performed on the final system using a variety of detectors (ion chambers, film, and neutron detectors) to quantify the leakage and the dose distribution properties for doubled-scattered and uniform-scanning beams.

Results: Our final MLC design consists of two banks of fifty 4.4 mm wide leaves made from a tungsten alloy. Steps in the side and the front of the leaves were used to prevent proton leakage through the gaps between adjacent leaves in the same bank and through closed leaf-pairs, respectively. Like the IBA snout system, the MLC can translate by 45 cm; at its most extended position the distal side of the leaves are 10 cm from isocenter. The leaves are angled in the direction perpendicular to the travel direction making them divergent for a nominal value of source position and translation. Multiple shielding structures, mostly related to the need for the MLC to rotate, were added to reduce leakage. Under the worst conditions using the highest energy and largest field, the measured leakage dose through the closed MLC was <1% for both proton and neutron leakage. The proton leakage was localized to small areas while the neutron dose covered a larger region. Lateral profiles of open fields show small, clinically insignificant, scattering from the edge of the collimator under some conditions.

Conclusion: A multileaf collimator has been designed and installed on a proton gantry. Measurements of leakage and the effect on dose distributions are at levels similar to MLCs used in conventional radiotherapy.