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

Influence of density variations on the proton range in therapy of ocular tumors

Meeting Abstract

  • J. Heufelder - BerlinProtonen am Helmholtz Zentrum Berlin, Charité – Universitätsmedizin Berlin, Berlin
  • A. Weber - BerlinProtonen am Helmholtz Zentrum Berlin, Charité – Universitätsmedizin Berlin, Berlin
  • D. Cordini - BerlinProtonen am Helmholtz Zentrum Berlin, Charité – Universitätsmedizin Berlin, Berlin
  • R. Stark - BerlinProtonen am Helmholtz Zentrum Berlin, Charité – Universitätsmedizin Berlin, Berlin

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog088

DOI: 10.3205/09ptcog088, URN: urn:nbn:de:0183-09ptcog0887

Published: September 24, 2009

© 2009 Heufelder et al.
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

Text

Background: Dose calculations for the proton therapy of ocular tumors are generally based on an eye model with a constant density. The calculations are thus reduced primarily to a simple geometrical range tracking algorithm, looking for the maximum range in a homogeneous eye model. As the clinical results obtained worldwide show, for nearly all patients with ocular tumors this method has proved to be adequate. Here however we present two cases with density variations within the eye: a retinoblastoma and another with medical silicone oil within the vitreous body. Converting the calculated geometrical range into a range shifter setting under the assumption of a constant density would in both cases lead to an incorrect proton range.

Material and methods: For the retinoblastoma, an eye model was reconstructed from the MRI data in OCTOPUS. The tumor was delineated using MRI and a treatment plan generated. A simplified eye model was generated in MCNPX. Initially, all structures (tumor, vitreous body, etc.) were set to the same density and stopping power. The incident proton energy was estimated to match the range calculated by OCTOPUS. From an existing CT of a retinoblastoma patient density and stopping power were derived, and assigned to the tumor within the MCNPX simulation. Now the simulation with the same incident proton energy but different tumor parameters was performed again leading to a more realistic proton range.

The silicone oil case was treated similarly. A treatment plan was calculated in OCTOPUS on the basis of a CT. Here the tumor was reconstructed from ophthalmological informations. Similar MCNPX simulations were performed, where by the stopping power and density from the tumor were kept constant, but those for the vitreous body being replaced by silicone oil in the patient's eye. Stopping power values were derived from the oil's material composition. The density was taken from the supplier's information and from the patient's CT by CT-number to density conversion. The differences in proton range were measured. To check the simulations, additional depth dose measurements in a water phantom with a pre-absorber phantom containing water as well as silicon oil were performed.

Results: The calcified retinoblastoma was treated like cortical bone with a density of 1.4 g/cm3;, according to the CT-numbers. With the given tumor size and shape, this led to a range reduction of about 2 mm.

With medical silicone oil the measured CT-number of 100 HU would lead to the assumption of a density of about 1.06 g/cm3; based on the CT calibration curve, resulting in a 2 mm proton range reduction. However the density of the silicone oil was 0.97 g/cm3;. The range was increased by 2 mm, being confirmed by the depth dose measurements.

Conclusions: In cases with non-standard density variations, the proton range may be significantly affected, necessitating an adaption of the treatment plan.