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

Dose deposited in the beam halo by a scanned protonpencil beam

Meeting Abstract

  • B. Clasie - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • H. Kooy - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • M. Fransen - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • N. Depauw - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, USA
  • J. Flanz - Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, 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. Doc09ptcog043

DOI: 10.3205/09ptcog043, URN: urn:nbn:de:0183-09ptcog0438

Veröffentlicht: 24. September 2009

© 2009 Clasie 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: Long-range secondary particles (neutrons and photons) and primary protons scattered through large angles contribute to the halo surrounding proton pencil beams. Is it desirable to include the dose deposited by these particles in the lateral direction in a pencil-beam dose algorithm. One way to model this is by a broad Gaussian distribution [see for example Pedroni et al. 2005]. This model is compared to measurements and used to determine the effect in absolute dose due to the beam halo in pencil beam scanning and correct Bragg peaks in a database for treatment planning.

Materials and methods: Measurements use a Wellhofer MatriXX detector (a 2D array of ionization chambers) for measurements of lateral dose distributions, a PTW Bragg peak chamber (a large parallel plate ionization chamber) for measurements of relative depth-dose distributions, and a PTW Markus chamber for measurements of absolute dose. This work uses the Monte Carlo simulation GEANT version 4.8.1.p01 for comparison to measurements of dose deposited in the center of fields with the beam scanned in concentric circles.

Results: While the dose in the halo is small compared to the primary dose deposited by the protons, it becomes significant when many pencil beams are delivered in a small region due to overlapping of the broad Gaussian functions, the sum of which can be 5% of the prescribed dose in fields delivered by pencil beam scanning. Without adequate modeling of the beam halo, treatment plans of homogenous irradiations can give rise to measurements that have a slope in the longitudinal direction and an overall difference compared to the absolute dose predicted. The beam halo affects measurements of depth-dose distributions that use large parallel plate ionization chambers. In these measurements, a narrow beam is centered on the chamber and the chamber is scanned in depth. Ideally, the sensitive volume intercepts the entire beam; however the integrated dose outside of an 8.4 cm diameter sensitive volume can be large (up to 8% of the total signal).

Conclusion: A Monte Carlo simulation is validated with measurements of the beam halo from a scanned proton beam and used in the development and testing of our treatment planning model. Results of the model compared to measurements will be reported.