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

Impact of the beam angle configuration on the quality and the robustness of a particle therapy plan

Meeting Abstract

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  • G. Cabal - Department of Radiation Oncology, University Hospital of Heidelberg, Heidelberg, Germany
  • S. Luan - Department of Computer Science, University of New Mexico, Albuquerque, USA
  • O. Jäkel - Heidelberg Ion Beam Therapy Center, University Hospital of Heidelberg, Heidelberg, Germany

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog032

DOI: 10.3205/09ptcog032, URN: urn:nbn:de:0183-09ptcog0322

Veröffentlicht: 24. September 2009

© 2009 Cabal 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: Particle therapy is well known for its capability to produce highly conformal plans with very good sparing of normal tissues and low integral doses. In this work we study the impact of beam angle selection on the quality and the robustness of a plan. We considered: (1) uncertainties in the CT to range calibration (2) set-up errors and (3) impact of beam angle selection on the plan quality without taking uncertainties into account. The final goal is to derive criteria which allow for an automated choice of treatment angles.

Materials: For our study we used an in house treatment planning model system designed for 2D studies. The patient model consisted of one CT slice with a skull base tumor and three critical structures: brain stem, optic nerves and eyes. The dose calculation algorithm and the optimization engine were based on published work [1], [2]. The sources of uncertainties taken into account were set-up errors and CT to range calibration uncertainties taken from published work [3], [4]. We studied the impact of these uncertainties in a single fraction and in a 20-fraction treatment plan. To quantify the robustness of a plan we used the fractions of the PTV volume receiving 95% of the prescribed dose or higher and the maximal and mean dose for the organs at risk. Plans with different angle configurations were optimized using the same criteria. The optimized plans were then subject to a patient set-up error and CT-to-Range calibration uncertainties.

Results: All plans were evaluated using DVH analysis and isodose comparison. The optimized nominal plans all showed good target coverage and no violation of the dose constraint for the OAR. We observed the following: (1) Even without uncertainties taken into account, beam angle selection significantly influences the overall quality of the plan, especially for the OAR: the volumes receiving medium and low doses can change considerably within just few degrees of beam angles. (2) The impact of set up errors in the plan was seen to be also dependent on the angle configuration selected. (3)In the case of the CT-to-Range calibration uncertainties, we observed a significant impact on the quality of the final plan for brain cases. (4)The effects of set-up errors reduce if one takes into account a multiple fraction treatment. The effects however are still significant.

Conclusions: Beam angle selection has a considerable impact on both quality and robustness of 2D treatment plans. Further research will be focused on: (1) the development of an algorithm for beam angle selection to optimize for both quality and robustness of a particle therapy treatment plan, (2) the impact of CT uncertainties for other anatomical sites and (3) extending the study to a 3D case.


References

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Bortfeld T, et al. An analytical approximation of the Bragg curve for therapeutic proton beams. Med Phys. 1997;24(12):2024-33.
2.
Lomax A. Intensity modulation methods for proton radiotherapy. Phys Med Biol. 1999;44:185-205.
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Jäkel O, et al. Relation between carbon ion ranges and x-ray CT numbers. Med Phys. 2001;28(4):701-3.
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Lomax A. Intensity modulated proton therapy and its sensitivity to treatment uncertainties 2: the potential effects of inter-fraction and inter-field motions. Phys Med Biol. 2008;53:1043-56.