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

Gaussian beam sigma and field number effects ondose conformity in intensity modulated proton therapy

Meeting Abstract

  • J. Farr - Westdeutsches Protonentherapiezentrum, Essen
  • D. Geismar - Westdeutsches Protonentherapiezentrum, Essen
  • A. Kaiser - Westdeutsches Protonentherapiezentrum, Essen
  • M. Stuschke - Strahlenklinik, Universitätsklinikum, Essen

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog059

doi: 10.3205/09ptcog059, urn:nbn:de:0183-09ptcog0592

Published: September 24, 2009

© 2009 Farr et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Purpose: This study considers the effects of varying proton beam dimensions (beam s using the Gaussian profile) for intensity modulated proton therapy on a series of geometric and clinical cases. In addition, the number of beams used for each beam s was also varied as a sensitivity parameter.

Method and materials: A 3–12 mm range of beam σ in air over the range 150–230 MeV were modeled in a pre-clinical treatment planning system with Monte Carlo (MC) dose calculation capability. To investigate the sensitivities of treatment quality from the beam σ a series of treatment plans of virtual geometric phantoms and example clinical cases were generated using 3, 6, 9, and 12 mm σ. The phantoms/cases were paired as 1) base of skull (BOS) 2) prostate and 3) lung. Following optimization and MC dose calculation, the dose volume histograms (DVH’s) were scored for: 95% volume dose (D95), 10% volume dose (D10), and the organ at risk (OAR) dose at 50% volume (DOR50). The following metric was used to grade the plans: Target Conformity = (D10-D95)/(D10-D95)3mm,OAR Hit = DOR50/DOR503mm, Quality Index (QI) = Target Conformality/OAR Hit. Lower QI scores (1–2) indicate potentially superior plans, although these were also reviewed from a clinical perspective.

Results: The results from the QI scores are self-consistent between all the paired phantom/clinical cases. The BOS and prostate cases/phantoms QI’s indicated a significant sensitivity to beam size, whereas the lung case/phantom did not. For the prostate case/phantom a low QI was only achieved by the 3 and 6 mm σ beams. In the case of the BOS phantom a QI decrease was observed by adding additional beams. For example, the combinations of 3 mm 1 field, 3 mm 2 fields, 6 mm 3 fields and 9 mm 5 fields all scored QI of unity. For the clinical BOS case, the combinations of 3mm 2 field, 3mm 3 fields, 3 mm 5 fields, 6mm 3 fields and 5 fields all scored QI of unity.

Conclusion: This study indicates that the treatment quality sensitivity to beam s is low for targets not immediately proximal to critical structures. For higher definition targets proximal to an OAR’s the study indicated that beam σ’s of ideally 3–6 mm, but possibly up to 9 mm may provide acceptable results depending on the number of beams used, the physical separation of the target and OAR, as well as the dose differential required between them. Lastly, in some cases where the addition of more fields for larger beam s is possible, comparable results in relation to fewer fields of smaller σ may be achieved.